Brad Plumer provides more details by digging into the new climate change report from the National Research Council:

The upshot? Earth is already seeing some abrupt changes, like the fast retreat of summer Arctic sea ice. There’s also a real risk that other rapid and drastic shifts could soon follow if the Earth keeps warming — including widespread plant and animal extinctions and the creation of large “dead zones” in the ocean. But other apocalyptic scenarios once thought plausible “are now considered unlikely to occur this century.” That includes shifts in Atlantic ocean circulation patterns that could radically alter Europe’s climate, as hyped in the disaster flick “The Day After Tomorrow.” Also unlikely this century: Collapsing ice sheets in West Antarctica that would push sea levels up very quickly, as well as sudden methane eruptions from the Arctic that could heat the planet drastically. Those problems are left to future generations.