| 英文摘要: | Climate change can affect well-being in poor economies more than previously shown if its effect on economic growth, and not only on current production, is considered. But this result does not necessarily suggest greater mitigation efforts are required. The impacts of climate change include, but are not limited to, increased heat stress, land loss due to sea level rise, and changes in agricultural productivity with consequences for the welfare of populations across the world. As the major cause of climate change is CO2 emissions, the value of the total welfare impact of the change in climate caused by one extra ton of CO2 emissions can be estimated and is known as the social cost of carbon (SCC). The SCC is of great importance for policymakers, as it provides guidance on how to value the benefits of CO2 reductions. The US government, for instance, provides SCC estimates to allow agencies to incorporate the value of the benefits of reducing CO2 emissions in the evaluation of regulatory actions1. Currently, SCC estimates are based on the assumption that climate change affects economic output — the amount of goods and services produced at present — but not the rate at which an economy grows over time. Although the difference may sound trivial, writing in Nature Climate Change, Frances Moore and Delavane Diaz2 show that considering a direct effect of climate change on economic growth would lead to an estimate of the SCC that could be several times larger than previously thought. Most estimates of the SCC are based on economic integrated assessment models (IAMs). There are many types of IAM, but all of them aim at studying the most relevant interactions between the human system and the earth system. Economic IAMs are characterized by a simple representation of the earth system, such as the climate system and the physical consequences of climate change, and a more complex representation of the economy. Although economic IAMs have been subject to criticism, especially because the assumed relationship between global warming and loss of economic output — as depicted by the damage function — is based mainly on guesswork3, 4, they remain influential in policy-making because of their SCC projections. Given their large impacts on policy, the robustness of SCC projections is of great importance. Earlier studies have shown that SCC projections are particularly sensitive to the weight given to the welfare of future generations (as captured by the discount rate), the damage function specification, and to the treatment of uncertainty, especially regarding the possibility of catastrophic events5, 6, 7. The main challenge is that for most of the above parameters, there is either no agreement among scientists or a lack of an empirical foundation. The damage function is notoriously difficult to estimate, as empirical data for global warming conditions higher than current ones are not available. Despite this, the damage functions used in IAMs are surprisingly similar between models, with almost all of them assuming that global warming does not affect economic growth, but only current economic output. Moore and Diaz2 showed how the picture can change if warming does affect economic growth. The impact of such an assumption is mainly reflected in the long-term welfare effects. Assuming that climate change only affects economic output implies that there are no, or hardly any, permanent effects of warming. After all, the growth rate of the economy is not affected. If warming affected economic growth, there would be permanent effects on the economy. Intuitively, a permanent effect of warming on the economy makes sense: destruction of ecosystems and deaths from weather extremes will probably have a long-term effect8. The authors are not alone in analysing the effect of warming on economic growth8, 9. Their contribution, however, is the use of empirical estimates of the impact of temperature shocks on the economy10 to calibrate the damage function. Their newly derived damage function hardly changes the picture for rich countries, but the effect on poor countries is alarming (Fig. 1). Without any climate policy, the impact of climate change on poor countries' economies would be a 40% income reduction by the end of the century, compared with a 12% income reduction according to the original damage function.
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