The Scariest Part of the New U.N. Climate Report? What Scientists Can’t Predict

The scariest part of a landmark new report on the science of climate change may be what scientists don’t know.

On Monday, the International Panel on Climate Change (IPCC), the U.N.’s climate–science body, released the final volume in a series of reports outlining experts’ latest understanding of the science of climate change. The report declared the science of climate change “unequivocal” and warned that even with urgent action we will continue to face a dramatic uptick in catastrophic events—from droughts to floods—that have become tell-tale signs of a rapidly warming world.

But, for all that scientists are able to lay out clearly and confidently, the part that gives me the greatest pause is the brief section on what you might call “known unknowns”—potential outcomes that scientists know could happen even if they don’t know when or understand exactly how. Near the middle of the 37-page summary for policymakers, which outlines the report’s key themes, 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 points-of-no-return and how little we know about those points.

Take the Atlantic Meridional Overturning Circulation (AMOC). This system of ocean currents may sound wonky, but it is a key regulator of temperatures in the Atlantic Ocean and, in turn, maintaining land temperatures, particularly in North America. In the latest report, the IPCC says that scientists have “medium confidence” that AMOC will not abruptly collapse before 2100. But, if it did, we would quickly face remade weather patterns, extreme stress on ecosystems, and distrupted “human activities.”

Another known unknown addressed by the IPCC is the magnitude of sea level rise. In high-emissions scenarios in which countries abandon climate commitments, global average sea levels are expected to rise up to one meter (about three feet) by 2100. But because the science of rapidly melting ice sheets remains difficult for scientists to understand, that number could also plausibly end up being two meters in the same time frame, a dramatic difference that could mean the difference between destruction and survival for coastal communities around the globe.

The report summarizes these known unknowns in dry, scientific language: “the likelihood of abrupt and/or irreversible changes increases with higher global warming levels. Similarly, the probability of low-likelihood outcomes associated with potentially very large adverse impacts increases with higher global warming levels.” In other words, the more it warms the more likely we are to experience unpredictable catastrophic changes.

Regardless of how its worded, the stakes could be world-changing: the loss of entire ecosystems, the rapid remaking of regional climates, and the destruction of entire regions.

Climate events of this magnitude are typically referred to as tipping points, singular climatic events that within an instant can reshape our understanding of climate systems. These tipping points have received attention because of how inherently scary they are, but they aren’t the only terrifying known unknowns.

It receives just a fleeting mention, but the new report lays out how the future becomes more difficult to predict as climate change proceeds. In short, climate change impacts that scientists might be able to project today will become more difficult to predict when they intersect with other climate effects. The report gives food insecurity as example. Food insecurity could drive changes in agricultural practices, which would in turn affect the climate.

“With further warming, climate change risks will become increasingly complex and more difficult to manage,” says the report. “Multiple climatic and non-climatic risk drivers will interact, resulting in compounding overall risk and risks cascading across sectors and regions.”

Flipping through three decades of IPCC reports, it’s easy to see how the science has evolved, growing more certain and more urgent with every passing report. The IPCC is not expected to publish another report for at least another six years. In that time, the science will evolve, as will the on-the-ground impacts of climate change that humans experience. It’s also likely that global warming will surpass the 1.5°C mark identified by scientists as a danger zone where we might expect some of these tipping points to occur. By then we should know more about these known unknowns, too. We can only hope that the knowledge brings relief, not further fear.