It is time to stop quivering in our boots in pointless fear of the future and just roll up our sleeves and build it.
- Ray Pierrehumbert

Friday, August 24, 2007

The Missing Feedback

People who are relatively sanguine about a climate sensitivity of 3 C per CO2 doubling often miss a number of points. The following are just about the climate system itself, not about the impacts of these changes on human infrastructure or the natural environment, nor about the direct disruptive effects of increasing CO2.
  1. That's a sensitivity, not a prediction. If we cause the atmospheric concentrations to more than double, the temperature perturbation goes up still more.
  2. There are other human perturbations besides CO2, some of them significant greenhouse gases, and all of them wildcards in relatively complicated climate dynamics
  3. It's the sensitivity of what has arbitrarily been called the "climate system" for the purposes of certain scientific reasoning processes. What we should be concerned about is the sensitivity to emissions, not the sensitivity to concentrations. The sensitivity to concentrations is pretty well constrained to be near 3 C per doubling, but we have a far less well-constrained idea of how much net emission corresponds to such a change in concentration.
The last point is widely underappreciated.

We know from paleoclimate evidence that there must be multiple feedbacks in the system. Here's how the argument goes:

Solar forcing is synchronous with much larger climate changes than it can account for directly. We see massive retreats of ice sheets more or less synchronized with peak summer insolation at the southern edge of the ice sheets. That is, when the earth's orbit wobbles so that the sunshine is greatest in, say, the middle of Canada, it is hard for the ice to maintain itself from one season to another, and the ice retreats. This makes qualitative sense.

But whoops! It doesn't make quantitative sense. The thermodynamics don't work out. A little ice should melt, and the ice should retreat a little, but it abruptly almost all goes away. Something gives the system a very big kick, the warming trend runs away, and the ice vanishes relatively abruptly.

SO we notice that CO2 increases synchronously with the ice retreat. Work that in, and it all works out. The extra CO2 enhances the greenhouse effect. The enhanced greenhouse effect rapidly warms the edge of the ice sheet. The ice sheets melt. The theory works out in detail, the models replicate the paleobotanical evidence, and Bob's your uncle.

OK, so problem solved, right? Well, no, not really. Nobody knows how the extra carbon got into the atmosphere. We don't really understand the fluxes of carbon in the system on the time scales of rapid natural climate change.

What we can be pretty sure of is that there is a mechanism whereby increased temperature leads to increased carbon, and we already have a mechanism whereby increased carbon leads to increased temperature. Now fortunately there must be some limit to the process or we would have all been throughly boiled and then steamed long before we had this conversation. On the other hand, that process must exist.

So we need to worry about whether that process has any extra ammunition.

In an excellent series of articles on Grist, Joe Romm points out the serious consequences of this error. Like me, Joe suspects land surface processes are the key, though he doesn't refer his argument to paleoclimate evidence. There are reasons based in isotopic evidence that argue against this, and the majority of the paleoclimate community therefore believes in an all ocean process as the missing feedback.

For now it doesn't matter. We are giving a tippy system which we don't fully understand an extra push; an extra carbon pulse is a very likely result.

This feedback is entirely unrepresented in most climate models. Joe's latest article refers to preliminary results from the first model to model the climate system and the carbon fluxes in a unified system. That news is not good.

I think there are all kinds of scientific reasons why coupling the carbon cycle into a climate model to create what is being called an ESM (Earth System Model) is premature. Many of the cirticisms unfairly leveled at GCMs and CGCMs will be leveled quiasi-fairly at ESMs. The results of these models are not at all trustworthy and it's very hard to say if and when they will be.

Presumably the Mojojojo set, who for some godforsaken idiotic peabrained psychotic reason would prefer to have more money in a lousy world than less money in a nice one, will argue that since ESMs are unreliable, they should be ignored. The contrary is true. If we cannot well constrain the carbon cycle, we need to account for the possibility that a great deal more carbon is lying in wait for us, be it under Siberia, or Greenland, or Antarctica, or the bottom of the Atlantic, or the swamps of Amazonia. Wherever it came from in the past, there might be more of it.

The feedback from temperature to carbon is not well understood but that does not mean it does not exist.

As usual the mojojojoists start with a nugget of scientific truth (the temperature inflection seems to lead the carbon inflection in the ice cores) and promote exactly the wrong conclusion. They conclude that it proves that CO2 does nothing.

(Of course, since they are determined to spew all the CO2 in their nefarious plot to destroy the Power Puff Girls and take over Townsville and then rule the world, every single piece of evidence anywhere is presumed to prove that CO2 does nothing).

In reality this is another piece of evidence that indicates that nature is likely to happily match our contributions to our impending calamity. That "temperature is the cause of the CO2 rise" argument does not prove the falsity of "CO2 is the cause of the temperature rise". They must both have been true in the past.

If they are both true now our boat could be much leakier than the usual understanding of a 3 C sensitivity indicates.

6 comments:

mz said...

Could it be even more drastic... that's a horrible thought.

But A technical comment on the blogpost(need not be published): colored text should be reserved for links. Use bold or italics or underline rather to accentuate. Now it's unclear if they are links or just accentuations and people go clicking on them in vain. Thanks.

Michael Tobis said...

Fair enough. Fixed. Thanks.

inel said...

Thanks for explaining this clearly.

I have two questions:
1 - I do not understand why a land and an ocean process could not work together to tip the balance. Isn't it more likely, with a complex system, that two or more processes could combine to launch us into uncharted waters?

2 - The limit to the shock warming /CO2 rise is only reassuring if it is not in itself a kick with the opposite sense, with the effect that temperatures drop rapidly, followed by CO2 levels … and this just makes me wonder, now that we have unlocked Pandora's box in the form of CO2 from fossil fuel burning, are we really able to predict anything in a future "unknown zone" based on past performances of (pre-human-ecosystem) climate?

As far as "the chicken came from an egg, therefore an egg cannot come from a chicken" argument is concerned (which is how I view the CO2-temperature spiel by those who argue and buy their way out of trouble of any kind) I would address both connections: more eggs make more chickens likely; more chickens make more eggs likely. Then, the key question is not which comes first, but what dependencies operate within that cycle to bring it to a reasonably stable state (of chickens and eggs, or of CO2 and temperatures).

I think it is difficult for anyone—but especially scientists—to talk about these uncertainties without fear of worrying people and being branded alarmists. However, that does not mean that the seriousness of scientists' concerns should be drowned out by those who scornfully reject the suggestion that things are bad and are on course to deteriorate if we continue with business as usual. "Business as never before" can be a positive stance we take upon ourselves to manage to our best ability, or it can be a negative, desperate situation in which we struggle to survive. The choice does not seem hard at all, especially in the light of "The Missing Feedback"

Michael Tobis said...

Inel; to your point 1, yes maybe, but Occam's razor applies. Typically the most economical explanation is the one to favor.

That said I agree with you. The isotopic evidence pretty clearly argues for the ocean participating, but a land-based mechanism is easier to understand.

To your point 2, a lot of people seem worried about this but none of them, to my knowledge, are actually climatologists. We simply don't have any postulated mechanisms for that sort of dyanmics globally. There was something of a theory that this might happen in Europe, but it was over-interpreted in the press, and the evidence supporting even that is getting weaker not stronger.

Hank Roberts said...

Nobody is modeling changes in biomass and changes in species composition of plankton yet, for climate models, right?

I'd bet the feedback is there. Fastest turnover of carbon on the planet, I think.

What happens when calcite and aragonite solubility prevents shell formation in the currently dominant species? Those only evolved in the same time span humans did, some 20,000 years --- enabling plankton to live beyond the narrow continental shelf habitats, as I recall, opening up a huge unexploited area.

Scholar finds 768 hits right now when limited to 2007 pages with climate model plankton
Most of them look interesting!

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Hank Roberts said...

Er, for example:
http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-2486.2007.01377.x

Global Change Biology
Volume 13 Issue 7 Page 1327-1334, July 2007

EVARISTO VÁZQUEZ-DOMÍNGUEZ, DOLORS VAQUÉ, JOSEP M. GASOL (2007)
Ocean warming enhances respiration and carbon demand of coastal microbial plankton
Global Change Biology 13 (7), 1327–1334.
doi:10.1111/j.1365-2486.2007.01377.x

The increase of anthropogenic CO2 during this century is expected to cause warming of large regions of the ocean. Microbes lead the biological role in the CO2 balance of marine ecosystems, their activity is known to be influenced by temperature, and it is important to constrain and quantify these effects on bacterial carbon use. Furthermore, if warming were to enhance the carbon demand (production + respiration) of planktonic microbes but would maintain their efficiency low (as it generally is), then most carbon consumed would end up respired. We designed a strategy in which we measured bacterial production and respiration throughout a seasonal cycle in a coastal Mediterranean site, and determined experimentally the effects of ca. 2.5 °C on these processes. We show that warming will increase nearly 20% the total carbon demand of coastal microbial plankton without any effect on their (commonly low) growth efficiency, which could generate a positive feedback between coastal warming and CO2 production.