November 14, 2006

Darn Drought Data

Filed under: Droughts, Precipitation

We have all heard the news that droughts will certainly become longer, more frequent, and more severe thanks to global warming. Higher temperatures will surely increase rates of potential evapotranspiration, and even if precipitation patterns remain unchanged, the odds will favor more droughts in the future. The Intergovernmental Panel on Climate Change (IPCC) states in the 2001 Summary for Policymakers that it is “Likely” that “Increased summer continental drying and associate risk of drought” has occurred in the later half of the 20th century and “Likely, over most mid-latitude continental interiors” to occur during the 21st century.

Figure 1 below shows the current state of affairs as of November 4, 2006, and generally, widespread drought in the mid-latitude continental interior is absent. In fact, as we look at the Great Plains, we find more areas in the “Extremely Moist” category than the “Extreme Drought” category. We would all agree that one snapshot of soil moisture conditions in the United States is not an adequate way to test the idea that global warming will lead to an increase in drought in mid-latitude continental interiors. What is needed, of course, is a longer perspective with drought information over hundreds of years.

Our wish for longer term information on drought in the United States has come true given a recent article in The Holocene. Celine Herweijer and her associates at the Lamont-Doherty Earth Observatory at Columbia University used a combination of proxy data (e.g., tree-ring data), historical accounts, limited instrumental records, and climate models to reconstruct the drought patterns of the past. Not surprisingly, they found “The two major long-lasting droughts of the 1930s and 1950s covered large areas of the interior and southern states and have long served as paradigms for the social and economic cost of sustained drought in the USA. Both had severe environmental and social impacts, in the Great Plains and southwest, respectively.” Note that the two major drought periods occurred 50 to 70 years ago, long before the celebrated global warming of the last three decades.

The team states “These events are not unique to the twentieth century” and that “three distinct periods of widespread and persistent drought stand out in these records for the latter half of the nineteenth century: 1856-1865, 1870-1877 and 1890-1896.” Note that these three drought periods occurred during the Little Ice Age which was anything but a period of warmer temperatures. Furthermore, Herweijer et al. note that “Analysis of mid- to late-nineteenth century instrumental and proxy records from the tropical Pacific reveal prolonged La Niña-like conditions during each of the persistent droughts:1856-1865, 1870-1877 and 1890-1896.” In case you have forgotten, El Niño is the condition that is associated with warm water in the eastern Pacific and La Niña is a period with exceptionally cold water in the eastern Pacific. So the droughts of the 19th century not only occurred during a cold century, but they occurred in relatively cold periods of that century (Figure 2).

Figure 2. Observed SST anomalies (°C) during the mid- to late- nineteenth century North American droughts: (a) 1856 to 1865 average, (b) 1870 to 1877 average and (c) 1890 to 1896 average (from Herweijer et al., 2006)

The team expands on this link to cold sea-surface temperatures (SSTs) stating “It is well known that changes in the configuration of tropical SSTs on interannual timescales can strongly influence extratropical precipitation: during La Niña winters there is reduced precipitation across much of the northern subtropics and mid-latitudes, with large deficits in particular in the southwest USA, extending into the Great Plains” and that “persistent drought conditions in the Great Plains were primarily influenced by the tropical part of the SST forcing, with a tendency for drought when the tropical Pacific SSTs are cold.” They also note that “During each of the mid- to late-nineteenth century droughts, the upper tropospheric geopotential heights are lowered in the tropics consistent with cooling at these latitudes.”

Next, the team explored droughts that may have occurred 1,000 years ago during what they called the Mediaeval period (not to be confused with the current Media Evil period). Coral provide proxy information about the past sea surface conditions, and they used a coral-based reconstruction to demonstrate “the potential link between a colder eastern equatorial Pacific and the persistent North American droughts of the Mediaeval period.” They found “that the present multiyear drought in the western USA pales in comparison with a ‘Mediaeval Megadrought’ that occurred from AD 900 to AD 1300. This drought reconstruction also shows an abrupt shift to wetter conditions after AD 1300, coinciding with the ‘Little Ice Age’, a time of globally cooler temperatures, and a return to more drought-prone conditions beginning in the nineteenth century.”

It becomes obvious that major droughts occur during warm and cold periods, but the evidence suggests that relatively cold periods in the tropical Pacific control prolonged droughts in the United States, whether at present, during the Little Ice Age, or during the Mediaeval Warm Period 1,000 years ago.

Since some climate models suggest El Niño will be more frequent than La Niña in the future, perhaps the outlook for future droughts is not so dreary!

Reference:

Herweijer, C., R. Seager and E.R. Cook. 2006. North American droughts of the mid to late nineteenth century: a history, simulation and implication for Mediaeval drought. The Holocene, 16, 159-171.




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