Tuesday, September 11, 2007

A NASA Q & A on Global Warming

Responsible NASA official: Dr. Michael D. King

On May 11, 2007, the Earth Observatory published Global Warming, a fact sheet outlining the basic principles behind the science of global warming. This follow-up article of questions and answers is based on questions from readers and other common questions about global warming. Many scientists and writers contributed to this article.

A - 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 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.

A - 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. (See “Has the Sun been more active in recent years?” for more on that topic.) 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.

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.

A - 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 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.

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.

A - 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.”

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.

A - The ozone hole and global warming are not the same thing, and neither one is the main cause of the other. The ozone hole is an area in the stratosphere above Antarctica where chemical reactions initiated by chlorofluorocarbons (CFCs) 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, 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.

A - 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 possibilities and a “range of likelihoods” 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.”

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. 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.

A - 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 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 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.

A - 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.

A - 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 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.”

A - No. Carbon dioxide levels are rising because we currently emit more carbon dioxide into the atmosphere than natural processes like photosynthesis and absorption into the oceans can remove. Therefore, stabilizing emissions at today’s rates will not stop global warming: our carbon dioxide “deposits” would still exceed natural “withdrawals.” Atmospheric carbon dioxide levels would continue to increase, and temperatures would continue to rise. To stop global warming, we will have to significantly reduce not just stabilize, emissions in coming decades.

A - Not right away. The Earth’s surface temperature does not react instantaneously to the energy imbalance created by rising carbon dioxide levels. This delayed reaction occurs 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 temperatures stopped rising.

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.

A - 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.

A - The Earth Observatory features many articles and satellite images and data about climate change:

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

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

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