October 19, 2006

Bogged Down in Soil Moisture

Filed under: Droughts, Precipitation

The Intergovernmental Panel of Climate Change (IPCC) provides itself a great deal of wiggle room in its predictions of future soil moisture levels. On one hand, IPCC predicts in the future that “The globally averaged mean water vapour, evaporation and precipitation increase.” That makes sense when one considers that warmer temperatures will cause an increase in evaporation, and the water that evaporates will ultimately fall from the sky. IPCC also predicts “Most tropical areas have increased mean precipitation, most of the sub-tropical areas have decreased mean precipitation, and in the high latitudes the mean precipitation increases.” Of course, IPCC predicts (and Gore et al. make the most of it) “Intensity of rainfall events increases.” But with respect to the United States, IPCC predicts “There is a general drying of the mid-continental areas during summer (decrease in soil moisture). This is ascribed to a combination of increased temperature and potential evapotranspiration that is not balanced by increases in precipitation.”

Yet another article has appeared in the literature that provides effectively zero empirical support for the prediction of decreased summer soil moisture levels. The latest article is entitled “Summer moisture availability across North America” and is published in the Journal of Geophysical Research by a team of scientists from the United States and United Kingdom. The van der Schrier et al. group reviewed the literature on drought studies in the United States and correctly identified that (a) many researchers use the very popular Palmer Drought Severity Index (PDSI) and (b) the PDSI has many substantial limitations. Among its many problems, they note “A significant drawback of the PDSI is that despite its intended value as a geographically comparable index, it can be poorly suited for investigation of moisture conditions across diverse climatological regions.”

Fair enough. The team convincingly argues that “The self-calibrating PDSI (SCPDSI)” “is more appropriate for geographical comparison of climates of in different regions.” Obviously, the PDSI has its fan base who might die on a cross defending their favorite drought index, but we are going with the flow on the arguments put forward by this team. They also quite fairly note in reviewing strengths and weaknesses of the PDSI versus the new SCPDSI that “Neither does the calculation of the index account for precipitation in the form of snow, but assumes all precipitation to be in the liquid phase and the calculation takes no account of changes in the potential water-holding capacity of the soils when the ground freezes. For these reasons the analysis we report here is restricted to exploring changes in summer moisture PDSI only.”

Great – recall that IPCC was predicting changes in soil moisture in the summer season — van der Schrier et al. seem to have brought a new partner to the dance, and we are very interested in what the new SCPDSI tells us about the great prediction for less summer soil moisture given the ongoing buildup of greenhouse gases.

van der Schrier et al. spend a great deal of time in the article comparing PDSI to SCPDSI and they found “that the PDSI distribution has a distribution which is skewed toward negative values. The distribution is broad, showing a large variance in the extreme events, and is strongly tailed toward positive PDSI values. The mean of the SCPDSI distributions is normally distributed and has a low variance of extreme events.” Great – SCPDSI has statistical properties that seem superior to the old PDSI.

Now for the bottom line. The SCPDSI shows that “The 1930s and 1950s stand out as persistent and exceptionally dry periods, with some clustering of dry years in the early 1960s, whereas the 1970s to 1990s were generally wet.” However, “No statistically significant trend (significance at the 95% level) was found in the mean summer SCPDSI over the 1901–2002 period, nor in the area percentage with moderate or severe moisture excess or deficit. Moreover, no coherent areas in the SCPDSI maps were found which showed a statistically significant trend over the 1901–2002 period.”

Key figures from the article are shown below, and simply stated, the new and wonderful SCPDSI shows that absolutely nothing is happening in terms of trends in summer soil moisture in the United States. IPCC’s “general drying of the mid-continental areas during summer” is just not happening yet in the United States.

Figure1. Mean summer self-calibrating PDSI for the contiguous United States. The values are the spatial average of all grid boxes, weighted by the cosine of the latitude. The solid line shows the 10-year low-pass-filtered values. Positive values are associated with wet conditions, negative values with dry conditions (from van der Schrier et al., 2006)

Figure 2. Indices of the changing area of drought and moisture excess. The areas of moderate (gray bars) or severe (black bars) (top) moisture excess or (bottom) deficiency are shown as percentages of the overall area (from van der Schrier et al., 2006).


van der Schrier, G., K. R. Briffa, T. J. Osborn, and E. R. Cook, 2006. Summer moisture availability across North America, Journal of Geophysical Research, 111, D11102, doi:10.1029/2005JD006745.

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