Global Warming Questions and Answers

by David Herring, Rebecca Lindsey, Holli Riebeek, Michon Scott, and Robert Simmon July 28, 2007

On May 11, 2007, the Earth Observatory published Global Warming, a fact sheet outlining the basic principles behind the science of global warming. We asked readers to submit questions about global warming after reading the fact sheet. Some of the most common questions follow. We acknowledge the help of many scientists and writers in responding to these questions.

  • What does NASA have to do with global warming?
  • Data show that the Earth has warmed and cooled throughout history, so what makes scientists think that the current warming trend is due to human influence?
  • Has the Sun been more active in recent years? Could an increase in solar activity account for some of the warming observed since the latter half of the twentieth century?
  • Haven’t satellites actually observed cooling temperatures in the atmosphere?
  • Are the ozone hole and global warming the same thing? Is one causing the other?
  • Are the unusual or extreme weather events in my area caused by global warming?
  • Are there natural processes that will amplify or limit global warming?
  • What if we are wrong about the severity of global warming? What if the problem isn’t as serious as predicted?
  • Why is global warming a problem? Won’t there be some benefits?
  • If we immediately stopped emitting greenhouses gases, would global warming stop?
  • What can we do about global warming?
  • Where can I learn more about global warming?

What does NASA have to do with global warming?

NASA employs the world's largest concentration of climate scientists. NASA's mission to study Earth involves monitoring atmospheric conditions, global temperatures, land cover and vegetation, ice extent, ocean productivity, and a number of other planetary vital signs with a fleet of space-based sensors. This information is critical in understanding how Earth’s climate works and how it is responding to change. In addition to collecting information about the Earth, NASA also builds global and regional climate models to predict the effects of global warming. NASA shares its climate data and information with the public and policy leaders freely and in a timely manner. And NASA works with its partner agencies and organizations—NOAA, USGS, the EPA, the Department of Energy, and many others—to ensure that its data are used for societal benefit.

Data show that the Earth has warmed and cooled throughout history, so what makes scientists think that the current warming trend is due to human influence?

The main reason that scientists think humans caused warming since 1950 is that none of the natural processes that influence Earth’s climate have changed enough during that time period to explain the warming.

Over the past thousand years, temperatures have been preserved in natural records like tree rings, ice cores, and coral reefs. Many independent estimates of temperatures from these sources show that while global average surface temperatures varied, at no time were they warmer or did they climb more quickly than during the latter half of the 20th century. Three things can alter global temperatures over this short period: changes in the Sun’s activity, volcanic eruptions, and human emissions of greenhouse gases and aerosols.

During the twentieth century, the average amount of energy coming from the Sun either remained constant or increased slightly. Major volcanic eruptions temporarily cooled temperatures by pumping reflective gases into the atmosphere. At the same time, the burning of fossil fuels pushed greenhouse gas levels higher than they have been for at least the past 700,000 years. Laboratory experiments have shown that carbon dioxide, methane, and other greenhouse gases absorb and re-radiate infrared energy, or heat, and satellite observations have shown that these gases have the same heat-trapping effect in the atmosphere. The dramatic rate of increase in greenhouse gases during the latter half of the 20th century matches the rate of temperature increase.

Even more telling is the way in which temperatures are rising. If the warming were caused by a more active Sun, then scientists would expect to see warmer temperatures in all layers of the atmosphere. Instead they have observed a warming at the surface and in the lower parts of the atmosphere and a cooling in the upper atmosphere. Something is trapping heat in the lower atmosphere, and that something is greenhouse gases.

Graph of temperature trends in the troposphere and stratosphere.

Finally, scientists are almost certain that warming during the last 50 years was caused by human activity because models can’t reproduce the observed temperature trend without including a rise in greenhouse gases.

  • References
  • Jansen, E., J. Overpeck, K.R. Briffa, J.-C. Duplessy, F. Joos, V. Masson-Delmotte, D. Olago, B. Otto-Bliesner, W.R. Peltier, S. Rahmstorf, R. Ramesh, D. Raynaud, D. Rind, O. Solomina, R. Villalba and D. Zhang. (2007). 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., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  • Jones, P. D., and Mann, M.E. (2004). Climate over past millennia. Reviews of Geophysics, 42, RG2002,

Has the Sun been more active in recent years? Could an increase in solar activity account for some of the warming observed since the latter half of the twentieth century?

Scientists are still debating whether or not the Sun’s activity increased during the latter half of the 20th century, but even the highest estimates of activity can’t account for the warming observed since about 1950. 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 about 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.

Graph of annual average sunspot number from 1610 to 1995.

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+ 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. doi:10.1038/nature05072.
  • Fröhlich, C. (2006). Solar irradiance variability since 1978: Revision of the PMOD composite during solar cycle 21. Space Sci. Rev. (in press)
  • Lindsey, R. (2003). Under a Variable Sun. Earth Observatory
  • Muscheler, R., Joos, F., Müller, S.A., Snowball, I. (2005). Climate: how unusual is today's solar activity? Nature, 436, E3-E4.
  • Rind, D., D. Shindell, Ju. Perlwitz, J. Lerner, P. Lonergan, J. Lean, and C. McLinden. (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, doi:10.1175/1520-0442(2004)017
  • 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.

Haven’t satellites actually observed cooling temperatures in the atmosphere?

No. To monitor atmospheric temperatures, climate scientists rely on measurements taken by a series of satellites dating back to 1979. Because each satellite operated differently, scientists have disagreed about how to correct the data for errors and how to merge all the satellite data into a long-term record.

Different techniques used to merge the data 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.”


*What does NASA have to do with global warming?

*What makes scientists think that the current warming trend is due to human influence?

*Has the Sun been more active in recent years?

*Haven’t satellites actually observed cooling temperatures in the atmosphere?

*Are the unusual or extreme weather events in my area caused by global warming?

*Are there natural processes that will amplify or limit global warming?

*What if we are wrong about the severity of global warming?

*Why is global warming a problem?

*If we immediately stopped emitting greenhouses gases, would global warming stop?

*What can we do about global warming?

*Where can I learn more about global warming?




Graphs comparing different versions of the microwave sounding unit tropospheric temperature record.

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 is most likely due to additional errors in the observational data sets that remain to be corrected and not to model errors.

Are the ozone hole and global warming the same thing? Is one causing the other?

No. The ozone hole is caused by chlorofluorocarbons (CFCs), which set off chemical reactions that destroy ozone molecules in the stratosphere above Antarctica. The principal contributor to recent global warming is 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, however. The CFCs that destroy ozone are also strong greenhouse gases. Although they are present in the atmosphere in very small concentrations (several hundred parts per trillion, compared to several hundred parts per million for carbon dioxide), 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 stratospheric ozone absorbs heat radiated to space by gases in the atmospheric layer (the upper troposphere) below it. The loss of ozone means a small amount of additional heat can escape into space.

Global warming is also predicted to have a modest impact on the ozone hole. CFCs only destroy ozone at extremely cold temperatures, below -80 degrees Celsius (-112 degrees Fahrenheit). Greenhouse gases absorb heat at a relatively low altitude, warming the surface but cooling the stratosphere. The cooler the stratosphere, the more rapidly ozone should be destroyed, resulting in a slightly larger ozone hole.

  • References
  • Allen, Jeannie. (February 2004). Tango in the Atmosphere: Ozone and Climate Change. Earth Observatory. Accessed: June 6, 2007.
  • Baldwin, M.P., Dameris, M., Shepherd, T.G. (2007, June 15). How will the stratosphere affect climate change? Science, 316 (5831), 1576-1577. doi: 10.1126/science.1144303
  • 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 University Press, Cambridge, United Kingdom and New York, NY, USA.
  • NASA. (2005). Ozone Hole Watch. Accessed: June 6, 2007.

Are the unusual or extreme weather events in my area caused by global warming?

Individual weather events, such as Hurricane Katrina or the European heat wave of 2003, are caused by a combination of factors, and teasing out the blame owed to natural variability and human-caused global warming is difficult. Climate does not directly dictate specific weather events. Rather, climate sets up a range of possibilitiesâ “and a range of likelihoodsâ” for weather events. As climate warms, heat waves, droughts, and severe storms will probably become more likely. But it is not possible to say that any individual heat wave, drought, or storm occurred solely “because of global warming.”

  Photograph of cattle and dust during the 2007 Tennessee drought.

A good example of the complexity is the European heat wave of 2003, in which an estimated 22,000 to 45,000 heat-related deaths occurred in August 2003. This heat wave resulted in part from a high-pressure system linked to clear skies and dry soils, which allowed more solar energy than normal to warm the land surface. Therefore, natural events beyond human control played a large role in this heat wave. However, a climate model that included human activities, such as land use and emissions, more accurately simulated the evolution of European climate than a climate model that only included natural influences such as volcanic activity and solar output. Therefore, both natural and human factors probably played a role.

  • References
  • Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M., and Miller, H.L., editors. (2007). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Accessed June 6, 2007.
  • Patz, J.A., Campbell-Lendrum, D., Holloway, T., Foley, J.A. (2005). Impact of regional climate change on human health. Nature, 438: 310-317. doi:10.1038/nature04188.

Are there natural processes that will amplify or limit global warming?

Yes. Changes in one part of the climate system trigger processes that may either amplify the initial change or counteract it. With a positive climate feedback, warming triggers a process that causes more warming. With a negative climate feedback, warming sets off a process that leads to cooling.

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 exponential: an increase in temperature is accompanied by an even bigger increase in radiated heat. The feedback does not prevent temperature from rising, but it allows the Earth to return to an equilibrated (balanced) state.

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, feedbacks—especially cloud feedbacks—are the biggest source of uncertainty in predictions of future climate.

  • References
  • 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.
  • Herring, D. (June 2002). Does the Earth have an iris analog? Earth Observatory. Accessed June 15, 2007.
  • Lindsey, R. (January 2007). Arctic Reflection Earth Observatory. Accessed June 15, 2007.
  • Riebeek, H. (March 2004). Will runaway water warm the world? Earth Observatory, Accessed June 15, 2007.
  • Soden, B., and Held, I. (2006). An assessment of climate feedbacks in coupled ocean-atmosphere models. Journal of Climate, 19(14), 3354-3360.

What if we are wrong about the severity of global warming? What if the problem isn’t as serious as predicted?

If models are wrong about the severity of global warming, it is because Earth’s climate is either more or less sensitive to change than we think it is. The biggest source of uncertainty in our understanding of climate sensitivity is climate feedbacks. Feedbacks are processes that either limit or amplify climate change once an external factor like a rise in greenhouse gases initiates change. (See “Are there natural processes that will amplify or limit global warming?” for a more complete discussion.)

Some argue that there may be as-yet-unidentified feedbacks in Earth’s climate system that will regulate global warming (negative feedbacks). If this is the case, they contend, then we should not waste money trying to mitigate global warming. However, most scientists believe that if there are hidden feedbacks, they are just as likely to amplify warming (positive feedbacks). In other words, there is just as much chance that the models are underestimating the severity of future warming as they are overestimating warming.

Given the potentially catastrophic effects of global warming, uncertainty is not a good reason to delay action. If we do reduce emissions and climate change turns out to be less serious than predicted, we still benefit from our efforts. By switching to renewable energy sources like solar and wind, we can reduce our dependence on oil (a limited resource) and improve our air quality.

  • References
  • 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.
  • Pierrehumbert, R. (2005). Natural Variability and Climate Sensitivity. Real Climate. Accessed June 14, 2007.
  • Schmidt, G. (2007, January). The Physics of Climate Modeling. Goddard Institute of Space Studies. Accessed June 14, 2007. (Reprinted from Physics Today, 72.)

Why is global warming a problem? Won’t there be some benefits?

The cost and benefits of global warming will vary greatly from area to area. For moderate climate change, the balance can be difficult to assess. But the larger the change in climate, the more negative the consequences will become. Global warming will probably make life harder, not easier, for most people. This is mainly because we have already built enormous infrastructure based on the climate we now have.

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, 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, population continues to grow most rapidly in flood-vulnerable, low-lying coastal zones across the globe;
  • The human population is large and growing, and it is more dependent on stable agricultural production than at any time in its history. Places where famine and food insecurity are greatest in today’s world (Africa) 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.”

  • References
  • 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 (p. 59-70). Malta: Progress Press Ltd.
  • Intergovernmental Panel on Climate Change. (2007). Summary for Policy Makers. In Climate Change 2007: Impacts, Adaptation and Vulnerability Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.

If we immediately stopped emitting greenhouses gases, would global warming stop?

Not right away. The Earth’s surface temperature does not react instantaneously to the energy imbalance created by rising carbon dioxide levels because a great deal of the excess energy is stored in the ocean, which has a tremendous heat capacity. Because of this lag (which scientists call "thermal inertia"), even the 0.6–0.9 degrees of global warming we have observed in the past century is not the full amount of warming we can expect from the greenhouse gases we have already emitted. Even if all emissions were to stop today, the Earth’s average surface temperature would climb another 0.6 degrees or so over the next several decades before global warming stopped and temperature stabilized.

The time lag is one reason why there is a risk in waiting to control greenhouse gas emissions until global warming becomes worse or its effects more serious and obvious. If we wait until we feel the amount or impact of global warming has reached an intolerable level, we will not be able to “hold the line” at that point; some further warming will be unavoidable.

The time lag in the climate’s complete response to greenhouse gases (or other climate disruptions) is not the only reason that we couldn’t expect global warming to stop right away. Carbon dioxide levels in the atmosphere are rising because the amount we emit exceeds the amount that natural processes like photosynthesis and absorption into the oceans can remove. Carbon dioxide has an atmospheric lifetime of decades to centuries, and so even if greenhouse gas emissions stopped today, it would take several hundred years before natural processes removed all the “extra” atmospheric carbon dioxide we have emitted since we began burning fossil fuels.

Because we currently emit more carbon dioxide than natural processes can absorb, stabilizing or capping, emissions at today’s rates will not stop global warming either: carbon dioxide “deposits” would continue to exceed “withdrawals.” So, atmospheric carbon dioxide levels would continue to increase, and temperatures would continue to rise. To really slow global warming, we will have to significantly reduce, not just stabilize, emissions in coming decades.

  • References
  • Hansen, J., Sato, Mki., Ruedy, R., Kharecha, P., Lacis, A., Miller, R.L., Nazarenko, L., Lo, K., Schmidt, G.A., Russell, G., Aleinov, I., Bauer, S., Baum, E., Cairns, B., Canuto, V., Chandler, M., Cheng, Y., Cohen, A., Del Genio, A., Faluvegi, G., Fleming, E., Friend, A., Hall, T., Jackman, C., Jonas, J., Kelley, M., Kiang, N.Y., Koch, D., Labow, G., Lerner, J., Menon, S., Novakov, T., Oinas, V., Perlwitz, Ja., Perlwitz, Ju., Rind, D., Romanou, A., Schmunk, R., Shindell, D., Stone, P., Sun, S., Streets, D., Tausnev, N., Thresher, D., Unger, N., Yao, M., and Zhang, S (2007). Dangerous human-made interference with climate: A GISS modelE study. Atmospheric Chemistry and Physics, 7, 2287-2312.
  • Hansen, J., Nazarenko, L., Ruedy, R., Sato, Mki., Willis, J., Del Genio, A., Koch, D., Lacis, A., Lo, K., Menon, S., Novakov, T., Perlwitz, J., Russell, G., Schmidt, G.A., and Tausnev, N. (2005). Earth's energy imbalance: Confirmation and implications. Science, 308, 1431-1435.
  • Schlesinger, W. H. (1997) Biogeochemistry: An Analysis of Global Change. San Diego: Academic Press.

What can we do about global warming?

It is not NASA’s mission to develop strategies or public policies for controlling global warming, but rather, to provide the scientific information that decision makers need to understand global warming and to assess the impact of strategies to mitigate it. Science tells us that to control global warming, we must reduce carbon dioxide and other greenhouse gas 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. Putting existing scientific and technological strategies into place and developing new ones can stimulate the economy, and will also generate significant near-term benefits in public health through air pollution reduction.

Among the many scientific and policy organizations who are working on the global warming challenge is the Carbon Mitigation Initiative, a university and industry partnership based at Princeton University. The group has laid out strategies that are based solely on existing technologies. Used in combination over the next 50 years, these strategies 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.) The strategies fall into four broad categories:

  • Increase the energy efficiency of our cars, homes, and power plants while lowering our consumption by adjusting our thermostats and driving fewer miles;
  • Capture the carbon emitted by power plants and store it underground;
  • Produce more energy from nuclear and renewable fuels—solar, wind, hydroelectric, and bio-fuels;
  • 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, the total emissions of any one of a number of entire countries! 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, 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 reduce carbon dioxide emissions.

  • References
  • Pacala, S., and Socolow, R. (2004) Solving the Climate Problem for the Next 50 Years with Current Technologies, Science, 305 (5686), 968-972.
  • 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.

Where can I learn more about global warming?

The Earth Observatory contains a wealth of articles and satellite images and data about climate change. A few other good sources of information include:


Thank you to the scientists and writers who contributed to these responses.

  • Scientists

  • Robert Cahalan, NASA Goddard Space Flight Center
  • G. James Collatz, NASA Goddard Space Flight Center
  • Anthony Del Genio, NASA Goddard Institute for Space Studies
  • Andrew Dessler, Texas A&M University
  • Forrest Hall, NASA Goddard Space Flight Center
  • Michael Mann, Pennsylvania State University
  • Paul Newman, NASA Goddard Space Flight Center
  • William Patzert, NASA Jet Propulsion Laboratory
  • Gavin Schmidt, NASA Goddard Institute for Space Studies
  • Brian Soden, University of Miami Rosenstiel School of Marine and Atmospheric Science
  • Tom Wigley, University Corporation for Atmospheric Research
  • Earth Observatory Writers

  • David Herring
  • Rebecca Lindsey
  • Holli Riebeek
  • Michon Scott
  • Robert Simmon


  Something under the Ice is Moving

by Michon Scott · design by Robert Simmon and Michon Scott · April 23, 2007

In May 2006, Helen Amanda Fricker was doing the kind of work that left her perfectly open to a distraction. A geophysicist at Scripps Institution of Oceanography, Fricker didn’t doubt that her research was important, but she admits it was a little mundane. In an effort to improve Antarctic ice shelf maps and models of Antarctic tides, she was mapping the continent’s coastline by looking at small changes in elevation detected from satellite.

  Page 2
  Aerial photo of ice shelf

Finding the precise outline of Antarctica can be tricky because the continent is fringed with ice shelves—thick slabs of ice, fed by glaciers, floating on the ocean surface. The shelves may hide the continent, but they do offer clues about where the land ends. The clue is that the floating part of an ice shelf moves up or down with ocean tides while the land-based part sits still.


The coast of Antarctica is fringed by ice shelves that are thousands of feet thick in places. Fed by glaciers, these massive slabs of ice float on the surface of the ocean. Their elevation rises and falls with the tides. (©2005 BrynJ.)

  Roosevelt Island on the Ross Ice Shelf

Fortunately for Fricker, she didn’t have to stand shivering on an Antarctic ice shelf with a handheld GPS receiver to pick up the clues. Subtle changes in ice-shelf elevation from rising and falling tides are visible to NASA’s GLAS (Geoscience Laser Altimeter System) sensor on the agency’s ICESat satellite. The altimeter bounces a laser pulse off the Earth’s surface and times how long the signal takes to come back. Differences in the signal’s return times for the same location indicate changes in elevation.

ICESat makes measurements for 33 days roughly every 4 months, collecting data over 70-meter-wide “footprints” every 175 meters along the satellite’s ground track. “So we got the same patch of real estate—or strip of real estate—surveyed every four months, and we could see how its elevation changed through time,” Fricker explains.

In late May 2006, Fricker was concentrating on West Antarctica, around the Whillans and Mercer Ice Streams on the Ross Ice Shelf. She looked for the small elevation changes that would mark grounding lines, the place where the ice shelves stopped resting on land and started floating on the ocean. Marking the grounding lines would improve tidal models, which would improve understanding of ice shelf behavior, which influences glaciers, which influence sea level. Important work with a long-term payoff, but not terribly exciting.

Then she found something she didn’t expect.

Fricker found an elevation change, but two things about it struck her as weird. For one, it was in the wrong place—near a feature known as Engelhardt Ice Ridge—inland from where the ice shelf grounding line should have been. For another, the elevation change was far bigger than the typical tidal movement of 1 or 2 meters (3 to 6.5 feet). Between October 2003 and November 2005, the area she was examining had dropped roughly 9 meters (nearly 30 feet). “I wasn’t expecting to find this at all,” Fricker recalls. “I was shocked.” Something under the ice had to be moving.

To figure out what was going on, Fricker needed a detailed, seamless map of Antarctica’s icy surface. Although precise, GLAS has a narrow view. A map of Antarctica made out of GLAS observations would look like the crisscrossing strings of a tennis racket. For a comprehensive view, she turned to the Mosaic of Antarctica (MOA), a detailed digital image of the continent based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensors on NASA’s Terra and Aqua satellites. “I plotted the place where I thought the elevation drop was occurring,” she says, “and lo and behold, it was sort of a flat region that showed up in MOA.”


The Ross Ice Shelf flows from land onto sea, encasing Roosevelt Island. In places where the thick ice hides the transition from land to floating ice shelf, scientists can locate the coastline by measuring changes in the ice shelf’s elevation. The floating part of the shelf rises and falls with the tides while the land-based parts sits still. (Image courtesy National Snow and Ice Data Center, Mosaic of Antarctica.)

  ICESat tracks

Fricker contacted Ted Scambos at the National Snow and Ice Data Center, the scientist who had spearheaded the development of the Mosaic of Antarctica. Was he interested in helping her pin down the source of the elevation change? He was.


The Geoscience Laser Altimeter System (GLAS) sensor on NASA’s ICESat satellite observes elevation over narrow swaths of the planet’s surface. This image shows GLAS orbit tracks (red lines) over Antarctica. (NASA image by the ICESat Science Team.)


Learning to Add and Subtract


Scambos and his colleagues had created the Mosaic of Antarctica map by layering hundreds of digital images of Antarctica taken by Aqua and Terra MODIS between late 2003 and early 2004. To help Fricker find the source of the elevation drop, he would use a similar technique. But instead of “adding” the layers of MODIS images together to improve the amount of detail in the map, he would “subtract” images captured at different times to figure out what had changed.

Scambos had been working with Robert Bindschadler of NASA’s Goddard Space Flight Center for the past two years, each scientist using data from different NASA sensors to explore the idea of “image differencing.” Fricker’s discovery was a perfect test case for them, and they jumped at the chance to apply their techniques to her find. Photo-like images can’t provide actual elevation change in meters or feet, but if they are captured under similar enough lighting conditions, differences in shadows or brightness between two images can verify that the slope of the surface, and therefore its elevation, has changed.

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  Differenced MODIS images

“With image differencing, we had to be even more constrained than MOA,” Scambos explains. A change in the Sun’s position at the time of an image would give a false impression of change on the continent’s surface. To keep the shadows consistent, the researchers only used images acquired at a certain time of day and a certain time of year (between late November and early December). Scambos then collected as many images fitting those criteria as possible, cleaning up any flaws created by blowing snow or clouds. Once he “subtracted” the images, he could see actual changes in the surface of the snow and ice.

“Near the grounding line, there were lots of changes that occurred, lots of crevasses [cracks in the ice] that you could see because the moving crevasse ridges and troughs had shifted between images,” explains Scambos. The MODIS images confirmed that the elevation inland of the ice shelf grounding line had changed over an oval-shaped area about 15 by 30 kilometers. The area had clearly dropped in elevation, consistent with Fricker’s ICESat interpretation.


Subtraction of satellite images captured at roughly the same time of day on different dates revealed changes in ice sheet elevation. Differences in shadows or brightness show where the slope of the ice surface changed. An image of part of the Whillans Ice Stream from 2002 (top) subtracted from an image from 2005 (middle) revealed that a section of the ice stream sank (bottom). Scientists inferred that water had drained from a subglacial lake, which they named Subglacial Lake Engelhardt. (Images courtesy Ted Scambos, National Snow and Ice Data Center.)

  Aerial photo downstream of Lake Helen

“We knew immediately we were onto something big,” Scambos says. The elevation change at the surface was only the visible sign of a change that took place beneath the thick layer of fast-moving ice that covered the area. Buried beneath the ice stream, the cause of the change couldn’t be directly observed. It had to be deduced.

Could it be a sediment shift? Elsewhere in Antarctica, scientists had previously documented that substantial amounts of sediment could move under the ice, but Fricker and her colleagues didn’t think that sediment was responsible here. Under the ice, the thick mud would likely have had an uneven surface. The area Fricker and her colleagues studied had a smooth surface, and it changed more quickly than sediment could likely move. One of geology’s oldest principles is that when a liquid fills a basin, its top surface is smooth and parallel to the horizon. “It took us about a day to convince ourselves it was water,” Scambos recalls.

“It’s been known for a very long time that there are lakes under Antarctica’s ice,” Fricker explains. While the surface of the stream is frigid, the underbelly is warmer, with geothermal heat and the friction of the ice’s movement producing meltwater. “There are 145 documented subglacial lakes, and people are discovering more and more.” The lake she and her collaborators found was similar in area to Lake Tahoe (although not nearly as deep). The lake sat in the vicinity of Siple Coast, under the Ross Ice Shelf.


This aerial photo was taken downstream from Subglacial Lake Engelhardt (nicknamed Lake Helen), looking toward the Ross Ice Shelf. Ridges on the ice surface are crevasses, or cracks in the ice. Shifting and deepening of crevasses that are visible in satellite images are indicators of ice movement. (Photo courtesy Christina Hulbe, Portland State University.)

Antarctica map showing subglacial lakes


They knew they had found a subglacial lake, and because the elevation had dropped, they knew the lake had drained. But where did the water go, and why did it move?

“After we found that first lake, we went ahead and mapped all of the ice streams around Siple Coast,” Fricker recounts. When they did, they found 14 areas under the ice where elevation rose, fell, or oscillated between February 2003 and June 2006. Drawing from nearby features , they proposed names for the four biggest areas: Subglacial Lake Engelhardt (the largest one), Subglacial Lake Conway, Subglacial Lake Mercer, and Subglacial Lake Whillans.


The lakes discovered by Fricker and her colleagues (white dots) add more information to a large body of data about Antarctica’s subglacial lakes (black dots), now numbering well over 100. (NASA map by Robert Simmon, based on data from the Radarsat Antarctic Mapping Project, Ted Scambos, Chris Shuman, and Martin J. Siegert.)

ICESat elevation change graph ICESat elevation change graph

The simplest explanation for simultaneous elevation changes was that water was moving between these lakes. Glaciologists had previously documented that movements of meltwater beneath the ice can change the ice sheet’s surface elevation. Any subglacial water is subject to tremendous force from the weight of the ice overhead. As the ice stream above the lakes shifts, pressure increases in one area, and the water squishes to another area. The water flow into the new lake increases the pressure there, and eventually, that lake drains into another. As the lakes fill and drain, the elevation of the ice sheet above them rises and falls.


Between October 23, 2003, and June 2, 2006, average elevation of the ice surface along this Geoscience Laser Altimeter System (GLAS) track dropped from roughly 53 meters to about 44 meters. After March 3, 2006, the elevation drop was negligible, indicating that the event that caused the sinking had ended. (NASA image by Robert Simmon, based on GLAS data courtesy Helen Amanda Fricker, Scripps Institution of Oceanography.)

  Pressure field, lake map

Based on elevation data, image differencing, and ice thickness measurements that suggested where the ice would be leaning most heavily on the water below, Fricker and her colleagues were able to deduce that a complicated network of waterways underlies this region of Antarctic ice. They were also able to describe how some of the subglacial lakes are connected to one another based on where the overlying ice exerted pressure.

“It’s fascinating to watch the water drain in one place and appear in another,” Fricker says. “It’s also incredible to think that you can actually get an idea of what’s going on in that subglacial environment just by looking at the surface. This is under a kilometer of ice we’re talking. We’re actually seeing what’s going on.”


This image incorporates ice pressure data and measurements from the Geoscience Laser Altimeter System (GLAS) sensor on NASA’s ICESat satellite. Rainbow colors show the range of elevation changes (either up or down) observed between 2003 and 2006, with red indicating the greatest change and purple indicating the smallest. Relative pressure exerted by the ice sheet appears in grayscale, with white indicating the greatest pressure. Pools of water are likeliest to form in areas of low pressure. The yellow bands indicate possible pressure “ridges” separating different pressure fields and, consequently, separate water basins. (Image courtesy Ted Scambos, National Snow and Ice Data Center.)


Water Greases the Skids


Yet the discovery of this complex, subglacial water network means more than water squishing around under Antarctic ice. The location of the network under a fast-flowing ice stream means that water likely plays an important role in how quickly the ice overhead moves on its path to the sea. “We know intuitively that water lubricates the ice stream and reduces the friction,” Fricker explains. “If you have more water under an ice stream, it will flow faster. We know there should be some influence on flow speeds, and ultimately that’s going to affect how fast the ice flows off the continent and therefore sea level rise.”

Given those expectations, what actually appeared to be going on beneath the Whillans Ice Stream was rather odd. “While one isolated lake lost about two cubic kilometers of water, all the other lakes gained about an equivalent amount of water,” says NASA scientist Robert Bindchadler. “But it’s not the same water because the lake that lost water is downstream of the other lakes and in a separate basin.” The amount of water that accumulated in the other lakes is nearly equal to all the water that pressure and geothermal heat could likely have produced under the Whillans Ice Stream during that period. “Yet—and here’s the really odd part—,” he emphasizes, “this ice stream is slowing down at a consistent rate of about 1 or 2 percent per year. Now if water is the lubricant that lets ice go fast, why, if there is more water, is the ice stream going slower? We have ideas how to answer this, but the seeming contradiction shows us that we don’t understand this system yet.”

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  Screen capture from NASA animation of subglacial lakes

It is a system that climatologists studying the impact of global warming on sea-level rise want to understand better. While the Antarctic and Greenland ice sheet systems are irrevocably tied to Earth’s climate in many ways, Fricker and her collaborators all point out that subglacial lakes have no direct relationship to Earth’s steadily rising temperatures. “We’ve actually got evidence that there were features similar to this in the 1980s on the same ice stream, from photographs.” But even though the lakes are not caused by climate change, their role in “greasing the skids” beneath ice sheets and glaciers will have to be factored into models of how the Earth’s icy landscapes will react to global warming.

That those models need a lot of improvement before they can produce more than just broad estimates of sea-level rise was made clear in February 2007. The same month that the scientific magazine Science published Fricker and her colleagues’ paper on subglacial lakes, the Intergovernmental Panel on Climate Change (IPCC) released a summary of its 2007 report. Even though the panel strongly affirmed the link between greenhouse gases and rising global temperatures, it had to qualify its predictions of future sea level because “understanding of these [ice-sheet] processes is limited, and there is no consensus on their magnitude.”


Water moving between subglacial lakes can explain elevation changes in ice stream surfaces. Lakes typically form in low-pressure areas under the ice. See animation for more. (Image courtesy Susan Twardy, NASA Goddard Space Flight Center.)

  Penguins on ice shelf

Scambos expects that ice sheet contributions to sea level will be significant, but he agrees with the IPCC’s decision to avoid specific predictions at this point. “We’re still learning about all of the things that are contributing to flow off these ice sheets,” he says. The discovery of a complex “plumbing” system beneath the ice sheet is clear evidence of that. “Stuffing this information into models will be a real challenge, but it is what we [snow and ice scientists] need to do before we can help out the next IPCC report.”

For Fricker, the obvious next question is whether this kind of subglacial plumbing system exists all over Antarctica. Fricker and Scambos both admit that they don’t know. But they plan to find out. “We’re going to map the whole ice sheet. We’re going to do a continent-wide survey and find out where all the active regions are,” she says.

The drive to have results in time to provide input for the next IPCC report, just a few years down the road, can make scientific progress feel like a race. While the map of Antarctica’s subglacial waterways is incomplete, Scambos says, “We’ve fired the starting gun.”

    References and Related Reading:
  • Fricker, H. A., Scambos, T., Bindschadler, R., and Padman, L. (2007). An active subglacial water system in West Antarctica mapped from space. Science, 315, 1544-1548.
  • Bell, R. E., Studinger, M., Shuman, C. A., Fahnestock, M. A., and Joughin, I. (2007). Large subglacial lakes in East Antarctica at the onset of fast-flowing ice streams. Nature, 445, 904-907.
  • Kohler, J. (2007). Lubricating lakes. Nature, 445, 830-831.
  • NASA: NASA Satellites Unearth Antarctic ‘Plumbing System,’ Clues to Leaks. February 15, 2007.
  • Intergovernmental Panel on Climage Change: Climate Change 2007: The Physical Science Basis: Summary for Policymakers (1.3 MB PDF).
  • Wingham, D. J., Siegert, M. J., Shepherd, A., and Muir, A. S. (2006). Rapid discharge connects Antarctic subglacial lakes. Nature, 440, 1033-1036.
  • Gray, L., Joughin, I., Tulaczyk, S., Spikes, V. B., Bindschadler, R., and Jezek, K. (2005). Evidence for subglacial water transport in the West Antarctic Ice Sheet through three-dimensional satellite radar interferometry. Geophysical Research Letters, 32, L03501.

Melting of ice sheets and crumbling of ice shelves will influence sea level as Earth’s climate warms. Understanding the influence of subglacial lakes on ice flow will help scientists better model the behavior of ice streams and ice shelves and to predict how they will respond to global warming. (©2005 BrynJ.)


Animation of Subglacial Lake Behavior



Water moving between subglacial lakes can explain elevation changes in ice stream surfaces. This animation shows modeled behavior of subglacial lakes. Depending on the pressure of overlying ice, water can pool in unusual places. Unlike a water body with no ice overhead, a subglacial lake might form on the top of a hill if it is surrounded by ice that exerts tremendous pressure. (Animation courtesy Susan Twardy, NASA Goddard Space Flight Center. The original files are available for download.)


The Role of Subglacial Lakes


A slab of ice larger than the continental United States smothers much of East Antarctica. A river of ice nearly 800 kilometers long, the Recovery Ice Stream, drains part of the East Antarctic Ice Sheet, sliding roughly 35 billion tons of ice into the ocean each year.

In some places, the ice stream lumbers along at 2 to 3 meters per year, but in others, it suddenly accelerates to 50 meters per year. It is as if the ice stream slips on a banana peel. Surprisingly, the acceleration doesn’t happen on a steep slope; it happens as the ice stream passes over a relatively flat area. What causes this sudden speed up?

  Composite image of subglacial lakes in Dronning Maud Land

Without satellite technology, the Recovery Ice Stream’s acceleration might be a mystery without a solution, but a team of researchers from Columbia University, NASA, and the Universities of New Hampshire and Washington combined satellite-based data from several sensors to peer across and below the ice in the area: radar data of ice velocity, photo-like images of the surface, and laser data of ice elevation. They also analyzed data from ground-based radar surveys collected between 1964 and 1966 on one of the few expeditions to traverse the area.


This image combines elevation data from the ICESat satellite and digital images from the Mosaic of Antarctica to show subglacial lakes in Dronning Maud Land, Antarctica, where the Recovery Ice Stream is located. (NASA image courtesy NASA Goddard Space Flight Center Science Highlights January 2006.)

  Recovery Ice Stream

The researchers identified four large lakes below the ice; their combined area is close to that of Antarctica’s largest subglacial water body, Lake Vostok. Their location coincides closely with an area where the Recovery Ice Stream widens and accelerates, offering a neat explanation of what enables the ice to move faster there.

Beneath an ice stream’s frigid surface, its underbelly is warmed and melted by geothermal heat and friction created as it passes over the bedrock below. The researchers deduced that once the underside of the ice stream moves over a lake, the friction disappears, and the ice speeds up. Some lake water freezes onto the bottom of the ice, but this ice is warm and soft compared to the rest of the ice stream. When the ice stream reaches the downstream side of the lake, the warmer, softer ice at its base allows the ice stream to glide over land at a substantially quicker pace.

“Large subglacial lakes that clearly initiate streaming flow in a major ice stream have never been so completely documented before,” explains one of the authors, Chris Shuman of NASA Goddard Space Flight Center. The location and influence of subglacial lakes on ice streams are among the processes that scientists must understand in order to accurately model ice streams and ice sheets and to predict their contributions to sea level.

  • Bell, R. E., Studinger, M., Shuman, C. A., Fahnestock, M. A., and Joughin, I. (2007). Large subglacial lakes in East Antarctica at the onset of fast-flowing ice streams. Nature, 445, 904-907.
  • Kohler, J. (2007). Lubricating Lakes. Nature, 445, 830-831.
  • Perkins, S. (2007). Fits and starts: What regulates the flow of huge ice streams? Science News. 171(13) Available online.
  • NASA: Antarctic Ice Sheet’s Hidden Lakes Speed Ice Flow Into Ocean, May Disrupt Climate. March 5, 2007.

The Recovery Ice Stream feeds the Filchner Ice Shelf and the Weddell Sea. Satellites revealed subglacial lakes that accelerate ice stream flow. (Image courtesy National Snow and Ice Data Center, Mosaic of Antarctica.)