Given the recent news that the rise in CO2 has greened the planet, I can see a new “skeptic” meme appearing. Essentially, I can see them arguing that both climate sensitivity is too high, and that nature will take up more of our emissions than we currently think. Consequently, they will argue that we will not warm much because climate sensitivity is low and because projections of atmospheric CO2 rises are too high. For example Judith Curry ended her post by saying
As Nic Lewis points out, this paper alters the RCP scenarios in terms of resulting atmospheric CO2 content; i.e. the RCP scenarios are significantly too high.
I actually don’t understand this particular issue all that well, so thought I’d write this in the hope that some of my commenters might know more than I do (not hard, I hear you say). I should start, however, by pointing out that I don’t think that this greening of the planet is such a surprise and I think it has been included in climate studies for quite some time already. The rise in atmospheric CO2 is about 46% of what we’ve emitted in total; this is called the airborne fraction. About one-quarter of our emissions have been absorbed by the biosphere, and greening of the planet is probably an expected outcome of this.The key issue is really how the airborne fraction will change as we continue to emit CO2. One of the authors of the paper that Nic Lewis mentions above is Pierre Friedlingstein. I found this presentation of his, which includes the figure on the right. It looks rather uncertain, but it seems that the airborne fraction only really starts to increase if we follow an emission pathway above RCP4.5.
So, maybe some of the higher emission pathways will lead to slower increases in atmospheric CO2 than we currently think, but then again, maybe not. If it is only likely to impact the higher emission pathways, this would be good, but not necessarily a reason to pay less attention to the potential risks associated with continuing to emit CO2. Of course, the uncertainty means that we can’t even be confident that this will indeed be the case, and there are other also factors, like ocean acidification, to take into account.
Okay, this post has got somewhat convoluted and isn’t really going anywhere, which is partly because I’m a bit tired, and partly because this is a topic about which I’m not that familiar. I’ll make one point as to why this may be less relevant than it might at first seem. At the moment, the airborne fraction is about 46%. If we were to halt emissions, we’d expect about 20-30% of our emissions to remain in the atmosphere for thousands of years – technically, the long-term enhancement in atmospheric CO2 will be equivalent to 20-30% of our emissions.
One interesting result from climate models is that if we halt emissions, atmospheric CO2 will reduce in such a way that global temperatures will – on average – stabilise (i.e., they will essentially stop rising). If we’re over-estimating the airborne fraction along some of the higher emission pathways, but not the long-term airborne fraction, this would seem to suggest that if we were to halt emissions, global temperatures would – rather than stabilising – continue to rise to the same equilibrium. In other words, if our estimates of the long-term airborne fraction is about right, then even if we’re over-estimating the airborne fraction along some of the higher emission pathways, our long-term warming projections are – I think – unchanged. At least, this would seem to be the case.
So, if this whole greening issue has long-term significance, then it would seem to require that we’re both over-estimating the airborne faction along some of the emission pathways, and over-estimating how much CO2 will remain in the atmosphere once we stop emitting. I’m not even sure if the former is a reasonable inference, but the latter seems a bit of a stretch; at the moment, at least. Anyway, I’ll stop there. If anyone has a better understanding of this than I do, or know where to find out more, comments would be appreciated.