IS THE American Southwest heading for a megadrought? A projection based on climate models suggests this could be the outcome as global warming alters the Pacific weather patterns that normally bring rain to the region (Science, vol 316, p 1181). Now tree rings are revealing just how dry things could get.
In a warren of rooms wedged under the University of Arizona’s football stadium, the Laboratory of Tree-Ring Research conjures up the past in more ways than one. It was here, in 1937, that astronomer A. E. Douglass established the science of dendrochronology after discovering a correlation between decreased sunspot activity and a narrowing of the growth rings in trees. Since then, generations of researchers have used tree rings to date wood and charcoal fragments from archaeological sites and to assess environmental changes across the centuries. That they are interrupted from time to time by fans cheering on the Arizona Wildcats is all part of the lab’s charm.
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Lab member Chris Baisan hands me a section of Douglas fir the size of a dinner plate, cut from a dead log he spotted last year in Harmon canyon, Utah (see Photo). He points to the centre. “The inner date here is AD 660,” he says. “The outer date looks like it’s in the 13th century.”
Older than the Magna Carta, the specimen is also noteworthy because it straddles one of the most intriguing periods in the global climate record. Beginning around 800, the Medieval Climate Anomaly was characterised by warm summers and mild winters in Europe. According to the samples Baisan has collected, the anomaly lasted roughly 500 years, bringing relentless drought to the upper Colorado river basin.
“It’s not that any one year was so extremely dry,” says David Meko, who led the Colorado river study, “but the lack of wet years over many years created a situation unlike anything we’ve seen in recent times.”
Meko’s study used tree rings from 11 sites in the Colorado basin to reconstruct a continuous record of rainfall in the region. By far the worst period was in the mid-1100s, when a “megadrought” persisted for six decades (Geophysical Research Letters, vol 34, p L10705).
If global warming produces a similar effect, the consequences could be devastating. The Colorado river supplies water to 30 million people from Los Angeles to Phoenix and is used to irrigate some of the US’s most productive farmland. More ominous still is the possibility that natural variations like the medieval anomaly could be superimposed on a gradual drying out that is already under way due to climate change.
“Will variations like we’ve seen in the past be superimposed on a generally drier climate? That would be a grim scenario”
“The systems we have now are not designed to handle such a serious drought,” says Ed Cook of Columbia University’s Lamont-Doherty Earth Observatory.
Meko’s team is now working to extend their tree ring reconstruction to even earlier epochs – an indication of how relevant palaeoclimate information is becoming to water managers planning for an uncertain future.
