Dec 1, 1999

Detection and Attribution of Recent Climate Change: A Status Report

T. P. Barnett, K. Hasselmann, M. Chelliah, T. Delworth, G. Hegerl, P. Jones, E. Rasmusson, E. Roeckner, C. Ropelewski, B. Santer, and S. Tett
Bulletin of the American Meteorological Society
  • Addresses the question of where we now stand with respect to detection and attribution of an anthropogenic climate signal
  • Reviews scientists's ability to estimate natural climate variability, against which claims of anthropogenic signal detection must be made 
  • The current situation suggests control runs of global climate models may give the best estimates of natural variability on a global basis, estimates that appear to be accurate to within a factor of 2 or 3 at multidecadal timescales used in detection work.
  • Estimates present uncertainties in both observations and model-simulated anthropogenic signals in near-surface air temperature
  • Finds that the uncertainty in model simulated signals is, in places, as large as the signal to be detected
  • Discusses two different, but complementary, approaches to detection and attribution in the context of these uncertainties
  • Applies one of the detection strategies, and finds that the change in near-surface, June through August air temperature field over the last 50 years is generally different at a significance level of 5% from that expected from model-based estimates of natural variability
  • States that greenhouse gases alone cannot explain the observed change
  • States that two of four climate models forced by greenhouse gases and direct sulfate aerosols produce results consistent with the current climate change observations, while the consistency of the other two depends on which model's anthropogenic fingerprints are used
  • Finds that most, but not all, results suggest that recent changes in global climate inferred from surface air temperature are likely not due solely to natural causes
  • States that at present it is not possible to make a very confident statement about the relative contributions of specific natural and anthropogenic forcings to observed climate change
  • Concludes that one of the main reasons is that fully realistic simulations of climate change due to the combined effects of all anthropogenic and natural forcings mechanisms have yet to be computed
  • Provides recommendations for reducing some of the uncertainties that currently hamper detection and attribution studies