Human Impact on Global Climate

CHAPTER OVERVIEW 24: Human Impact on Global Climate Learning Objectives Describe how humans have changed greenhouse gas concentrations in the Earth's atmosphere and how scientists have identified this as the cause of recent changes in climate. Discuss how humans have changed Earth's energy balance through changes in albedo. Characterize feedbacks to climate change from changes in temperature, carbon cycling, and albedo. Explain predictions for future changes in temperature, precipitation, storm events, ice cover and permafrost. Define ocean acidification and how it will change with changes in atmospheric carbon dioxide concentrations. Describe changes in plant productivity, species interactions, and diseases that may occur with changes in climate. 24.1: Anthropogenic Climate Change 24.2: Implications of Climate Change Summary This chapter discusses recent changed in climate caused by human activity and how scientists have attributed those changes to increases in greenhouse gas concentrations in the atmosphere from human activities. Section 24.1 also describes changes in the earths reflectivity (albedo) due to humans and feedbacks to global climate. Section 24.2 characterizes future climate changes expected due to the changes in greenhouse gas concentrations in the atmosphere including changes in temperature, precipitation, storm events, ice cover and permafrost, sea level, ocean acidification, plant productivity, species interactions, and disease prevalence. For more information on Earth's energy balance, atmospheric and oceanic circulation, natural climate forcings, and past climate change, see the chapter on "The Physical Environment". 24: Human Impact on Global Climate is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.

When we talk about anthropogenic climate change, we are generally thinking of the industrial era, which really got going when we started using fossil fuels (coal to begin with) to drive machinery and trains. That was around the middle of the 18th century. The issue with fossil fuels is that they involve burning carbon that was naturally stored in the crust over hundreds of millions of years. Some climate scientists argue that anthropogenic climate change actually goes back much further than the industrial era, and that humans began to impact the climate by clearing land to grow grains in Europe and the Middle East around 8,000 years BCE and by creating wetlands to grow rice in Asia around 5,000 years BCE. Clearing forests for crops is a type of climate-forcing because the CO2 storage capacity of the crops is generally lower than that of the trees they replace, and creating wetlands is a type of climate forcing because the anaerobic bacterial decay of organic matter within wetlands produces CH4.

In fact, whether anthropogenic climate change started with the agricultural revolution or the industrial revolution is not important,

because the really significant climate changes didn't start until the early part of the 20th century, and although our activities are a

major part of the problem, our increasing numbers are a big issue as well. Figure

from around 5 million, when we first started growing crops, to about 18 million when wetland rice cultivation began, to over 800

million at the start of the industrial revolution, to over 7,700 million in 2019. A big part of the incredible growth in our population

is related to the availability of the cheap and abundant energy embodied in fossil fuels, which we use for transportation, heating and

: World population growth over the past 12,000 years. Based on estimates by the History Database of the Global

Environment (HYDE) and the United Nations. This visualization is from OurWorldinData.org and is liscensed under CC-BY-SA by

A rapidly rising population, the escalating level of industrialization and mechanization of our lives, and an increasing dependence

on fossil fuels for transportation and energy generation have driven the anthropogenic climate change of the past century. The trend

of mean global temperatures since 1880 is shown in Figure

. For approximately the past 55 years, the temperature has

increased at a relatively steady and rapid rate, especially compared to past changes. The average temperature now is approximately

1.0°C higher than before industrialization, and two-thirds of this warming has occurred since 1975.

: Global mean annual temperatures for the period from 1880 to 2019. The value for 2019 is projected, based on only 6

months of data. 1998 and 2016 were strong El Niño years. © Steven Earle. CC BY. Based on data from NASA.

Changes in land surface or ocean surface temperatures can be expressed as temperature anomalies. A temperature anomaly is the

difference in average temperature measurement from a predetermined datum (or baseline). This datum is the average temperature

of a particular date range, for example, 1951 to 1980. Another common datum is the last century (1900-2000). Therefore, an

anomaly of 1.25 for 2015 (last century datum) means that the average temperature for 2015 was 1.25 greater than the 1900-

2000 average. In 1950, the temperature anomaly was -0.28 , so this is -0.28 lower than the 1900-2000 average (Lindsey,

2009). These temperatures are annual average surface temperatures. Figure

shows the global mean temperature estimates for

the period of 1880 to 2020 using the datum of the 1951-1980 average temperature.

: Land-ocean temperature index, 1880 to 2020, with a base time 1951-1980. The solid black line is the global annual

mean and the solid red line is the five-year lowess smooth. The blue uncertainty bars (95% confidence limit) account only for

[From https://berkeleyearth.org/global-temperature-report-for-2020/]

Since 1880, average global surface temperatures have trended upward and most of that warming has occurred since 1970 (see this NASA animation). Since the ocean is absorbing a lot of the additional trapped heat, surface temperatures include both land surface and ocean temperatures (Hansen et ak., 2011). This video figure also shows worldwide temperature changes since 1880. The more blue, the cooler; the more yellow and red, the warmer.

Scientific Consensus: Global Climate Change is Real The Intergovernmental Panel on Climate Change (IPCC)--established by the United Nations Environment Programme and the World Meteorological Organization in 1988--is responsible for reviewing the scientific literature on climate change and issuing periodic reports on several topics, including the scientific basis for understanding climate change, our vulnerability to observed and predicted climate changes, and what we can do to limit climate change and minimize its impacts. The IPCC uses this information to evaluate current impacts and future risks, in addition to providing policymakers with assessments. These assessments are released about once every every six years. The most recent report, the 6th Assessment, was released in 2023. Hundreds of leading scientists from around the world are chosen to author these reports. Over the history of the IPCC, these scientists have reviewed thousands of peer-reviewed, publicly available studies. The scientific consensus according to the last IPCC assessment is clear: global climate change is real and humans are very likely the cause for this change. Additionally, the major scientific agencies of the United States, including the National Aeronautics and Space Administration (NASA) and the National Oceanic and Atmospheric Administration (NOAA), also agree that climate change is occurring and that humans are driving it. In 2010, the US National Research Council concluded that "Climate change is occurring, is very likely caused by human activities, and poses significant risks for a broad range of human and natural systems". Many independent scientific organizations have released similar statements, both in the United States and abroad. This doesn't necessarily mean that every scientist sees eye to eye on each component of the climate change problem, but broad agreement exists that climate change is happening and is primarily caused by excess greenhouse gases from human activities. The scientific consensus is clear: through alterations of the carbon cycle, humans are changing the global climate by increasing the effects of something known as the greenhouse effect.

Comparing modeling results to the historical climate record shows that, in general, climate changes prior to the Industrial

Revolution in the 1700s can be explained by natural causes, such as changes in solar energy, volcanic eruptions, and natural

changes in greenhouse gas (GHG) concentrations (Figure

). However, recent changes in climate, especially warming since the

mid-20th century, cannot be explained by natural causes alone and models need to include the effects of human activities,

especially our combustion of fossil fuels, to explain the current warming (Figure

: This graph shows the predicted temperatures from two climate models and observed temperatures from 1880 to

2020. The first model considered only natural factors that could influence temperature and is represented by the green line

(bottom). It shows some fluctuations in temperature by no overall increase or decrease. The second model considered both human

and natural factors and is represented by the orange line (top). It shows an overall increase in temperature. Actual observations

(black, jagged line; middle) more closely match the second model. Overall, temperature has increased about 1.2 degrees Celsius

(2.1 degrees Fahrenheit) since pre-industrial times. Image relabeled from Efbrazil (CC-BY-SA).

Human Impact on Greenhouse Gases Recall that greenhouse gases are probably the most significant drivers of the climate. The greenhouse gases that affect Earth include carbon dioxide, methane, water vapor, nitrous oxide, and ozone. Approximately half of the radiation from the sun passes through these gases in the atmosphere and strikes the Earth. This radiation is converted into thermal radiation on the Earth's surface, and then a portion of that energy is re-radiated back into the atmosphere as thermal energy. Greenhouse gases absorb much of the thermal energy near the Earth's surface so the more greenhouse gases there are in the atmosphere, the more thermal energy is retained by the Earth. Water vapor is the most abundant greenhouse gas and also the most important in terms of its contribution to the natural greenhouse effect, despite having a short atmospheric lifetime. Some human activities can influence local water vapor levels. However, on a global scale, the concentration of water vapor is controlled by temperature, which influences overall rates of evaporation and precipitation. Therefore, the global concentration of water vapor is not substantially affected by direct human emissions.

: The relative climate-warming effects of the various greenhouse gases. (Effects listed in red are all related to fossil

© Steven Earle. CC BY. Based on data in IPCC AR 5, 2014.

, which is based on data from the fifth assessment report of the IPCC, issued in 2014, shows the relative contributions

of various long-lasting anthropogenic GHGs to current climate forcing, based on the changes from levels that existed in 1750.

Human activity releases carbon dioxide and methane, two of the most important greenhouse gases, into the atmosphere in several

ways. The biggest anthropogenic contributor to warming is CO2, which accounts for 56% of positive forcing. The primary

mechanism that releases carbon dioxide is the burning of fossil fuels, such as gasoline, coal, and natural gas (Figure

accounts for 32%, and the halocarbon gases (mostly leaked from older air-conditioning appliances that still contain CFCs) and

nitrous oxide (N2O) (from burning fossils fuels) account for 6% each. CO2 emissions come mostly from coal- and gas-fired power stations, motorized vehicles (cars, trucks, and aircraft), and industrial operations (e.g., smelting). CH4 emissions come from

production of fossil fuels (escape from coal mining and from gas and oil production and processing), livestock farming (mostly

beef), landfills, waste water, and wetland rice farming. N2O is derived almost entirely from the combustion of fossil fuels. In

summary, most (by far) of our current GHG emissions come from fossil fuel production and use.

: The burning of fossil fuels in industry and by vehicles releases carbon dioxide and other greenhouse gases into the

atmosphere (credit: "Smoke Plume" by Pöllö is licensed under CC BY 3.0).

Carbon dioxide (CO2) is the primary greenhouse gas that is contributing to recent global climate change. CO2 is a natural component of the carbon cycle, involved in such activities as photosynthesis, respiration, volcanic eruptions, and ocean-atmosphere exchange. Human activities, primarily the burning of fossil fuels and changes in land use, release very large amounts of CO2 to the atmosphere, causing its concentration in the atmosphere to rise. Deforestation, cement manufacture, animal agriculture, the clearing of land, and the burning of forests are other human activities that release carbon dioxide.

Scientists look at patterns in data and try to explain differences or deviations from these patterns. The atmospheric carbon dioxide

data reveal a historical pattern of carbon dioxide increasing and decreasing, cycling between a low of 180 ppm and a high of 300

). Scientists have concluded that it took around 50,000 years for the atmospheric carbon dioxide level to increase

from its low minimum concentration to its higher maximum concentration.

: Graph of change in temperature in degrees Celsius (top, blue line) and carbon dioxide concentration in parts per

million by volume (bottom, green line) measured from the Vostok, Antarctica ice core. These have been associated for over

400,000 years. As carbon dioxide concentration increased, so did temperature. As carbon dioxide concentration decreased, so did

temperature. These data were collected in 1999. Since then, carbon dioxide concentrations have increased to 409.8 ppm (2019

average). Image and caption (modified) by NOAA/Autopilot (CC-BY-SA).

However, starting recently, atmospheric carbon dioxide concentrations have increased beyond the historical maximum of 300 ppm

). Atmospheric CO2 concentrations have increased by 45% since pre-industrial times, from approximately 280

parts per million (ppm) in the 18th century to 409.8 ppm in 2019 (Figures

: Monthly mean carbon dioxide concentration in parts per million (ppm) measured at Mauna Loa Observatory,

Hawaii. The carbon dioxide data on Mauna Loa constitute the longest record of direct measurements of CO2 in the atmosphere. The fluctuating red line represents the monthly mean values, centered on the middle of each month. Carbon dioxide concentrations

dip every summer due to increased photosynthesis. The smoother black line represents the same, after correction for the average seasonal cycle. Image and caption (modified) by NOAA (public domain).

: This graph, based on the comparison of atmospheric samples contained in ice cores and more recent direct

measurements, provides evidence that atmospheric CO2 has increased since the Industrial Revolution. On the x-axis are the years

before today (0 = 1950). It begins 400,000 years prior to 1950. On the y-axis is the carbon dioxide level in parts per million.

Carbon dioxide levels have fluctuated over the years, but they never exceeded 300 parts per million until 1950. In 2018, carbon

dioxide levels reached 409.8 ppm. This graph is a few years old and shows the current CO2 level at 400 ppm; Credit: Vostok ice

core data/J.R. Petit et al.; NOAA Mauna Loa CO2 record.

The current CO2 level is higher than it has been in at least 800,000 years, based on evidence from ice cores that preserve ancient atmospheric gases. The current increases in atmospheric carbon dioxide have happened very quickly--in a matter of hundreds of years rather than thousands of years. What is the reason for this difference in the rate of change and the amount of increase in carbon dioxide? A key factor that must be recognized when comparing the historical data and the current data is the presence of modern human society; no other driver of climate change has yielded changes in atmospheric carbon dioxide levels at this rate or to this magnitude. Human activities currently release over 30 billion tons of CO2 into the atmosphere every year. While some volcanic eruptions released large quantities of CO2 in the distant past, the U.S. Geological Survey (USGS) reports that human activities now emit more than 135 times as much CO2 as volcanoes each year. This human-caused build-up of CO2 in the atmosphere is like a tub filling with water, where more water flows from the faucet than the drain can take away.

The following video shows how atmospheric CO2 has varied recently and also over the last 800,000 years as determined by many CO2 monitoring stations (shown on the insert map). It is also instructive to watch the CO2 variation of the Keeling portion of the video by latitude. This shows that most of the human sources of CO2 are in the Northern Hemisphere.

Methane Although this concentration is far less than that of CO2, methane (CH4) is 28 times as potent a greenhouse gas. Methane is produced when bacteria break down organic matter under anaerobic conditions and can be released due to natural or anthropogenic processes. Anaerobic conditions can happen when organic matter is trapped underwater (such as in rice paddies) or in the intestines of herbivores. Anthropogenic causes now account for 60% of total methane release. Examples include agriculture, fossil fuel extraction and transport, mining, landfill use, and burning of forests. Methane can also be released from natural gas fields and the decomposition that occurs in landfills. Specifically, raising cattle releases methane due to fermentation in their rumens produces methane that is expelled from their GI tract. Methane is more abundant in Earth's atmosphere now than at any time in at least the past 650,000 years, and CH4 concentrations increased sharply during most of the 20th century. They are now more than two and-ahalf times pre-industrial levels (1.9 ppm), but the rate of increase has slowed considerably in recent decades. Another source of methane is the melting of clathrates. Clathrates are frozen chunks of ice and methane found at the bottom of the ocean. When water warms, these chunks of ice melt and methane is released. As the ocean's water temperature increases, the rate at which clathrates melt is increasing, releasing even more methane. This leads to increased levels of methane in the atmosphere, which further accelerates the rate of global warming. This is an example of the positive feedback loop that is leading to the rapid rate of increase of global temperatures.

Other Greenhouse Gases Nitrous oxide (N2O) is produced through natural and human activities, mainly through agricultural activities and natural biological processes. Fuel burning and some other processes also create N2O. Concentrations of N2O have risen approximately 18% since the start of the Industrial Revolution, with a relatively rapid increase towards the end of the 20th century. [3] In contrast, the atmospheric concentration of N2O varied only slightly for a period of 11,500 years before the onset of the industrial period, as shown by ice core samples. Ground-level ozone (O3), which also has a short atmospheric lifetime, is a potent greenhouse gas. Chemical reactions create ozone from emissions of nitrogen oxides and volatile organic compounds from automobiles, power plants, and other industrial and commercial sources in the presence of sunlight. In addition to trapping heat, ozone is a pollutant that can cause respiratory health problems and damage crops and ecosystems. Chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), together called F-gases, are often used in coolants, foaming agents, fire extinguishers, solvents, pesticides, and aerosol propellants. Unlike water vapor and ozone, these F-gases have a long atmospheric lifetime, and some of these emissions will affect the climate for many decades or centuries. Human Induced Changes in Albedo When sunlight energy reaches Earth it can be reflected or absorbed. The amount that is reflected or absorbed depends on Earth's surface and atmosphere. Light-colored objects and surfaces, like snow and clouds, tend to reflect most sunlight, while darker objects and surfaces, like the ocean and forests, tend to absorb more sunlight. The term albedo refers to the amount of solar radiation reflected from an object or surface, often expressed as a percentage. Earth as a whole has an albedo of about 30%, meaning that 70% of the sunlight that reaches the planet is absorbed. Sunlight that is absorbed warms Earth's land, water, and atmosphere. Albedo is also affected by aerosols. Aerosols are small particles or liquid droplets in the atmosphere that can absorb or reflect sunlight. Unlike greenhouse gases (GHGs), the climate effects of aerosols vary depending on what they are made of and where they are emitted. Those aerosols that reflect sunlight, such as particles from volcanic eruptions or sulfur emissions from burning coal, have a cooling effect. Those that absorb sunlight, such as black carbon (a part of soot), have a warming effect. In addition, human activities have generally increased the number of aerosol particles in the atmosphere. Overall, human-generated aerosols have a net cooling effect offsetting about one-third of the total warming effect associated with human greenhouse gas emissions. Reductions in overall aerosol emissions can therefore lead to more warming. However, targeted reductions in black carbon emissions can reduce warming. Human changes in land use and land cover have changed Earth's albedo. Processes such as deforestation, reforestation, desertification, and urbanization often contribute to changes in climate in the places they occur. These effects may be significant regionally, but are smaller when averaged over the entire globe. Climate Feedbacks When sea ice melts, as it has done in the Arctic Ocean at a disturbing rate over the past decade, the albedo of the area affected changes dramatically, from around 80% down to less than 10%. This is a positive feedback because much more solar energy is absorbed by the water than by the pre-existing ice, and the temperature increase is amplified. The same applies to ice and snow on land, but the difference in albedo is not as great. When ice and snow on land melt, sea level rises. Sea level is also rising because the oceans are warming and that increases their volume. A higher sea level means a larger proportion of the planet is covered with water, and since water has a lower albedo than land, more heat is absorbed and the temperature goes up a little more. Since the last glaciation, sea-level rise has been about 125 m; a huge area that used to be land is now flooded by heat-absorbent seawater. During the current period of anthropogenic climate change, sea level has risen only about 20 cm, and although that doesn't make a big change to albedo, sea-level rise is accelerating.

: A degrading permafrost site on the north coast of Alaska.

Most of northern Canada has a layer of permafrost that ranges from a few centimeters to hundreds of meters in thickness; the same

applies in Alaska, Russia, and Scandinavia. Permafrost is a mixture of soil and ice (Figure

amount of trapped organic carbon that is released as CO2 and CH4 when the permafrost breaks down. Because the amount of

carbon stored in permafrost is in the same order of magnitude as the amount released by burning fossil fuels, this is a feedback

mechanism that has the potential to equal or surpass the forcing that has unleashed it.

In some polar regions, including northern Canada, permafrost includes methane hydrate, a highly concentrated form of CH4 trapped in solid form. Breakdown of permafrost releases this CH4. Even larger reserves of methane hydrate exist on the sea floor, and while it would take significant warming of ocean water down to a depth of hundreds of meters, this too is likely to happen in the future if we don't limit our impact on the climate. There is strong isotopic evidence that the Paleocene-Eocene thermal maximum was caused, at least in part, by a massive release of sea-floor methane hydrate. There is about 45 times as much carbon in the ocean (as dissolved bicarbonate ions, HCO3-) as there is in the atmosphere (as CO2), and there is a steady exchange of carbon between the two reservoirs. But the solubility of CO2 in water decreases as the temperature goes up. In other words, the warmer it gets, the more of that oceanic bicarbonate gets transferred to the atmosphere as CO2. That makes CO2 solubility another positive feedback mechanism.

Vegetation growth responds positively to both increased temperatures and elevated CO2 levels, and so in general, it represents a negative feedback to climate change because the more the vegetation grows, the more CO2 is taken from the atmosphere. But it's not quite that simple because when trees grow bigger and more vigorously, forests become darker (they have lower albedo) so they absorb more heat. Furthermore, climate warming isn't necessarily good for vegetation growth; some areas have become too hot, too dry, or even too wet to support the plant community that was growing there, and it might take centuries for something to replace it successfully.

All of these positive (and negative) feedbacks work both ways. For example, during climate cooling, growth of glaciers leads to higher albedos, and formation of permafrost results in storage of carbon that would otherwise have returned quickly to the atmosphere.

References Callendar, G. S. (1938). The artificial production of carbon dioxide and its influence on temperature. Quarterly Journal of the Royal Meteorological Society, 64(275), 223-240. https://doi.org/10.1002/qj.49706427503 Earle, S. (2015). Physical geology (OER textbook). BCcampus OpenEd. Easterling, D. R., & Wehner, M. F. (2009). Is the climate warming or cooling? Geophysical Research Letters, 36, L08706. https://doi.org/10.1029/2009GL037810 Foster, G., & Rahmstorf, S. (2011). Global temperature evolution 1979-2010. Environmental Research Letters, 6(4), 044022. https://doi.org/10.1088/1748-9326/6/4/044022 Hansen, J., Sato, M., Kharecha, P., et al. (2011). Earth's energy imbalance and implications. Atmospheric Chemistry and Physics, 11, 13421-13449. https://doi.org/10.5194/acp-11-13421-2011 International Panel on Climate Change (IPCC). (2014). Climate change 2014: Synthesis report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. IPCC. Karl, T. R., & Knight, R. W. (1998). Secular trends of precipitation amount, frequency, and intensity in the United States. Bulletin of the American Meteorological Society, 79(2), 231-241. https://doi.org/10.1175/1520-0477(1998)079<0231:STOPAF>2.0.CO;2 Kosaka, Y., & Xie, S.-P. (2013). Recent global-warming hiatus tied to equatorial Pacific surface cooling. Nature, 501(7467), 403- 407. https://doi.org/10.1038/nature12534 Lindsey, R. (2009). Climate and Earth's energy budget. NASA Earth Observatory. http://earthobservatory.nasa.gov (Accessed September 14, 2016) Oreskes, N. (2004). The scientific consensus on climate change. Science, 306(5702), 1686. https://doi.org/10.1126/science.1103618 Santer, B. D., et al. (2007). Identification of human-induced changes in atmospheric moisture content. Proceedings of the National Academy of Sciences, 104(39), 15248-15253. https://doi.org/10.1073/pnas.0702872104 Zemp, M., Frey, H., Gärtner-Roer, I., et al. (2015). Historically unprecedented global glacier decline in the early 21st century. Journal of Glaciology, 61(228), 745-762. https://doi.org/10.3189/2015JoG15J017 Further "Reading" For more information, watch this six-minute video on climate change by two professors at a North Carolina State University.

Contributors and Attributions Modified by Kyle Whittinghill (University of Vermont) from the following sources: Connie Rye (East Mississippi Community College), Robert Wise (University of Wisconsin, Oshkosh), Vladimir Jurukovski (Suffolk County Community College), Jean DeSaix (University of North Carolina at Chapel Hill), Jung Choi (Georgia Institute of Technology), Yael Avissar (Rhode Island College) among other contributing authors. Original content by OpenStax (CC BY 4.0; Download for free at http://cnx.org/contents/185cbf87-c72...f21b5eabd@9.87). Kyle Whittinghill (University of Pittsburgh) "Physical Geology" by Steven Earle used under a CC-BY 4.0 international license. Download this book for free at http://open.bccampus.ca Contributed by Melissa Ha and Rachel Schleiger Faculty (Biological Sciences) at Yuba College & Butte College Contributed by Paul Webb Professor (Biology) at Rodger Williams University Contributed by Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher Faculty (Geology) at Salt Lake Community College Sourced from OpenGeology from An Introduction to Geology Chapter Chapter 15: Global Climate Change 12.4 Climate Change from Essentials of Environmental Science by CK-12 and Kamala Dorsner (licensed under CC BY 4.0.) Climate and the Effects of Global Climate Change from General Biology by OpenStax (licensed under CC-BY) Climate Change from Environmental Biology by Matthew R. Fisher (licensed under CC-BY) Carbon Cycle from Biology by John W. Kimball (licensed under CC-BY) 24.1: Anthropogenic Climate Change is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts. 10.4: Climate Change by Matthew R. Fisher is licensed CC BY 4.0. Original source: https://openoregon.pressbooks.pub/envirobiology. 44.5: Climate and the Effects of Global Climate Change by OpenStax is licensed CC BY 4.0.

24.2: Implications of Climate Change Past and Present-day GHG Emissions Will Affect Climate Far into the Future Many greenhouse gases stay in the atmosphere for long periods of time. As a result, even if emissions stopped increasing, atmospheric greenhouse gas concentrations would continue to remain elevated for hundreds of years. Moreover, if we stabilized concentrations and the composition of today's atmosphere remained steady (which would require a dramatic reduction in current greenhouse gas emissions), surface air temperatures would continue to warm. This is because the oceans, which store heat, take many decades to fully respond to higher greenhouse gas concentrations. The ocean's response to higher greenhouse gas concentrations and higher temperatures will continue to impact climate over the next several decades to hundreds of years.

: Temperature increases have been most pronounced in northern latitudes and over land masses. The colors represent

the temperature difference between the 2011-2020 average and the 1951-1980 baseline, with warmer colors (yellow, orange, red)

representing increases, and cool colors (green, blue) representing decreases. The image uses longer term averages of at least a

decade to smooth out climate variability due to factors such as El Niño. Grey areas in the image have insufficient data for

rendering. Image and caption (modified) from NASA's Scientific Visualization Studio/Eric Fisk (public domain).

Future Temperature Changes Climate models project the following key temperature-related changes: Average global temperatures are expected to increase by 2°F to 11.5°F by 2100, depending on the level of future greenhouse gas emissions, and the outcomes from various climate models. By 2100, global average temperature is expected to warm at least twice as much as it has during the last 100 years. Ground-level air temperatures are expected to continue to warm more rapidly over land than oceans. Some parts of the world are projected to see larger temperature increases than the global average. These changes will impact our food supply, water resources, infrastructure, ecosystems, and even our own health. The magnitude and rate of future climate change will primarily depend on the following factors: The rate at which levels of greenhouse gas concentrations in our atmosphere continue to increase,

How strongly features of the climate (e.g., temperature, precipitation, and sea level) respond to the expected increase in greenhouse gas concentrations, Natural influences on climate (e.g., from volcanic activity and changes in the sun's intensity) and natural processes within the climate system (e.g., changes in ocean circulation patterns).

Patterns of precipitation and storm events, including both rain and snowfall are likely to change. However, some of these changes

are less certain than the changes associated with temperature. Because warm air is able to hold more water than cold air, the general

global trend over the past century has been one of increasing precipitation (Figure

: Global precipitation anomalies compared with the average over the period from 1901 to 2012.

["Precipitation Worldwide, 1901-2013" by NASA. Public domain.]

Projections show that future precipitation and storm changes will vary by season and region. Some regions may have less precipitation, some may have more precipitation, and some may have little or no change. The amount of rain falling in heavy precipitation events is likely to increase in most regions, while storm tracks are projected to shift towards the poles. Climate models project the following precipitation and storm changes:

Global average annual precipitation through the end of the century is expected to increase, although changes in the amount and intensity of precipitation will vary by region. The intensity of precipitation events will likely increase on average. This will be particularly pronounced in tropical and highlatitude regions, which are also expected to experience overall increases in precipitation. The strength of the winds associated with tropical storms is likely to increase. The amount of precipitation falling in tropical storms is also likely to increase. Annual average precipitation is projected to increase in some areas and decrease in others.

Occurrence and intensity of extreme weather events such as hurricanes, precipitation, and heatwaves are increasing (Lindsey, 2009; International Panel on Climate Change (IPCC), 2014). Since the 1980s, hurricanes, which are generated from warm ocean water, have increased in frequency, intensity, and duration and connections to a warmer climate are likely. Since 1910, average precipitation has increased by 10% in the contiguous United States, and much of this increase is associated with heavy precipitation events like storms (Karl & Knight, 1998). However, the distribution is not even and more precipitation is projected for the northern United States while less precipitation is projected for the already dry southwest (Lindsey, 2009). Further, heatwaves have increased and rising temperatures are already affecting crop yields in northern latitudes (International Panel on Climate Change (IPCC), 2014). Increased heat allows for greater moisture capacity in the atmosphere, increasing the potential for more extreme events (Santer et al., 2007).

One of the other risks for coastal populations, besides sea-level rise, is that climate warming is also associated with an increase in

the intensity of tropical storms (e.g., hurricanes or typhoons), which almost always bring serious flooding from intense rain and

storm surges. Some recent examples are New Orleans in 2005 with Hurricane Katrina, and New Jersey and New York in 2012 with

: Damage to the Casino Pier, Seaside Heights, New Jersey, from Hurricane Sandy, November 2012

["Hurricane Sandy New Jersey Pier" © Master Sgt. Mark C. Olsen/U.S. Air Force/New Jersey National Guard. CC BY.]

Tropical storms get their energy from the evaporation of warm seawater in tropical regions. In the Atlantic Ocean, this takes place

between 8° and 20° N in the summer. Figure

shows the variations in the sea-surface temperature (SST) of the tropical

Atlantic Ocean (in blue) versus the amount of power represented by Atlantic hurricanes between 1950 and 2008 (in red). Not only

has the overall intensity of Atlantic hurricanes increased with the warming since 1975, but the correlation between hurricanes and

sea-surface temperatures is very strong over that time period.

: Relationship between Atlantic tropical storm cumulative annual intensity and Atlantic sea-surface temperatures

["Atlantic sea-surface T vs. tropical storm power" © Steven Earle. CC BY.

Based on data from Papers, Data, and Graphics Pertaining to Tropical Cyclone Trends and Variability.]

For several weeks in July and August of 2010, a massive heat wave affected western Russia, especially the area southeast of

Moscow, and scientists have stated that climate change was a contributing factor. Temperatures soared to over 40°C, as much as

12°C above normal over a wide area, and wildfires raged in many parts of the country. Over 55,000 deaths are attributed to the heat

and to respiratory problems associated with the fires. A summary of the impacts of climate change on natural disasters is given in

. The major types of disasters related to climate are floods and storms, but the health implications of extreme

temperatures are also becoming a great concern. In the decade 1971 to 1980, extreme temperatures were the fifth most common

natural disasters; by 2001 to 2010, they were the third most common.

: Numbers of various types of disasters between 1971 and 2010 (WMO Atlas of Mortality and Economic Losses from

Weather, Climate and Water Extremes, 2014. CC BY-NC-ND).

We've all experienced the effects of climate change over the past decade. However, it's not straightforward for climatologists to

make the connection between a warming climate and specific weather events, and most are justifiably reluctant to ascribe any

specific event to climate change. In this respect, the best measures of climate change are those that we can detect over several

decades, such as the temperature changes shown in Figure

Future Ice, Snowpack, and Permafrost Arctic sea ice is already declining. The area of snow cover in the Northern Hemisphere has decreased since 1970. Permafrost temperature has increased over the last century, making it more susceptible to thawing. Over the next century, it is expected that sea ice will continue to decline, glaciers will continue to shrink, snow cover will continue to decrease, and permafrost will continue to thaw. For every 2°F of warming, models project about a 15% decrease in the extent of annually averaged sea ice and a 25% decrease in September Arctic sea ice. The coastal sections of the Greenland and Antarctic ice sheets are expected to continue to melt or slide into the ocean. If the rate of this ice melting increases in the 21st century, the ice sheets could add significantly to global sea level rise. Glaciers are expected to continue to decrease in size. The rate of melting is expected to continue to increase, which will contribute to sea level rise. Glaciers are ice on top of the land. Alpine glaciers, ice sheets, and sea ice are all melting. Almost all major alpine glaciers are shrinking, deflating, and retreating and the rate of ice mass loss is unprecedented (never observed before) since the 1940's when quality records for most began.Before anthropogenic warming, glacial activity was variable with some retreating and some

advancing (Zemp et al., 2015). The extent of spring snow cover has decreased. Satellites have recorded that Antarctica is melting at

118 gigatons per year and Greenland is melting at 281 gigatons per year (1 gigaton is over 2 trillion pounds) (Figure

: Decline of Antarctic ice mass from 2002 to 2016.

In addition, the extent of sea ice is shrinking. Sea ice is ice floating in the ocean (not on land like a glacier). Most sea ice is at the North Pole which is only occupied by the Arctic Ocean and sea ice (Lindsey, 2009; International Panel on Climate Change (IPCC), 2014). Below, the NOAA animation shows how perennial sea ice has declined from 1987 to 2015. The oldest ice is white and the youngest (seasonal) ice is dark blue. The amount of old ice has declined from 20% in 1985 to 3% in 2015.

Watch 25 Years of Arctic Sea Ice Disappear in 1 Mi Climate Central

This loss of ice is leading to increases in the global sea level. On average, the sea is rising at a rate of 1.8 mm per year. However, between 1993 and 2010 the rate of sea level increase ranged between 2.9 and 3.4 mm per year. A variety of factors affect the volume of water in the ocean, including the temperature of the water (the density of water is related to its temperature) and the amount of water found in rivers, lakes, glaciers, polar ice caps, and sea ice. As glaciers and polar ice caps melt, there is a significant contribution of liquid water that was previously frozen.

A number of global events have occurred that may be attributed to climate change during our lifetimes. Explore melting glaciers at

NASA's interactive Global Ice Viewer. Glacier National Park in Montana is undergoing the retreat of many of its glaciers, a

phenomenon known as glacier recession. In 1850, the area contained approximately 150 glaciers. By 2010, however, the park

contained only about 24 glaciers greater than 25 acres in size. One of these glaciers is the Grinnell Glacier (Figure

Gould. Between 1966 and 2005, the size of Grinnell Glacier shrank by 40 percent. Similarly, the mass of the ice sheets in Greenland and the Antarctic is decreasing: Greenland lost 150-250 km3 of ice per year between 2002 and 2006. In addition, the

size and thickness of the Arctic sea ice is decreasing.

: The effect of global warming can be seen in the continuing retreat of Grinnel Glacier. The mean annual temperature

in the park has increased 1.33 °C since 1900. The loss of a glacier results in the loss of summer meltwaters, sharply reducing

seasonal water supplies and severely affecting local ecosystems (credit: modification of work by USGS).

Future Sea Level Change Warming temperatures contribute to sea level rise by expanding ocean water, melting mountain glaciers and ice caps, and causing portions of the Greenland and Antarctic ice sheets to melt or flow into the ocean. Since 1870, global sea level has risen by about 8 inches. Estimates of future sea level rise vary for different regions, but global sea level for the next century is expected to rise at a greater rate than during the past 50 years. The contribution of thermal expansion, ice caps, and small glaciers to sea level rise is relatively well-studied, but the impacts of climate change on ice sheets are less understood and represent an active area of research. Thus, it is more difficult to predict how much changes in ice sheets will contribute to sea level rise. Greenland and Antarctic ice sheets could contribute an additional 1 foot of sea level rise, depending on how the ice sheets respond.

Sea-level is rising 3.4 millimeters (0.13 inches) per year and has risen 0.19 meters (7.4 inches) from 1901 to 2010 (Figure

This is thought largely to be from both the melting of glaciers and thermal expansion. Thermal expansion means that as objects

such as solids, liquids, and gases heat up, they expand in volume. Since 1970, the melting of glaciers and thermal expansion

account for 75% of the sea-level rise (International Panel on Climate Change (IPCC), 2014).

: Sea height variation (mm) over time. Sea height has increased about 3.3 millimeters per year on average since 1993.

Data is from satellite sea level observations by the NASA Goddard Space Flight Center. Image by NASA (public domain).

Projections for sea level rise to the end of this century vary widely. This is in large part because we do not know which of the above

) we will most closely follow, but many are in the range from 0.5 m to 2.0 m. One of the

problems in predicting sea level rise is that we do not have a strong understanding of how large ice sheets, such as Greenland and

Antarctica, will respond to future warming. Another issue is that the oceans don't respond immediately to warming. For example,

with the current amount of warming, we are already committed to a future sea level rise of between 1.3 m and 1.9 m, even if we

could stop climate change today. This is because it takes decades to centuries for the existing warming of the atmosphere to be

transmitted to depth within the oceans and to exert its full impact on large glaciers. Most of that committed rise would take place

over the next century, but some would be delayed longer. And for every decade that the current rates of climate change continue,

that number increases by another 0.3 m. In other words, if we don't make changes quickly, by the end of this century we'll be

locked into 3 m of future sea level rise. In a 2008 report, the Organization for Economic Co-operation and Development (OECD)

estimated that by 2070 approximately 150 million people living in coastal areas could be at risk of flooding due to the combined

effects of sea level rise, increased storm intensity, and land subsidence. The assets at risk (buildings, roads, bridges, ports, etc.) are

in the order of $35 trillion ($35,000,000,000,000). Countries with the greatest exposure of population to flooding are China, India,

Bangladesh, Vietnam, U.S.A., Japan, and Thailand. Some of the major cities at risk include Shanghai, Guangzhou, Mumbai,

Kolkata, Dhaka, Ho Chi Minh City, Tokyo, Miami, and New York.

: Projected sea-level increases to 2100, showing likely range (grey) and possible maximum

[Adapted by Steven Earle from "Past and Projected Changes in Global Sea Level Rise"

based on data from Parris et al. 2012 with input from NASA Jet Propulsion Laboratory.]

Regional and local factors will influence future relative sea level rise for specific coastlines around the world (Figure

example, relative sea level rise depends on land elevation changes that occur as a result of subsidence (sinking) or uplift (rising).

Relative sea level rise also depends on local changes in currents, winds, salinity, and water temperatures, as well as proximity to

thinning ice sheets. Assuming that these historical geological forces continue, a 2-foot rise in global sea level by 2100 would result

in the following relative sea level rise:

2.3 feet at New York City 2.9 feet at Hampton Roads, Virginia 3.5 feet at Galveston, Texas 1 foot at Neah Bay in Washington state

: The United States government paid for the residents of Isle De Jean Charles, an island south of Louisiana (that is

also part of Louisiana), to relocate when it became inhabitable due to sea level rise. Image by Karen Apricot (CC-BY-SA).

Future Ocean Acidification Since 1750, about 40% of the new anthropogenic carbon dioxide has remained in the atmosphere. The remaining 60% gets absorbed by the ocean and vegetation. Therefore, the ocean has absorbed about 30% of new anthropogenic carbon dioxide. When carbon dioxide gets absorbed in the ocean, it creates carbonic acid which makes the ocean more acidic. Ocean acidification is the process of ocean waters decreasing in pH. Oceans become more acidic as carbon dioxide (CO2) emissions in the atmosphere dissolve in the ocean. This change is measured on the pH scale, with lower values being more acidic. The pH level of the oceans has decreased by approximately 0.1 pH units since pre-industrial times, which is equivalent to a 25% increase in acidity (see the EPA website for a more detailed explanation). The pH level of the oceans is projected to decrease even more by the end of the century as CO2 concentrations are expected to increase for the foreseeable future. Ocean acidification adversely affects many marine species, including plankton, mollusks, shellfish, and corals. As ocean acidification increases, the availability of calcium carbonate will decline. Calcium carbonate is a key building block for the shells and skeletons of many marine organisms. Acidification combined with warmer temperature and lower oxygen levels is expected to have severe impacts on marine ecosystems and human-used fisheries, possibly affecting our ocean-derived food sources (International Panel on Climate Change (IPCC), 2014). Corals require the right combination of temperature, light, and the presence of calcium carbonate (which they use to build their skeletons). As atmospheric carbon dioxide (CO2) levels rise, some of the excess CO2 dissolves into ocean water, reducing its calcium carbonate saturation. Calcium carbonate saturation has already been reduced considerably from its pre-industrial level, and model projections suggest much greater reductions in the future. If atmospheric CO2 concentrations double, coral calcification rates are projected to decline by more than 30%. If CO2 concentrations continue to rise at their current rate, corals could become rare on tropical and subtropical reefs by 2050. Under projections for the future, it is very unlikely that calcium carbonate saturation levels will be adequate to support coral reefs in any U.S. waters. Plant Productivity Watch this NASA video to discover the mixed effects of global warming on plant growth. While scientists found that warmer temperatures in the 1980s and 1990s caused an increase in plant productivity, this advantage has since been counteracted by more frequent droughts. Mismatched Interactions In addition to some abiotic conditions changing in response to climate change, many organisms are also being affected by the changes in temperature. Temperature and precipitation play key roles in determining the geographic distribution and phenology of plants and animals. Phenology is the study of the effects of climatic conditions on the timing of periodic life cycle events, such as flowering in plants or migration in birds. Researchers have shown that 385 plant species in Great Britain are flowering 4.5 days sooner than was recorded earlier during the previous 40 years. In addition, insect-pollinated species were more likely to flower earlier than wind-pollinated species. The impact of changes in flowering date would be mitigated if the insect pollinators emerged earlier. This mismatched timing of plants and pollinators could result in injurious ecosystem effects because, for continued survival, insect-pollinated plants must flower when their pollinators are present. Likewise, migratory birds rely on day length cues, which are not influenced by climate change. Their insect food sources, however, emerge earlier in the year in response to warmer temperatures. As a result, climate change decreases food availability for migratory bird species. Spread of Disease This rise in global temperatures will increase the range of disease-carrying insects and the viruses and pathogenic parasites they harbor. Thus, diseases will spread to new regions of the globe. This spread has already been documented with dengue fever, a disease the affects hundreds of millions per year, according to the World Health Organization. Colder temperatures typically limit the distribution of certain species, such as the mosquitoes that transmit malaria, because freezing temperatures destroy their eggs. West Nile virus and Lyme disease are two examples that already directly affect North Americans, while dengue fever could be an issue in the future (dengue became a "nationally notifiable condition" in the United States in 2010). Not only will the range of some disease-causing insects expand, the increasing temperatures will also accelerate their lifecycles, which allows them to breed and multiply quicker, and perhaps evolve pesticide resistance faster. In addition to dengue fever, other

diseases are expected to spread to new portions of the world as the global climate warms. These include malaria, yellow fever, West Nile virus, zika virus, and chikungunya. Climate change does not only increase the spread of diseases in humans. Rising temperatures are associated with greater amphibian mortality due to chytridiomycosis (see Invasive Species). Similarly, warmer temperatures have exacerbated bark beetle infestations of coniferous trees, such as pine an spruce. Climate Change Affects Everyone Our lives are connected to the climate. Human societies have adapted to the relatively stable climate we have enjoyed since the last ice age which ended several thousand years ago. A warming climate will bring changes that can affect our water supplies, agriculture, power and transportation systems, the natural environment, and even our own health and safety. Carbon dioxide can stay in the atmosphere for nearly a century, on average, so Earth will continue to warm in the coming decades. The warmer it gets, the greater the risk for more severe changes to the climate and Earth's system. Although it's difficult to predict the exact impacts of climate change, what's clear is that the climate we are accustomed to is no longer a reliable guide for what to expect in the future References Callendar, G. S. (1938). The artificial production of carbon dioxide and its influence on temperature. Quarterly Journal of the Royal Meteorological Society, 64(275), 223-240. https://doi.org/10.1002/qj.49706427503 Earle, S. (2015). Physical geology (OER textbook). BCcampus OpenEd. Easterling, D. R., & Wehner, M. F. (2009). Is the climate warming or cooling? Geophysical Research Letters, 36, L08706. https://doi.org/10.1029/2009GL037810 Foster, G., & Rahmstorf, S. (2011). Global temperature evolution 1979-2010. Environmental Research Letters, 6(4), 044022. https://doi.org/10.1088/1748-9326/6/4/044022 Hansen, J., Sato, M., Kharecha, P., et al. (2011). Earth's energy imbalance and implications. Atmospheric Chemistry and Physics, 11, 13421-13449. https://doi.org/10.5194/acp-11-13421-2011 International Panel on Climate Change (IPCC). (2014). Climate change 2014: Synthesis report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. IPCC. Karl, T. R., & Knight, R. W. (1998). Secular trends of precipitation amount, frequency, and intensity in the United States. Bulletin of the American Meteorological Society, 79(2), 231-241. https://doi.org/10.1175/1520-0477(1998)079<0231:STOPAF>2.0.CO;2 Kosaka, Y., & Xie, S.-P. (2013). Recent global-warming hiatus tied to equatorial Pacific surface cooling. Nature, 501(7467), 403- 407. https://doi.org/10.1038/nature12534 Lindsey, R. (2009). Climate and Earth's energy budget. NASA Earth Observatory. http://earthobservatory.nasa.gov (Accessed September 14, 2016) Oreskes, N. (2004). The scientific consensus on climate change. Science, 306(5702), 1686. https://doi.org/10.1126/science.1103618 Santer, B. D., et al. (2007). Identification of human-induced changes in atmospheric moisture content. Proceedings of the National Academy of Sciences, 104(39), 15248-15253. https://doi.org/10.1073/pnas.0702872104 Zemp, M., Frey, H., Gärtner-Roer, I., et al. (2015). Historically unprecedented global glacier decline in the early 21st century. Journal of Glaciology, 61(228), 745-762. https://doi.org/10.3189/2015JoG15J017 Suggested Supplementary Reading Intergovernmental Panel on Climate Change. 2013. 5th Assessment: Summary for Policymakers. NASA. 2018. Global Climate Change: Vital Signs of the Planet. This website by NASA provides a multi-media smorgasbord of engaging content. Learn about climate change using data collected by NASA satellites and more.

Contributors and Attributions Modified by Kyle Whittinghill and Melissa Ha from the following sources: Contributed by Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher Faculty (Geology) at Salt Lake Community College Sourced from OpenGeology from An Introduction to Geology Chapter Chapter 15: Global Climate Change "Physical Geology" by Steven Earle used under a CC-BY 4.0 international license. Download this book for free at http://open.bccampus.ca 12.4 Climate Change from Essentials of Environmental Science by CK-12 and Kamala Dorsner (Essentials of Environmental Science by Kamala Dorsner is licensed under CC BY 4.0.) Climate and the Effects of Global Climate Change from General Biology by OpenStax (licensed under CC-BY) Climate Change from Environmental Biology by Matthew R. Fisher (licensed under CC-BY) Carbon Cycle from Biology by John W. Kimball (licensed under CC-BY) 24.2: Implications of Climate Change is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts. 10.4: Climate Change by Matthew R. Fisher is licensed CC BY 4.0. Original source: https://openoregon.pressbooks.pub/envirobiology. 44.5: Climate and the Effects of Global Climate Change by OpenStax is licensed CC BY 4.0.

25: Light and Photosynthesis Learning Obj Learning Objectives Define photosynthesis and photoautotrophs Describe properties of light important for photosynthesis Outline the steps in both the light-dependent and light-independent reactions in photosynthesis Explain photorespiration and the three major photosynthetic pathways used by plants Photoautotrophs Plants, algae, and certain bacteria (cyanobacteria and green and purple sulfur bacteria) are among the organisms capable of performing photosynthesis (Figure ). Because they use light to manufacture their own food, they are called photoautotrophs (literally, "self-feeders using light", Greek autos = self and trophe = nutrition). Other organisms, such as animals, fungi, and most other bacteria, are termed heterotrophs ("other feeders"), because they must rely on the sugars produced by photosynthetic organisms for their energy needs. A third very interesting group of bacteria synthesize sugars, not by using sunlight's energy, but by extracting energy from inorganic chemical compounds; hence, they are referred to as chemoautotrophs.

Figure : Photoautotrophs including (a) plants, (b) algae, and (c) cyanobacteria synthesize their organic compounds via photosynthesis using sunlight as an energy source. Cyanobacteria and planktonic algae can grow over enormous areas in water, at times completely covering the surface. In a (d) deep sea vent, chemoautotrophs, such as these (e) thermophilic bacteria, capture energy from inorganic compounds to produce organic compounds. The ecosystem surrounding the vents has a diverse array of animals, such as tubeworms, crustaceans, and octopi that derive energy from the bacteria. (credit a: modification of work by Steve

Hillebrand, U.S. Fish and Wildlife Service; credit b: modification of work by "eutrophication&hypoxia"/Flickr; credit c: modification of work by NASA; credit d: University of Washington, NOAA; credit e: modification of work by Mark Amend, West Coast and Polar Regions Undersea Research Center, UAF, NOAA).

Photosynthesis is the process on which photoautotrophs rely to capture the energy in solar radiation (the "photo-" part) as highenergy electrons and use it to produce the carbon-carbon bonds of carbohydrate molecules (the "-synthesis" part). The carbon used to make these molecules comes from the carbon dioxide (CO2) in the atmosphere. Those carbohydrates are the energy source that heterotrophs use to power the synthesis of ATP via cellular respiration. Because photosynthesis removes carbon from the atmosphere and incorporates it into organic molecules which eventually become the plant's leaves, stems, roots, and fruits, photosynthesis is sometimes said to fix carbon. Fix, in this sense, means to secure or sequester rather than to repair. Properties of Light Recall that light travels in waves and that light is made up of particles are called photons. The length of the wave is measured from one peak to the next and is called the wavelength, which differs for different colors of light (Figure . Within the visible wavelengths of light, the longest wavelengths are red light; outside the visible range of wavelengths, even longer wavelengths include infrared radiation, microwaves, and radio waves. Shorter visible wavelengths include blue and purple light, and beyond the visible range even shorter wavelengths include UV light, X-rays, and Gamma rays.

Figure : Visible spectrum. "EM spectrum revised" by Philip Ronan is licensed under CC BY-SA 3.0.

Steps of Photosynthesis The photons in light provide the energy that drives photosynthesis. The chemical formula is the same for the two types of simple sugars produced by photosynthesis: glucose and fructose: C6H12O6. The equation that summarizes photosynthesis is: water + carbon dioxide -> oxygen, water, and simple sugars 12H20 + 6CO2 -> 6O2 + 6H2O + C6H12O6 This balanced equation tells us that 12 molecules of water plus 6 molecules of carbon dioxide, in the presence of chlorophyll, accessory pigments, and light, produces 6 molecules of oxygen gas, returns 6 molecules of water back to the cell, and produces one molecule of a simple sugar like glucose or fructose. Photosynthesis takes place in two sequential stages: the light-dependent reactions and the light independent-reactions (Calvin cycle). In the light-dependent reactions, energy from sunlight is absorbed by chlorophyll and that energy is converted into stored chemical energy. Light-dependent reactions require water and produce oxygen and energy in the form of ATP and NADPH. In the light-independent reactions, the chemical energy harvested during the light-dependent reactions drive the assembly of sugar molecules from carbon dioxide. Therefore, although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions to function. In addition, several enzymes of the light-independent reactions are activated by light. ATP and NAPDH move energy from light-dependent reactions to light-independent reactions. In all autotrophic eukaryotes, photosynthesis takes place inside the chloroplast. Figure illustrates the components inside the chloroplast (a membrane bound organelle in plant cells) where the light-dependent and light-independent reactions take place.

Figure : Photosynthesis takes place in two stages: light-dependent reactions and the light-independent reactions (Calvin cycle or Calvin-Benson cycle). Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. In the process, water is used and oxygen is produced. Energy from ATP and NADPH are used to power the Calvin cycle, which produces GA3P from carbon dioxide. ATP is broken down to ADP and Pi, and NADPH is oxidized to NADP+. The cycle is completed when the light reactions convert these molecules back into ATP and NADPH.

Light-Dependent Reactions There are two types of chlorophyll, a green pigment that captures light for photosynthesis, Chlorophyll a and Chlorophyll b.

Figure : Chlorophyll A & B Absorption Spectrum. Image by byr7 is licensed under CC BY 2.0

The graph above shows % absorbance of different wavelengths by these two chlorophylls (Figure ). The Y axis (the vertical one) shows the percentage of the light that is absorbed (rather than reflected). High levels of absorption mean that the chlorophyll molecule uses that wavelength of light for energy. Low absorption means that the molecule does not use that wavelength, and is thus reflected away. The X axis indicates the wavelength of light in nanometers (nm) and the bar at the top represents the color of the light at the wavelength shown. The blue line is a typical absorption curve for chlorophyll a, while the green line shows chlorophyll b. Both chlorophyll a and b absorb blue and red light wavelengths and reflect green. Chlorophyll a has a peak in the violet and red regions and chlorophyll b in the blue and orange regions. Notice how their absorbance is very low in the green region. That's why we think of chlorophyll as green, and why we perceive leaves, which have chlorophyll as the predominant pigment, as green.

Figure : Carotenoids absorption spectrum. Image by byr7 is licensed under CC BY 2.0.

The graph above shows the absorbance of carotenoid pigments, which are an accessory pigment in photosynthesis and present throughout the growing season (Figure ). Carotenoids absorb light in the green range, but reflect in yellow and red. We don't see these pigments during the growing season because they are much lower in concentration than the chlorophylls, so the green reflected light overwhelms the orange, and we see green. But when the chlorophyll fades in the fall the orange can be seen in beautiful fall leaf colors.

Figure : Photosynthesis takes place in chloroplasts, which have an outer membrane and an inner membrane. The space between the outer and inner membranes is called the intermembrane space. Stacks of thylakoids called grana form a third membrane layer. The thylakoids form stacks called grana. The liquid inside the inner membrane is called the stroma, and the space inside the thylakoid is called the thylakoid lumen.

The chloroplast has a double membrane envelope (composed of an outer membrane and an inner membrane) with an interior called the stroma (Figure ). Within the stroma are coin-like thylakoids and stacks of thylakoids are called grana. The thylakoids are also surrounded by a membrane, called the thylakoid membrane. The thylakoid membrane encloses an internal space called the thylakoid lumen. Embedded in the thylakoid membrane is chlorophyll, as well as the accessory pigments, and numerous proteins that make up the electron transport chain (Figure . In the light-dependent reactions protein complexes and pigment molecules work together to produce NADPH and ATP.

Figure : A diagram of the electron transport chain. Image by Jen Valenzuela (CC-BY).

When light hits a pigment molecule in the thylakoid membrane, the energy from the light photon promotes (pushes up) an electron in one of the pigment's atoms to a higher orbital as seen in the cartoon and energy is gained (Figure . The electron can't stay in that higher orbital indefinitely, and when it drops back to its home orbital it releases the energy it absorbed from light, denoted as energy loss. This released energy can be passed to another pigment molecule. This resonance energy travels down the antenna complex to the reaction center, where the captured energy pulls electrons out of water molecules, and water is split into oxygen gas, hydrogen ions, and electrons. This process of one pigment capturing the photon's energy and passing that energy onto adjacent pigment molecules is the crucial step in energy transformation that takes place in photosynthesis. This is the step that takes light energy and converts it into chemical energy -- one of the only known biological processes that allows this type of energy transformation. Recall that the overall equation for photosynthesis is: water + carbon dioxide -> oxygen, water, and simple sugars 12H20 + 6CO2 -> 6O2 + 6H2O + C6H12O6 This equation is made up of two parts called half-reactions. The half-reaction for the Light Reaction is as follows: 12H2O -> 6O2 + 24e- + 24H+ Light-Independent reactions The Light-Independent Reactions is the second part of photosynthesis. It takes place in the stroma of the chloroplast. Unlike the Light-Dependent Reaction, it does not require light. In the Light-Independent Reactions, two compounds, NADPH and ATP, carry the energy from light that was originally transformed into hydrogen ions and electrons through the splitting of water. The NADPH and ATP, along with carbon dioxide from the atmosphere, enter a process called the Calvin Cycle, where the energy is used to fix carbon into a molecule abbreviated G3P as shown in Figure . This process requires the help of an important protein abbreviated RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) that catalyzes the step in the process where the carbon from atmospheric CO2 is incorporated into an organic molecule. RuBisCO is the most abundant protein in leaves and, given the number of leaves in the world, likely the most abundant protein on the planet.

Figure : The light-independent reactions (Calvin cycle) has three stages. In stage 1, the enzyme RuBisCO adds carbon dioxide to RuBP, which immediately splits, producing two three-carbon 3-PGA molecules. In stage 2, two NADPH and two ATP are used to reduce 3-PGA to GA3P. In stage 3, RuBP, the molecule that starts the cycle, is regenerated so that the cycle can continue. One ATP is used in the process. Only one carbon dioxide molecule is incorporated at a time, so the cycle must be completed three times to produce a single three-carbon GA3P molecule, and six times to produce a six-carbon glucose molecule.

RuBisCO catalyzes a reaction between CO2 and RuBP (ribulose-1,5-bisphosphate). For each CO2 molecule that reacts with one RuBP, two molecules of another compound, 3-phosphoglycerate (3-PGA), form. 3-PGA has three carbon atoms and one phosphate. Each turn of the cycle involves only one RuBP and one carbon dioxide and forms two molecules of 3-PGA. The number of carbon atoms remains the same, as the atoms move to form new bonds during the reactions (3 atoms from 3 CO2 + 15 atoms from 3 RuBP = 18 atoms in 3 atoms of 3-PGA). This process is called carbon fixation, because CO2 is "fixed" from an inorganic form into organic molecules. Six molecules of both ATP and NADPH are used to convert the six molecules of 3-PGA into six molecules of a chemical called glyceraldehyde 3-phosphate (G3P). The G3P produced by the carbon fixation process is called a triose phosphate, meaning it is a 3-carbon sugar (triose) with phosphorus and oxygen atoms (phosphate) attached. Triose phosphate moves out of the chloroplast into the mesophyll cell's cytoplasm, where two of these three-carbon molecules are combined to produce the 6-carbon molecules glucose and fructose. The glucose and fructose molecules then combine to form sucrose, a 12-carbon organic molecule. Sucrose is important because it is the sugar that is transported by the phloem throughout the plant to provide energy and building blocks for other organic molecules like starch and cellulose. Interestingly, at this point, only one of the G3P molecules leaves the light-independent reactions and is sent to the cytoplasm to contribute to the formation of other compounds needed by the plant. Because the G3P exported from the chloroplast has three carbon atoms, it takes three "turns" of the cycle to fix enough net carbon to export one G3P. But each turn makes two G3P, thus three turns make six G3P. One of these six is exported while the remaining five G3P molecules remain in the cycle and are used to regenerate RuBP, which enables the system to prepare for more CO2 to be fixed. Three more molecules of ATP are used in these regeneration reactions. Recall that the overall equation for photosynthesis is:

water + carbon dioxide -> oxygen, water, and simple sugars 12H20 + 6CO2 -> 6O2 + 6H2O + C6H12O6 The half-reaction for the Light-Independent Reactions is: 24H+ + 24e- + 6CO2 -> C6H12O6 + 6H2O Photorespiration and other Photosynthetic Pathways Different plant species have adaptations that allow them to do different variations of the light-independent reactions. These are called photosynthetic pathways. Plants are classified as C3, C4, or CAM depending on their use of these pathways, but note that some plants can switch photosynthetic pathways depending on environmental conditions. The process for light-independent reactions described in the previous section was the C3 pathway: the compound formed during fixation (3-PGA) has three carbon atoms. Before discussing the details of the C4 pathway, it is important to understand the circumstances that led to these adaptations. As its name suggests, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) catalyzes two different reactions (Figure ).. The first is adding CO2 to ribulose-1,5- bisphosphate (RuBP) -- the carboxylase activity. The second is adding O2 to RuBP -- the oxygenase activity. The oxygenase activity of RuBisCO forms the three-carbon molecule 3-phosphoglycerate (3-PGA), just as in the light-independent reactions, and the two-carbon molecule glycolate. The glycolate enters peroxisomes, where it uses O2 to form intermediates that enter mitochondria where they are broken down to CO2. So this process uses O2 and ATP and liberates CO2 as aerobic cellular respiration does, which is why it is called photorespiration. It undoes the work of photosynthesis, which is to build sugars, and wastes ATP which could be more useful to the plant in other ways.

Figure : Rubisco is a two-faced enzyme.

Which action of RuBisCO predominates depends on the relative concentrations of O2 and CO2 with high CO2, low O2 favoring the carboxylase action and high O2, low CO2 favoring the oxygenase action. The light reactions of photosynthesis liberate oxygen, and more oxygen dissolves in the cytosol of the cell at higher temperatures. Therefore, high light intensities and high temperatures (above ~ 30°C) favor the second reaction and result in photorespiration. If concentration of CO is high enough, assimilation will overcome photorespiration. Consequently, to minimize the amount of photorespiration plants employ Le Chatelier's principle ("Equilibrium Law") and increase concentration of carbon dioxide near RuBisCO. One solution to increased concentrations of carbon dioxide in leaf tissues is for plants to open their stomata to release O2 and obtain CO2. However, if conditions are hot or dry, this will result in too much water loss (transpiration). For this reason, C3 plants do best in cool, moist areas. Rice and potatoes are examples of C3 plants. Plants can also increase the concentration of carbon dioxide near RuBisCO by temporarily bonding carbon dioxide with phosphoenolpyruvic acid (PEP, C ) using carboxylase enzyme; this results in C molecules, like malate or malic acid, with four carbons in the skeleton. When plant needs it, that C splits into pyruvate (C ) plus carbon dioxide, and the release of that carbon dioxide will increase its concentration. On the final step, pyruvate plus ATP react to restore PEP; recovery of PEP does cost ATP. This entire process is called the "C pathway" (Figure ). These plants are called C4 plants. C4 plants have structural changes in their leaf anatomy so that synthesizing the four-carbon sugar (the C4 pathway) and resuming the light-independent reactions (C3 pathways) are separated in different parts of the leaf with RuBisCO sequestered where the CO2

level is high and the O2 level low. After entering through stomata, CO2 diffuses into a mesophyll cell (Figure ) where it undergoes the C4 pathway. Being close to the leaf surface, these cells are exposed to high levels of O2, but they have no RuBisCO so cannot start photorespiration (nor the light-independent reactions). Then the four carbon molecules are transported in to bundle sheath cells (Figure ) deep in the leaf, so atmospheric oxygen cannot diffuse easily to them. In the bundle sheath cells, the four-carbon compound is broken down into carbon dioxide, which enters the light-independent reactions (C3 pathway) to form sugars, and pyruvic acid, which is transported back to a mesophyll cell where it is converted back into PEP.

Figure : C plants (left) conduct the C4 pathway in the mesophyll cells and the Calvin cycle (C3 pathway) in the bundle sheath cells, meaning they spatially separate the two . CAM plants (right) conduct the C4 pathway at night and the Calvin cycle

(C3 pathway) during the day, resulting an a temporal separation of the two.

Figure : Cross section and diagram of a C4 plant, showing mesophyll cells surrounding bundle sheath cells in concentric

circles. The bundle sheath cells are larger and have more chloroplasts than in other plants. This arrangement is called Kranz anatomy (wreath anatomy). Vascular bundles contain xylem (vessel elements and tracheids) and phloem (sieve-tube and companion cell complexes). A vascular parenchyma is also shown in the vascular bundle. Waxy suberin borders the bundle sheath cells. The chloroplasts in the mesophyll cells are distinct from those in the bundle sheath cells, and the chloroplasts are thus called dimorphic. Sclerenchyma fibers are just above and below the vascular bundle, and epidermal cells surround the entire leaf. Image by Kelvinsong (CC-BY-SA).

These C4 plants are well adapted to (and likely to be found in) habitats with high daytime temperatures and intense sunlight. Because they use the C4 pathway to prevent photorespiration, they do not have to open their stomata to the same extent as C3 plants and can thus conserve water. Some examples crabgrass, corn (maize), sugarcane, and sorghum. Although comprising only ~3% of the angiosperms by species, C4 plants are responsible for ~25% of all the photosynthesis on land. CAM stands for crassulacean acid metabolism because it was first studied in members of the plant family Crassulaceae. CAM plants also do the C4 pathway. However, instead of segregating the C4 and C3 pathways in different parts of the leaf, CAM plants separate them in time instead (Figure , Table ). As a result, CAM plants do not need to open their stomata in the daytime to reduce photorespiration because they have already formed a four-carbon molecule at night that can be broken down to release carbon dioxide during the day.

Table : Activities of CAM plants at night and in the morning.

CAM plants take in CO2 through their open stomata (they tend to have reduced numbers of them). The CO2 joins with PEP to form the four-carbon oxaloacetic acid. This is converted to four-carbon malic acid that accumulates during the night in the central vacuole of the cells.

The stomata close (thus conserving moisture as well as reducing the inward diffusion of oxygen). The accumulated malic acid leaves the vacuole and is broken down to release CO2. The CO2 is taken up into the light-independent reactions (C3 pathway).

CAM plants thus thrive in conditions of high daytime temperatures, intense sunlight, and low soil moisture. Some examples of

), pineapples, all epiphytic bromeliads, sedums, and the "ice plant" that invade the

Figure : Cultivated cacti in the Singapore Botanic Gardens. Image by Calvin Teo (CC-BY-SA).

Summary This chapter defines photosynthesis, autotrophs, and heterotrophs and describes key properties of light for the reactions of photosysnthesis. The chapter outlines the important steps in the photosynthesis reactions and differentiates between the lightdependent reactions and the light-independent reactions in photosynthesis. Finally, the chapter explains the concepts of photorespiration and compares the three major photosynthetic pathways used by plants (C3, C4, and CAM). Contributors and Attributions Modified by Kyle Whittinghill (University of Pittsburgh) from the following sources 11.2: Light and Photosynthesis by Tom Michaels, Matt Clark, Emily Hoover, Laura Irish, Alan Smith, and Emily Tepe. Original source: The Science of Plants: Understanding Plants and How They Grow by Tom Michaels; Matt Clark; Emily Hoover; Laura Irish; Alan Smith; and Emily Tepe Some of the explanations and pictures used in this section came from Introduction to photosynthesis and other pages at that McDaniel College site. 8.1 Overview of Photosynthesis, 8.2 The Light-Dependent Reactions of Photosynthesis, and 8.3 Using Light Energy to Make Organic Molecules from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org. 3.1 Discovery of Photosynthesis and 3.3 Enzymatic Stage from Introduction to Botany by Alexey Shipunov 13.3: Photosynthesis Overview and Equation, 13.5: The Light-dependent Reactions, 13.6: Light-independent Reactions, and 13.7: Photorespiration and Photosynthetic Pathways by Melissa Ha, Maria Morrow, & Kammy Algiers is licensed CC BY-NC 4.0. 16.2E Photorespiration and C4 Plants from Biology by John W. Kimball (licensed CC-BY) 25: Light and Photosynthesis is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts. 4.1.3: Photosynthesis Overview and Equation by Melissa Ha, Maria Morrow, & Kammy Algiers is licensed CC BY-NC 4.0. 11.2: Light and Photosynthesis by Tom Michaels, Matt Clark, Emily Hoover, Laura Irish, Alan Smith, and Emily Tepe is licensed CC BYNC 4.0. Original source: https://open.lib.umn.edu/horticulture. 4.1.6: Light-independent Reactions by Melissa Ha, Maria Morrow, & Kammy Algiers is licensed CC BY-SA 4.0. 4.1.7: Photorespiration and Photosynthetic Pathways by Melissa Ha, Maria Morrow, & Kammy Algiers is licensed CC BY-SA 4.0.

26: TEAM - PA Ecology Funding: Ecology for All! was developed thanks to a grant from the Commonwealth of Pennsylvania's PA GOAL program. Funding for PA GOAL was provided by the Governor's Emergency Education Relief Fund (GEER) by the state Department of Education (PDE) through the Office of Commonwealth Libraries (OCL). Additional funding was also provided by the Gettysburg College Johnson Center for Creative Teaching and Learning and the Office of the Provost's Open Educational Research grant initiative at the University of Pittsburgh. Acknowledgments: We thank Drs. Rebecca Ekert, David Outomuro Priede, and Marion Holmes for providing feedback on the book. At the University of Pittsburgh Cathy Barr, Lynn Rago, Matthew Rager and Vivian Lin provided support with the administration of the grant. At Gettysburg College, Katie Weigle, Laura Runyan, and Dawn Helsing provided administrative support and Janelle Wertzberger and Mary Elmquist provided expertise on open educational resources. Drs. Walt Carson and Martin Turcotte generously supported the involvement of graduate students on the project. We are also incredibly thankful to all of the authors who have contributed to the open access resources that we relied on for this project. In particular, we benefited greatly from previously developed open textbooks including Biology 2e on OpenStax (by M.A. Clark, M. Douglas, and J. Choi), Conservation Biology in Sub-Saharan Africa (J.W. Wilson and R.B. Primack), and Spreadsheets Exercises in Ecology and Evolution (T.M. Donovan and C. Weldon). Last, but certainly not least, we are grateful to the staff at Libretexts for building this open educational resource plaftorm and for their patience with our many, many emails throughout the development of this textbook.

Nathan Brouwer is a lecturer in the Department of Biological Sciences at the University of Pittsburgh, where he teaches intro biology, computational biology, biostatistics, and scientific writing. Prior to graduate school he served as a Peace Corps Volunteer with the National Agricultural Research Institute of The Gambia, West Africa. A firstgeneration PhD, he obtained his doctorate from the University of Pittsburgh studying the impacts of invasive species and overabundant deer on the native plants of western Pennsylvania. He did his post-doctoral work at the National Aviary of Pittsburgh with Dr. Steve Latta, where he modeled long-term monitoring data of tropical bird communities.

Hayden Dubniczki worked on Ecology for All! while an undergraduate student at Gettysburg College, where she earned a BS in Environmental Science. She is pursuing a Master of Environmental Management at Duke University's Nicholas School of the Environment. Her program concentration is Coastal Environmental Management.

Natasha (Tasha) Gownaris (she/her) is an Assistant Professor of Environmental Studies at Gettysburg College. At Gettysburg, Tasha teaches Ecology, Marine Ecology, Marine and Freshwater Fisheries, and Oceanography. Her research focuses on seabird foraging ecology and diet and the response of these species to climate change. She earned her BS in Biology and Environmental Studies at Gettysburg College in 2009 and a PhD in Marine Sciences from Stony Brook University in 2015. Before joining the Gettysburg faculty, she spent three years as a postdoctoral researcher at the University of Washington, where she studied Magellanic penguin demography and chick growth.

Leah Nath is an Environmental Studies major at Gettysburg College (Class of '26), where she is interested in both the natural sciences and the humanities. She is working to edit and improve the accessibility of the Ecology for All! textbook and, as a student who has used the book herself, to add new learning tools for future students. Castilleja (Cassie) Olmsted (she/her) is a graduate student at the University of Pittsburgh. Cassie conducts field research in Pennsylvania on the impact of natural and anthropogenic disturbance on forest soil seed banks, and on the role of traditional agriculture in preserving genetic diversity in South America. She doublemajored in Biology and

Spanish at Earlham College. Dan Wetzel (he/his) is a lecturer and academic advisor in the Department of Biological Sciences at the University of Pittsburgh, where he teaches ecology, biostatistics, and scientific writing in addition to meeting with hundreds of biology students each year. Dan earned a PhD at the University of Kentucky and a MS degree from Georgia Southern University, both focused on understanding individual differences in levels of parental care in birds. While he still dabbles in biological research, he's currently studying the perception and usage of office hours in large introductory science courses.

Kyle Whittinghill is a lecturer and academic advisor in the Department of Geology and Environmental Science at the University of Pittsburgh. Kyle teaches Environmental Science, Environmental Geochemistry, Statistics for Earth, Ecosystem Modeling and Environmental Science and Communication in Geosciences. Kyle also an instructor for the University of Pittsburgh's summer Wyoming Field Studies in Ecology course. Additionally, Kyle has edited and written an introductory environmental science book on LibreText, Environmental Science. Andy Wilson (he/him/his) is an Associate Professor in the Environmental Studies Department at Gettysburg College, where

he teaches Ecology, Wildlife Ecology, Conservation Biology, and Geographic Information Systems. A firstgeneration college student, Andy earned his PhD at Penn State, before which he worked as an ecologist for 10 years at the British Trust of Ornithology, a nongovernmental organization. His research focuses on large-scale studies of birds, often involving large numbers of citizen scientists, and the use of new technologies in Ecology. Taylor Zallek is a graduate student at the University of Pittsburgh where he studies the implications of rapid evolutionary change during experimental invasions on plant communities. Taylor has previously taught as a lecturer and lab instructor for

undergraduate ecology courses in the University of Pittsburgh's Department of Biological Sciences. He has a Bachelor's degree from St. John's University and a Master's from Michigan Tech. 26: TEAM - PA Ecology is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by LibreTexts.