Yesterday, Climatewire (subscription required) released a peer review letter on EPA's Social Cost of Greenhouse Gases (SC-GHG), which got a fair bit of press last year, because EPA's metric was $190/ton, even though the Biden administration was using the $51/ton figure originally developed by the Obama administration. The peer reviewers' views can be distilled down to two major points:
- It's a really solid piece of work that provide a solid technical foundation for the SC-GHG.
- It's really solid only as far as it goes – and it leaves out a lot.
The core issue is that EPA has not attempted to include in the SC-GHG those climate impacts that we do not yet have good means of measuring. These include the impact of climate change on changes in precipitation patterns, which recent events suggest is probably a rather important impact.
How agencies should handle uncertainty is a recurring issue and one that I've addressed multiple times, but it's hard to imagine a context in which the issue has greater significance than in attempting to calculate the SC-GHG. One of the peer reviewers succinctly summarized the issue:
While EPA acknowledges that there are uncertainties, simply not using any value because the analysis is uncertain does not avoid the problem but instead chooses a value of zero.
I am sympathetic to this position. From a purely logical point of view, if we know that a cause has an impact, it seems better to provide the best possible measurement of that impact rather than to act as though it doesn't exist. On the other hand, there is a political dimension to all this and releasing a revised SC-GHG that's open to criticism for not taking uncertainty into account could undermine people's reliance on the SC-GHG.
I take some comfort in the fact that the peer review panel seemed fundamentally both supportive of the SC-GHG as a whole and also reasonably confident that EPA's approach provides a basis for incorporating other impacts into the SC-GHG as methodological advances permit.
I take no comfort from the clear indication that, at a substantive level, the costs imposed by climate change are almost certainly more substantial than we are currently able to measure and are also likely to increase over time.
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