T. M. L. Wigley, B. D. Santer

Climate Dynamics

Published date November 8, 2012

A probabilistic quantification of the anthropogenic component of twentieth century global warming

  • Examines in detail the statement in the 2007 IPCC Fourth Assessment Report that “Most of the observed increase in global average temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations"
  • Uses a quantitative probabilistic analysis to evaluate this IPCC statement
  • For forcing by greenhouse gases (GHGs) only
  • Finds that there is a greater than 90% probability that the greenhouse gas forcing is larger than the total amount (not just “most”) of the observed warming over 1950–2005
  • States that this is because, following current best estimates, negative aerosol forcing has substantially offset the GHG-induced warming
  • Considers the expected warming from all anthropogenic forcings using the same probabilistic framework
  • Provides evidence that the IPCC estimate for the upper bound of indirect aerosol forcing is almost certainly too high
  • Results show that the expected warming due to all human influences since 1950 (including aerosol effects) is very similar to the observed warming
  • Finds that including the effects of natural external forcing factors has a relatively small impact on the 1950–2005 results, but improves the correspondence between model and observations over 1900–2005