Dec 19, 2007

Identification of external influences on temperatures in California

Céline Bonfils, Philip B. Duffy, Benjamin D. Santer, Tom M. L. Wigley, David B. Lobell, Thomas J. Phillips, Charles Doutriaux
Climatic Change
  • Uses nine different observational datasets to estimate California-average temperature trends during the periods 1950–1999 and 1915–2000
  • Compares observed results to trends from a suite of climate model simulations of natural internal climate variability
  • Finds that, on the longer (86-year) timescale, increases in annual-mean surface temperature in all observational datasets are consistently distinguishable from climate noise
  • On the shorter (50-year) timescale, results are sensitive to the choice of observational dataset
  • Finds that for both timescales, the most robust results are large positive trends in mean and maximum daily temperatures in late winter/early spring, as well as increases in minimum daily temperatures from January to September
  • Finds that these trends are inconsistent with model-based estimates of natural internal climate variability, and thus require one or more external forcing agents to be explained
  • Finds that the warming of Californian winters over the twentieth century is associated with human-induced changes in large-scale atmospheric circulation