Blog Archive

Sunday, March 9, 2014

Tamino: California Drought

by tamino, Open Mind, March 10, 2014

Despite recent rains, California is still in the midst of crippling drought. In a recent opinion piece by Martin Hoerling the case is made that essentially, man-made climate change has nothing at all to do with the present California drought.
Thus, the scientific evidence does not support an argument that human-induced climate change has played any appreciable role in the current California drought.
I disagree.
Martin Hoerling is a smart guy, and I wouldn’t call him a “denier.” But we all know how easy it is for smart people to be stupid. It’s even possible for those who aren’t deniers, to deny. I think that’s exactly what Matt Hoerling has done.
There’s a lot of huffing and puffing in the article, but his argument basically boils down to this:
We can also say with high confidence that no appreciable trend toward either wetter or drier conditions has been observed for statewide average precipitation since 1895.
The ironic part is that Hoerling makes this the centerpiece of his argument in spite of emphasizing that drought is not just about precipitation.
It’s true that there’s no significant trend in California precipitation since 1895. Here’s precipitation anomaly, which reduces the counfounding influence of the annual cycle (it tends to be wetter during winter, especially January):
The estimate trend from linear regression (shown by the thick blue line) is decreasing, but not statistically significant even if you don’t correct for autocorrelation. If you do correct for autocorrelation, it’s definitely not statistically significant.
But as Hoerling himself states, drought isn’t just about precipitation. If we look at an actual measure of drought — the Palmer Drought Severity Index, or PDSI — then there is a decreasing trend (which means, toward more and/or more extreme drought) which is statistically significant, even after correcting for autocorrelation:
The blue line shows the trend estimated by linear regression, the red line shows a lowess smooth. Yes, there’s a trend toward more drought in California. Yes, it’s statistically significant. Matt Hoerling’s claim that there’s “no appreciable trend … for statewide average precipitation” is true, but misleading. What’s worse, he reallyought to know better.
A similar, but weaker, trend shows up in a common measure of short-term drought, the Palmer Z-index:
Note that this last January (the last month for which data are presently available from the National Climate Data Center) is the lowest on record. We can see the trend more clearly if we just plot the smoothed version, together with the estimated linear trend.
After correcting for autocorrelation, the linear trend is not quite significant at the 95% confidence level. It’s “only” significant at the 94.8% confidence level. Statistically speaking, it’s one of those things that make you go “Hmmm…”
But instead of making Matt Hoerling go “Hmmm…” it seems to have made him go wrong. Or perhaps he hasn’t even looked at the PDSI or Z-index — in which case, what the hell is he doing pontificating about the California drought? He tries his best to assert that it’s definitely not the case that “human-induced climate change has played any appreciable role in the current California drought.” It sure reads to me as though he isn’t saying “Hold on, let’s not be so sure,” like a genuine skeptic— instead he’s asserting that he knows it ain’t so — like a denier.
What’s worse, for Californians, is that the snowpack in the Sierra Nevada mountains is so low that one can expect drought conditions to persist, possibly even worsen, throughout the year. An extreme example (yes it’s cherry-picked, in order to make a point) is the data from the SNOTEL site at Independence Creek:
Each line represents the snow-water-equivalent (SWE) in the snowpack throughout a single year. The thick red line is the SWE for this year. Notice anything striking about it? Here’s a close-up of the first three months, to make this year’s snowpack easier to compare to previous years’:
Given the current state of affairs, the outlook for California’s water supply this year isn’t just “not good,” it’s grim.
Maybe Marty Hoerling thinks he’s being the “honest broker” and arguing for some sort of “restraint” in assigning a role of man-made global warming. If he thinks that, he’s fooling himself, because he really doesn’t give the impression “hold on, let’s not be so sure,” he gives me the impression “there’s no connection at all and I’m sure of it.” And in that, he’s just plain wrong.
In my opinion, he’s also being irresponsible. Tremendously irresponsible. Man-made climate change is a huge threat to California, and this year’s drought is a perfect example why. It should be an alarm to “Wake up and smell the coffee,” but Matt Hoerling is saying “There is no coffee.”
One of the saddest aspects of this is that the present situation was predicted. The prediction was stunningly accurate — but I do realize that it’s possible to make a prediction which is stunningly accurate because you got lucky. Nonetheless, a stunningly accurate prediction is powerful evidence, and if correct, it means thatman-made global warming was actually the cause of the present California drought, because man-made global warming certainly did cause the disappearance of Arctic sea ice. According to the scientists who made the prediction, prospects for the future are not good. I’d go so far as to say “grim.”

No comments: