26 December 2014

Identifying Human Influences on Atmospheric Temperature


Some of the clearest evidence to date of a discernible human influence is found using a multimodel archive to obtain fingerprints of atmospheric temperature change. These fingerprints are estimates of the climate responses to external forcing by the combined effects of human activities, volcanoes and changes in the irradiance from the Sun. The primary components of external forcing over the past century are human caused increases in well-mixed greenhouse gases, depletion of the ozone layer and changes in atmospheric burdens of various aerosol particles. These fingerprints therefore mainly reflect human influences on climate. This detection and attribution study uses satellite-based measurements of tropospheric and stratospheric temperature change and output from 20 climate models participating in phase 5 of the Coupled Model Intercomparison Project. From these estimates of the signal pattern in response to combined anthropogenic and natural external forcing (the fingerprint) and the noise of internally generated variability, signal-to-noise (S/N) ratios can be calculated to quantify the strength of the fingerprint in the observations relative to fingerprint strength in natural climate noise. For changes in lower stratospheric temperature between 1979 and 2011, S/N ratios vary from 26 to 36, depending on the choice of observational dataset. In the lower troposphere, the fingerprint strength in observations is smaller, but S/N ratios are still significant at the 1% level or better, and range from three to eight. We find no evidence that these ratios are spuriously inflated by model variability errors. After removing all global mean signals, model fingerprints remain identifiable in 70% of the tests involving tropospheric temperature changes. Despite such agreement in the large-scale features of model and observed geographical patterns of atmospheric temperature change, most models do not replicate the size of the observed changes. On average, the models analyzed underestimate the observed cooling of the lower stratosphere and overestimate the warming of the troposphere. Although the precise causes of such differences are unclear, model biases in lower stratospheric temperature trends are likely to be reduced by more realistic treatment of stratospheric ozone depletion and volcanic aerosol forcing.

B. D. Santer
Santer, BD.  2013.  "Identifying Human Influences on Atmospheric Temperature."  Proceedings of the National Academy of Sciences of the United States of America 26-33, doi:10.1073/pnas.1210514109.