Peter Fawcett has a terrific paper in Nature this week on southwestern megadrought. I’ve been “upstream” (as the science journos like to say) for a while, having been along when Peter and others did some of their very first field work in the Valle Grande in Northern New Mexico back in 2003, and I’ve been talking to Peter (and occasionally writing about the project) ever since. So as a personal matter, this one has been fun on a lot of levels.
It’s also a fascinating paper, using lake sediments to recreate climate signal during Pleistocene. Peter and his colleagues found a couple of very important things:
- a tight correlation between warmer periods and epic droughts
- an apparent collapse in summer rains
They also tie their findings to Richard Seager’s 2007 Science paper on the possibility of huge droughts here as a result of climate change. Which is where, from an inner science perspective, this gets interesting to me. Because Seager told me and other reporters he thinks the connection between their work and his is small. From the story (sub/ad req):
A key to Fawcett’s research is evidence he says shows the region’s prolific summer rains shut down during the ancient megadroughts.
That led Columbia University’s Seager to caution that the new research sheds little light on what we can expect in a warming world, when rising greenhouse gases rather than orbital variations are driving climate changes.
“The thing is that the Fawcett megadroughts were … forced by orbital variations, not CO2, and are caused by drops in summer precipitation, not winter as for the projected future, so I think the relevance of the Fawcett paper to the future is slight,” Seager said in an e-mail.
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Different mechanisms, sure, but what if they happen at the same time? I haven’t read the paper, but if it’s clear that the natural cycles are orbitally forced, presumably there should be some predictability.
Also, what about the Holocene droughts in the SW, in particular the big medieval one? Same thing? And what about those recent cave results (which cover more recent times)? Article too short!
IIRC the drying Seager refers to is basically a consequence of the storm track moving north, an inevitable result of warming, but I suppose it’s possible that related effects farther south could have consequences (good or bad) for the summer monsoon.
Finally, you mention the need for confirmation by other lake sediment cores, but is any such thing in the works?
Article always too short. That’s why I’m writing a book. 🙂
Substantially longer than any of the Holocene droughts.
I asked around, found lots of people who said other proxies from the time period would be good, but no one has one in the works.
I see the abstract has a bit more, from what I can gather postulating that the monsoon basically just goes away with a warmer climate. On the one hand orbital forcings currently have a slight long-term cooling influence, but on the other hand if the summer monsoon is sensitive to the indicated degree of warming from any source rather than to something more subtle then we obviously have trouble on the same time-scales as from AGW. So while Seager’s objection is valid as far as it goes, I wonder if he missed the point.
Re the monsoon, any of stopping, going elsewhere, or just failing to get as far north would be consistent with the paper, which of course highlights the limitation of the single site.
Thanks for your response, John. Hopefully this paper will motivate the gathering of additional data.
Where did the article get placed, BTW?
It was on the front page, with big pictures. We’ve been documenting this project for a long time, so we had shots of him in the field, in the refrigeration unit with mud cores, and we were out in 2004 when they did the drilling.
I didn’t know what they’d find, just thought the science process was cool, which is why I stayed with the story for a long time. Plus Peter’s a great guy.
In mostly unrelated news, I see that the big monsoon is also looking a little delicate. Another model failure, too!
Joe Romm has an updated post on the big picture.
In your Albuquerque Journal article, differences between past and present are put off to orbital variations. Orbital variations do not explain the historic droughts in the US Southwest over the course of the last two thousand years.
Paul – Thanks for the comment, and sorry for the lack of clarity on that point. Fawcett invokes orbital variations as the mechanism for the Pleistocene droughts he’s studying in the Valle Grande record. His paper is silent on the Holocene droughts to which you’re referring (they record doesn’t cover that time period), which are shorter in duration and involve different mechanisms.
I think using the term drought to refer to what happened during those interglacials is a little confusing, since a temporary condition rather than a long-term climate change is implied.
OK, I’ve now done a first and second read of the paper (which Peter kindly sent me), and I’m really wondering what Seager meant. Fawcett et al. point to the poleward movement of the subtropics as the cause of the monsoon failure, but Seager seems to be saying that because the movement was caused by orbital variations rather than increased CO2 it won’t have the same effect on the monsoon going into the future. That makes no sense to me. His objection would make sense if his modeling work showed the monsoon continuing, but that’s not what he seems to be saying.
This is sufficiently interesting to me that I’m going to spend the time to really understand what’s going on, although I have to say a quick count of the quantity of papers involved nearly dissuaded me.
Also, it does indeed look as if the “megadrought” term was inflicted on Fawcett et al. in order to sex up the paper, noting that the term only appears in the title and abstract. Calling it a megadrought is rather like calling the wet conditions of the Amazon rain forest a megapluvial, when it facts it’s a different climate state.