evidence: Even at this late date, expressions of unrealistic hope continue to abound. Hardly a day seems to pass without my reading that it’s time to "roll up our sleeves" and "save the planet"; that the problem of climate change can be "solved" if we summon the collective will. Although this message was probably still true in 1988, when the science became fully clear, we’ve emitted as much atmospheric carbon in the past thirty years as we did in the previous two centuries of industrialization. The facts have changed, but somehow the message stays the same. [...] Our atmosphere and oceans can absorb only so much heat before climate change, intensified by various feedback loops, spins completely out of control. Some scientists and policymakers fear that we’re in danger of passing this point of no return if the global mean temperature rises by more than two degrees Celsius (maybe more, but also maybe less). The I.P.C.C.—the Intergovernmental Panel on Climate Change—tells us that, to limit the rise to less than two degrees, we not only need to reverse the trend of the past three decades. We need to approach zero net emissions, globally, in the next three decades.