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Human-caused climate change signal emerges from the noise

November 29, 2012
DOE/Lawrence Livermore National Laboratory
By comparing simulations from 20 different computer models to satellite observations, climate scientists have found that tropospheric and stratospheric temperature changes are clearly related to human activities.

The globe on the left displays the atmospheric temperature changes simulated by a computer model developed at the National Center for Atmospheric Research in Boulder, Colorado. The globe on the right shows satellite estimates of temperature change produced by scientists at Remote Sensing Systems in Santa Rosa, Calif. As the two globes rotate, the temperature changes are visible in three different atmospheric layers. The outermost layer is the lower stratosphere. The middle layer is the mid- to upper- troposphere and the innermost layer is the lower troposphere. Blue colors indicate cooling; red colors denote warming. As human activities change the chemical composition of the atmosphere, increasing the levels of well-mixed greenhouse gases and depleting stratospheric ozone, the stratosphere cools and the troposphere warms. The maximum cooling of the stratosphere occurs at the location of the Antarctic ozone hole. Because both globes are resting on a virtual mirror, the viewer sees not only the temperature changes over the Northern Hemisphere, but also the behavior of temperature in the atmosphere over Antarctica.
Credit: Image courtesy of DOE/Lawrence Livermore National Laboratory

By comparing simulations from 20 different computer models to satellite observations, Lawrence Livermore climate scientists and colleagues from 16 other organizations have found that tropospheric and stratospheric temperature changes are clearly related to human activities. 

The team looked at geographical patterns of atmospheric temperature change over the period of satellite observations. The team's goal of the study was to determine whether previous findings of a "discernible human influence" on tropospheric and stratospheric temperature were sensitive to current uncertainties in climate models and satellite data.

The troposphere is the lowest portion of earth's atmosphere. The stratosphere sits just above the troposphere, between 6 and 30 miles above earth's surface.

The satellite temperature data sets were produced by three different research groups, and rely on measurements of the microwave emissions of oxygen molecules. Each group made different choices in processing these raw measurements, and in accounting for such complex effects as drifts in satellite orbits and in instrument calibrations.

The new climate model simulations analyzed by the team will form the scientific backbone of the upcoming 5th assessment of the Intergovernmental Panel on Climate Change, which is due out in 2014.

In both satellite observations and the computer model simulations of historical climate change, the lower stratosphere cools markedly over the past 33 years. This cooling is primarily a response to the human-caused depletion of stratospheric ozone. The observations and model simulations also show a common pattern of large-scale warming of the lower troposphere, with largest warming over the Arctic, and muted warming (or even cooling) over Antarctica. Tropospheric warming is mainly driven by human-caused increases in well-mixed greenhouse gases.

"It's very unlikely that purely natural causes can explain these distinctive patterns of temperature change," said Laboratory atmospheric scientist Benjamin Santer, who is lead author of the paper appearing in the Nov. 29 online edition of the journal, Proceedings of the National Academy of Sciences.

"No known mode of natural climate variability can cause sustained, global-scale warming of the troposphere and cooling of the lower stratosphere."

The team analyzed results from climate model simulations with specified historical changes in human and natural external factors, and from simulations with projected 21st century changes in greenhouse gases and anthropogenic aerosols. They also looked at simulations with no changes in external influences on climate, which provide information on the year-to-year and decade-to-decade "noise" of internal climate variability, arising from such natural phenomena as the El Nino/Southern Oscillation and the Pacific Decadal Oscillation.

The team used a standard "climate fingerprint" method to search for the model signal pattern (in response to human influences, the sun and volcanoes) in the satellite observations. The method quantifies the strength of the signal in observations, relative to the strength of the signal in natural climate noise.

Other contributors include researchers from Remote Sensing Systems of Santa Rosa; the Centre for Australian Weather and Climate Research, Melbourne, Australia; the Canadian Centre for Climate Modeling and Analysis, Victoria, Canada; the National Oceanic and Atmospheric Administration (NOAA) Geophysical Fluid Dynamics Laboratory, Princeton; the University of Colorado, Boulder; the Massachusetts Institute of Technology, Cambridge; the U.K. Met. Office Hadley Centre, Exeter, U.K.; the Centre National de la Recherche Scientifique, Toulouse, France; North Carolina State University; the National Climatic Data Center, Asheville; Lawrence Berkeley National Laboratory; the National Center for Atmospheric Research, Boulder; the University of Adelaide, South Australia; the University of Reading, U.K.; and the Center for Satellite Applications and Research, Camp Springs.

Story Source:

The above post is reprinted from materials provided by DOE/Lawrence Livermore National Laboratory. Note: Materials may be edited for content and length.

Journal Reference:

  1. Benjamin D. Santer, Jeffrey F. Painter, Carl A. Mears, Charles Doutriaux, Peter Caldwell, Julie M. Arblaster, Philip J. Cameron-Smith, Nathan P. Gillett, Peter J. Gleckler, John Lanzante, Judith Perlwitz, Susan Solomon, Peter A. Stott, Karl E. Taylor, Laurent Terray, Peter W. Thorne, Michael F. Wehner, Frank J. Wentz, Tom M. L. Wigley, Laura J. Wilcox, and Cheng-Zhi Zou. Identifying human influences on atmospheric temperature. PNAS, November 29, 2012 DOI: 10.1073/pnas.1210514109

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DOE/Lawrence Livermore National Laboratory. "Human-caused climate change signal emerges from the noise." ScienceDaily. ScienceDaily, 29 November 2012. <>.
DOE/Lawrence Livermore National Laboratory. (2012, November 29). Human-caused climate change signal emerges from the noise. ScienceDaily. Retrieved November 28, 2015 from
DOE/Lawrence Livermore National Laboratory. "Human-caused climate change signal emerges from the noise." ScienceDaily. (accessed November 28, 2015).

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