Sumitted by Mark Goldes (from www.ucsusa.org site): Earth's surface has undergone unprecedented warming over the last
century, particularly over the last two decades. Astonishingly, every
single year since 1992 is in the current list of the 20 warmest years
The natural patterns of climate have been altered. Like detectives,
science sleuths seek the answer to "Whodunnit?" — are humans part of
the cause? To answer this question, patterns observed by meteorologists
and oceanographers are compared with patterns developed using
sophisticated models of Earth's atmosphere and ocean. By matching the
observed and modeled patterns, scientists can now positively identify
the "human fingerprints" associated with the changes. The fingerprints
that humans have left on Earth's climate are turning up in a diverse
range of records and can be seen in the ocean, in the atmosphere, and
at the surface.
n its 2001 report, the Intergovernmental Panel on Climate Change
stated, "There is new and stronger evidence that most of the warming
observed over the last 50 years is attributable to human activities." 
Carbon dioxide from fossil fuel burning and land clearing has been
accumulating in the atmosphere, where it acts like a blanket keeping
Earth warm and heating up the surface, ocean, and atmosphere. As a
result, current levels of carbon dioxide in the atmosphere are higher
than at any time during the last 650,000 years. [4,5,6]
Background: Driving the Climate ("Forcing")
Climate is influenced by many factors, both natural and human. 
Things that increase temperature, such as increases in heat-trapping
emissions from cars and power plants or an increase in the amount of
radiation the sun emits, are examples of "positive" forcings or
drivers. Volcanic events and some types of human-made pollution, both
of which inject sunlight-reflecting aerosols into the atmosphere, lower
temperature and are examples of "negative" forcings or drivers. Natural
climate drivers include the sun's energy output, aerosols from volcanic
activity, and changes in snow and ice cover. Human climate drivers
include heat-trapping emissions from cars and power plants, aerosols
from pollution, and soot particles.
Much as the Air Force develops computer programs to simulate aircraft
flight under different conditions, climate scientists develop computer
programs to simulate global climate changes under different conditions.
These programs use our knowledge of physical, chemical, and biological
processes that occur within Earth's atmosphere and oceans and on its
land surfaces. Mathematical models allow scientists to simulate the
behavior of complex systems such as climate and explore how these
systems respond to natural and human factors.
Fingerprint 1: The Ocean Layers Warm
world's oceans have absorbed about 20 times as much heat as the
atmosphere over the past half-century, leading to higher temperatures
not only in surface waters but also in water 1,500 feet below the
surface. [8,9] The measured increases in water temperature lie well outside the bounds of natural climate variation.
Fingerprint 2: The Atmosphere Shifts
Recent research shows that human activities have lifted the boundary
of Earth's lower atmosphere. Known as the troposphere (from the Greek
tropos, which means "turning"), this lowest layer of the atmosphere
contains Earth's weather. The stable layer above is called the
stratosphere. The boundary that separates the two layers, the
tropopause, is as high as nine miles above the equator and as low as
five miles above the poles. In an astounding development, a 2003 study
showed that this tropopause has shifted upward over the last two
decades by more than 900 feet.  The rising tropopause marks another human fingerprint on Earth's climate.
In their search for clues, scientists compared two natural drivers of
climate (solar changes and volcanic aerosols) and three human drivers
of climate (heat-trapping emissions, aerosol pollution, and ozone
depletion), altering these one at a time in their sophisticated models.
Changes in the sun during the twentieth century have warmed both the
troposphere and stratosphere. But human activities have increased
heat-trapping emissions and decreased stratospheric ozone. This has led
to the troposphere warming more because the increase in heat-trapping
emissions is trapping more of Earth's outgoing heat. The stratosphere
has cooled more because there is less ozone to absorb incoming sunlight
to heat up the stratosphere. Both these effects combine to shift
the boundary upward. Over the period 1979-1999, a study shows that
human-induced changes in heat-trapping emissions and ozone account for
more than 80 percent of the rise in tropopause height.  This is yet another example of how science detectives are quantifying the impact of human activities on climate.
Fingerprint 3: The Surface Heats Up
show that global average temperature has risen by 1.4 degrees
Fahrenheit in the last 100 years, with most of that happening in the
last three decades. [1,2]
By comparing Earth's temperature over that last century with models
comparing climate drivers, a study showed that, from 1950 to the
present, most of the warming was caused by heat-trapping emissions from
human activities .
In fact, heat-trapping emissions are driving the climate about three
times more strongly now than they were in 1950. The spatial pattern of
where this warming is occurring around the globe indicates
human-induced causes. Even accounting for the occasional short-lived
cooling from volcanic events and moderate levels of cooling from
aerosol pollution as well as minor fluctuations in the sun's output in
the last 30 years, heat-trapping emissions far outweigh any other
current climate driver. Once again, our scientific fingerprinting
identifies human activities as the main driver of our warming climate.
Human Causes, Human Solutions
The identification of humans as the main driver of global warming
helps us understand how and why our climate is changing, and it clearly
defines the problem as one that is within our power to address. Because
of past emissions, we cannot avoid some level of warming from the
heat-trapping emissions already present in the atmosphere, some of
which (such as carbon dioxide and nitrous oxide) last for 100 years or
more. However, with aggressive emission reductions as well as
flexibility in adapting to those changes we cannot avoid, we have a
small window in which to avoid truly dangerous warming and provide
future generations with a sustainable world. This will require
immediate and sustained action to reduce our heat-trapping emissions
through increased energy efficiency, expanding our use of renewable
energy, and slowing deforestation (among other solutions). Melanie Fitzpatrick (Earth and Space Sciences and Atmospheric Sciences at the University of Washington and UCS consultant) prepared this summary with input from Brenda Ekwurzel (Union of Concerned Scientists) and reviews by Philip Mote (Climate Impacts Group at the University of Washington and Washington's state climatologist) , Richard Gammon (Chemistry, Oceanography, and Atmospheric Sciences at the University of Washington) and Peter Frumhoff (Union of Concerned Scientists). (c) 2006 UCS
U.S. National Aeronautics and Space Administration (NASA) Goddard
Institute for Space Studies. 2006. Global temperature trends: 2005
summation. New York, NY. Online at http://data.giss.nasa.gov/gistemp/2005.
2. U.S. National Oceanic and
Atmospheric Administration (NOAA) National Climate Data Center. 2006.
Climate of 2005 - annual report. Asheville, NC. Online at http://www.ncdc.noaa.gov/oa/climate/research/2005/ann/global.html.
3. Intergovernmental Panel on Climate
Change. 2001. Climate change 2001: The scientific basis. Cambridge, UK:
Cambridge University Press.
4. EPICA. 2004. Eight glacial cycles from an Antarctic ice core. Nature 429:623-628.
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