There was a recent paper about the Evolution of global temperature over the past two
million years. The reason it is controversial is not because it produces a global temperature reconstruction for the last 2 million years, but because it attempts to determine climate sensitivity; in particular the Earth System Sensitivity (ESS). This is essentially the climate sensitivity after the all feedbacks have responded, even the ones that operate on very long timescales.
The analysis in this paper suggests the ESS is around 9oC (range – 7 to 13oC) which, as this Realclimate post points, is wrong. The one pretty obvious problem is pointed out in this post by James Annan. The ESS is almost certainly state dependent. It will probably be higher if the initial state is one with large ice sheets (such as during a glacial period) than it will be during a period with small ice sheets (such as now). Therefore using the transition between a glacial and an inter-glacial will probably over-estimate the ESS for today.
There is a more fundamental problem, though. Determining the ESS by regressing CO2 forcing against temperature essentially assumes that changes in CO2 is the primary driver. As I think this Realclimate post is pointing out, the transition from a glacial to an inter-glacial is triggered by changes in orbital forcing; in particular a large increase in solar insolation at high Northern latitudes (although the net change in global solar forcing is small).
This is thought to then trigger a retreat of the ice sheets which then produces some warming, releasing CO2, changing vegetation, and changing circulation patterns. This then produces further warming, causing the ice sheets to retreat further, releasing more CO2, etc. In other words, it is complex situation with multiple drivers of warming and, hence, assuming that the primary driver is CO2 is probably wrong.
So, using changes in CO2 and changes in temperature to suggest an ESS of around 9oC is almost certainly wrong. However, there is another factor that doesn’t seem to be considered, that I wanted to discuss here (mainly because it confuses me slightly). If you look at projections for cumulative emissions and atmospheric CO2, doubling atmospheric CO2 (280ppm to 560ppm) would require emitting about 1200 GtC (about double what we’ve emitted to date).
According to Archer et al. (2009), the long-term enhancement in atmospheric CO2 will be equivalent to somewhere between 20% and 30% of our total emissions. Therefore if we do emit a total of 1200 GtC (and double atmospheric CO22), the atmospheric CO2 concentration will then tend (over hundreds of years) towards a long-term (millenial timescales) concentration of around 400ppm – 450ppm. So, if this is correct, the long-term warming is not determined by the maximum atmospheric concentration that we reach, but by the atmospheric concentration to which we will tend (somewhere between 400ppm and 450ppm if the maximum is around 560ppm). An ECS of 3oC would then suggest a committed warming of around 2oC, and an ESS of 5 – 6oC would suggest a committed warming of somewhere between 2.5 and 3oC (all numbers approximate and there are, of course, ranges for all quantities).
However, and this is where I get confused, there is another paper that consider permafrost carbon feedback, and which Andy Skuce discusses here. This paper suggests that even if we shutdown anthropogenic emissions after reaching 560ppm, the release of CO2 because of thawing permafrost would be sufficient to maintain atmospheric CO2 at the level when anthropogenic emissions ceased (assuming an ECS of 3oC).
This has become somewhat convoluted and I’m no longer quite sure what my point was. I had thought that we would expect atmospheric CO2 to drop if we were to halt emissions, but it seems that there is a chance that permafrost feedbacks could be sufficient to fix atmospheric CO2, even if we were to halt all anthropogenic emissions. I wrote this thinking that I had some point to make, but I’m no longer sure what that is. Maybe someone who understands this better than I do can clarify our current understanding as to what would happen if we were to halt all anthropogenic emissions.