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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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."
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.
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.
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.