But we don't have a strong handle on exactly how path-independent this relationship is. One way of estimating it is via something that we call the Zero Emissions Commitment, or ZEC, which we can measure in Earth System models by abruptly stopping CO2 emissions and looking at how warming responds. - ThreadSky

ckoven.bsky.social • 114 days ago

But we don't have a strong handle on exactly how path-independent this relationship is. One way of estimating it is via something that we call the Zero Emissions Commitment, or ZEC, which we can measure in Earth System models by abruptly stopping CO2 emissions and looking at how warming responds.

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ckoven.bsky.social•114 days ago

Our best estimate of ZEC is that it is roughly zero, which aligns with path independence. I.e. if the temperature-to-cumulative-emissions relationship were perfectly path dependent, then it would follow that ZEC must be exactly zero.

ckoven.bsky.social•114 days ago

doh! typo in this. it should read, "if the temperature-to-cumulative-emissions relationship were perfectly path independent..."

ckoven.bsky.social•114 days ago

But ZEC isn't the only potential path dependency that we might care about. For example, when, if we reduce emissions and achieve net zero, would we expect warming to stop? Al Gore focused on this point at the end of a recent TED talk.

ckoven.bsky.social•114 days ago

Or what about the response to negative emissions? Some day, future generations might try to clean up the mess we have left, by pulling CO2 out of the air and storing it somewhere. How would the climate respond to that? Would it follow the same proportionality to cumulative emissions backwards?

ckoven.bsky.social•114 days ago

We can try to estimate some of these potential path dependencies using metrics derived from the flat10MIP experiments. So, in addition to ZEC, we quantified a set of other path dependencies using the experiment where emissions gradually go through net zero, all the way back to cumulative net zero.

ckoven.bsky.social•114 days ago

Like ZEC, if the warming-to-cumulative-emissions relationship were exactly path-independent, then each of these would be exactly zero. So any nonzero value is a measure of path dependence. Using the flat10MIP experiments, we can compare all of these metrics, to see how they relate to each other.

ckoven.bsky.social•114 days ago

The answer is that the models do show path dependencies, and they are all correlated, but the correlation weakens on longer timescales. So, for example, globally, temperature would stop rising anywhere from decades before to decades after net zero, and the timing is closely correlated with ZEC.

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