Excess Greenhouse Gases may be Responsible for Drought

© Eugene S. Takle, 2003

Slowly changing temperature patterns in the tropical Pacific Ocean have long been known to influence precipitation patterns over specific regions around the Northern Hemisphere. For instance warm sea-surface temperatures in the eastern Pacific Ocean, known as El Niño, are associated with extended periods of excess rainfall in the southern US. And cool sea-surface temperatures in the same region (known as La Nina) are correlated with drought in this region. These are naturally recurring patterns that are not likely related to the buildup of atmospheric greenhouse gases.

The period from 1998-2002 produced a persistent pattern of cool sea-surface temperatures in the eastern tropical Pacific Ocean. Although it existed longer than the normal La Nina, this pattern was not unprecedented in the climate record. Hoerling and Kumar (2003) noticed, however, that accompanying this pattern was a highly unusual persistent and strong sea-surface warming in the western Pacific and Indian Oceans.

Global precipitation patterns during this period showed the expected drier-than-normal conditions in the southern and southeast US, but also persistent drought in southern Europe and central-southwest Asia from Iraq to Pakistan.

Hoerling and Kumar (2003) made 51 runs with three global climate models using these observed persistent sea surface conditions to see what peculiarities would develop in the atmosphere and in the climate over continental areas. They found that these two patterns of ocean temperatures - warm in the western Pacific and Indian Ocean and cool in the eastern Pacific - in combination were likely responsible for the globe-spanning drought (see also Kerr, 2003).

So what was the cause of the unusual warming in the western Pacific that conspired with a more-or-less normally occurring La Niña to produce such widespread drought? Hoerling and Kumar (2003) point to evidence from Knutson et al (1998, 1999) that warming in this region is at least partially due to increased atmospheric greenhouse gas concentrations. It is therefore likely that such globe-spanning droughts in the lower mid-latitudes may be more frequent occurrences in the future.

References

Hoerling, M., and A. Kumar, 2003: The perfect ocean for drought. Science, 299, 691-694.

Kerr, R. A., 2003: A perfect ocean for four years of globe-girdling drought. Science, 299, 636.

Knutson, T. R., T. L. Delworth, K. W. Dixon, and R. J.Stouffer, 1999: J. Geophys. Res., 104, 30,981.

Knutson, T. R., and S. Manabe, 1998: J. Climate, 11, 2273.