Wednesday, May 24, 2006

Global warming risk 'much higher'

OK, back to some climate science to keep poor Rufus happy (it's not for nothing that I've kept the "Empty Blog" name - even scientists have holidays). Readers have been clamouring for my opinions on this story. And by "readers", I really do mean both of you, separately, which is something of a record :-) Perhaps I should wait until I get to see the papers themselves in a few days, but it's more fun to speculate in advance with no knowledge of the facts :-)

Actually, I will not comment in detail on the science - it looks like two groups have found some observational (historical) evidence of a significantly positive carbon cycle feedback, meaning that any particular emissions pathway will lead to a higher atmospheric CO2 concentration than was previously expected. I will assume for now that this science is solid enough in itself, and merely discuss the presentation of the results.

According to the BBC article, one of the two groups has presented its results in terms of redefining climate sensitivity to account for this feedback, giving a range of 1.6-6.0C rather than the previous 1.5-4.5 (IPCC TAR). If this is really what they have done, it seems to be a confused and unhelpful approach to me. Climate sensitivity is traditionally defined as the equilibrium temperature rise associated with a doubling of atmospheric CO2. As such, it is completely independent of questions about the origins of that CO2, eg how anthropenic emissions vary over time, what proportion of anthropogenic emissions are disolved in the ocean, any other feedbacks in the carbon cycle etc. Of course, one can define anything one likes, but the only way I can see to interpret the new definition would be to say something along the lines of "If we were to consider an emissions pathway that would result in a steady-state doubling of CO2 under the assumption that the carbon cycle feedback does not exist, then the resulting temperature change in the real world (accounting for this feedback) would be likely to be in the range 1.6-6.0C" or some similar verbiage. That's a bit of a mouthful, and it's not clear exactly in what way it would be useful.

What the research does suggest (assuming it holds up under scrutiny) is that for any particular emissions pathway, the resulting CO2 level is likely to be somewhat higher than was previously thought. That does mean that the transient SRES results would have to get bumped up a bit (but to compensate, perhaps the highest ones could be "updated" to more plausible values anyway). Equivalently, for any particular stabilisation level, permitted emissions would have to be lower. I'm not sure of the magnitude of the difference between this and previous work though - AIUI there has already been some research suggesting a positive carbon cycle feedback - it seems that this might be more along the lines of data firming up this as-yet-uncertain hypothesis, than something strikingly new on its own. The BBC journalist talks up this research as a "challenge" to "the consensus view of the Intergovernmental Panel on Climate Change (IPCC)" but I think it would be more realistic to describe it as a slight nudge.

9 comments:

Adam said...

I was wondering about this. Climate sensitivity is a measure of the temperature increase caused by a doubling of CO2, right?

This takes into account various forcings and feedbacks?

If feedbacks along the lines of altered CO2 levels come into play, these do not affect the climate sensitivity, but the path taken to reach that doubling?

Does this mean that no CO2 level affecting feedbacks are incorporated in sensitivity calculations?

coby said...

Climate sensitivity to 2x CO2 takes into account water vapor feedbacks and sea ice feedbacks. It does not consider carbon cycle feedbacks (permafrost melt or oceanic methane releases, carbon from soil, biosphere reactions) or ice sheet changes.

This is an easily confused issue. The take home message is that if *we* double CO2, the actual temp rise will likely be higher than just what climate sensitivity to 2xCO2 would tell us. It also does not give the very long term view of what the effect of a vanishing Greenland ice sheet will have over however many centuries that may take.

James Annan said...

Yes Adam, that's right - we account for the physical feedbacks (eg clouds, water vapour, sea ice) but not anything which will change the CO2 level.

Coby,

if *we* double CO2 is already a rather fuzzy concept that requires the use of some moderately sophisticated modelling anyway - merely pumping out another 600GT in total of carbon as CO2 (ie the pre-industrial atmopheric carbon content) certainly will not do it, as much of what we emit is disolved in the ocean. So this new definition of climate sensitivity doesn't actually help us - for a given emissions scenario, we have to do the detailed calculations anyway. Plus, the physical climate doesn't care whether the CO2 is "our" CO2 or from "natural" sources.

coby said...

True enough, but I think it is reasonable to say that at this point the attribution for the CO2 rise is 100% anthropogenic, no? But I take your point, we don't seem to have much of an idea as to how much current natural sinks will continue to absorb, or even how land use changes will play out.

James Annan said...

Anthropogenic attribution for CO2 rise is probably about 180% currently :-) But maybe that should be 175%, with the remaining 5% due to the temperature feedback (wild guess). If land use changes are partly motivated by climate changes, is that a direct anthropogenic input or a feedback? The can of worms may turn out to be a bottomless pit if you look hard enough :-)

The bottom line for me is that we can measure atmospheric CO2 both now and historically, so this is a convenient variable to use in various ways (eg input to standard climate models). We can't directly measure the proportion of any change that that is due to us, and which proportion is due to various feedbacks (we can estimate through models, but any nonlinearity may make the whole question rather ill-posed anyway).

The papers are supposed to be published tomorrow, but I'm away for the day and have no access at home. So it will be next week before I can comment on what they actually say :-)

Adam said...

Thanks Coby & James. Two further questions:

"Climate sensitivity to 2x CO2 takes into account water vapor feedbacks and sea ice feedbacks. It does not consider carbon cycle feedbacks (permafrost melt or oceanic methane releases, carbon from soil, biosphere reactions) or ice sheet changes."

Will ice sheet changes feedback in ways other than albedo?

"We can't directly measure the proportion of any change that that is due to us, and which proportion is due to various feedbacks (we can estimate through models, but any nonlinearity may make the whole question rather ill-posed anyway)."

I thought that carbon isotope ratios enabled "us" (I don't think I'm up to the task ;) ) to see what proportion of CO2 is man made? Or does feedback produced CO2 produce the same result as forcing CO2 (for wnat of a better phrase)?

James Annan said...

Will ice sheet changes feedback in ways other than albedo?

Well, the proportion of land and ocean will change slightly, as will the elevation (with possible effects on atmospheric circulation) :-) but basically the main effect is albedo, and there isn't enough ice to melt for this to be a big influence on the global scale in the future (between the last glacial maximum and present day is a different matter).

I thought that carbon isotope ratios enabled "us" (I don't think I'm up to the task ;) ) to see what proportion of CO2 is man made?

To a fair precision that is true, but I'm not sure it would enable us to determine exactly what proportion of this year's increase is directly from last year's emissions, and what proportion is actually carbon that was emitted 20 years ago, sequestered into the terrestrial biosphere, then emitted again due to last year's warming.

adam said...

"To a fair precision that is true, but I'm not sure it would enable us to determine exactly what proportion of this year's increase is directly from last year's emissions, and what proportion is actually carbon that was emitted 20 years ago, sequestered into the terrestrial biosphere, then emitted again due to last year's warming."

Thanks again. Over a course of several years, and factoring in estimates of CO2 emitted by industry, could it be calculated how much came from man made sources and how much from feedbacks?

NB I'll switch to the new group soon to ask these questions.

James Annan said...

No doubt it can be estimated. But I'm not the bet person to do that (incidentally, a quick check suggests my 180% is some way off the mark - it should be more than 200%, not less).

And just to re-emphasise in case someone misinterprets what I'm saying, the issue isn't whether the increase in CO2 is essentially "all us" but to what extent it can be partitioned up into direct emissions, vs changes in sinks (which can be further partitioned into those due to natural and anthropogenically-forced climate change, and other anthropogenic land-use changes etc...).