Mar 12, 2018

Model Assessment of Observed Precipitation Trends Over Land Regions: Detectable Human Influences and Possible Low Bias in Model Trends

Thomas R. Knutson and Fanrong Zeng
AMS Journal of Climate
  • Assesses precipitation trends for 1901-2010, 1951-2010 and 1981- 2010 over relatively well-observed global land regions for detectable anthropogenic influences and for consistency with Coupled Model Intercomparison Project 5 (CMIP5) historical simulations
  • Finds that the CMIP5 historical All-Forcing runs are broadly consistent with the observed trend pattern (1901-2010), but with an apparent low trend bias tendency in the simulations
  • Finds that despite this bias, observed and modeled trends are statistically consistent over 59% of the analyzed area
  • Finds that in over 20% (9%) of the analyzed area, increased (decreased) precipitation is partly attributable to anthropogenic forcing
  • These inferred human-induced changes include: increases over regions of the north-central U.S., southern Canada, Europe, and southern South America; and decreases over parts of the Mediterranean region and northern tropical Africa
  • Trends for the shorter periods (1951-2010 and 1981-2010) do not indicate a prominent low trend bias in the models, as found for the 1901-2010 trends
  • An atmosphere-only model, forced with observed sea surface temperatures and other climate forcing agents, also under-predicts the observed precipitation increase in the northern hemisphere extratropics since 1901
  • States that the CMIP5 All-Forcing ensemble’s low bias in simulated trends since 1901 is a tentative finding which, if borne out in further studies, suggests that precipitation projections using these regions/models could overestimate future drought risk, and underestimate future flooding risk