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Global Warming: Human Fingerprints
Global Warming: Human FIngerprints
Author: Melanie Fitzpatrick Source: Union of Concerned Scientists
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 on record.[1,2] 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.
In 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." [3] 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. [7] 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
The 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. [10] 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.
[10] This is yet another example of how science detectives are quantifying the impact of human
activities on climate.
Fingerprint 3: The Surface Heats Up
Measurements 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 [3]. 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 Union of Concerned Scientists
References
1. 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.
5. Petit, J.R., J. Jouzel, D. Raynaud, N.I. Barkov, J.-M. Barnola, I. Basile, M. Bender, J. Chappellaz,
M. Davis, G. Delaygue, M. Delmotte, V.M. Kotlyakov, M. Legrand, V.Y. Lipenkov, C. Lorius, L.
Pépin, C. Ritz, E. Saltzman, and M. Stievenard. 1999. Climate and atmospheric history of the past
420,000 years from the Vostok ice core, Antarctica. Nature 399:429-436.
6. Siegenthaler, U., T.F. Stocker, E. Monnin, D. Lüthi, J. Schwander, B. Stauffer, D. Raynaud, J.-M.
Barnola, H. Fischer, V. Masson-Delmotte, and J. Jouzel. 2005. Stable carbon cycle-climate
relationship during the late Pleistocene. Science 310:1313-1316.
7. Hansen, J., L. Nazarenko, R. Ruedy, M. Sato, J. Willis, A. Del Genio, D. Koch, A. Lacis, K. Lo, S.
Menon, T. Novakov, J. Perlwitz, G. Russell, G.A. Schmidt, and N. Tausnev. 2005. Earth's energy
imbalance: Confirmation and implications. Science 308:1431-1435.
8. Barnett, T.P., D.W. Pierce, K.M. AchutaRao, P.J. Gleckler, B.D. Santer, J.M. Gregory, and W.M.
Washington. 2005. Penetration of human-induced warming into the world's oceans. Science 309:
284-287.
9. Levitus, S., J. Antonov, and T. Boyer. 2005. Warming of the world ocean, 1955-2003.
Geophysical Research Letters 32. Online at http://www.agu.org (doi:10.1029/2004GL021592).
10. Santer, B.D., M.F. Wehner, T.M.L. Wigley, R. Sausen, G.A. Meehl, K.E. Taylor, C. Ammann, J.
Arblaster, W.M. Washington, J.S. Boyle, and W. Bruggemann. 2003. Contribution of anthropogenic
and natural forcing to recent tropospheric height changes. Science 301:479-483.
Author: Melanie Fitzpatrick Source: Union of Concerned Scientists
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 on record.[1,2] 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.
In 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." [3] 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. [7] 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
The 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. [10] 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.
[10] This is yet another example of how science detectives are quantifying the impact of human
activities on climate.
Fingerprint 3: The Surface Heats Up
Measurements 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 [3]. 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 Union of Concerned Scientists
References
1. 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.
5. Petit, J.R., J. Jouzel, D. Raynaud, N.I. Barkov, J.-M. Barnola, I. Basile, M. Bender, J. Chappellaz,
M. Davis, G. Delaygue, M. Delmotte, V.M. Kotlyakov, M. Legrand, V.Y. Lipenkov, C. Lorius, L.
Pépin, C. Ritz, E. Saltzman, and M. Stievenard. 1999. Climate and atmospheric history of the past
420,000 years from the Vostok ice core, Antarctica. Nature 399:429-436.
6. Siegenthaler, U., T.F. Stocker, E. Monnin, D. Lüthi, J. Schwander, B. Stauffer, D. Raynaud, J.-M.
Barnola, H. Fischer, V. Masson-Delmotte, and J. Jouzel. 2005. Stable carbon cycle-climate
relationship during the late Pleistocene. Science 310:1313-1316.
7. Hansen, J., L. Nazarenko, R. Ruedy, M. Sato, J. Willis, A. Del Genio, D. Koch, A. Lacis, K. Lo, S.
Menon, T. Novakov, J. Perlwitz, G. Russell, G.A. Schmidt, and N. Tausnev. 2005. Earth's energy
imbalance: Confirmation and implications. Science 308:1431-1435.
8. Barnett, T.P., D.W. Pierce, K.M. AchutaRao, P.J. Gleckler, B.D. Santer, J.M. Gregory, and W.M.
Washington. 2005. Penetration of human-induced warming into the world's oceans. Science 309:
284-287.
9. Levitus, S., J. Antonov, and T. Boyer. 2005. Warming of the world ocean, 1955-2003.
Geophysical Research Letters 32. Online at http://www.agu.org (doi:10.1029/2004GL021592).
10. Santer, B.D., M.F. Wehner, T.M.L. Wigley, R. Sausen, G.A. Meehl, K.E. Taylor, C. Ammann, J.
Arblaster, W.M. Washington, J.S. Boyle, and W. Bruggemann. 2003. Contribution of anthropogenic
and natural forcing to recent tropospheric height changes. Science 301:479-483.