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By Root 1056 0

outcomes of climate change is that the Hadley Cell circulation will weaken slightly and expand. This appears not only in a broad range of climate model projections for the future, but also from historical data extending three decades into the past.244 The effect of this is the spawning of more clouds and rain in the tropics, but even drier conditions and a poleward expansion of the two desert blast zones straddling both hips of the equator. Precipitation futures are notoriously difficult to project, but this is one of those things about which all the climate models agree. Put simply, many of the world’s wet places will become even wetter, and its dry places even drier.

Rainfall will increase around the equator, but decrease across the Mediterranean, Middle East, southwestern North America, and other dry zones. Rivers will run fuller in some places and lower in others. One highly regarded assessment tells us to get ready for 10%-40% runoff increases in eastern equatorial Africa, South America’s La Plata Basin, and high-latitude North America and Eurasia, but 10%-30% runoff declines in southern Africa, southern Europe, the Middle East, and western North America by the year 2050.245 Through the language of statistics, these models are telling us to brace for more floods and droughts like the ones in Iowa and California.

The Great Twenty-first-Century Drought?

Part of the explanation for the many floods and droughts that happened around the world in 2008 was that it was a La Niña year, meaning that sea surface temperatures (SSTs) in the eastern half of the tropical Pacific Ocean cooled off. This triggered, among other things, dry conditions over California, contributing to its ongoing drought (her counterpart, El Niño, is associated with warm SSTs and wetter conditions there). Through connections between the sloshing ocean and the atmosphere, this “Little Girl” had impacts on human water supply that reverberated worldwide.

My UCLA colleague Glen MacDonald, an expert in the study of prehistoric climate change, is deeply concerned that something like the 2008 La Niña could happen again—but persisting for decades rather than months. In fact, MacDonald and his students believe the American Southwest, in particular, could be struck by a drought worse than anything ever seen in modern times. From shrunken tree-rings and other prehistoric natural archives, they have assembled a growing body of evidence that the region suffered at least two extended “Perfect Droughts” (coined by MacDonald to describe periods when Southern California, northern California, and the upper Colorado River Basin all experienced drought simultaneously) during medieval times.246 These Perfect Droughts were as bad as or worse than the Dust Bowl but lasted much longer, persisting as long as five to seven decades (the Dust Bowl lasted barely one). These prehistoric data tell us that this heavily populated region is capable of experiencing droughts far worse than anything experienced since the first European explorers arrived.

One reason for these massive prehistoric droughts was that between seven hundred and nine hundred years ago temperatures rose. The increase was similar to what we are beginning to see now but not so high as what climate models are projecting by 2050. The reason for the medieval temperature rise (fewer volcanic eruptions plus higher solar brightness) was different from what’s happening today, but it nonetheless provides us with a glimpse of how our planet might respond to greenhouse warming.247

Not only did the medieval climate warming increase the drying of soils directly, it may also have altered an important circulation pattern in the Pacific Ocean, by shifting relatively cool water masses off the western coast of North America for many decades at a time (this would be a prolonged negative phase of the so-called “Pacific Decadal Oscillation,” an El Niño- like oscillation in the northern Pacific that currently vacillates over a 20-30-year time scale). This likely created pressure systems driving rain-bearing storm tracks north,