Dot Earth Blog: A Closer Look at Extreme Drought in a Warming Climate

The extraordinarily hot, dry summer in the American heartland has seen higher temperatures, so far, than any single year during the devastating 1930s dry spell described in the Son House tune in the video above [performed by my old band]. As corn crops wither and food prices rise, this has resulted in a steady string of visits to Iowa and other sun-baked states by politicians, including both Rep. Paul Ryan and President Obama today.

It has also resulted in a stream of coverage and commentary on the relationship of this and other recent drought episodes to global warming. It?s worth digging a little deeper and putting the heated discussions of the moment into global and historical context.

An article and related infographic in the Review section of the Sunday Times crystallized the issues that attend discussions of human-driven global warming in the context of drought.

The Op-Ed article, ?100-Year Forecast: Drought,? was by three of the co-authors of a new Nature Geoscience paper on the 2000-2004 drought in the American West, which was the worst drought in that region ? by their estimate ? ?in 800 years. In the article, they posited that such droughts, ?once a rare calamity, have become more frequent and are set to become the ?new normal?? if emissions of greenhouse gases are not curbed.

At the same time, the related graphic, a timeline of 21 centuries of deviations from normal precipitation in New Mexico, showed that the authors? notion of a ?new normal? could easily be said to be the ?old normal? given how much of recent history has been extremely dry in that state. (I posted this thought on Twitter over the weekend.)

I sent an e-mail query to the the authors of the Op-Ed piece, including this thought (e-mail shorthand adjusted):?

While I, like you, would love to see global greenhouse-gas emissions slow, I have trouble reconciling your greenhouse-focused Op-Ed message with the facts ? which are that the West is prone to potent sustained?drought?periods, some mega, and much of the development of the region took place in an anomalously wet half century.?In other words, isn?t what you warn of as a ?new normal? actually the ?old normal?, perhaps with a new mix of drivers? Whether through the nature of the West?s climate or a greenhouse push (or both), the region is poised for trouble (which will almost assuredly be amplified should greenhouse trajectories stay what they are).

Christopher A. Williams, a Clark University geographer who was one of the Op-ed authors and an author of the Nature Geosciences paper on recent western drought, replied (shorthand adjusted):

The connection between elevated greenhouse gases and drought in the American West is that greenhouse-induced warming is very likely to increase the frequency and severity of drought conditions in the region. The recent prevalence of today?s droughts may already be caused in part by elevated greenhouse gases. But, indeed, greenhouse-induced warming is not the sole cause of droughts. Natural (non-human-induced) variability is still likely the dominant cause of today?s droughts, and clearly was for megadroughts evidenced in the paleoclimate record. Still, human-caused growth in greenhouse gases is expected to become a new driver of things that have been seen in the distant past as century-long megadroughts, an alarming future by most any measure. In short, projections warn of how continued growth in greenhouse gases will make the American west even drier, with today?s extreme droughts becoming the average condition of the future, and the future?s droughts being far worse than the worst droughts of today.

It?s also worth looking back to research on the drought history of sub-Saharan Africa.?In 2009, an important analysis of lake-bed sediments in Ghana revealed a millenniums-long record of?extreme droughts in West Africa, some a century long.

This finding sparked a valuable discussion among scientists of how to consider, and communicate, drought risks related to human-driven global warming when truly monstrous dry spells ? dwarfing anything in modern experience ? are an underlying norm. (The same issues attend discussions of extreme storms.)

The discussion in that post spilled into the American West and includes valuable input on various realities, one being that ? in both southern Africa and the southwestern United States ? the factors triggering megadroughts remain unclear, while rising greenhouse gas concentrations could well tip the balance toward such an outcome.

Here?s a description of the uncertainty from?Richard Seager,?a senior research scientist at the Lamont Doherty Earth Observatory of Columbia University long focused on greenhouse warming and extreme drought:

In the case of southwest North America I am also impressed by the size of hydroclimate changes on centennial/millennial time scales that had nothing to do with humans. That said, human-induced climate changes there, and elsewhere, will, over the coming decades, reach the amplitude of known past and natural persistent variations, if the models are correct.

Although there are hypotheses, well-documented events such as the centurieslong period of elevated aridity in the [Southwest] during medieval times have no known cause. Many climate records around the globe also show impressive medieval hydroclimate anomalies. An inability to explain these past changes is, I think, a clear indication of the inadequacy of state-of-the-art climate science.

But, the most pressing problem we now face is to predict climate in the near-term future ? years to decades ? and that requires an ability to 1) predict the evolving response to rising greenhouse gases and 2) predict the natural evolution of the climate system on interannual to decadal time scales (ENSO/PDV, AMO ..) and the impacts both 1) and 2) have on climate over land. We are only now beginning to attempt that?.

Finally, with drought and climate change in mind, Keith Kloor conducted an invaluable review of studies of the normal nature of American megadrought last month.

He quoted this important line from a 2010 Proceedings of the National Academy of Sciences paper, ?A 1,200-year perspective of 21st century drought in southwestern North America,? by Connie A. Woodhouse of the University of Arizona and others:

As far as we know, there is no reason why droughts of the duration, severity, and spatial extent experienced in the medieval period could not occur in the future. Even without the anticipated increased warming in the 21st century, droughts of the magnitude of the medieval droughts would present enormous challenges to water management agencies. Worst-case droughts of the 20th century, unlike those of the paleo record, do not contain episodes of many consecutive decades without high [water] flows, so critical for refilling of reservoirs.

In other words, efforts to develop human communities in drought-prone areas ? whether in poverty-mired regions of Africa or the prosperous American West ? had better be resilient to what Joe Romm calls ?dust-bowlification? with or without a greenhouse push.

Source: http://feeds.nytimes.com/click.phdo?i=5dbc5332b6eb269d4444b34d0ef7783c

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