23 July 2012

Krugman vs. Research, Who You Gonna Believe?

In yesterday's NYT Paul Krugman writes:
[R]eally extreme high temperatures, the kind of thing that used to happen very rarely in the past, have now become fairly common. Think of it as rolling two sixes, which happens less than 3 percent of the time with fair dice, but more often when the dice are loaded. And this rising incidence of extreme events, reflecting the same variability of weather that can obscure the reality of climate change, means that the costs of climate change aren’t a distant prospect, decades in the future. On the contrary, they’re already here, even though so far global temperatures are only about 1 degree Fahrenheit above their historical norms, a small fraction of their eventual rise if we don’t act.

The great Midwestern drought is a case in point. This drought has already sent corn prices to their highest level ever. If it continues, it could cause a global food crisis, because the U.S. heartland is still the world’s breadbasket. And yes, the drought is linked to climate change: such events have happened before, but they’re much more likely now than they used to be.

Now, maybe this drought will break in time to avoid the worst. But there will be more events like this. Joseph Romm, the influential climate blogger, has coined the term “Dust-Bowlification” for the prospect of extended periods of extreme drought in formerly productive agricultural areas. He has been arguing for some time that this phenomenon, with its disastrous effects on food security, is likely to be the leading edge of damage from climate change, taking place over the next few decades; the drowning of Florida by rising sea levels and all that will come later.

And here it comes.
Krugman's claims raise an obvious question: Have US droughts actually become more common on climate time scales? Especially US Midwest droughts?

Instead of looking at the musings of a "climate blogger" (as entertaining as that may be) like Krugman does, let's instead look at scientific research that has examined trends in US droughts. A crazy idea, I know. Fortunately, scientists have examined empirical data on the frequency and severity of drought on climate time scales.

Here is Andreadis and Lettenmaier (2006) in GRL (PDF):
[D]roughts have, for the most part, become shorter, less frequent, less severe, and cover a smaller portion of the country over the last century.
A longer excerpt:
We used a constructed time series of soil moisture and runoff over the continental U.S. to examine trends in soil moisture and runoff, and drought characteristics related to these variables for the period 1925–2003. Over much of the country, there has been a wetting trend, reflected in a predominance of upward trends in both model-derived soil moisture and runoff. These trends are generally consistent with increases in precipitation during the latter half of the 20th century observed over most of the U.S. [Groisman et al., 2004], and are in general agreement with results from other studies [Dai et al., 2004; Milly et al., 2005]. Furthermore, trends in the simulated runoff are similar to those in observed records of streamflow at a set of index stations that have been minimally affected by anthropogenic activities. Trends in most drought characteristics are similar to those in soil moisture and runoff, that is, droughts have, for the most part, become shorter, less frequent, less severe, and cover a smaller portion of the country over the last century. The main exception is the Southwest and parts of the interior of the West ...
About that recent breathless NOAA press release and subsequent media frenzy ... Have a look at comments by John Neilsen-Gammon and also Cliff Mass, both of whom pushed the button.

PS. Here is the necessary disclaimer to ward off those, like Krugman, who use the notion of "deniers" to shout down inconvenient voices: Climate change is real, humans have a significant impact on the planet, and mitigation and adaptation policies both make sense, as I argue in The Climate Fix. None of that justifies treating climate science like astrology.