| 英文摘要: | Emissions pledges from the United States and China have re-energized the push for a global climate agreement. Anna Petherick considers how serious the promises are. This year begins with a sense that the winds of change are picking up speed. As the climate community orientates its efforts towards creating the first legally binding global agreement since 1997, in Paris-Le Bourget in November, there is plenty to fill its sails. Late last year, promises by rich countries to put up money for adaptation in poor ones trickled in, and global mega-emitters stated fresh and more rigorous mitigation intentions for the next 15 years. The political unit with the third largest emissions, the European Union, said it would reach 2030 with emissions 40% below 1990 levels. In the more common language of projected change relative to 2005, this translates into cuts of 43% for sectors within the auspices of the EU's emissions trading system, and 30% for sectors outside of it1 — which is good news, for sure, but by no means a lurch in Europe's historical narrative. A lurch did come, however, from the two biggest emitters. The United States, which never ratified Kyoto, announced a 2025 target of greenhouse-gas-emission reductions 26–28% lower than 2005 levels2. And China, the world's number one emitter, which ratified but is not restricted by Kyoto, said it would try to stop increasing its annual emissions past 2030, perhaps earlier. In short, two countries that account for nearly half of humanity's greenhouse-gas emissions — and that have been thorns in the side of architects of global climate frameworks — have signalled a willingness to shift course. Most importantly, they did this together. That said, many aspects of the joint announcement are unclear. The details are smudgy, if not entirely absent, on how either country will meet its target, what (if anything) will pressure them to stay on track, and whether even bull's-eye achievement will be enough to fend off various environmental consequences. Of these three areas of smudginess, a glance at the build up to the deal helps clarify the first two. Until a few months ago, China clung doggedly to discussing only the carbon intensity of its economy, not absolute emissions. The new deal schedules peak emissions for carbon dioxide, but does not give them a ceiling. The commitment to peak in a decade and a half appears demanding — by some reports3 requiring China to add enough zero-emissions electricity to power Australia, annually, until 2030. Sticking to it will still lead to a world in which China's emissions dwarf those of others. Dabo Guan, a researcher at the University of East Anglia, in Norwich, England, estimates that they will hit thrice those of the US in 2030. China's intention to peak in 2030 is reflected in the emissions path produced from an 'accelerated effort' group of policies that have been modelled by a team at Tsinghua University, in Beijing, and the Massachusetts Institute of Technology4 (see Fig. 1). In this model, achieving such a trajectory requires a carbon tax with gradual increases towards $38 per ton in 2030, a 10% tax on coal, an 8% tax on crude oil, additional charges on electricity consumption to finance renewable options, and meeting the government's current plan to generate 58 GW from nuclear power in 2020, plus taking it up a notch thereafter to hit 450 GW by 2050.
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