There’s always a feeling of tristesse when they start pulling down the circus tents and loading the last of the elephants into their trailers. The last day of AGU feels a bit like that. AGU puts one much in mind of those medieval faires, or the Jokkmokk Vintermarknad, where people gathered (and still gather, in the latter case) from time to time to exchange goods and the latest news. Our own faire is a marketplace of ideas, though you can buy some nifty stuff here,too. Like a medieval faire, this is a social event as well — a time of feasting and revels, of renewing old friendships, and of making new ones. Happily, any brawls we have here are rather genteel ones.
But, it’s not over ’til it’s over especially in view of the fact that I was chairing (and giving the last talk at) the very last session of the whole shooting match — on evolution of extrasolar Large Earths. A dedicated group of extrasolar types stayed around for the fun. Closer to home, though, I dropped in on the session on Pliocene climate and the session on geoengineering.
The Pliocene was the latest warm time in the Northern Hemisphere before the great glaciations of the Pleistocene closed in. To some extent, as we increase the atmosphere’s CO2 content, we are traveling backward in time so far as climate is concerned. Hence the Pliocene, which ended about two million years ago, has attracted a lot of attention as an analog climate for what may lie ahead. It’s not a perfect analogy, but the challenge of understanding Pliocene climate provides another test of the operation of model physics in a warm climate. Another interesting feature of the Pliocene is that some paleoceanographic data indicates that the tropics were subject to a permanent El Nino configuration, with much more zonally symmetric Pacific temperatures.
Mark Chandler presented a talk raising the concern that explaining the warm Pliocene climate seems to require an assumption of high climate sensitivity (well above the IPCC mid-range). M. A. Medina-Elizalde discussed some new high-resolution data on the temperature of the late Pliocene tropical Pacific. This included alkenone proxy data as well as Mg/Ca. Something that particularly struck me about this data is that the Late Pliocene shows a pronounced 100,000 year cycle in tropical Pacific sea surface temperature. Since the Northern Hemisphere ice sheets had not yet formed at this time, they could not be playing a role in amplifying the effect of the eccentricity cycle. Being purely speculative, I’d suspect we’re seeing some kind of CO2 modulation connected with things going on in the Southern Ocean, or perhaps connected with partial land ice cover in coastal Antarctica. Anarctica was already glaciated at this time. There was also a modelling talk by M. Vizcaino, evaluating several factors proposed to have accounted for Pliocene warmth. The ones that seem to contribute the most to conditions unfavorable for Northern Hemisphere glaciation are elevated CO2, the orbital configuration, and a permanent El Nino.
I skipped the geoengineering talks that rehashed material already covered at the Harvard geoengineering workshop, but there were some new things. The authors of the talks I went to were all quite cautious and were careful to point out the many possible hazards of geoengineering. There was very little new attention, however, to the biggest issue, which is what happens to the planet if you have to suddenly stop the sulfate injection, and then hit the planet with 200 years worth of greenhouse forcing all in two decades. There was plenty to be concerned about, though. In my previous geoengineering post, I pointed out concern that a geoengineered world would have lower precipitation than the normal world, even if you got the temperature right. Kevin Trenberth presented additional support for this, based on analysis of response to volcanic eruptions. There was some concern expressed that these transient results were not representative of the equilibrated response. However, Alan Robock, in a paper subtitled Cooling but Drought, presented simulations that confirmed a sharp precipitation drop in a geoengineered world, and G. Bala re-examined his earlier simulations done with Ken Caldeira to confirm that the effect was there, but overlooked in their analysis.
Of particular interest to me were two papers presenting the first geoengineering simulations carried out with fully coupled dynamic ocean-atmosphere models. This is especially interesting in view of the importance of sea ice response in evening out the difference between the tropics-heavy solar radiation reduction vs. the more uniform CO2 radiative forcing. At the Harvard geoengineering workshop, David Battisti stated that mixed layer ocean simulations of geoengineering were of dubious utility, because they lack the most important processes governing sea ice formation and retreat. The two fully coupled simulations were presented by D.J. Lunt and co-workers, and C.M. Amman and co-workers. Sure enough, these simulations show that geoengineering is much less effective at restoring the natural temperature pattern than was suggested to be the case in the earlier simulations. In particular, if one tunes the global mean to have the right value, one fails to save the Arctic perennial sea ice. This is not a way to save the polar bears, as it has been sold, nor is it a reliable way to save Greenland. Another concern comes from atmospheric chemistry. In a talk substituting for a cancellation, the NCAR group showed that stratospheric warming in a geoengineered world increased ozone destruction — by a factor of 2-3 in the Arctic — even if one took into account the downward trend in stratospheric chlorine coming from the gradual reduction in CFC content of the atmosphere.
I continue to think that geoengineering is a big and unfortunate distraction, but since the cat is out of the bag, it is good that some people are doing the work to head off rosy and over-optimistic projections of sulfate geoengineering as a magic bullet that could substitute for the hard but necessary work of mitigation of CO2 emissions.
What was really most exciting to me in today’s sessions (and last night’s Sagan lecture, presented by Ralph Lorenz) was all the great thinking about solar system and extrasolar planetary climate. The missions in the planning stage for Solar System exploration are really something to look forward to. Now that people think Europa has a thick ice crust (maybe 20km or more) there’s less talk of drilling through with mini-subs, but there’s an orbiter planned, and possibly a lander with a seismometer,which would settle a lot of questions about the nature of Europa’s ice crust. And Ralph Lorenz’s work on hot-air balloon missions to Titan is really cool (about 110K, to be precise).
That wraps it up for this year. I’m sitting in the airport waiting for the red-eye to Chicago. The spontaneously organized session on extrasolar planetary evolution was so productive I’m thinking of organizing a Union session on the subject for next year.
With best wishes to all for Happy Holidays!