Warming-Enhanced Texas Drought Is Once in “500 or 1,000 Years … Basically Off the Charts,”

http://thinkprogress.org/romm/2011/11/30/378412/texas-drought-historic-off-the-charts-says-state039s-climatologist/

By Stephen Lacey on Nov 30, 2011 at 2:30 pm

From October of 2010 through this September 2011, Texas saw its driest year on record, according to the National Oceanic and Atmospheric Administration.

But these historic dry conditions stretch back even further than that.

After examining tree-ring data going back to 1550, researchers at Columbia University found that this year’s drought was only rivaled once in the last 461 years. According to the Palmer Drought Severity Index, a system for measuring wet and dry conditions, the last time Texas experienced a drought this bad was in 1789.

The state’s climatologist, John Nielsen-Gammon, explained the historical significance of the ongoing drought in an interview with CBS:

“This is basically off the charts. Based on past history, you wouldn’t expect to see this happening in maybe 500 or 1,000 years. One more year and we’re already talking about a drought more severe than anything we’ve ever had. And this will become for them, the drought of record.”

The drought, which Nielsen-Gammon says could stretch over a number of years, has devastated cotton crops, livestock, pumpkin crops, and, as the below CBS story points out, Christmas trees.

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As Texas climatologist Katherine Kayhoe put it in an email to Climate Progress, dumping ever-increasing amounts of greenhouse gases into the atmosphere is setting the conditions for turning extreme-weather events into history-setting catastrophes:

We often try to pigeonhole an event, such as a drought, storm, or heatwave into one category: either human or natural, but not both. What we have to realise is that our natural variability is now occurring on top of, and interacting with, background conditions that have already been altered by long-term climate change.

As our atmosphere becomes warmer, it can hold more water vapor. Atmospheric circulation patterns shift, bringing more rain to some places and less to others. For example, when a storm comes, in many cases there is more water available in the atmosphere and rainfall is heavier. When a drought comes, often temperatures are already higher than they would have been 50 years ago and so the effects of the drought are magnified by higher evaporation rates.

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Although the drought is linked to La Nina, it is also exacerbated by climate warming, Trenberth adds. Human climate change adds “about a 1 percent to 2 percent effect every day in terms of more energy. So after a month or two this mounts up and helps dry things out. At that point all the heat goes into raising temperatures. So it mounts up to a point that once again records get broken. The extent of the extremes would not have occurred without human climate change.”Although the drought is linked to La Nina, it is also exacerbated by climate warming, Trenberth adds. Human climate change adds “about a 1 percent to 2 percent effect every day in terms of more energy. So after a month or two this mounts up and helps dry things out. At that point all the heat goes into raising temperatures. So it mounts up to a point that once again records get broken. The extent of the extremes would not have occurred without human climate change.”

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