January 9, 2007

The Lessons of Mid-Holocene Droughts

Filed under: Droughts, Precipitation

We have been told over and over that the buildup of greenhouse gases will vastly alter climate all over the world. The planet will be warmer, precipitation will be greater, droughts and floods will savage civilization, and everything will be worse than we could ever believe. In the case of the central United States, we have been warned repeatedly that higher atmospheric concentrations of greenhouse gases will lead to a substantial increase in the duration, severity, and areal extent of droughts in the American heartland.

A recent issue of Geophysical Research Letters includes two articles that shed light on the future (and past) of droughts in the central United States. The first article was produced by five climatologists from academic units at Purdue, Minnesota, Wisconsin, and Oregon. Diffenbaugh et al. focused on the Mid-Holocene period between 8,000 and 3,000 years ago — a period for which the proxy record clearly shows drier-than-present conditions throughout the central United States. They note that “Proxy data also indicate that changes in summer precipitation played a major role in shaping mid-Holocene moisture balance in North America.” Back then, summers were a lot drier than today, despite any lack of elevated concentration of greenhouse gases.

To explore why the central United States was drier than today during the mid-Holocene, the team used “a high-resolution nested climate modeling system to test the relative roles of large- and fine-scale processes in shaping the response of summer aridity to mid-Holocene insolation forcing and insolation-induced changes in SST.” Basically, they used a regional climate model (called RegCM3) and a global general circulation model (called CCSM3) to examine how changes in insolation (solar radiation) and sea-surface temperatures (SST) may have contributed to the drying of central US. Quite notably to us at World Climate Report, they set the atmospheric CO2 level to 280 ppm, which is 100 ppm less than concentrations today.

The four maps below all look quite similar, and they certainly all show a drying in the central US irrespective of which model was used or whether they altered solar radiation or sea surface temperatures. They found that “Summer precipitation anomalies (mid-Holocene minus control) are negative over the central U.S., with differences of up to -2 mm/d stretching from southern Texas through the upper Midwest.” They state “The similarity in the results of the two experiments (Insolation and Insolation + SST) implies that mid-Holocene insolation forcing dominates the response of summer precipitation over the continental U.S.” and “Additionally, we find that this pattern is dictated primarily by changes in large-scale processes.” They find a few inconsistencies in their results, and suggest “This raises the possibility that the ocean dynamics calculated by the coupled GCM are not entirely correct.”


Change in summer precipitation: (a) Response to changes in insolation and sea surface
temperatures (Insolation + SST minus Control) calculated in the RegCM3 simulations; (b) Response to changes in insolation only (Insolation minus Control) calculated in the RegCM3 simulations; (c) Response to changes in insolation and sea surface temperatures (Insolation + SST minus Control) calculated in the CAM3 simulations; (d) Response to changes in insolation only (Insolation minus Control) calculated in the CAM3 simulations (from Diffenbaugh et al., 2006).

The second article in the recent issue comes from three scientists at the University of Illinois who examined oxygen-isotopic composition (d18O) from layered deposits at the bottom of Steel Lake in north-central Minnesota. The high-resolution calcite d18O values are shown to be sensitive to drought in the area, so once again, we, via proxy, have a 3,100 year record of drought. Tian et al. begin their article noting “The detrimental societal, economic, and environmental repercussions of 20th-century drought episodes are evident in the North American mid-continent. However, the intensity, duration, and frequency of 20th-century drought are dwarfed by those inferred from paleoclimatic records in some areas.”

Among many interesting findings, they note that the “1930s Dust Bowl was minor compared to that during many other time intervals both before and after 300 AD. Thus drought events of greater magnitude than the aridity extremes of the 20th century were not uncommon during the late Holocene.” They also note “A striking feature of the record is the mean-state shift around 300 AD; drought was prevalent and long-lasting before 300 AD.” Regarding variability of drought, they state “Results indicate much greater variability before 1900 AD than after” and “In all, ~90% of the variability values during the last 3100 years were greater than the average of the 20th century.” They warn that “This low variability was atypical of the last 3100 years, and it probably should not be expected as the prevailing state of variability for the future.” Finally, they find that drought in north-central Minnesota is significantly related to sea surface temperatures in the North Pacific and to output from the sun.

World Climate Report has addressed the issue of drought many times in the past. We fully realize that the central United States will experience severe drought in the future, and we also realize when this occurs, the global warming fraternity will have a field day. But it is more than interesting that we are told drought will become more common as greenhouse gases buildup in the future, and that variability will increase giving us an undesirable combination of droughts and floods. But as we look at these two recent articles, we see that during the 20th century, and during a time of substantial increase in greenhouse gas concentration, droughts were actually less severe than during the mid-Holocene and variability decreased as well.

References

Diffenbaugh, N. S., M. Ashfaq, B. Shuman, J. W. Williams, and P. J. Bartlein (2006), Summer aridity in the United States: Response to mid-Holocene changes in insolation and sea surface temperature, Geophysical Research Letters, 33, L22712, doi:10.1029/2006GL028012.

Tian, J., D. M. Nelson, and F. S. Hu (2006), Possible linkages of late-Holocene drought in the North American midcontinent to Pacific Decadal Oscillation and solar activity, Geophysical Research Letters, 33, L23702, doi:10.1029/2006GL028169.




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