Does it matter whether the current southwestern US drought is caused by anthropogenic climate change? Or, to be slightly more precise, in what ways does it or does it not matter? This isn’t a rhetorical question. Let me know what you think in the comments below.
The question arises anew in the context of the package of climate change and drought papers in PNAS this week. One of the most interesting of the bunch is by Richard Seager and Gabriel Vecchi, looking at greenhouse warming and the climate now and in the future here in the southwest.
Drawing on the IPCC AR4 model runs, Seager and Vecchi repeat a well-made point – that the poleward expansion of the planet’s arid regions is a well-understood feature of expected greenhouse climate change, and that we here in the southwest are likely to be swept up:
Among the 24 models participating in AR4, the broad agreement that SWNA (southwestern North America) will dry in the current century arises because subtropical drying and expansion are fundamental features of a warming climate. Indeed, it occurs even in idealized global atmosphere models with no surface inhomogeneities whatsoever when the opacity to longwave radiation is increased.
So is that what we already see happening? Has the shift to a new greenhouse-induced drier normal already begun?
The problem in answering that question, they argue, is that natural variability on interannual to decadal scales is large, as a result of the influence of large scale sea surface temperature patterns , and teasing out the climate change signal from that natural variability is not yet possible:
Due to the presence of large amplitude decadal variations of presumed natural origin, observations to date cannot confirm that this transition to a drier climate is already underway.
In answer to my own introductory question, I would argue that, for purposes of societal response to the drought over the 21st century here in the southwest, it does not matter. The steps we need to take as a society to adapt to a greenhouse-forced change in climate are largely the same as those we need to take to adjust to our longstanding misunderstanding of the range and depth of natural variability. Either way, we’re forced to make decisions in the face of fundamental and irreducible uncertainty with a big downside risk of a lot less water. Societal systems robust to decadal-scale droughts of the type seen in the tree ring record will also be robust to greenhouse-induced climate change.