Last updated October 10, 2018

Identification of external influences on temperatures in California

  • 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

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