NewsApril 11, 2013

2012 Drought Worse Than Dust Bowl Era & Unpredictable

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Andrew Freedman

By Andrew Freedman

The extreme 2012 Central Great Plains drought was more intense than the Dust Bowl era droughts of the 1930s, according to a new federal assessment of the origins and predictability of the drought, released on Thursday. The team of 19 atmospheric scientists, led by the National Oceanic and Atmospheric Administration (NOAA), found that global warming may have played a relatively small contributing role by helping to make the drought slightly warmer, and hence drier, than it otherwise might have been. 

Temperature anomalies during July 2012, the hottest month on record in the U.S.
Credit: NOAA.

The biggest “proximate cause” of the drought remains unidentified, the report found, and it was most likely the result of random natural weather and climate variability. However, critics argue that the analysis left out many important factors, such as the unusually low snow pack leading up to the spring of 2012, and studies that show how global warming influences the odds of heat waves.

The period of May through August 2012 was the driest such period in the Central Great Plains since 1895, eclipsing the records set during the Dust Bowl era of the 1930s, the report found. The drought caused the largest single-year loss in corn production since 1866, and the full economic impact of the drought, which in many areas is still ongoing, has not been tallied yet, but is likely to be greater than $12 billion. 

The report was aimed at determining whether the drought, which climate forecasters failed to foresee, was in fact predictable. The conclusion: It is likely that forecasters would be just as blindsided if the same precursor conditions that existed in early spring 2012 were to present themselves again. 

“This report’s appraisal is of an extreme event having limited potential for skillful long-lead predictability,” the report said.

However, experimental techniques are being tested that could improve such forecasts in the future, the report said.

The report may leave more open questions than answers, given that it found that no known source of natural climate variability can shoulder most of the blame for the drought, nor can man-made global warming, which over the long run is projected to make droughts more likely in some parts of the U.S., particularly the Southwest.

The report found that the Central Great Plains drought was distinct from the drought that developed in late 2010 over the Southern Plains, rather than an extension of that event. The report identified the beginning of the Central Plains drought as May 2012, when Mother Nature suddenly shut off the spigot of moisture that typically flows northward at that time of year from the Gulf of Mexico, where it usually triggers beneficial rains across the nation’s breadbasket. 

With High Pressure dominating the region and banishing Gulf moisture to other parts of the country, Omaha, Neb. saw no rain at all during July of last year, and other locations across the drought region also saw little if any precipitation during what is typically a wet time of year, the report found. Even tornadoes steered clear of the Plains, with the least active July on record.

“High pressure is just bad news when it comes to producing rains in summer,” said Martin Hoerling, lead author of the report and a NOAA meteorologist, during a conference call with reporters.

According to the report, computer modeling showed that slowly evolving ocean cycles, such as El Niño and La Niña in the Pacific Ocean, as well as other cycles in the Atlantic Ocean, did not appear to play a significant role in causing the drought. During 2012, La Niña, which is a periodic cooling of equatorial Tropical Pacific Ocean waters, was eyed as a possible key contributor to the drought, but this report casts doubt on that view. 

Instead, Hoerling said that unusually warm waters in the North Atlantic were “suspicious” since some research shows this can favor drought in the Plains. The report also found that the likelihood of severe drought events in the Central Great Plains has increased during the past 10 to 15 years, in response to global sea surface temperature changes, but this is of limited use when making a forecast for an individual summer.

“There was a change in the large-scale, slowly evolving climate that made drought severity more likely” in the past decade or so, Hoerling said, but nothing that pointed to a severe drought in 2012 specifically. In order to determine whether climate change made the 2012 drought more likely to occur, scientists need to conduct a separate analysis, and so this report offers a somewhat limited perspective on the role of climate change in last year’s drought. Instead, the report mainly dealt with the immediate causes of the drought, which led the team to focus on short-term weather and climate variability.

Map of rainfall departures from average for July 2012 across the Central Plains.
Credit: NOAA.

However, the scientists involved in the report said the warming climate means that current and especially future drought events are likely to be hotter and more severe than historical events, and that this one may have been somewhat hotter as well. July 2012, for example, was the hottest month of any month on record in the lower 48 states.

The Texas drought in 2010-2011 was the most severe one-year drought in that state’s history, as well as the hottest, and the Texas state climatologist, John Nielsen-Gammon, has said that some portion of that extreme heat was likely due to the long-term warming trend. 

“Temperatures within the last couple of years have been nudging up against or exceeding those that occurred during the Dust Bowl drought of the 1930s,” said Richard Seager, a researcher at Columbia University’s Lamont-Doherty Laboratory and a contributor to the new report. 

According to Seager, the main factor that controls summer temperatures in the central Great Plains is the amount of precipitation that falls during the season. Wetter summers tend to be cooler than dry summers, and the variability in precipitation has helped mask the long-term warming trend from man-made global warming. 

To find that global warming signal in a particular summer in the Central Great Plains, Seager said, “You’re looking for a tree within a forest of natural variability.”

Kevin Trenberth, a researcher at the National Center for Atmospheric Research and a frequent critic of Hoerling's work, blasted the report's conclusions in an online commentary. “The question never addressed is what does global warming and human influences bring” to the High Pressure areas typically associated with extreme heat and drought events, Trenberth said. 

The report, Trenberth said, “fails completely to say anything about the observed soil moisture conditions, snow cover, and snow pack during the winter prior to the event in spite of the fact that snow pack was at record low levels in the winter and spring.”

Arun Kumar, of NOAA’s Climate Prediction Center, which is responsible for issuing seasonal climate outlooks for the U.S., said that while climate change may be increasing the long-term risk of heat waves and drought, this knowledge is hard to translate into forecasting the probability of specific droughts in a particular region at a particular point in time. 

The bigger factor in seasonal climate forecasting, Kumar said, “is what natural variability is going to do.”

One thing about the report is especially noteworthy: meteorologists could not pinpoint an end to the drought, because it is still ongoing. As of April 9, about 51 percent of the lower 48 states is still experiencing some form of drought. 

Editor's Note: This story was updated on Friday morning to include the view of Kevin Trenberth, a scientist who was not involved in the NOAA report.

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http://Extreme Weather 101: Drought & Our Changing Climate