IPCC: Sea Level Rise and Extreme Sea Level Events are Inevitable

After a review of 7,000 scientific studies, the Intergovernmental Panel on Climate Change has concluded that the world economy is on track to increase the mean sea level by about 2.8 feet, dramatically reduce the extent of warm-water coral reefs and raise the frequency of marine heatwaves by a factor of 50, all by the end of the century. These changes are possible under a business-as-usual emissions scenario, with more alterations expected in the centuries to come. 

Change is already apparent in the Arctic, as U.S. Secretary of State Mike Pompeo noted in May. Relative to 1979, Arctic sea ice extent has decreased for all months of the year, and September sea ice reductions are estimated in the range of 13 percent per decade, IPCC concluded. The warming of the Arctic also opens the way for the melting of vast regions of permafrost, which contains as much as twice the amount of carbon in the atmosphere today. 

Though the atmosphere has warmed, the rate has been substantially slowed by the oceans, according to IPCC. The ocean has served as a giant heat sink, absorbing an estimated 90 percent of the excess heat added to the biosphere since 1970. This has had knock-on effects, including a likely doubling of the incidence rate of marine heat waves, IPCC said. 

The ocean has also absorbed as much as a third of the carbon dioxide produced since the 1980s, causing it to become more acidic. In future years, a combination of warming, ocean acidification and deoxygenation may affect fish stocks and other marine resources, IPCC warned. Almost all warm-water coral reefs are expected to suffer significant losses of area and local extinctions under all emissions scenarios, and the composition of their species will likely change.

Thermal expansion combined with glacial melt will raise the sea level under all scenarios, IPCC forecasts. Under a business as usual scenario, sea level is expected to rise by about three feet through the end of this century and continue rising for centuries thereafter, reaching about 12 feet by 2300 - enough to make New Orleans an island. (There is considerable upside potential: based on the most recent research, the possibility of sea level rise exceeding six feet by the end of this century cannot be excluded.) By contrast, in a scenario where GHG emissions are mitigated, sea level rise will likely reach1.5 feet by 2100 and remain under three feet over the centuries to come, IPCC predicts. 

Sea level rise will also create new impacts for coastal communities on a shorter timescale. The IPCC predicts that “extreme sea level events that [occured] once per century in the recent past are projected to occur at least once per year at many locations by 2050 in all scenarios.”

Environmental activist group Greenpeace called for political action. “World leaders have the opportunity now to signal their seriousness by adopting a strong Global Ocean Treaty to protect vulnerable ecosystems in the nearly 50% of the planet covered by international waters," said senior oceans campaigner Arlo Hemphill of Greenpeace USA.