Texas Drought Update: Third, with a Bullet - June 3, 2011
For the past several weeks, media have been asking me whether this is one of the worst droughts Texas has ever seen. So far, my answer has been "not yet, but if it stays dry like this..." Now it's been dry long enough, and kept getting drier long enough, and gotten far enough into the growing season, that my answer from here on out is "yes, this is one of the worst Texas droughts ever."
To put it into historical perspective, I've compiled a graphic showing the year and intensity of the twenty worst droughts for this time of year (end of May) since the start of statewide record-keeping in 1895, as measured by the most common drought indicator, the Palmer Drought Severity Index (PDSI).
The PDSI is based on a combination of rainfall and temperature, and is most sensitive to droughts that have lasted about six to twelve months. The present drought has been around long enough that its intensity is well represented by the PDSI.
Texas is subdivided into ten so-called climate divisions. As of now, four out of ten are in PDSI-based "extreme drought" (PDSI < -4.0, for you drought connoisseurs). Only two years have been worse: 1918 and 1956.
The National Climatic Data Center recently released its preliminary rainfall totals for May. Here is the (ugly) tale of the tape:
May: 9th driest (1895-present)
Apr-May: 2nd driest
Mar-May: driest ever
Feb-May: driest ever
Jan-May: 2nd driest
Dec-May: driest ever
Nov-May: 2nd driest
Oct-May: driest ever, also driest 8 consecutive months ever
Sept-May: 3rd driest
Aug-May: 3rd driest
We rank behind 1918 and 1956 because they were preceded by other dry years, making the water shortages that much more acute. So I think it's fair to say, based on the objective evidence, that for this time of year this is the third-worst drought on record for Texas.
What are the future prospects? Not so good. The short-range and medium-range forecasts are dry for Texas. Over the longer range, the national drought outlook has much of Texas pegged for "slight improvement", under the premise that if it's already exceptionally dry it can't get much worse. But I took a look at the droughts shown in the figure, and compared the May drought severity to August drought severity, and found that only about a fifth showed substantial improvement. The rest were about evenly split between staying about the same and becoming considerably worse.
Summertime temperatures in Texas are tied closely to present and past rainfall. Moist ground doesn't heat up as much as dry ground. So it should be no surprise that Houston Intercontinental Airport reached 100 degrees F yesterday, earlier than ever before. Unless Texas gets doused by a lot of tropical moisture soon, this is going to be a long, hot summer.
According to a recent op-ed by Bill McKibben (see here), "the drought [in Texas and adjoining parts of Oklahoma and New Mexico] is worse than that of the Dust Bowl". I find myself disliking this op-ed more and more each time I read it. The quoted statement is one of the most offensive. Of course it's worse than that of the Dust Bowl: the Dust Bowl drought doesn't even make the top twenty for Texas because most of Texas wasn't severely affected by it except for parts of the Texas Panhandle. But it's not worse than 1956, and it's not worse than 1918.
McKibben wishes us to believe that detecting climate change in extreme weather is obvious. As I've pointed out before, the drought here in Texas is not independent of the floods elsewhere: if some places don't get enough rain, others get too much. The simultaneous occurrence of droughts and floods is normal. The same weather pattern that made the Pakistani floods possible in summer 2010 also brought the Russian drought. Now, the intensity might be unusual, or the frequency might be unusual, but global warming does not get credit for the simultaneity.
So let's look back again at the historical droughts. What sort of trend do you see in the diagram above? Not much. Mainly, it's erratic. Perhaps they've become more frequent recently, but droughts have been coming in clusters since 1895. If perchance one of the recent drought years hadn't happened, it would appear that drought was becoming less frequent. So certainly there's no significant trend, nothing that anybody but a fool would conclude was being caused by global warming.
The long-term precipitation trend in Texas is actually positive. It's increasing by about 10% per century, and the trend is statistically significant. This is despite a weak projected decrease in precipitation under global warming. So we really don't know whether Texas will get more rainfall in the future, or less.
Temperatures in Texas, however, are steadily rising. This affects droughts because it causes plants, ponds, and streams to dry out faster. And remember, dry ground makes for hot air in the summer. So this drought is going to be a couple of degrees more brutal, day in and day out, because of global warming.
In wintertime, global warming feels kind of nice. In summertime during drought, it's not nice at all.
From the Houston Chronicle, June 3, 2011
John Nielson-Gammon, Climate Abyss