Moving Beyond the Hockey Stick

Michael Mann was in town speaking at the University of New Mexico yesterday. I wrote about his talk (sub. req., lots of quotes from the story below the fold) and got to spend some useful time talking to him about big picture climate issues. (Thanks, Mike!)

I didn't have much space, so the story was short, and I chose to focus more on regional climate issues than hockey sticks. I'm kinda sick of hockey sticks, and it was interesting to see that Mann's recent work, the stuff he chose to talk about in his UNM talk and the stuff I've been reading recently, is moving beyond the stick in ways that I think are incredibly useful.

(click through for more)

When I first started blogging about climate here, I spent a lot of time defending the Mann, Bradley and Hughes hockey stick paleoclimate reconstruction against what I saw as unwarranted attacks. It was kinda fun, and I learned a lot, but it's increasingly seemed to me a distraction, for two reasons.

First, the rather arcane statistical debate over the validity of the hockey stick research as evidence of "unprecedented warming" misses the point that there have been lots of other reconstructions that show the same thing. We can throw out the MBH hockey stick (I don't think we have to, but we could) and our knowledge of paleclimate over the past couple of millenia would be at roughly the same place. There's still uncertainty over the strength of internal variability over the last couple thousand years, but the 20th century is obviously anomolous.

Second, from my perspective as a user of the information the climate weenies create, knowing past temperature history on a northern hemisphere or global scale is interesting, but I need to know what's happened here in the past and could happen here in the future - paleoclimate and future projections on a regional scale are critical.

From my story (the snip is a quick couple of graphs of hockey stick history):

Michael Mann faces a tough problem.

A global climate change researcher, Mann sees clear evidence that the planet is warming, in a way that's unprecedented over the past 2,000 years.

But can we expect New Mexico's climate to get wetter or drier as a result? Scientists don't know enough yet to say, Mann said during a talk Friday afternoon at the University of New Mexico.

[snip]

During his talk, Mann highlighted studies he and a team of researchers have done looking at Pacific sea surface temperatures over the last 1,000 years.

For New Mexicans, Pacific temperatures are critical because they are linked to wet spells and droughts.

Warm water temperatures— known as El Niño— bring wet winters. Cool water temperatures— La Niña— bring drought.

The question for global warming researchers trying to understand effects in the Southwest is "How might El Niño change?" Mann said.

Mann's work suggests an odd relationship between global temperatures and the Pacific Ocean in the past. When the planet warms, the complexities of wind and ocean currents cause the equatorial Pacific to cool, and vice versa.

The result is that when the global temperature warmed in the past, La Niñas and drought have happened here, the scientists found.

Whether that means we can say global warming will cause more drought here is an open question, Mann said. "It's a more difficult problem."

The paper is Mann, Cane. Zebiak and Clement from the Feb. 1, 2005, Journal of Climate:

Previously published empirical results demonstrating a statistically significant tendency toward El Niño conditions in response to past volcanic radiative forcing are reproduced in the model experiments. A combination of responses to past changes in volcanic and solar radiative forcing closely reproduces changes in the mean state and interannual variability in El Niño in past centuries recorded from fossil corals. The dynamics of El Niño thus appear to have played an important role in the response of the global climate to past changes in radiative forcing.

That means volcanic forcing - global cooling - is correlated with El Niño, which means wet for us here in New Mexico. By implication, then, warmer global temperatures have in the past meant more La Niña's, and therefore a drier New Mexico. This fits with Ed Cook's drought reconstructions last year in Science that seemed to point toward widespread western drought during globally warm times. (In fact, it was through the Cook paper that I first ran across the Mann et al. work on this.)

Mann's pretty cautious about this. The oceans, which drive all this, are darn complicated, and we're pushing them into uncharted territory with greenhouse forcing. So he was quick to point out that we don't know the answer yet about what happens next. But this is the sort of work that is critical now, as far as I'm concerned. The hockey stick wars are just an annoying sideshow.

Posted by John Fleck at October 08, 2005 12:19 PM