Climate sensitivity, agricultural productivity and the social cost of carbon in FUND

I have reproduced below only the concluding section of the academic journal article, as the careful statistics would be impenetrable to most readers here. Basically, the article is a complex "proof" of the statement that the likely benfits of global warming outweigh the costs. Global warming is good for us!

By Kevin D. Dayaratna, Ross McKitrick & Patrick J. Michaels
Environmental Economics and Policy Studies volume 22, pages 433–448 (2020

Discussion: IAMs as if–then statements

IAMs cannot provide a single, canonical social cost of carbon. As Weyant (2017) notes, they are best thought of as elaborate “if–then” statements. Researchers must decide on their preferred premises, and the IAMs provide the implied SCC range. As shown herein, user judgment is unavoidable, and a researcher prescribing an SCC for policy purposes must be able to defend the “if” statements that give rise to it.

It is already well known that if the appropriate discount rate is 5% or higher, then the SCC will be relatively small compared to 2.5% or 3% cases. We do not propose to resolve herein the ethical arguments over time preference; instead, we note that once climate sensitivity is changed to an empirically constrained distribution, the choice of discount rate matters a lot less.

While some studies have considered ranges of ECS values, the IAM literature as a whole has been wedded to climate model-based distributions with modal values around 3 °C and thick upper tails extending above 6 °C. However, there is now a substantial climatological literature showing that distributions with modal values below 2 °C and small upper tails match historical (post-1850) data better. The debate over which distribution best describes the real climate system must ultimately be resolved within the climatology literature, but economists need to be aware that it exists and the outcome has significant ramifications for SCC estimates. If ECS values like those estimated in Lewis and Curry (2018) turn out to be approximately correct, then the FUND model indicates that CO2 is for all practical purposes not a negative global externality through mid-century. Even if we consider possible catastrophic tipping points, the possibility of reaching such a threshold any time in the next 1000 years diminishes substantially.

IAM practitioners should therefore study the empirically constrained ECS estimates rather than relying exclusively on model-derived distributions. Kiehl (2007) noted the puzzle that climate models can differ in their implied ECS by a factor of 3 yet all fit the historical surface temperature record equally well. One of the compensating parameterizations emphasized by Lewis and Curry (2018) is aerosol cooling: a model with high ECS paired with strong aerosol cooling fits the surface trend as well as one with low ECS and weak aerosol cooling. The Lewis and Curry (2018) empirical ECS distribution is conditioned on the IPCC’s updated estimates of observed historical aerosol forcing, lending it increased credibility. Specifically, the IPCC’s preferred estimate of aerosol forcing (cooling) has declined over time, which leads to a lower preferred ECS estimate in empirical energy balance models. The methodology of Christy and McNider (2017) provides an independent and model-free check on this approach. Also, while climate models with high ECS values can be made to fit the surface warming trend, they have shown demonstrably excess warming elsewhere, especially in the troposphere over the tropics (Fu et al. 2011; McKitrick and Christy 2018). We therefore believe that the LC18 results in Table 2 are more credible than the ones conditioned on the Roe–Baker distribution.

Another if–then statement concerns CO2 fertilization of agriculture. If adding CO2 to the air has no effect on plant growth, then the assumption in DICE and PAGE that the effect is non-existent is appropriate. However, there is overwhelming evidence that CO2 increases do have a beneficial effect on plant growth, so models that fail to take these benefits into account overstate the SCC. Indeed, the initial studies on which the FUND parameterizations were based cautioned against ignoring this line of benefit (Kane et al. 1992; Tsigas et al. 1997). The recent literature on global greening and the response of agricultural crops to enhanced CO2 availability suggests that the productivity boost is likely stronger than that parameterized in FUND. If the effect is 30% stronger, and if the Lewis and Curry ECS distribution is valid, then the mean social cost of carbon is negative even at discount rates as low as 2.5% at least through mid-century.

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