Charlie KovenNew paper published, with @ecoclimatelab and @benmsanderson We asked how the climate and carbon cycles would respond if we were to smoothly transition from positive to negative CO2 emissions. https://doi.org/10.1088/1748-9326/acab1a
We released a preprint of this in August, and explained some of the findings in a twitter thread then, all of which hold for the published version too: https://twitter.com/cdkoven/status/1563170016236748800
I will also be presenting this research this afternoon at #AGU22 https://agu.confex.com/agu/fm22/meetingapp.cgi/Paper/1059666 if you are here come see my talk!
The key question we were after was whether the proportionality of global warming to cumulative CO2 emissions holds even if the cumulative emissions start to decrease. This proportionality is called the Transient Climate Response to Emissions, or TCRE.
Earlier research with idealized experiments had suggested this proportionality does not hold, but future scenarios that transition to negative emissions do show this proportionality. So we were curious what was going on. We designed an idealized experiment that avoids any large abrupt changes to emissions, which we suspected might cause the systematic deviations from proportionality.
The key result is that the proportionality of global warming to cumulative CO2 emissions does hold under negative emissions, even for large amounts of it. The only deviation from this TCRE proportionality is one that would also occur under net zero emissions. That small amount of committed warming or cooling is called the Zero Emissions Commitment, or ZEC.
The surprising result is that ZEC starts to show up in the temperature to cumulative emissions relationship before reaching net zero. This has a lot of implications, including that TCRE and ZEC together govern peak warming levels, that if ZEC is negative, then peak warming may happen before we get to net zero, and that ZEC is a better measure of the deviation from TCRE proportionality than it is a measure of warming that occurs after reaching net zero.
Zack Subin 🏳️🌈@ckoven interesting. Can you unpack that a bit? Is there an expectation that ZEC would in fact be negative or is this just hypothetical?
@zack_subin thanks! No, we don’t know whether ZEC is positive or negative. The IPCC AR6 assessed that the best estimate of ZEC is zero, with low confidence in its sign, and that is still the best estimate. But the point here is that the uncertainty in ZEC is highly correlated with the uncertainty in the relative timing between net zero and peak warming.