It is time to stop quivering in our boots in pointless fear of the future and just roll up our sleeves and build it.
- Ray Pierrehumbert

Thursday, October 29, 2009

Texas Drought Abates

It's actually been raining quite a bit these last few weeks, and the outlook is for a wet winter on account of El Nino. Things are pleasant and green again, and we're hopeful for an especially colorful and cheerful spring next year. Nevertheless, it was the worst drought on record for several counties near Austin, though our own county (Travis) did not quite experience that.

There's a news report here and the state climatologist's report (PDF) here.

The image, from the state climatologist's report, shows 24 month precipitation surplus/deficit expressed as standard deviations at the peak of the drought. Darkest red shows 3 standard deviations, and corresponds to the areas of record-setting drought. At that moment, (late August) San Antonio had received less than two feet of rain over the preceding two years. In addition to the rainfall deficit, this drought was exacerbated by unusual heat.

Regarding climate change, the report is willing to take the bull by the (long) horns:
The drought intensified to exceptional status during late spring and summer 2009.
This intensification was associated with an upper‐level jet stream pattern featuring
troughs on the west and east coasts of the United States and a ridge across the
central United States. This weather pattern persisted for close to two months,
inhibiting convective activity and causing late spring storms to move farther north
than usual. The unusual, persistent jet stream pattern simultaneously led to
drought and heat in Texas and rain and cool weather in the Midwest and Northeast.
The extent to which this particular jet stream pattern was a random event or was
driven by particular patterns of sea surface temperatures is not known at this time.

Global warming has been identified as a possible cause of future extensive droughts
in the subtropics, including the southwestern United States. Computer models on
average project a precipitation decline of 5% over the next forty years. However,
long‐term precipitation trends across Texas remain upward. It is possible that the
present drought is the beginning of a long-term decline in rainfall, but it is also
possible that precipitation will remain steady or continue to increase. Based

on present scientific knowledge, it is not possible to say whether global warming
contributed to the present rainfall deficit. Indeed, whatever large‐scale processes
led to the overall upward precipitation trend may have caused rainfall to be greater
than it otherwise would have been.

Global warming has, however, contributed slightly to the severity of the present
drought through higher temperatures. Global temperatures have increase by about
0.7 °C over the past century, and long‐term temperature trends across Texas are
now at or above the sustained warm temperatures of the 1950s. It seems
reasonable to assume that present temperatures in Texas are on average about
1 °F warmer than they would have been in the absence of global warming. This has

increased potential evaporation and water demands by livestock and humans.
Thus, if a similar precipitation deficit had developed in the absence of global
warming, it would not have been quite as severe.

I'm not sure this is exactly the best way to describe the situation, ("global warming has contributed", argh...) but all in all I guess it's a good compromise between getting overly technical, being too vague, and also between being too strident, and being too indifferent.

1 comment:

John Fleck said...

As I wrote recently (, I've come to realize that in places like Texas and New Mexico, where you and I live, the emphasis on the question of whether it will rain more or less is less than helpful. It's the final paragraph here that matters the most, as all of the modeling and increasingly the on-the-ground data suggest that increased evaporation is the dominant variable in our drought equation going forward.