Something I’ve mentioned here quite regularly is the idea that warming depends roughly linearly on cumulative (total) emissions. This is slightly counter intuitive, in that warming depends logarithmically on atmospheric CO2 concentration. The reason is essentially that it incorporates climate sensitivity (which depends on changing atmospheric concentrations) and carbon cycle feedbacks, into a single quantity. It seems that the airborne fraction is expected to increase so as to compensate for the logarithmic dependence on atmospheric CO2 concentration. In others words, the expectation is that if we double how much we’ve emitted, we’ll more than double the human contribution to the atmospheric CO2 concentration.
There are a number of papers that have considered this and the general result is that it appears to be a reasonable relationship for most realistic future emission pathways, although it might over-estimate the warming from the highest emission pathway. The quantity is called the transient response to cumulative carbon emissions (TCRE) and is thought to have a range of 0.8 to 2.5oC per 1000GtC for real emission pathways, and 1 to 2oC per 1000GtC, for a 1% per year CO2 only emission pathway. The reason for the difference is simply that the real emission pathways include non-CO2 GHGs, while the TCRE is defined in terms of the CO2 emissions only.The reason I’m telling you this is because Nic Lewis has a guest post on Climate Etc. in which he is suggesting that the TCRE is quite a bit lower than other estimates suggest. The figure on the right shows his analysis (solid lines) and the IPCC values (dashed lines). Nic Lewis appears to be suggesting a best estimate for the TCRE of 1.15oC per 1000 GtC, or 0.9oC per 1000GtC if the forcing is CO2 only. His analysis suggests much less warming (along the same emission pathways as used by the IPCC).
The reason, I think, for his result, is pretty straightforward. His underlying model has a low TCR (about 1.35oC) and he is assuming that the carbon cycle feedbacks are on the low end of the range. The carbon cycle feedbacks essentially relate to how the carbon sinks respond to increased CO2 and to increased temperatures. If they’re on the low side, then the sinks are not significantly influenced by increased CO2 levels and warming, and the airborne fraction will remain roughly constant. Hence, the logarithmic nature of CO2 is not compensated for by an increasing airborne fraction. So, the overall warming is reduced because of the lower TCR and because of the weaker carbon cycle feedbacks. A double-whammy.
As I understand it, this all within the realms of possibility, so it could well be what happens. However, discovering that one can develop a plausible model that suggests that warming will be on the low side, is not really evidence that it will be. Also, bear in mind that there is probably something like a range of ± 0.5oC on either side of values presented by Nic Lewis. so even his lower estimates doesn’t rule out greater than 2oC even along an RCP6 emission pathway.Okay, I’ve managed to bumble through this post a bit. What I wanted to highlight was that, as usual, Nic Lewis’s work is being highlight as being observationally based. However, the figure on the left (from Matthews et al. (2009)) shows TCRE determined from 20th century observations (based on warming and CO2 emissions relative to 1900-1909). The range varies from about 1oC to over 2oC per 1000 GtC (depending on the time period considered), with a best estimate of about 1.5oC per 1000GtC. It is pretty similar to the IPCC range I mentioned earlier, and quite a bit more than Nic Lewis estimate of around 1.15oC per 1000GtC. Also, the break in Nic Lewis’s graphs seems to suggest that he thinks we will go from a TCRE of probably around 1.5oC per 1000GtC, to one around 0.5oC per 1000 GtC, starting about now. A little odd as many others seem to think that global warming is probably going to start accelerating.
I guess we will know in the next few decades if Nic Lewis’s suggestion is correct. On the other hand, we’ll also know, in the next few decades, if the 2oC budget really is around 300GtC. Given how we appear to be unwilling to do anything to actually cut emissions, I’m hoping Nic Lewis is correct. I’m not hopeful, though. I also think it would be better to consider all the evidence, not just select what gives us what we’d like to see, but maybe that’s just me.