evidence: The questions this endeavor raises are foundational, even though the parts per million of atmospheric carbon dioxide seem so minuscule and the predicted temperature increases don’t seem, in a daily context, to be so daunting. And yet. Just what is the sweet spot for the Earth’s global average temperature—or, rather, the temperature we want the Earth to have? Keep the warming to about a 2°C rise? Should the parts per million (ppm) of atmospheric CO2 be 350? 450? We’re already pushing 400 ppm. At 450 we might avoid warming the planet above the 2°C mark. But that’s a 50-50 proposition if we rely solely on reducing emissions, according to Tom Wigley of the National Center for Atmospheric Research, and, he says, "any pathway to 450 looks rather optimistic." The Royal Society says that "it seems increasingly likely that concentrations will exceed 500 ppm by mid-century and may approach 1000 ppm by 2100." Such levels could lead to civilization-ending global warming.