Perhaps most important, disappearing ice sheets are harbingers. Humans have built societies around specific, relatively narrow temperature ranges, which determine countless features of our habitat. It is hard enough to adapt to natural variability without also forcing rapid shifts, some expectable and some surprising, in humanity’s climatic circumstances.

Another warning is the increasingly dire state of the Great Barrier Reef. For all the warming that has occurred on land, the oceans have absorbed far more of the excess energy that human-caused greenhouse emissions have trapped on Earth. Spiking ocean temperatures and increasing acidification are clear signs of carbon dioxide’s growing impact. High ocean temperatures led to two mass coral die-offs in 2016 and 2017. Now, according to new research published April 3 in Nature, scientists are finding that the reef is not repopulating as might have been hoped. Following the 2016-2017 catastrophe, new corals are down 89 percent. The reef will never be the same — and it needs recovery time just to return to minimal health. Yet experts expect bleaching events to occur more and more often.