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Summary
One concrete goal adopted by some policy-makers is to reduce the risks associated with climate change by preventing the mean global temperature from rising by more than 2°C above preindustrial levels (1). Climate models indicate that achieving this goal will require limiting atmospheric carbon dioxide (CO2) concentrations to less than 450 parts per million (ppm), a level that implies substantial reductions in emissions from burning fossil fuels (2, 3). So far, however, efforts to curb emissions through regulation and international agreement haven't worked (4); emissions are rising faster than ever, and programs to scale up “carbon neutral” energy sources are moving slowly at best (5). On page 1330 of this issue, Davis et al. (6) offer new insights into just how difficult it will be to say farewell to fossil fuels.