‘final’ climate report

RELEASED

THE SCARIEST PART OF A LANDMARK NEW climate report may be what scientists don’t know. On March 20, the U.N. International Panel on Climate Change (IPCC) released the final volume in a series of reports outlining experts’ latest understanding of climate science. It declared the science “unequivocal” and warned that even with urgent action we will face a dramatic uptick in catastrophic events—from droughts to floods—that have become telltale signs of a rapidly warming world. But for all that scientists can confidently say, most worrisome are the “known unknowns”—potential outcomes scientists know could happen even if they don’t know exactly when or how. Near the middle of the 37-page summary for policymakers, scientists explain the “likelihood and risks of unavoidable, irreversible or abrupt changes.”

It’s striking both how close we may be to crossing a number of these points of no return, and how little we know about them. Take the Atlantic Meridional Overturning Circulation (AMOC). This system of ocean currents is a key regulator of Atlantic Ocean temperatures and, in turn, maintaining land temperatures, particularly in North America. The IPCC says scientists have “medium confidence” that the AMOC will not abruptly collapse before 2100. But if it did, it would remake weather patterns and disrupt “human activities.”

Another known unknown is the scale of sea-level rise. In high-emissions scenarios in which countries abandon climate commitments, global average sea levels are expected to rise up to 1 m (about 3 ft.) by 2100. But because the science of rapidly melting ice sheets remains difficult for scientists to understand, that number could also end up being 2 m in the same time frame—which could mean the difference between survival and destruction for coastal communities.

The more the planet warms, the more likely we are to experience unpredictable catastrophic changes. The stakes could be world-changing: the loss of ecosystems, thec s rapid remaking of regional climates, and the destruction of entire regions. Climate events of this magnitude are referred to as tipping points: singular climatic events that can instantly reshape our understanding of climate systems. But they aren’t the only terrifying known unknowns.

The new repor