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Unresolved questions
about Earth's climate
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
- Solar Irradiance.
The sun has a well-known 11-year irradiance cycle that produces
about .1% 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.
- 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 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
- 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.
- 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.
- 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.
- 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.
- 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.2 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.
1Claus
Frohlich and Judith Lean, “Solar radiative output and its
variability: evidence and mechanisms,” The Astronomy and
Astrophysics Review, 2004, doi:10.1007/s00159-004-0024-1.
2IPCC Fourth Assessment
Report, Summary for Policymakers, p. 7
http://climate.nasa.gov/index.cfm\
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