Tuesday, August 28, 2012

Paper finds droughts & floods due to natural variability, not man-made greenhouse gases

A paper published in the Journal of Climate finds regional precipitation trends from 1977-2006 were related to natural variability of sea surface temperatures, not man-made greenhouse gases or aerosols. The finding contradicts claims by alarmists that mankind has "loaded the dice" for more droughts, floods, and extreme weather. In addition, the paper finds climate models are very poor at predicting both the intensity and patterns of regional rainfall, "especially a simulated increase in rainfall over the tropical Pacific and southeastern Australia that are opposite in sign to the actual drying in these areas."

Regional Precipitation Trends: Distinguishing Natural Variability from Anthropogenic Forcing

Martin Hoerling
NOAA/Earth System Research Laboratory, Boulder, Colorado
Jon Eischeid and Judith Perlwitz
NOAA/Earth System Research Laboratory, and Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado

In this study, the nature and causes for observed regional precipitation trends during 1977–2006 are diagnosed. It is found that major features of regional trends in annual precipitation during 1977–2006 are consistent with an atmospheric response to observed sea surface temperature (SST) variability. This includes drying over the eastern Pacific Ocean that extends into western portions of the Americas related to a cooling of eastern Pacific SSTs, and broad increases in rainfall over the tropical Eastern Hemisphere, including a Sahelian rainfall recovery and increased wetness over the Indo–West Pacific related to North Atlantic and Indo–West Pacific ocean warming. It is further determined that these relationships between SST and rainfall change are generally not symptomatic of human-induced emissions of greenhouse gases (GHGs) and aerosols. The intensity of regional trends simulated in climate models using observed time variability in greenhouse gases, tropospheric sulfate aerosol, and solar and volcanic aerosol forcing are appreciably weaker than those observed and also weaker than those simulated in atmospheric models using only observed SST forcing. The pattern of rainfall trends occurring in response to such external radiative forcing also departs significantly from observations, especially a simulated increase in rainfall over the tropical Pacific and southeastern Australia that are opposite in sign to the actual drying in these areas.

Additional experiments illustrate that the discrepancy between observed and GHG-forced rainfall changes during 1977–2006 results mostly from the differences between observed and externally forced SST trends. Only weak rainfall sensitivity is found to occur in response to the uniform distribution of SST warming that is induced by GHG and aerosol forcing, whereas the particular pattern of the observed SST change that includes an increased SST contrast between the east Pacific and the Indian Ocean, and strong regional warming of the North Atlantic Ocean, was a key driver of regional rainfall trends. The results of this attribution study on the causes for 1977–2006 regional rainfall changes are used to discuss prediction challenges including the likelihood that recent rainfall trends might persist.


  1. A new paper published in nature cites the paper above and summarizes the findings as

    "The long-term SST trend is part of the global warming; however, many of the observed decadal to multidecadal SST variations are absent in greenhouse-gas- (GHG) and aerosol-forced coupled model simulations 13, implying that these SST variations are unforced, natural variations whose phase or timing and spatial patterns may depend on the initial conditions of the models and thus they are generally irreproducible."

  2. http://www.theregister.co.uk/2012/09/14/climate_models_too_many_droughts/