The ozone hole is an area in the stratosphere above Antarctica where chlorine and bromine gases from human-produced chlorofluorocarbons (CFCs) and halons have destroyed ozone molecules.

Global warming is the rise in average global surface temperature caused primarily by the build-up of human-produced greenhouses gases, mostly carbon dioxide and methane, which trap heat in the lower levels of the atmosphere.

There are some connections between the two phenomena.

For example, the CFCs that destroy ozone are also potent greenhouse gases, though they are present in such small concentrations in the atmosphere (several hundred parts per trillion, compared to several hundred parts per million for carbon dioxide) that they are considered a minor player in greenhouse warming. CFCs account for about 13% of the total energy absorbed by human-produced greenhouse gases.

The ozone hole itself has a minor cooling effect (about 2 percent of the warming effect of greenhouses gases) because ozone in the stratosphere absorbs heat radiated to space by gases in a lower layer of Earth’s atmosphere (the upper troposphere). The loss of ozone means slightly more heat can escape into space from that region.

Global warming is also predicted to have a modest impact on the Antarctic ozone hole. The chlorine gases in the lower stratosphere interact with tiny cloud particles that form at extremely cold temperatures — below -80 degrees Celsius (-112 degrees Fahrenheit). While greenhouse gases absorb heat at a relatively low altitudes and warm the surface, they actually cool the stratosphere. Near the South Pole, this cooling of the stratosphere results in an increase in polar stratospheric clouds, increasing the efficiency of chlorine release into reactive forms that can rapidly deplete ozone.

1. References:

2. Allen, Jeannie. (2004, February 10). Tango in the Atmosphere: Ozone and Climate Change. Earth Observatory. Accessed: September 14, 2010.
3. Baldwin, M.P., Dameris, M., Shepherd, T.G. (2007, June 15). How will the stratosphere affect climate change? Science, 316 (5831), 1576-1577.
4. Intergovernmental Panel on Climate Change, (2007). Summary for Policymakers. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller (eds.)]. Cambridge, United Kingdom, and New York, New York: Cambridge University Press.
5. NASA. Ozone Hole Watch. Accessed: September 14, 2010.

Controlling Emissions
Controlling emissions is a large, complex, and potentially expensive problem that no single strategy will solve. On the other hand, the costs of uncontrolled global warming will probably also be significant. Many economists have concluded that putting existing scientific and technological strategies into place and developing new ones may stimulate the economy, and would also generate significant near-term benefits in public health through air pollution reduction.

The Carbon Mitigation Initiative, a university and industry partnership based at Princeton University, has identified strategies—based solely on existing technologies—that used in combination over the next 50 years, would keep the amount of carbon dioxide in the atmosphere from more than doubling the pre-industrial level. (Many scientists believe doubled carbon dioxide levels will cause a dangerous interference with the climate.) These strategies are:

1. Increase the energy efficiency of our cars, homes, and power plants while lowering our consumption by adjusting our thermostats and traveling fewer miles;
2. Capture the carbon emitted by power plants and store it underground;
3. Produce more energy from nuclear, natural gas, and renewable fuels—solar, wind, hydroelectric, and bio-fuels;
4. Halt deforestation and soil degradation worldwide, while reforesting more areas.

Some of those strategies will have to be put into place by governments and industry, but individuals can also do a lot on their own. On average, individual Americans emit 19 tons of carbon dioxide annually while driving our cars and heating our homes—more than people in any other country. If we can reduce our personal emissions by just 5 percent, total U.S. emissions would drop by 300 million tons. That reduction could be easily achieved by replacing appliances and light bulbs with more efficient ones, planning our automobile trips more carefully, driving more fuel-efficient cars, taking fewer flights, and so on.

By learning about global warming, by communicating with elected officials about the problem, and by making energy-conscious decisions, individuals will play a meaningful role in what must be a global effort to respond to global warming.

Adapting to Climate Change
Climate has been fluctuating throughout Earth’s history, and recently, humans have become one of the factors contributing to climate change. Changes related to human activity are already being felt. Even if we were to stop greenhouse gas emissions today, additional climate change from emissions already in the atmosphere would be inevitable. For this reason, many governments and industries are beginning to adapt policies, disaster response plans, or infrastructure to prepare for anticipated changes. While some adaptations are difficult and expensive, many are relatively inexpensive and offer immediate benefits.

Adaptation strategies vary from region to region, depending on the greatest threat posed by climate change locally. For example, coastal regions facing rising sea levels and increased coastal erosion might eliminate incentives to develop high-risk coastlines and encourage a “living buffer” of sand dunes and forest between the ocean and infrastructure. New York City has already integrated climate change into the process it uses to plan future development, reducing the need for expensive retrofitting later.

Local governments may adjust disaster response plans to accommodate changes in weather patterns. The city of Philadelphia recently implemented an emergency response plan to limit the health impact of increasingly frequent heat waves on its population. Philadelphia officials estimate that their heat response plan has already reduced heat-related deaths.

More extreme and expensive adaptations may become necessary in some regions. Thawing permafrost and increased storms, windiness and coastal erosion are now putting at least 166 communities at risk in Alaska. Moving each community to safer areas will cost an estimated 30 to 50 million dollars per village, estimates the U.S. Army Corps of Engineers. Six communities have already decided to relocate.

For individuals, governments, and businesses, adapting to climate change requires understanding and accepting the risks of regional climate change, assessing the immediate and long-term costs and benefits of adaptation strategies, and implementing adaptations that bring the most benefits relative to the cost and risk.

Geoengineering
Though risky and unproven, geoengineering could provide another near-term strategy for slowing global warming until carbon emissions can be reduced enough to prevent catastrophic climate change. In this context, geoengineering means deliberately altering the atmosphere, land, or ocean to counter the effects of global warming.

Many geoengineering schemes have been proposed, but all can be reduced to two main strategies: reduce the amount of greenhouse gases in the atmosphere (increase the amount of infrared radiation escaping to space) or reduce the amount of solar energy the Earth system absorbs. Two of the most common examples of these geoengineering strategies involve removing carbon from the atmosphere by adding fertilizer to selected regions of the ocean to increase phytoplankton growth and reflecting more sunlight by injecting tiny, non-absorbing particles (aerosols) into the upper atmosphere (stratosphere).

While both of these geoengineering examples might counter global warming for a time, they could also have significant drawbacks. Increased fertilizers and/or phytoplankton growth could have unintended consequences on ocean ecosystems, including increased ocean dead zones and toxic blooms. Adding aerosols to the upper atmosphere could modify the chemistry of the upper atmosphere, affecting ozone and thereby having possible unintended impacts on the lower atmosphere.

Because the impact of geoengineering on the complex global climate system hasn’t been extensively studied, any large-scale geoengineering strategy could have serious unexpected consequences. As a result, most scientists consider geoengineering only as a last-resort, emergency measure.

1. References

2. America’s Climate Choices. (2010, May). Adapting to the impacts of climate change. National Research Council of the National Academies. Accessed July 16, 2010.
3. Intergovernmental Panel on Climate Change. (2007). Summary for Policymakers. In: Climate Change 2007: Mitigation of Climate Change
   Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
4. Pacala, S., and Socolow, R. (2004) Solving the Climate Problem for the Next 50 Years with Current Technologies. Science, 305 (5686), 968-972.
5. Parkinson, C. L. (2010). Coming Climate Crisis? Consider the Past, Beware the Big Fix. Lanham, Maryland: Rowman & Littlefield Publishers.
6. Robock, A., Marquardt, A., Kravitz, B, and Stenchikov, G. (2009, October 2). Benefits, risks, and costs of stratospheric geoengineering. Geophysical Research Letters, 36, L19703.

7. Further Reading

8. The Carbon Mitigation Initiative, is a collaboration between Princeton University, BP, and Ford to find solutions to the global warming problem.
9. The Energy Star Website, published by the U.S. Department of Energy and the U.S. Environmental Protection Agency provides information for individuals and businesses on making energy-conscious choices.

It’s natural to question whether we and future generations will regret our efforts to reduce greenhouse gas emissions if it turns out global warming isn’t as bad as predicted. But the best science we have to guide us at this time indicates that the chance that warming will be much larger than the best estimate is greater than the chance that it will be much smaller.

Climate scientists know that there is plenty they don’t know about the way the Earth system works. Some of the physical processes that models describe are thoroughly well-established—the melting point of ice, for example, and the law of gravity.

Other physical processes are less perfectly known: when the air temperature is not far below 0 Celsius, for example, will water vapor condense into liquid or ice? Either is possible, depending on atmospheric conditions.

To understand how uncertainty about the underlying physics of the climate system affects climate predictions, scientists have a common test: they have a model predict what the average surface temperature would be if carbon dioxide concentrations were to double pre-industrial levels.

They run this simulation thousands of times, each time changing the starting assumptions of one or more processes. When they put all the predictions from these thousands of simulations onto a single graph, what they get is a picture of the most likely outcomes and the least likely outcomes.

The pattern that emerges from these types of tests is interesting. Few of the simulations result in less than 2 degrees of warming—near the low end of the IPCC estimates—but some result in significantly more than the 4 degrees at the high end of the IPCC estimates.

This pattern (statisticians call it a “right-skewed distribution”) suggests that if carbon dioxide concentrations double, the probability of very large increases in temperature is greater than the probability of very small increases.

Our ability to predict the future climate is far from certain, but this type of research suggests that the question of whether global warming will turn out to be less severe than scientists think may be less relevant than whether it may be far worse.

1. References:

2. Intergovernmental Panel on Climate Change, Core Writing Team. (2007). Chapter 3: Climate change and its impacts in the near and long term under different scenarios. In Pachauri, R. & Reisinger, A. (Eds.), Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Geneva, Switzerland: IPCC.
3. Ramanathan, V., & Xu, Y. (2010). The Copenhagen Accord for limiting global warming: Criteria, constraints, and available avenues. Proceedings of the National Academy of Sciences, 107(18), 8055.
4. Realclimate.org. (2007, October 26). The certainty of uncertainty. Accessed June 21, 2010.
5. Roe, G. H., & Baker, M. B. (2007). Why Is Climate Sensitivity So Unpredictable? Science, 318(5850), 629-632.
6. Stainforth, D. A., Aina, T., Christensen, C., Collins, M., Faull, N., Frame, D. J., Kettleborough, J. A., et al. (2005). Uncertainty in predictions of the climate response to rising levels of greenhouse gases.

People in some temperate zones may benefit from milder winters, more abundant rainfall, and expanding crop production zones. But people in other areas will suffer from increased heat waves, coastal erosion, rising sea level, more erratic rainfall, and droughts.

The crops, natural vegetation, and domesticated and wild animals (including seafood) that sustain people in a given area may be unable to adapt to local or regional changes in climate. The ranges of diseases and insect pests that are limited by temperature may expand, if other environmental conditions are also favorable.

The problems seem especially obvious in cases where current societal trends appear to be on a “collision course” with predictions of global warming’s impacts:

• at the same time that sea levels are rising, human population continues to grow most rapidly in flood-vulnerable, low-lying coastal zones;
• places where famine and food insecurity are greatest in today’s world are not places where milder winters will boost crop or vegetation productivity, but instead, are places where rainfall will probably become less reliable, and crop productivity is expected to fall;
• the countries most vulnerable to global warming’s most serious side effects are among the poorest and least able to pay for the medical and social services and technological solutions that will be needed to adapt to climate change.

In its summary report on the impacts of climate change, the Intergovernmental Panel on Climate Change stated, “Taken as a whole, the range of published evidence indicates that the net damage costs of climate change are likely to be significant and to increase over time.”

(For specific information on the projected impacts of climate change in the United States, see the National Assessment Report by the U.S. Global Change Research Program.)

1. Related Resources

2. United Nations Environment Programme, Division of Early Warning and Assessment. (2006). Emerging Challenges: New Findings, in P. Harrison (Ed.), Global Environment Outlook Year Book 2006 (59-70). Malta: Progress Press Ltd.
3. McGranahan, G., Balk, D., and Anderson, B. (2007) The rising tide: assessing the risks of climate change and human settlements in low elevation costal zones. Environment and Urbanization, 19 (1), 17-37.
4. Intergovernmental Panel on Climate Change. (2007). Summary for Policy Makers. In Climate Change 2007: Impacts, Adaptation and Vulnerability, Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller (eds.)]. Cambridge, United Kingdom, and New York, New York: Cambridge University Press.

Studies do show that solar variability has significantly influenced past climate changes. For example, a decrease in solar activity is thought to have triggered the Northern Hemisphere’s Little Ice Age between approximately 1650 and 1850, when temperatures dipped low enough that rivers that don’t freeze in today’s human-warmed climate froze over.

Scientists use substitutes (proxies) like records of sun spots, which have been kept since Galileo’s time, or carbon in tree rings to estimate the amount of energy the Sun has sent to Earth. Though not perfect, these estimates give a rough approximation of how much the Sun’s activity has varied over time. Scientists are still debating over how reliable proxies are in determining the Sun’s past activity, but current estimates indicate that the Sun is probably now as active as or more active than it has ever been during the past 8,000 years.

Records of sunspots dating back to 1610 provide an indication of energy output from the Sun. In general, more sunspots (blue peaks) mean more intense solar activity and more energy received by the Earth. On average, there are more sunspots now than during the Maunder Minimum 350 years ago, but the increase in solar activity alone is not sufficient to explain the temperature increases that have occurred since 1950. (Graph by Robert Simmon, based on data from Hoyt and Schatten, 1997.)

A shorter, but more detailed record comes from NASA satellites, which have been recording the Sun’s activity from space since 1978. The measurements, however, come from six different satellites, each with its own bias. It is difficult to combine the measurements from these satellites into a single 25-year-plus record to get a trend of solar activity. Different scientific teams have attempted to create a continuous record from the satellite data. Each long-term record shows the rise and fall of two 11-year sunspot cycles, but they differ from one another in the average trend over the full period. When stitched together one way, the satellites seemed to record a slight increase in solar activity, but in other analyses, solar activity remained constant.

Regardless, even when scientists assume that solar activity is increasing based on proxy data and the satellite record, they can’t account for all of the warming observed at the end of the twentieth century. Climate models can only reproduce the warming observed since 1950 when a rise in greenhouse gases is built into the system.

References
Foucal, P., Frölich, C., Spruit, H., and Wigley, T. (2006). Variations in solar luminosity and their effect on the Earth’s climate. Nature, 443, 161-166.

Fröhlich, C. (2007). Solar Irradiance Variability Since 1978. In Solar Variability and Planetary Climates (pp. 53-65).

Hoyt, D. V., and Schatten K. H. (1997). Group Sunspot Numbers: A New Solar Activity Reconstruction. Solar Physics, 179 (1), 189-219.

Lean, J. L. (2009). Cycles and trends in solar irradiance and climate. Wiley Interdisciplinary Reviews: Climate Change, 1(1), 111-122.

Lindsey, R. (2003). Under a Variable Sun. NASA’s Earth Observatory. Accessed June 20, 2010.

Muscheler, R., Joos, F., Müller, S.A., Snowball, I. (2005). Climate: how unusual is today’s solar activity? Nature, 436, E3-E4.

Rind, D., Shindell, D., Perlwitz, Ju., Lerner, P., Lonergan, P., Lean, J., and McLinden, C. (2004). The relative importance of solar and anthropogenic forcing of climate change between the Maunder Minimum and the present. Journal of Climate, 17, 906-929.

Solanki, S.K., Ususkin, I.G., Kromer, B., Schössler, M., Beer, J. (2004). Unusual activity of the Sun during recent decades compared to the previous 11,000 years. Nature, 431, 1084-1087.

The second reason that scientists think the current warming is not from natural influences is that, over the past century, scientists from all over the world have been collecting data on natural factors that influence climate—things like changes in the Sun’s brightness, major volcanic eruptions, and cycles such as El Niño and the Pacific Decadal Oscillation. These observations have failed to show any long-term changes that could fully account for the recent, rapid warming of Earth’s temperature.

Reconstructions of global temperature that include greenhouse gas increases and other human influences (red line, based on many models) closely match measured temperatures (dashed line). Those that only include natural influences (blue line, based on many models) show a slight cooling, which has not occurred. The ability of models to generate reasonable histories of global temperature is verified by their response to four 20th-century volcanic eruptions: each eruption caused brief cooling that appeared in observed as well as modeled records. (Graph adapted from Hegerl and Zwiers et al., 2007.)

Finally, scientists know that carbon dioxide is a greenhouse gas and that it is released into the air when coal and other fossil fuels burn. Paleoclimate data show that atmospheric carbon dioxide levels are higher than they have been in the past 800,000 years. There is no plausible explanation for why such high levels of carbon dioxide would not cause the planet to warm.

Air bubbles trapped in Antarctic ice preserve an 800,000-year record of atmospheric carbon dioxide levels, which naturally varied from about 180 to about 280 parts per million. Once humans began burning large quantities of coal and oil in the 19th century, concentrations rose to 315 parts per million by 1958 (when direct measurements of carbon dioxide in the Antarctic atmosphere began) to 380 parts per million in 2007. (NASA graph by Robert Simmon, based on data from Keeling et al., 2008.)

1. References

2. Hegerl, G. C., Zwiers, F. W., Braconnot, P., Gillett, N. P., Luo, Y., Orsini, J. A., Nicholls, N., et al. (2007). Chapter 9: Understanding and attributing climate change. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B. , Tignor, M., and Miller, H.L. (eds.)] Cambridge and New York: Cambridge University Press.
3. Jansen, E., Overpeck, J., Briffa, K.R. , Duplessy, J.-C , Joos, F., Masson-Delmotte, V., Olgao, D., et al. (2007). Chapter 6: Paleoclimate. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B. , Tignor, M., and Miller, H.L. (eds.)] Cambridge and New York: Cambridge University Press.
4. Lean, J. L., & Rind, D. H. (2008). How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006. Geophysical Research Letters, 35(18).
5. Lockwood, M., & Fröhlich, C. (2008). Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature. II. Different reconstructions of the total solar irradiance variation and dependence on response time scale. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 464(2094), 1367-1385.
6. Lüthi, D., Le Floch, M., Bereiter, B., Blunier, T., Barnola, J., Siegenthaler, U., Raynaud, D., et al. (2008). High-resolution carbon dioxide concentration record 650,000–800,000 years before present. Nature, 453(7193), 379-382. [Download 800,000-Year CO₂ Data]
7. Steele, L. P., Krummel, P. B., & Langenfelds, R. L. (2007). Atmospheric CO2 concentrations from sites in the CSIRO Atmospheric Research GASLAB air sampling network (August 2007 version). In Trends: A Compendium of Data on Global Change, Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, US Department of Energy, Oak Ridge, TN, USA.

Most of the time, the observatory experiences “baseline” conditions and measures clean air which has been over the Pacific Ocean for days or weeks. We know this because the CO₂ analyzer usually gives a very steady reading which varies by less than 3/10 of a part per million (ppm) from hour to hour. These are the conditions we use to calculate the monthly averages that go into the famous 50-year graph of atmospheric CO₂ concentration.

We only detect volcanic CO₂ from the Mauna Loa summit late at night at times when the regional winds are light and southerly. Under these conditions, a temperature inversion forms above the ground, and the volcanic emissions are trapped near the surface and travel down our side of the mountain slope. When the volcanic emissions arrive at the observatory, the CO₂ analyzer readings increase by several parts per million, and the measured amounts become highly variable for periods of several minutes to a few hours. In the last decade, this has occurred on about 15% of nights between midnight and 6 a.m.

These periods of elevated and variable CO₂ levels are so different from the typical measurements that is easy to remove them from the final data set using a simple mathematical “filter.”

NOAA’s Earth Science Research Laboratory program also measures CO₂ in weekly flask samples taken at over 60 remote locations around the world. The Mauna Loa Observatory baseline CO₂ concentrations agree very well with flask measurements taken at a similar latitude around the world, which confirms that the volcanic CO₂ does not affect our final results. These measurements all show significant increases in CO₂ over the last few years. (You can find this information up through the end of 2007 on our website at www.esrl.noaa.gov/gmd/ccgg/.)

Although the volcanically contaminated CO₂ data are removed from the global averages, they are still scientifically useful. I have published two papers in which I used the non-baseline, volcanically contaminated, data to reconstruct a 50-year history of CO₂ emissions from Mauna Loa volcano. This is by far the longest continuous record of volcanic CO₂ anywhere in the world. It shows that the CO₂ emissions are greatest right after each eruption and then decrease at a well-defined inverse-exponential rate in the following years.

For example, right after the 1984 eruption, the volcano was producing over 100,000 metric tons of CO₂ per year (equivalent to a town of 40,000 people). It has decreased steadily ever since and is now at only about 1,000 metric tons per year, the lowest it has been since our record began in 1958. These measurements are valuable to volcanologists trying to model the evolution of subsurface magma chambers throughout an eruption cycle. You can read more about this topic at www.mlo.noaa.gov/programs/esrl/volcanicco2/volcanicco2.html.

Over the past decade, different “merging” techniques resulted in different long-term temperature trends, not all of which showed the warming that climate models predicted should have occurred. Some early analyses even suggested that parts of the troposphere (lower atmosphere), where warming was expected, had cooled. The lack of an unequivocal warming trend in the troposphere was sometimes used to challenge both the reality of human-induced global warming as well as the reliability of climate models.

To help resolve the discrepancies, the U.S. Climate Change Science Program undertook a comprehensive review of surface and atmospheric temperature observations and trends. The group identified and corrected errors in early versions of satellite and weather-balloon data, and concluded “For recent decades, all current atmospheric data sets now show global average warming that is similar to the surface warming.”

Early versions of atmospheric temperature data sets from satellites suggested that the lower atmosphere had warmed little if at all since 1978 (blue line). Newer versions (green and orange lines) show a warming trend that is similar to the warming measured at the Earth's surface. Blue line is Version D data from University of Alabama in Huntsville (UAH), developed in 1999. Green line is UAH Version 5.3, developed in 2010. Orange line is Remote Sensing Systems (RSS) version 3.2, developed in 2008. (Graph by Robert Simmon, based on UAH data from the National Space Science and Technology Center, and RSS data from Remote Sensing Systems, sponsored by the NOAA Climate and Global Change Program.)

Some uncertainties remain, however, particularly in the tropics. While all the long-term atmospheric data sets now show a warming trend, they do not all show the amplified warming (greater warming of the atmosphere than the surface) that models predict. According to the U.S. Climate Change Science Program report, this remaining uncertainty may be due to additional errors in the observational data sets (the explanation favored by the report authors) or to limitations in the models’ ability to simulate the impact of global warming on different atmospheric layers.

1. References

2. Karl, T. R., Hassol, S. J., Miller, C. D., and Murray, W. L., editors. (2006).
   Temperature Trends in the Lower Atmosphere: Steps for Understanding and Reconciling Differences. A Report by the Climate Change Science Program and the Subcommittee on Global Change Research, Washington, DC. Accessed June 8, 2007.
3. Remote Sensing Systems. (2007). Description of MSU and AMSU Data Products. Accessed June 27, 2007.
4. Schmidt, G. (2005). Et Tu LT? Real Climate. Accessed June 6, 2007.
5. The University of Alabama in Huntsville (2007). The National Space Science and Technology Center. Accessed June 15, 2007.

In addition to collecting information about the Earth, NASA also builds global and regional climate models to understand the causes and effects of climate change, including global warming. NASA shares its climate data and information with the public and policy leaders freely and in a timely manner. As part of the U.S. Climate Change Science Program, NASA works with other agencies—including the National Oceanic and Atmospheric Administration, the U.S. Geological Survey, the Environmental Protection Agency, the Department of Energy, and many others—to conduct research and to ensure climate science results are available to all users to address a broad range of societal needs.

• Further Reading

• NASA Earth Observing System Project Science Office Educational Publications
• NASA Global Modeling and Assimilation Office Overview
• NASA Goddard Institute for Space Studies Climate Research

The most fundamental negative (cooling) feedback is that the Earth radiates heat into space based on its temperature. The relationship between temperature and radiated heat is such that an increase in temperature is accompanied by an even bigger increase in radiated heat. The feedback does not prevent temperature from rising, but it slows the rate of temperature increase (or decrease) that a given energy imbalance can cause. The radiative feedback allows the Earth to achieve a new balanced (equilibrated) state in response to a change in surface temperature.

The other key feedbacks are water vapor, snow and ice, and clouds. Warming temperatures increase the amount of water vapor in the atmosphere. Because water vapor is a powerful greenhouse gas, it amplifies warming. Decreases in snow and ice make the Earth less reflective to incoming sunlight, also amplifying warming. Changes in clouds may either amplify or limit global warming, depending on where (latitude and altitude) and when (time of year) changes occur. Nearly all climate models scientists use today predict that net cloud feedbacks will either be neutral or positive (warming), but such predictions are still uncertain.

Numerous other feedbacks also exist. Warmer temperatures may decrease the rate at which the ocean absorbs carbon dioxide. Global currents that distribute heat among the world’s oceans may change because of temperature and salinity changes. Expansion or contractions of global vegetation can influence the reflection and absorption of incoming sunlight, the flow of energy and moisture between the surface and the air, and the carbon cycle. With the exception of not knowing precisely how much humans will do to control greenhouse gas emissions in coming decades, the strength of climate feedbacks—especially cloud feedbacks—is the biggest source of uncertainty in predictions of future climate.

1. References

2. Bony, S., Colman, R., Kattsov, V., Allan, R., Bretherton, C., Dufresne, J., Hall, A., Hallegatte, S., Holland, M., Ingram, W., Randall, D., Soden, B., Tselioudis, G., and Webb, M. (2006). How well do we understand and evaluate climate change feedback processes? Journal of Climate, 19(15), 3345-3482. doi: 10.1175/JCLI3819.1.
3. Soden, B., and Held, I. (2006). An assessment of climate feedbacks in coupled ocean-atmosphere models. Journal of Climate, 19(14), 3354-3360. doi: 10.1175/JCLI3799.1

• Further Reading

• Lindsey, R. (2007). Arctic Reflection, NASA Earth Observatory
• Riebeek, H. (2004). Will Runaway Water Warm the World?, NASA Earth Observatory