| 英文摘要: | Climate and carbon debts are one way to identify contributions to climate change. But they must be seen as part of a larger body of research assessing international responsibility. Delegates will meet in Paris in December for the latest round of international negotiations aiming to design a new global climate policy framework. Once again, there will be debate over the amount different countries should contribute to the international climate policy effort. Research can play a key role in informing this debate, but should be wary of crossing the fine — and crucial — line between policy-relevant research and political debate. The UN Framework Convention on Climate Change (UNFCCC) established that countries' actions should be guided by “common but differentiated responsibilities and respective capabilities”1. There has been a substantial scientific debate on differentiation (often referred to as 'burden sharing' or 'effort sharing') since the early 1990s. In particular, a 1997 proposal by Brazil to assign mitigation targets (to developed countries) based on historical contributions to climate change triggered a body of research2, 3, 4, 5. One of the key insights from this body of work is that, in addition to scientific and data uncertainties, several methodological choices — scientific and policy related — have significant impacts on calculated warming contributions. In Nature Climate Change, Damon Matthews6 adds to this field7 by applying the concept of carbon and climate debts (and credits) to a range of countries. Previous studies typically present the contributions to historical warming as shares of total global man-made impact2, 3, 4. Matthews, however, quantifies contributions using carbon and climate debts, defined as the amount by which contributions have deviated from hypothetical equal per capita shares over time. He focuses on the “over- or under-contribution” of each country to warming relative to this benchmark. Matthews' measurement implies that if per-capita emissions were equal, there would be no debts or credits. It also implies that any country under-contributing is owed some right (a credit) to emit more or to receive some form of compensation. This ignores the common responsibility that would exist if all countries had equal per capita emissions, as well as different capabilities. Matthews argues that the carbon and climate debt concept “offer[s] a new lens with which to examine historical disparities among countries with respect to their contributions to climate warming”. However, this magnifies certain disparities, leaving others out of the frame, and focuses on the differentiated responsibilities to the detriment of the common responsibility. It is also potentially problematic to use calculations of causal contributions directly to infer moral responsibility8. In particular, Matthews' perspective that some countries have undercontributed could reignite the political posturing that has blocked progress on earlier occasions. Calculating historical contributions can be difficult. One crucial choice is how far back in time to include historical emissions, which are used to calculate the warming contributions. This is closely related to the question of when policymakers should have known that climate change is a serious problem. Furthermore, which year should be chosen for evaluating the climate response, and what indicators of climate impact should be used remain undecided. Although emissions, concentrations and radiative forcing are essential and much used, people are more concerned about changes to temperature, precipitation and sea level, not only as global annual means, but also in terms of regional and temporal variability. A broad set of components have disturbed the climate9, short- and long-lived, causing both warming and cooling effects (Fig. 1), and it is not obvious which of these to include in calculations of the contributions of countries. The set of gases regulated by the Kyoto Protocol is one option. But what about SO2, which causes cooling? Should climate credits be given for air pollution?2, 4 These choices have large impacts on the calculated warming contributions.
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