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Life on earth plays a critical role in regulating the earth's physical, chemical, and geological properties, from influencing the chemical composition of the atmosphere to modifying climate.
About 3.5 billion years ago, early life forms (principally cyanobacteria) helped create an oxygenated atmosphere through photosynthesis, taking up carbon dioxide from the atmosphere and releasing oxygen (Schopf 1983; Van Valen 1971). Over time, these organisms altered the composition of the atmosphere, increasing oxygen levels, and paved the way for organisms that use oxygen as an energy source (aerobic respiration), forming an atmosphere similar to that existing today.
Carbon cycles on the planet between the land, atmosphere, and oceans through a combination of physical, chemical, geological, and biological processes (IPCC 2001). One key way biodiversity influences the composition of the earth's atmosphere is through its role in carbon cycling in the oceans, the largest reservoir for carbon on the planet (Gruber and Sarmiento, in press). In turn, the atmospheric composition of carbon influences climate. Phytoplankton (or microscopic marine plants) play a central role in regulating atmospheric chemistry by transforming carbon dioxide into organic matter during photosynthesis. This carbon-laden organic matter settles either directly or indirectly (after it has been consumed) in the deep ocean, where it stays for centuries, or even thousands of years, acting as the major reservoir for carbon on the planet. In addition, carbon also reaches the deep ocean through another biological process -- the formation of calcium carbonate, the primary component of the shells in two groups of marine organisms coccolithophorids (a phytoplankton) and foraminifera (a single celled, shelled organism that is abundant in many marine environments). When these organisms die, their shells sink to the bottom or dissolve in the water column. This movement of carbon through the oceans removes excess carbon from the atmosphere and regulates the earth's climate.
Over the last century, humans have changed the atmosphere's composition by releasing large amounts of carbon dioxide. This excess carbon dioxide, along with other 'greenhouse' gases, is believed to be heating up our atmosphere and changing the world's climate, leading to 'global warming'. There has been much debate about how natural processes, such as the cycling of carbon through phytoplankton in the oceans, will respond to these changes. Will phytoplankton productivity increase and thereby absorb the extra carbon from the atmosphere? Recent studies suggest that natural processes may slow the rate of increase of carbon dioxide in the atmosphere, but it is doubtful that either the earth's oceans or its forests can absorb the entirety of the extra carbon released by human activity (Falkowski et al. 2000).
1.1: Atmosphere and Climate Regulation - Biology
Throughout its long history, Earth has warmed and cooled time and again. Climate has changed when the planet received more or less sunlight due to subtle shifts in its orbit, as the atmosphere or surface changed, or when the Sun&rsquos energy varied. But in the past century, another force has started to influence Earth&rsquos climate: humanity.
(NASA astronaut photograph ISS022-E-6674.)
What is Global Warming?
Global warming is the unusually rapid increase in Earth&rsquos average surface temperature over the past century primarily due to the greenhouse gases released by people burning fossil fuels.
How Does Today&rsquos Warming Compare to Past Climate Change?
Earth has experienced climate change in the past without help from humanity. But the current climatic warming is occurring much more rapidly than past warming events.
Why Do Scientists Think Current Warming Isn&rsquot Natural?
In Earth&rsquos history before the Industrial Revolution, Earth&rsquos climate changed due to natural causes unrelated to human activity. These natural causes are still in play today, but their influence is too small or they occur too slowly to explain the rapid warming seen in recent decades.
How Much More Will Earth Warm?
Models predict that as the world consumes ever more fossil fuel, greenhouse gas concentrations will continue to rise, and Earth&rsquos average surface temperature will rise with them. Based on plausible emission scenarios, average surface temperatures could rise between 2°C and 6°C by the end of the 21st century. Some of this warming will occur even if future greenhouse gas emissions are reduced, because the Earth system has not yet fully adjusted to environmental changes we have already made.
How Will Earth Respond to Warming Temperatures?
The impact of global warming is far greater than just increasing temperatures. Warming modifies rainfall patterns, amplifies coastal erosion, lengthens the growing season in some regions, melts ice caps and glaciers, and alters the ranges of some infectious diseases. Some of these changes are already occurring.
References and Related Resources
1.1: Atmosphere and Climate Regulation - Biology
Biodiversity influences climate at local, regional and global levels, yet most climate models do not take biodiversity into consideration because its variables and effects are too diverse and complex to compute. Two recent studies, however, demonstrate the importance of being able to consider the response of vegetation to elevated levels of carbon dioxide in climate models as we try to predict our climate future.
Photo credit: David Darricau
Scientists at the Carnegie Institution for Science found that carbon dioxide’s direct effects on vegetation contribute to global warming. Through the pores called stomata in their leaves, plants take in carbon dioxide from the atmosphere that they use for photosynthesis. They then give off water through the stomata in a process called evapotranspiration which cools the plant just as perspiration cools human beings. Evapotranspiration also cools the surrounding air—a tree can transpire up to ten gallons of water on a hot day. But when carbon dioxide levels increase, plants’ stomata shrink, releasing less water into the air and reducing the cooling effect.
Carnegie scientists Long Cao and Ken Caldeira doubled the level of carbon dioxide in their model and found that globally the reduced evapotranspiration was responsible for 16 percent of the land warming the rest was due to CO2’s heat-trapping effects. In North America and Asia, more than 25 percent of the warming was due to the impact of increased CO2 on vegetation. “There is no longer any doubt that carbon dioxide decreases evaporative cooling by plants and that this decreased cooling adds to global warming,” said Cao. “This effect would cause significant warming even if carbon dioxide were not a greenhouse gas.”
Another effect of the doubled CO2 is increased runoff from the land as more precipitation bypasses the plant’s evapotranspiration system and makes its way directly into streams and rivers.
The Carnegie study did not take into consideration other effects of increased carbon dioxide such as an increase in leaf area, variations in vegetative distribution and resulting changes in albedo (the reflectivity of Earth’s surfaces which affects how much solar radiation is absorbed). These aspects were fixed in their model. But they cited earlier research on increased CO2 that showed that cooling due to increased leaf area produced an overall cooling effect over land, and that a decrease in albedo due to the expansion of coniferous forests resulted in land warming.
“These results really show that how plants respond to carbon dioxide is very important for making good climate predictions,” said Caldeira. “So if we want to improve climate predictions, we need to improve the representation of land plants in the climate models.
A new NASA study that did take plant growth into consideration found that doubling the level of CO2 resulted in a cooling effect. The model used by Lahouari Bounoua of the Goddard Space Flight Center in Greenbelt, Md., was innovative in its consideration of a reaction that plants have to increased CO2 called “down-regulation.” Down-regulation is the process that enables plants to use water and nutrients more efficiently when there is increased CO2, so that they are able to maintain previous levels of photosynthesis, which can ultimately boost leaf growth.
The increased leaf area resulted in more evapotranspiration globally, and thus created a cooling effect. The amount of cooling in the study measured -0.6 degrees C (-1.1 F) over land, compared to models that didn’t include down-regulation.
Bounoua stressed, however, that the cooling was not enough to offset the warming trends that are predicted.
Climate models usually factor in a doubling of CO2 to simulate global warming, and scientists generally agree that under this scenario, temperatures would increase from 2 to 4.5 degrees C (3.5 to 8.0 F). Bounoua’s model found a warming of 1.94 degrees C globally without the inclusion of down-regulation. The range in temperature results from unknowns about various “feedbacks,” i.e. how the various systems on Earth such as clouds, plant growth, methane release, the water cycle, albedo, etc. might respond to warming and interact with each other.
Bounoua and her colleagues also looked at how plant growth is stimulated by warmer temperatures, increased precipitation in some areas, and the plants’ more efficient use of water and nutrients when CO2 is doubled. The results suggest that in the long term, increases in vegetation due to elevated CO2 might reduce temperatures after CO2 levels stabilize.
“As we learn more about how these systems react, we can learn more about how the climate will change,” said the study’s co-author Forrest Hall. “Each year we get better and better.”
New climate models are being designed to consider dynamic global vegetation that allows plant types to shift interactively with climate, and ecosystem demography that accounts for how communities of diverse plants might respond to climate change over time.
Time for a change
In the US, the phaseout enjoys support from both major political parties, and there are already substitutes available for new refrigerators and air conditioners. One substitute that is already in many models of refrigerators is isobutane. Known in the industry as R-600a, it’s inexpensive, it has almost no ozone depletion potential, and it has a small global warming potential (three instead of R-134a's 1,430). Already, manufacturers have begun switching to the new refrigerant.
But the switch took decades longer than it needed to. Major chemical companies like DuPont, which produced CFCs, fought CFC regulations until the companies had replacement HFCs ready and patented, and a report by Inside Climate News shows they worked to slow the HFC phaseout, too. Isobutane is cheap, not patentable, and widely available—it’s most commonly known as a fuel for camp stoves.
Almost a decade ago, isobutane and other hydrocarbon refrigerants seemed poised for use in the market. In 2011, Underwriters Laboratories (UL) gave the refrigerants the go-ahead, and the EPA followed soon after. But then UL slashed their limits, citing a risk of fire if hydrocarbon refrigerants were to leak in a small room that also contained an open flame, like from a gas-fired water heater. Other experts have said the change was unwarranted and that UL’s fire risk scenario was extremely unlikely to occur. But in 2017, UL raised the limit again, and the EPA followed. Appliances using isobutane have begun to trickle onto the market.
Ultimately, the fire issue may be moot as some grocery stores have begun switching to using carbon dioxide as a refrigerant. Though it requires higher pressures throughout the cooling system, carbon dioxide is not detrimental to the ozone layer. And its global warming potential? One.
Evidence for Global Climate Change
Since scientists cannot go back in time to directly measure climatic variables, such as average temperature and precipitation, they must instead indirectly measure temperature. To do this, scientists rely on historical evidence of Earth’s past climate.
Antarctic ice cores are a key example of such evidence. These ice cores are samples of polar ice obtained by means of drills that reach thousands of meters into ice sheets or high mountain glaciers. Viewing the ice cores is like traveling backwards through time the deeper the sample, the earlier the time period. Trapped within the ice are bubbles of air and other biological evidence that can reveal temperature and carbon dioxide data. Antarctic ice cores have been collected and analyzed to indirectly estimate the temperature of the Earth over the past 400,000 years ([link]a). The 0 °C on this graph refers to the long-term average. Temperatures that are greater than 0 °C exceed Earth’s long-term average temperature. Conversely, temperatures that are less than 0 °C are less than Earth’s average temperature. This figure shows that there have been periodic cycles of increasing and decreasing temperature.
Before the late 1800s, the Earth has been as much as 9 °C cooler and about 3 °C warmer. Note that the graph in [link]b shows that the atmospheric concentration of carbon dioxide has also risen and fallen in periodic cycles note the relationship between carbon dioxide concentration and temperature. [link]b shows that carbon dioxide levels in the atmosphere have historically cycled between 180 and 300 parts per million (ppm) by volume.
[link]a does not show the last 2,000 years with enough detail to compare the changes of Earth’s temperature during the last 400,000 years with the temperature change that has occurred in the more recent past. Two significant temperature anomalies, or irregularities, have occurred in the last 2000 years. These are the Medieval Climate Anomaly (or the Medieval Warm Period) and the Little Ice Age. A third temperature anomaly aligns with the Industrial Era. The Medieval Climate Anomaly occurred between 900 and 1300 AD. During this time period, many climate scientists think that slightly warmer weather conditions prevailed in many parts of the world the higher-than-average temperature changes varied between 0.10 °C and 0.20 °C above the norm. Although 0.10 °C does not seem large enough to produce any noticeable change, it did free seas of ice. Because of this warming, the Vikings were able to colonize Greenland.
The Little Ice Age was a cold period that occurred between 1550 AD and 1850 AD. During this time, a slight cooling of a little less than 1 °C was observed in North America, Europe, and possibly other areas of the Earth. This 1 °C change in global temperature is a seemingly small deviation in temperature (as was observed during the Medieval Climate Anomaly) however, it also resulted in noticeable changes. Historical accounts reveal a time of exceptionally harsh winters with much snow and frost.
The Industrial Revolution, which began around 1750, was characterized by changes in much of human society. Advances in agriculture increased the food supply, which improved the standard of living for people in Europe and the United States. New technologies were invented and provided jobs and cheaper goods. These new technologies were powered using fossil fuels, especially coal. The Industrial Revolution starting in the early nineteenth century ushered in the beginning of the Industrial Era. When a fossil fuel is burned, carbon dioxide is released. With the beginning of the Industrial Era, atmospheric carbon dioxide began to rise ([link]).
W.S. was supported by USDA NIFA through the Minnesota Agricultural Experiment Station (Project# MIN-13-124), the Minnesota Department of Agriculture (Contract No. 138815), the Minnesota Wheat Research & Promotion Council (Projects# 00070003 and 00076909), and the Minnesota Soybean Research & Promotion Council (Projects# 00070622 and 00078080). D.A.W. acknowledges funding from the NSERC Discovery program and support from the Research School of Biology at the Australian National University. D.A.W. was also supported in part by the United States Department of Energy Contract No. DE-SC0012704 to Brookhaven National Laboratory. The authors declare no conflicts of interest.