Based on tree-ring calculations of drought severity before instrumental records, federal climate experts have concluded that 1789 was the only year in almost five centuries when Texas’ summer drought matched conditions in 2011. There was only one other year in almost five centuries when Texas’ summer drought was as severe as it was in 2011, federal climate experts have concluded. Instrumental weather records used to measure drought severity don’t go back much before the 20th century. (In Texas, they date to 1895.)
To establish a longer-range record, scientists at Columbia University’s Lamont-Doherty Earth Observatory have analyzed tree-ring data and calculated how drought conditions dating back hundreds of years (to 1550 in Texas) ranked on the standard Palmer Drought Severity Index (PDSI). Positive numbers on the PDSI represent wet conditions and negative numbers indicate dry conditions. The more severe a drought is, the lower its PDSI number. Texas’ average PDSI this past summer (June through August) was -5.37 – the lowest, indicating the most severe drought conditions, since the start of the instrumental record in 1895. And according to the federal government’s National Climatic Data Center, there was apparently only one other year during the last 461 years when Texas had a drought so severe.
Going back to 1550, the tree-ring reconstructions reveal that only in 1789 was Texas’ PDSI number so low, the center reported recently. (For our readers who don’t readily recall key historical dates, 1789 was the year when George Washington was inaugurated as the United States’ first president and also when the French Revolution started.) Here’s part of the National Climatic Data Center’s report:
The tree-ring record can put the droughts of the last century across Texas, including 2011, into a much longer perspective. The frequency of severe one-year statewide droughts appears not to have significantly changed between the “paleo” period (1550-1894) and the instrumental period (after 1895). Both the instrumental and reconstructed PDSI records indicate that “severe” or “extreme” statewide summer drought (PDSI below -3) occurred in about 1 in 15 years. “Extreme” statewide summer droughts (PDSI below -4) such as 2011 and 1956 are seen in about 1 in 40 years in both the instrumental and reconstructed records.
So how does the 2011 summer PDSI (-5.37) compare to the worst one-year paleo-droughts? We first need to consider that the tree rings are imperfect recorders of past drought, and so the reconstructed values have confidence intervals (or “error bands”) associated with them. When this error band is taken into account, there is only one value in the paleo record, 1789 (-5.14), that can be said to be equivalent to the 2011 observed value. Thus, 2011 appears to be unusual even in the context of the multi-century tree-ring record.
The current drought in Texas has been unprecedented relative to the century-long observed record in a number of ways: the record-low precipitation, the extreme summer heat, and the enormous wildfires. The tree-ring record of PDSI confirms that, in a much longer context, the 2010-2011 Texas drought is an extraordinary event. And it appears no relief is in sight, the federal Climate Prediction Center said last week in its Winter Outlook for December through February: With La Niña in place Texas, Oklahoma, New Mexico and parts of surrounding states are unlikely to get enough rain to alleviate the ongoing drought. Texas, the epicenter of the drought, experienced its driest 12-month period on record from October 2010 through September 2011
– Bill Dawson