A new study finds more than 80 percent of the world’s known coal reserves and about half of gas reserves must remain unburned in order to avoid the most catastrophic impacts of climate change. The report published in the journal Nature outlines specific limits, saying 92 percent of U.S. coal supplies, most Canadian tar sands oil, and all oil and gas in the Arctic are “unburnable” if the world is to keep an increase in global temperatures below the agreed-upon cap of 2 degrees Celsius.