NOAA report unable to pinpoint causes of the historic 2012 U.S. drought

The extreme 2012 drought in the Central Great Plains of the U.S. was more intense than any drought since record keeping began in 1895, says a new NOAA assessment of the historic drought, released Thursday. However, the study was unable to pinpoint the cause of the drought. Other major global droughts in recent years have been linked to global warming and/or natural variation in patterns of sea surface temperatures, but these factors were seemingly not important in causing the drought of 2012, said the team of 19 atmospheric scientists, led by Martin Hoerling of the NOAA Modeling, Analysis, Predictions and Projections Program (MAPP). Their study attributed the drought to a random natural variation in the jet stream, which caused it to become "stuck" far to the north in Canada. Since rain-bearing low pressure systems travel along the jet stream, the northwards displacement of the jet stream resulted in abnormally dry conditions over the Central U.S. "This is one of those events that comes along once every couple hundreds of years," said Hoerling.



Key findings of the report
The researchers focused on a six-state region--Wyoming, Kansas, Nebraska, Colorado, Missouri and Iowa, and found that the amount of precipitation in 2012 was only 53% of the long-term average. This was the driest year since record keeping began in 1895, surpassing the previous record driest years of 1934 and 1936, during the great Dust Bowl drought.

The researchers called the 2012 drought a "flash drought"--it developed suddenly in May, and was unrelated to the 2011 drought over Texas and surrounding states. The 2011 drought had a separate and well-understood trigger (a change in the jet stream and storm tracks, due to a La Niña event in the Eastern Pacific.)

The 2012 drought was not predicted by long-range weather forecast models. The new report concluded that our ability to predict drought is limited, but some new experimental techniques could improve future drought forecasts. For example, NOAA's long-range GFDL forecast model and the European EUROSIP model correctly anticipated the summer 2012 heat and dryness over the Central U.S. in projections made as early as January 2012.


Figure 1. Drought-damaged corn in a field near Nickerson, Nebraska, Aug. 16, 2012. The great U.S. drought of 2012 was the most extensive U.S. drought since the 1930s Dust Bowl. Over a six-state region--Wyoming, Kansas, Nebraska, Colorado, Missouri and Iowa--precipitation during 2012 was only 53% of the long-term average., making it the driest year since record keeping began in 1895. Damage from the 2012 drought is at least $35 billion, and probably much higher. The associated heat wave killed 123 people, and brought the U.S. its second hottest summer on record. (AP Photo/Nati Harnik)

Criticism of the report
Dr. Kevin Trenberth of the National Center for Atmospheric Research was critical of the report's conclusions. In comments posted in Joe Romm's blog at climateprogress.org, Dr. Trenberth said that the study failed 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" and "no attempt was made to include soil moisture, snow cover anomalies, or vegetation health" in the climate model runs performed.

I would have liked to have seen the paper mention the growing body of research that has linked unusually early May snow melt in the Northern Hemisphere and Arctic sea ice loss in recent years to unusual summertime jet stream patterns, like the jet stream pattern observed during 2012. A March 2013 paper by scientists at the Potsdam Institute for Climate Impact Research (PIK) in Germany found that under special conditions, the atmosphere can start to resonate like a bell. This causes the jet stream pattern to freeze in place and amplify, leading to months-long periods of weather extremes. They showed that warming of the Arctic due to human-caused climate change might be responsible for this resonance phenomenon, which became twice as common during 2001 - 2012 compared to the previous 22 years. One of the more extreme examples of this resonance occurred during the summer of 2012, and could have been the cause of the 2012 drought.

Other blogs on the report
Yes, Climate Change Is Worsening U.S. Drought — NOAA Report Needlessly Confuses The Issue by Joe Romm at climateprogress.org

Global Warming Not Significant in 2012 Drought: Report by Andrew Freedman of climatecentral.org

Have a great weekend, everyone!

Jeff Masters