Rolling up the circus tent: Dispatch #7

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.

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