Climate Change Study Strengthens Link to Human Activities

New research shows some of the clearest evidence yet of a discernible human influence on atmospheric temperature.

Published online in the Nov. 29 early edition of the Proceedings of the U.S. National Academy of Sciences, the study compared 20 of the latest climate models against 33 years of satellite data. When human factors were included in the models, they followed the pattern of temperature changes observed by satellite. When the same simulations were run without considering human influences, the results were quite different.

“We can only match the satellite record when we add in human influences on the atmosphere,” said Michael Wehner, a research scientist at Lawrence Berkeley National Laboratory (Berkeley Lab) Computational Research Division and a coauthor of the article, which involved colleagues from 16 other organizations and was led by Benjamin Santer, an atmospheric scientist at Lawrence Livermore National Laboratory (LLNL).

Because the differences were so marked between models run with and without human influences, “we can conclude that these differences are unlikely to be due to natural causes with a high degree of certainty. In fact, in statistical terms, we are far more certain of this finding than we are of the existence of the Higgs-Boson,” said Wehner.

Select figure to enlarge: Geographical patterns of observed and simulated trends (in degrees Celsius per decade) from 1979 to 2011. Abbreviations stand for the lower stratosphere (TLS), the mid- to upper troposphere (TMT), and the lower troposphere (TLT). The observations are measurements of microwave emissions made by microwave sounding units (MSUs) on polar-orbiting satellites. MSU-based temperature data came from three different observational groups: Remote Sensing Systems (RSS), the University of Alabama at Huntsville (UAH), and the Center for Satellite Applications and Research (STAR) in Maryland. (Courtesy Benjamin Santer, LLNL)

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.

The goal of the study was to determine whether previous findings of a “discernible human influence” on tropospheric and stratospheric temperature were affected by 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 six and 30 miles above earth’s surface.)

To help eliminate the influence of naturally occurring climate variability, the team ran two different kinds of models: Models that included historical and projected increases in carbon dioxide and “aerosols,” such as smoke and dust, and one without. The second set of models acted as a sort of baseline that allowed researchers to filter out the effects of phenomena such as the El Niño/Southern Oscillation and the Pacific Decadal Oscillation. This “noise” can obscure the “signal” scientists are searching for in satellite data and models. Using this information, researchers were able to distinguish the clearest signal yet linking human factors to satellite-observed temperature changes between 1979 and 2011.

Analyzing geographical patterns of atmospheric temperature change over the 33-year period of satellite observations, researchers found that the lower stratosphere cools markedly in both observations and computer models. This cooling is primarily a response to the human-caused depletion of stratospheric ozone, due mostly to the once-widespread use of chlorofluorocarbons (CFCs) in spray can propellants and coolants, among other uses.

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, primarily carbon dioxide.

“It’s very unlikely that purely natural causes can explain these distinctive patterns of temperature change,” said LLNL’s Santer. “No known mode of natural climate variability can cause sustained, global-scale warming of the troposphere and cooling of the lower stratosphere.”

Wehner's work was supported by the Regional and Global Climate Modeling Program and the Earth System Modeling Program of the Office of Biological and Environmental Research in the Department of Energy Office of Science.

Other contributors to the study, outside Berkeley Lab and LLNL, 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; 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.