As our leaders gather in New York for a UN meeting on climate change, it's fair to ask which measures to date have contributed the most to slow global warming. Because no one has really thought to ask this question, The Economist recently tried to find out.

In the chart above, The Economist ranks 20 policies and courses of action according to how much they have done to reduce the atmosphere's stock of greenhouse gases. Figures were pulled from governments, the EU, and UN agencies.

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The first three items — the Montreal Protocol, global hydropower, and global nuclear power — take up the lion's share. Shockingly, China's one-child policy is ranked fourth. Renewables, vehicle emissions testing, and Brazilian forest preservation rank high as well, but from there the items dwindle into sliver-like — but cumulatively important — actions. Fascinatingly, even the collapse of the Soviet Union made the list.

In the supporting Economist article, the authors offer explanations and a ton of caveats. In addition to providing commentary on the first several items on the list, they add that:

[Policies] to slow or reverse deforestation are more important than one might expect. Trees absorb carbon as they grow and release it when they are cut down. According to a recent study in Science, declining deforestation in Brazil meant that the country produced 3.2 billion tonnes less atmospheric carbon dioxide between 2005 and 2013 than it would have if the tree-felling had continued unabated. That is 400m tonnes a year. The slowdown in deforestation in tropical countries is one of the reasons that the conversion of forests to farmland now accounts for only 11% of greenhouse-gas emissions globally, much less than 20 years ago.

The other reason for deforestation's dramatically reduced share of total emissions, though, is that industrial emissions of carbon dioxide have continued to grow rapidly. The rise is not as fast as it might have been. Rules that make vehicles more efficient and improve the energy efficiency of buildings and appliances have done more than might be expected. America has been setting standards for vehicle greenhouse-gas emissions and fuel efficiency since the mid 1970s; the current rules are forecast to reduce carbon-dioxide emissions by 6 billion tonnes in 2012-25, meaning by about 460m tonnes a year. America's Department of Transportation reckons that overall such rules have reduced carbon-dioxide emissions by a cumulative 14 billion tonnes. Europe's equivalent regulations for passenger cars and light trucks do less (European vehicles were more efficient to start with) but are still respectable; being adopted by overseas manufacturers who want to sell cars in Europe gives them an unquantified extra clout.

Be sure to read the entire article, where you'll also find a chart showing which policies will have the highest impact on climate change mitigation in 2020.

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