There’s an interesting new paper by Katharine Ricke and Ken Caldeira called Maximum warming occurs about one decade after a carbon dioxide emission. The basic result of the paper is that the median time between the emission of some CO2 and the maximum warming from that emission is 10.1 years (90% range from 6.6 – 30.7 years). This leads the authors to conclude that
Our results indicate that benefit from avoided climate damage from avoided CO2 emissions will be manifested within the lifetimes of people who acted to avoid that emission. While such avoidance could be expected to benefit future generations, there is potential for emissions avoidance to provide substantial benefit to current generations.
The main figure from the paper is below, and shows how the maximum warming occurs after about 10 years and that the warming is about 2oC per 1000GtC.
I must admit that it took me a little while to understand what was going on in this paper as I was surprised that the peak warming occurred so soon after the emission. In fact, the paper starts with
It is a widely held misconception that the main effects of a CO2 emission will not be felt for several decades…… Indeed, a co-author on this paper has previously said that ‘it takes several decades for the climate system to fully respond to reductions in emissions’2. Such misconceptions extend beyond the scientific community and have played roles in policy discussions. For example, former US Energy Secretary Steven Chu has been quoted as saying, ‘It may take 100 years to heat up this huge thermal mass so it reaches a uniform temperature …
Although, I think I now understand this paper, I think the comment above is possible a little unfair and a little unfortunate. I think what’s being shown in this paper is related to what I discussed in this post and what Steve Easterbrook discusses here. In the Ricke & Caldeira (2014) paper, I think they considered abrupt CO2 emissions but also included the carbon cycle in their models. Therefore, what I think is happening is that the abrupt emission causes a certain rise in atmospheric CO2. Subsequently, however, the atmospheric CO2 decays at a rate that essentially ensures that the resulting temperature rise will be closer to the transient response to the initial atmospheric CO2 concentration, than the equilibrium response to this initial CO2 concentration. Consequently, this also occurs quite quickly (about a decade, rather than many decades).
So, basically, what this paper is illustrating is that any particular emission will be responsible for warming to the transient temperature of the initial CO2 concentration that this emission produces, and that it will reach this temperature quickly. Therefore, this means that any reduction in emissions can have an effect on quite a short timescale, and also – presumably – means that any calculation of the social cost of carbon should take into account that emissions today can produce their maximal warming within a decade. However, I do think that there are a few things that one should bear in mind.
If we consider a particular instance in time, there will be a particular atmospheric CO2 concentration and the temperature will – at that instant – be the transient response to that CO2 concentration. Any emission will increase the atmospheric concentration and will, consequently, increase the warming to the transient response to the new atmospheric concentration. However, if we want to fix the temperature at that level, we cannot emit any more CO2 at all (i.e., we would need to reduce our emissions to zero). This seems incredibly drastic and highly unrealistic. Even if we want to fix the atmospheric CO2 concentrations at the current level, we’d need to reduce emissions by a factor of 2 almost instantly, another factor of 2 within 2-3 decades, and to a level one-tenth of the original rate within 100 years. Again, incredibly drastic and possibly also unrealistic.
So, even though what this paper is illustrating is probably correct (any particular emission will only be responsible for warming to the transient response to the CO2 concentration that this emission produces) I think we do have to be careful of thinking that the transient response is all that matters. At any instant in time, I suspect that it is virtually impossible to prevent us from continuing to warm to the equilibrium response to the current atmospheric CO2 concentration (and, given slow feedbacks, even beyond this). So, when we think of future warming, the equilibrium response and the thermal inertia of the climate system are relevant, even if each emission is only formally responsible for rapid warming to the transient response to the CO2 concentration it produces.
This post has got rather long a little convoluted. I also hope I’ve understood the Ricke & Caldeira (2014) paper properly. If anyone disagrees, feel free to let me know. I do think the result is interesting and what they conclude is reasonable; any emission reduction can have an impact on quite a short timescale and could, as they say, benefit those alive today, rather than only benefiting future generations. So, yes, any emission reduction has an impact, but the sooner we start reducing our emissions, the bigger the impact. We should, however, bear in mind that we can’t really reverse what we’ve already locked-in.