Taken from: http://climate.nasa.gov/causes
Most scientists agree the main cause of the current global warming trend is human
expansion of the "greenhouse effect" -- warming that results when the atmosphere traps
heat radiating from Earth toward space.
Certain gases in the atmosphere behave like the glass on a greenhouse, allowing sunlight
to enter, but blocking heat from escaping. Long-lived gases, remaining semi-permanently in
the atmosphere, which do not respond physically or chemically to changes in temperature
are described as "forcing" climate change whereas gases, such as water, which respond
physically or chemically to changes in temperature are seen as "feedbacks."
Gases that contribute to the greenhouse effect include:

Water vapor. The most abundant greenhouse gas, but importantly, it acts as a
feedback to the climate. Water vapor increases as the Earth's atmosphere warms,
but so does the possibility of clouds and precipitation, making these some of the
most important feedback mechanisms to the greenhouse effect.

Carbon dioxide (CO2). A minor but very important component of the atmosphere,
carbon dioxide is released through natural processes such as respiration and
volcano eruptions and through human activities such as deforestation, land use
changes, and burning fossil fuels. Humans have increased atmospheric CO 2
concentration by a third since the Industrial Revolution began. This is the most
important long-lived "forcing" of climate change.

Methane. A hydrocarbon gas produced both through natural sources and human
activities, including the decomposition of wastes in landfills, agriculture, and
especially rice cultivation, as well as ruminant digestion and manure management
associated with domestic livestock. On a molecule-for-molecule basis, methane is a
far more active greenhouse gas than carbon dioxide, but also one which is much
less abundant in the atmosphere.

Nitrous oxide. A powerful greenhouse gas produced by soil cultivation practices,
especially the use of commercial and organic fertilizers, fossil fuel combustion, nitric
acid production, and biomass burning.

Chlorofluorocarbons (CFCs). Synthetic compounds of entirely of industrial origin
used in a number of applications, but now largely regulated in production and
release to the atmosphere by international agreement for their ability to contribute to
destruction of the ozone layer. They are also greenhouse gases .
Not enough greenhouse effect: The planet Mars has a very thin atmosphere, nearly all
carbon dioxide. Because of the low atmospheric pressure, and with little to no methane or
water vapor to reinforce the weak greenhouse effect, Mars has a largely frozen surface that
shows no evidence of life.
Too much greenhouse effect: The atmosphere of Venus, like Mars, is nearly all carbon
dioxide. But Venus has about 300 times as much carbon dioxide in its atmosphere as Earth
and Mars do, producing a runaway greenhouse effect and a surface temperature hot
enough to melt lead.
On Earth, human activities are changing the natural greenhouse. Over the last century the
burning of fossil fuels like coal and oil has increased the concentration of atmospheric
carbon dioxide (CO2). This happens because the coal or oil burning process combines
carbon with oxygen in the air to make CO2. To a lesser extent, the clearing of land for
agriculture, industry, and other human activities have increased concentrations of
greenhouse gases.
The consequences of changing the natural atmospheric greenhouse are difficult to predict,
but certain effects seem likely:

On average, Earth will become warmer. Some regions may welcome warmer
temperatures, but others may not.

Warmer conditions will probably lead to more evaporation and precipitation overall,
but individual regions will vary, some becoming wetter and others dryer.

A stronger greenhouse effect will warm the oceans and partially melt glaciers and
other ice, increasing sea level. Ocean water also will expand if it warms, contributing
further to sea level rise.

Meanwhile, some crops and other plants may respond favorably to increased
atmospheric CO2, growing more vigorously and using water more efficiently. At the
same time, higher temperatures and shifting climate patterns may change the areas
where crops grow best and affect the makeup of natural plant communities.
The role of human activity
In its recently released Fourth Assessment Report, the Intergovernmental Panel on Climate
Change, a group of 1,300 independent scientific experts from countries all over the world
under the auspices of the United Nations, concluded there's a more than 90 percent
probability that human activities over the past 250 years have warmed our planet.
The industrial activities that our modern civilization depends upon have raised atmospheric
carbon dioxide levels from 280 parts per million to 379 parts per million in the last 150
years. The panel also concluded there's a better than 90 percent probability that humanproduced greenhouse gases such as carbon dioxide, methane and nitrous oxide have
caused much of the observed increase in Earth's temperatures over the past 50 years.
They said the rate of increase in global warming due to these gases is very likely to be
unprecedented within the past 10,000 years or more. The panel's full Summary for
Policymakers report is online at http://www.ipcc.ch/pdf/assessmentreport/ar4/syr/ar4_syr_spm.pdf.
Solar irradiance
It's reasonable to assume that changes in the sun's energy output would cause the climate
to change, since the sun is the fundamental source of energy that drives our climate
system.
Indeed, studies show that solar variability has played a role in past climate changes. For
example, a decrease in solar activity is thought to have triggered the Little Ice Age between
approximately 1650 and 1850, when Greenland was largely cut off by ice from 1410 to the
1720s and glaciers advanced in the Alps.
But several lines of evidence show that current global warming cannot be explained by
changes in energy from the sun:

Since 1750, the average amount of energy coming from the Sun either remained
constant or increased slightly.

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 cooling in the upper atmosphere, and a warming at the surface and in
the lower parts of the atmosphere. That's because greenhouse gasses are trapping
heat in the lower atmosphere.

Climate models that include solar irradiance changes can’t reproduce the observed
temperature trend over the past century or more without including a rise in
greenhouse gases.
Taken from: http://climate.nasa.gov/uncertainties
This website presents a data-rich view of climate and a discussion of
how that data fits together into the scientists' current picture of our
changing climate. But there's a great deal that we don't know about
the future of Earth's climate and how climate change will affect
humans.
For convenience and clarity, climate scientists separate things that
affect climate change into two categories: forcings and feedbacks
(see sidebar at right).
Also, climate scientists often discuss "abrupt climate change," which
includes the possibility of "tipping points" in the Earth's climate.
Climate appears to have several states in which it is relatively stable
over long periods of time. But when climate moves between those
states, it can do so quickly (geologically speaking), in hundreds of
years and even, in a handful of cases, in only a few decades. These
rapid 'state changes' are what scientists mean by abrupt climate
change. They are much more common at regional scales than at the
global scale, but can be global. State changes have triggers, or
"tipping points," that are related to feedback processes. In what's
probably the single largest uncertainty in climate science, scientists
don't have much confidence that they know what those triggers are.
Below is an explanation of just a few other important uncertainties
about climate change, organized according to the categories forcing
and feedback. This list isn't exhaustive. It is intended to illustrate the
kinds of questions that scientists still ask about climate.
Forcings
1. Solar Irradiance. The sun has a well-known eleven-year
irradiance cycle that produces a .08% variation in output.1
Solar irradiance has been measured by satellite daily since
the late 1970s, and this known solar cycle is incorporated into
climate models. There is some evidence from proxy
measurements-sunspot counts going back centuries,
measurements from ancient trees, and others-that solar
output varies over longer periods of time, too. While there is
currently no evidence of a trend in solar output over the past
half century, because there are no direct observations of solar
output prior to the 1970s, climate scientists do not have much
confidence that they understand longer-term solar changes. A
number of U.S. and international spacecraft study the sun.
2. Aerosols, dust, smoke, and soot. These come from both
human and natural sources. They also have very different
effects on climate. Sulfate aerosols, which result from burning
coal, biomass, and volcanic eruptions, tend to cool the Earth.
Increasing industrial emissions of sulfates is believed to have
Forcings and Feedbacks
Climate forcings are the
initial drivers of a climate
shift. Solar irradiance is
one example of a forcing. If
the sun generates more
light, the Earth will warm.
Climate feedbacks are
processes that change as a
result of a change in
forcing, and cause
additional climate change.
An example of this is the
"ice-albedo feedback." As
the atmosphere warms,
sea ice will melt. Ice is
highly reflective, while the
underlying ocean surface is
far less reflective. The
darker ocean will absorb
more heat, getting warmer
and making the Earth
warmer overall. A feedback
that increases an initial
warming is called a
"positive feedback." A
feedback that reduces an
initial warming is a
"negative feedback." The
ice-albedo feedback is a
very strong positive
feedback that has been
included in climate models
since the 1970s.
Resources
For answers to frequently
asked questions about
global climate change, visit
the FAQ section of this
website.
caused a cooling trend in the Northern Hemisphere from the
1940s to the 1970s. But other kinds of particles have the
opposite effect. The global distribution of aerosols has only
been tracked for about a decade from the ground and from
satellites, but those measurements cannot yet reliably
distinguish between types of particulates. So aerosol forcing
is another substantial uncertainty in predictions of future
climate.
Feedbacks
3. Clouds. Clouds have an enormous impact on Earth's climate,
reflecting back into space about one third of the total amount
of sunlight that hits the Earth's atmosphere. As the
atmosphere warms, cloud patterns may change, altering the
amount of sunlight absorbed by the Earth. Because clouds
are such powerful climate actors, even small changes in
average cloud amounts, locations, and type could speed
warming, slow it, or even reverse it. Current climate models
do not represent cloud physics well, so the Intergovernmental
Panel on Climate Change has consistently rated clouds
among its highest research priorities. NASA and its research
partners in industry, academia, and other nations have a
small flotilla of spacecraft and aircraft studying clouds and the
closely related phenomenon of aerosols.
4. Carbon cycle. Currently, natural processes remove about
half of each year's human carbon dioxide emissions from the
atmosphere, although this varies a bit year to year. It isn't well
understood where this carbon dioxide goes, with some
evidence that the oceans are the major repository and other
evidence that land biota absorbs the majority. There is also
some evidence that the ability of the Earth system to continue
absorbing it may decline as the world warms, leading to faster
accumulation in the atmosphere. But this possibility isn't well
understood either. The planned Orbiting Carbon Observatory
mission will mark NASA's first attempt to answer some of
these questions via space observations.
5. Ocean circulation. One very popular hypothesis about
climate change is that as the Earth as a whole warms, ocean
circulation in the Atlantic will change to produce cooling in
Western Europe. In its most extreme form, this hypothesis
has advancing European ice sheets triggering a new ice age.
A global-warming induced ice age is not considered very
likely among climate scientists. But the idea highlights the
importance of ocean circulation in maintaining regional
climates. Global ocean data sets only extend back to the early
1990s, so there are large uncertainties in predictions of future
ocean changes.
6. Precipitation. Human civilization is dependent upon where
and when rain and snow fall. We need it for drinking water
and for growing our food. Global climate models show that
precipitation will generally increase, but not in all regions.
Some regions will dry instead. Scientists and policymakers
would like to use climate models to assess regional changes,
but the models currently show wide variation in their results.
For just one example, some models forecast less precipitation
in the American southwest, where JPL is, while others
foresee more precipitation. This lack of agreement on even
the direction of change makes planning very difficult. There's
much research to be done on this question.
7. Sea level rise. In its 2007 Fourth Assessment Report, the
Intergovernmental Panel on Climate Change used new
satellite data to conclude that shrinkage of ice sheets may
contribute more to sea level rise than it had thought as
recently as 2001. The panel concluded that it could not
"provide a best estimate or an upper bound for sea level rise"
over the next century due to their lack of knowledge about
Earth's ice. There are 5-6 meters worth of sea level in the
Greenland ice sheet, and 6-7 meters in the West Antarctic Ice
Sheet, while the much larger East Antarctic Ice Sheet is
probably not vulnerable to widespread melting in the next
century. Many hundreds of millions of people live within that
range of sea level increase, so our inability to predict what
sea level rise is likely over the next century has substantial
human and economic ramifications.