- There is a difference between the residence time for a single CO2 molecule (years) and the time over which an enhancement in atmospheric CO2 would decay (centuries, or longer).
- CO2 continually cycles between different resevoirs (atmosphere, oceans, biosphere, lithosphere) and there are a number of different cycles, with different timescales.
- The most rapid process involves the cycling of CO2 between the atmosphere and oceans, and the atmosphere and the biosphere (hundreds of years). The slowest cycles involve reactions with calcium carbonate (10s of thousands of years), weathering (hundreds of thousands of years) and the emission of CO2 back into the atmosphere through volcanic activity.
- The long-term, quasi-stable atmospheric CO2 concentration is essentially set by the slowest of the carbon cycles. Hence, the time it will take to return to equilibrium, after an enhancement in atmospheric CO2, is hundreds of thousands of years.
- Even though the time it would take to return to equilibrium after an ehancement is hundreds of thousands of years, the initial decay – if we stopped emitting – would be quite fast. However, that new CO2 has been added to the system means that some fraction will remain in the atmosphere for a very long time.
- Given what we’ve already emitted, and expect to emit in the future, it is likely that at least 20% of our emissions will remain in the atmosphere for thousands of years.
So, the last point above is what I wanted to discuss here. We’ve currently emitted about 550GtC since the mid-1800s. We’re currently emitting just under 10GtC per year. If we carry one as we are, we could reach 1000GtC by the middle of this century. We would expect 20-25% of this to remain in the atmosphere for millenia (although, this doesn’t mean 20-25% of the specific molecules, but that the enhancement in atmospheric concentration would be equivalent to 20-25% of what we’ve emitted).
Currently, about 45% of what we’ve emitted remains in the atmosphere and atmospheric CO2 is at about 400ppm. If we were to halt all emissions now, we would expect (over a few hundred years), the enhancement in atmospheric concentration to approximately halve (i.e., go from 400ppm to about 340ppm). If we get to the point where we’ve emitted 1000GtC and then stopped emitting, atmospheric CO2 would decay (again over a few hundred years) from around 550ppm to around 380ppm. If we emit even more than 1000GtC, then the atmospheric concentration would remain even higher than 380ppm for thousands of years.An important point, which is discussed in this paper on irreversible climate change, is that the radiative forcing depends logarithmically on atmospheric CO2. If halting all emission reduces the enhancement in CO2 by a factor of 2, it doesn’t mean that it reduces the forcing by a factor of 2. Consequently, what is expected (as shown by the figure on the left) is that if we halt emissions, the CO2 concentrations will drop, but the temperature will not.
This is essentially because if CO2 concentrations were fixed at the peak, then we’d continuing warming to equilibrium. Halting emissions leads to a reduction in atmospheric CO2, but does not lead to much of a reduction in temperature; we’d essentially remain at – or close to – the transient response to the peak CO2 concentration. The significance of this is that, without some additional way to remove atmospheric CO2, the most we can do is halt all emissions, which would be expected to simply stop future warming, but not lead – on average – to any cooling; on century timescales, at least. Also, as the bottom panel in the figure shows, even though warming may stop, thermal expansion of the oceans does not.
What the above also implies that even if we reduce emissions substantially, but not halt it completely, we’d expect continued warming; simply fixing the concentration at the peak would probably require reductions of 80-90%. So, we would expect 20-25% of our total emissions to remain in the atmosphere for thousands of years and – without finding a way to draw extra CO2 out of the atmosphere – the best we can do is to halt all emissions, which would essentially prevent future warming, but would not produce any cooling.
Okay, this post isn’t as fluent and clear as I would have liked. Should probably not have started it late last night, but I’ll post it as is. I should credit Tom Curtis for bringing the Solomon et al. (2008) paper to my attention. I should also highlight Eli’s post that also discusses this topic and has a very nice animation at the end. Comments and corrections welcome.