While most projections in the sixth IPCC assessment of global climate science track anticipated developments under five emissions scenarios, there is one notable exception: a single chart projecting future sea level rise that deviates significantly as early as this decade.
The chart, titled “Global mean sea level change relative to 1900” and presented on page 29 of the IPCC’s Summary for Policymakers, represents an abrupt increase in sea levels by as much as two metres by the end of the century—a development that would likely devastate coastal regions that are home to hundreds of millions of people.
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It is one of several unlikely but potentially highly consequential climate events that the latest science summary says “cannot be ruled out.” Other transformative impacts of global warming are now effectively locked in, even in the lowest emissions scenarios.
Projections of sea level rise illustrate both features. “It is virtually certain that global mean sea level will continue to rise over the 21st century,” the IPCC reports—regardless of which emissions scenario humanity follows. “In the longer term, sea level is committed to rise for centuries to millennia due to continuing deep ocean warming and ice sheet melt, and will remain elevated for thousands of years.”
But in the future represented by that exceptional dotted line, rising seas could blow past the maximum projection of up to a metre this century under the highest of the five emissions scenarios. Depending on as-yet-poorly understood dynamics, collapsing Antarctic ice sheets could nearly double the levels of ocean rise during this century and, by 2300, could lift sea levels by 15 metres or more. Such a development would leave coastlines unrecognizable.
Similar uncertainty pervades projections for the Atlantic Meridional Overturning Circulation (AMOC)—the northern portion of a vast global circulation of sea water that distributes enormous quantities of heat from the tropics to northern latitudes and includes the Gulf Stream. The current has been seen to be weakening in recent decades, and that “is very likely to [continue] over the 21st century” under all scenarios, including one that achieves net-zero emissions by 2050.
The AMOC has failed in the distant past. Were it to happen again, “abrupt shifts in weather patterns” could include a sharply chillier northern Europe, a drier continental Europe, and weaker monsoons in Africa and Asia.
But while scientists are “highly” confident that the AMOC will keep weakening through the rest of this century, they are much less certain about how much it will falter. They express only “medium” confidence that it will avoid collapse completely.
By contrast, increasing acidification and declining oxygen levels in sea water, both of which threaten marine life, are “virtually certain” and “irreversible on centennial to millennial time scales,” regardless of emissions trajectory. Likewise, mountain and polar glaciers will continue melting for centuries to come (“very high” confidence), as will permafrost (“high” confidence). Both developments will proceed faster under higher emissions scenarios than lower ones.
The difference may be decisive for Arctic sea ice, which is projected to stabilize under the two conceptualized lower-emission pathways, but will vanish entirely in the summer by later this century under higher emissions.
Then there are some known unknowns, events with potentially big impacts that can’t be predicted with precision—and can’t be controlled.
“For example,” the IPCC observes, “a sequence of large explosive volcanic eruptions within decades has occurred in the past.” Should that recur, there could be a temporary pause in heating as ash and particulates shade the planet, reducing temperatures and precipitation for several years. That, however, would only briefly “mask” the underlying warming trend.
Other areas of remaining scientific uncertainty with much at stake are the effect of carbon feedbacks from melting permafrost and wildfires, and the risk of wholesale ecosystem transformation as forests burn or die off. It is also possible, the IPCC concedes, that warming will exceed the ranges projected for the various emission scenarios.
While any of the most alarming outcomes may still be unlikely, the fact that they cannot be ruled out adds to the urgency of zeroing out carbon emissions sooner rather than later.
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