There is no longer any question of preventing climate change. Some 98 percent of working climate scientists agree that the atmosphere is already warming in response to human greenhouse-gas emissions, and the most recent research suggests that we are on a path toward what were once considered “worst case” scenarios.

How much warmer must it get before things really go to hell? “Climate sensitivity” remains a subject of intense investigation, and what counts as hellish is a matter of judgment, but United Nations climate negotiators have settled on a goal to limit atmospheric carbon dioxide to 450 parts per million, which would cause the global mean temperature to peak no more than 3.6°F above preindustrial levels. If it gets much hotter than that, we will most likely be confronted by levels of drought and severe storms for which humanity has no precedent. That sounds bad enough—and indeed, postindustrial temperatures have already risen by as much as 1.6°—but there’s increasing reason to believe, as James Hansen and many other climate scientists do, that severe effects will arrive well below 450 ppm, and possibly below today’s level of 396 ppm. Danger is much closer than we thought.

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