In DepthBehind the Numbers

Data Check: How a figure key to new HFC pact was born

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Science  28 Oct 2016:
Vol. 354, Issue 6311, pp. 402
DOI: 10.1126/science.354.6311.402

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When world leaders reached a deal last month in Kigali to curb the use of hydrofluorocarbons (HFCs)—planet-warming chemicals widely used in air conditioners and refrigerators—many boasted the move would prevent nearly 0.5°C in warming by 2100. That is a big number, given that the Paris climate agreement commits nations to keeping the total global temperature increase to less than 2°C. If the HFC number is correct, it will make the Paris goal easier to achieve. But there's a bit more scientific uncertainty surrounding that half-degree claim than the politicians let on. And although scientists routinely acknowledge such uncertainty, "that's not what politicians do," says Durwood Zaelke, president of the Institute for Governance and Sustainable Development in Washington, D.C. Here's a look at how the half-degree figure was born, and what it might mean for the planet.