Shock horror: in a press release from the American Geophysical Union, the following headline promoting a new journal article appeared:
No rise of atmospheric carbon dioxide fraction in past 160 years
Sounds pretty dramatic, doesn’t it? So is everything we thought we knew about the changing composition of the atmosphere wrong? Only if you just read the headline.
As explained at Skeptical Science, the “airborne fraction” is the fraction of two quantities: total human emissions of CO2, and the amount that actually remains in the atmosphere. As is well known, some fraction of the extra CO2 we’ve emitted has been absorbed by nature, notably the oceans (which have become more acid (or, more accurately, less alkaline) in response). Obviously, measuring this fraction, and forecasting what it will do in the future, has major implications for future atmospheric CO2 levels, and thus climate. It has been feared that the natural carbon sinks will be able to absorb less of our CO2 as time goes on, and thus the airborne fraction will increase.
The new paper that caused this fuss estimates the airborne fraction over the past 160 years, and concludes that the it has remained reasonably constant, at around 43%. That is, for every kilogram of CO2 emitted in 1850, about 430 grams of CO2 stayed up. For every kilogram emitted in 2010, about 430 grams stayed up. There is some evidence (see Skeptical Science for discussion) that this fraction may be slowly increasing, but the data is noisy and thus rather ambiguous.
It seems that this estimate is a little bit lower than that reported in the Fourth IPCC report of around 55%, but in line with other recent estimates.
The net result of this is that the growth in atmospheric CO2 concentrations might be a little bit slower than predicted, if the atmospheric fraction continues to stay at its present value. But even if you make that rather heroic assumption, there will still be quite enough CO2 and other greenhouse gases in the atmosphere to radically disrupt the climate.
A chemical engineer wouldn’t be surprised by this because the rate of absorption would be expected to go up as the concentration in the atmosphere increases. The real issues are the effects of rising CO2 on the greenhouse behavior of the atmosphere and the marine ecology – Both remain sources of concern.
Robert Merkel said:
All this paper is saying is that the earths sinks have so far remained robust in their absorptive ratio ie 57% of airborne CO2 is being absorbed by nautical or arboreal sinks whilst 43% of CO2 is remaining ambient in the atmosphere.
But the absolute value of CO2 in the atmosphere is going up through industrial combustion of carbon fuels. So absorbing the same fraction of a greater total quantity still leaves a greater quantity of CO2 in the atmosphere, available to trap heat.
So say in 1900 the industrialized countries emitted one billion tons of CO2 the earth’s sinks were absorbing 57%, that would leave an additional 430 million tons in the atmosphere to trap heat. Fast forward to 2000 when industrialized countries emitted 20 billion tons of CO2, and keeping the same 43% absorption ratio, that would leave 8.6 billion tons in the atmosphere.
The interesting question is what happens to atmospheric concentrations of CO2 when the earths sinks start to lose their absorptive capacity. Presumably this would cause a rapid increase in CO2 concentration. Although we apparently have not reached that point yet.
Its probable that sinks will lose their absorbing capacity and sources will increase their emitting propensity. The author of the paper, Knorr, is adamant that his conclusions do not justify sanguinary attitude towards global warming.
Quite right Jack, and to that one might add that as temperatures rise, once can positively foresee the decline in the efficacy of sinks. Most obviously, warmer oceans are not as efficaceous at holding Co2 as cooler oceans. Similarly, if warmer temperatures lead to a loss of vegetation and the associated biota, as they certainly could in places like the Amazon Basin, one can well anticipate these becoming net emitters.
Finally, one can anticipate that the sinks will at some point become saturated. There’s only so much CO2 that plants can hold and use and once they have it the associated growth will tail off.
Quite right, everyone.
My point in the last paragraph was only that the latest numbers for the airborne fraction seem a little lower than earlier estimates; if the modelling has been done assuming an airborne fraction of 0.55, if it turns out to be stable at 0.43 the results will probably be a little lower.
That the Earth’s carbon sinks have not (yet) begun to lose absorptive capacity is, I would have that was quite significant good news. At least, compared to the alternative.
PS. There seems to be one error.
Of the carbon emitted in 1850, much less than 43% remains. Of the carbon emitted in 2010, much more than 43% remains – probably closer to 99% of it!
More accurate perhaps to say that of the total carbon emitted by humans, about 43% is still in the atmosphere, and that this was true in 1850 and is still true today.
Fair point Ben.
Robert, I’m wondering about the IPCC graph you linked to. I always thought that the ratio was 40% up and 60% down moving to 45% up and 55% down. Maybe I came in late, but this set of slides from Pep Cannadell in 2007 (see slide eight) shows just that. So I reckon that in terms of airborne fraction instead of 40% moving to 45% over time the new research is saying that it’s 43% steady all the time. If the sceptics can make anything out of that, they are hallucinating.
Cannadell’s slide 7 (relates to what Ben said @ 5) is interesting in terms of the problem we are facing. Clearly the emissions really took off with the industrialisation after WW2. I used to think that 5gt of anthropogenic carbon (18.35 of CO2e) could be absorbed benignly by sinks. This is assumed in Stern, not sure about IPCC. If you divide this by 9 million people for 2050 you get about 2t per person pa of CO2e.
Monbiot in 2006 took account of a 2003 study that indicated the sink would be reduced to 2.7gt of carbon by 2030. That’s 1.1t per person of Co2e in 2050 (check India out). But it’s clear that while ever we are emitting above the natural pre-industrial level, atmospheric CO2 is going to increase. So the task becomes very much more difficult.
On my numbers, if absorption does continue to be proportional to airborne CO2, then flat-lining of emissions would be followed by flat-lining of airborne CO2. At a higher than pre-industrial level, certainly, but at least it won’t keep climbing once emissions peak.
Of course, we may still have to deal with consequences of driving the sinks harder. Possibly including stuff like this: “Freak Current Takes Gulf Stream to Greenland” http://bit.ly/4LuIo2
BenA,
That is the most interesting article I’ve seen in a while. Maybe the conveyor is really weakening.
While boreal forests may in fact have some potential to increase their carbon absorbtion through above ground biomass, their below ground C is direly threatened, and temperate and tropical forests haven’t got much of a chance in hell to retain the C stores they have had historically. So yeah, from direct observation and very well understood science of forests, the fraction is definitely trending the wrong way.
BenA,
The full article on the North Atlantic current. A lot more information, particularly with relation to pressure cells. Extremely interesting.
http://www.dailykos.com/story/2010/1/6/822520/-Freak-Current-Takes-Gulf-Stream-to-Greenland
Brian, the denialists probably think “airborne fraction” means the proportion of CO2 in the atmosphere…