Two Degrees is Dangerous

A study by James Hansen et al published in Atmospheric Chemistry and Physics takes issue with the conventional two degree Celsius limit on warming:

The modeling, paleoclimate evidence, and ongoing observations together imply that 2 °C global warming above the preindustrial level could be dangerous. Continued high fossil fuel emissions this century are predicted to yield (1) cooling of the Southern Ocean, especially in the Western Hemisphere; (2) slowing of the Southern Ocean overturning circulation, warming of the ice shelves, and growing ice sheet mass loss; (3) slowdown and eventual shutdown of the Atlantic overturning circulation with cooling of the North Atlantic region; (4) increasingly powerful storms; and (5) nonlinearly growing sea level rise, reaching several meters over a timescale of 50–150 years.

Update:  Another study, published in Nature Geoscience, finds that current human carbon dioxide emissions are as much as ten times that of the Palaeocene-Eocene Thermal Maximum of 55 million years ago. Also, Joe Romm explains how carbon dioxide concentration can go up even if emissions level out.

Update (March 23):  Joe Romm notes the muted warning of Hansen's paper--switched from "highly dangerous" in the draft to "could be dangerous". But there are previous studies indicating that “[t]he natural state of the Earth with present carbon dioxide levels is one with sea levels about 70 feet higher than now.” Romm says that what's new is the understanding of how fast sea levels could rise.

Since there are a growing number of experts who consider that 10 feet of sea level rise this century is a possibility, it would be unwise to ignore the warning. That said, on our current emissions path we already appear to be headed toward the ballpark of four to six feet of sea level rise in 2100 — with seas rising up to one foot per decade after that. That should be more than enough of a “beyond adaptation” catastrophe to warrant strong action ASAP.

Update (March 30):  A study published in Nature supports the idea that sea levels could rise 3 feet by 2100 and 45 feet by 2500 just from the contributions of the Antarctic ice sheet.

Update (April 2):  More reporting on the Nature study.

Update (September 2, 2019):  A study published in Nature gives evidence that when Earth was two to three degrees Celsius warmer, sea levels were about 50 feet higher.

[The] study found that during [the mid Piacenzian Warm Period -- some 3.264 to 3.025 million years ago], global mean sea level was as high as 16.2 meters (with an uncertainty range of 5.6 to 19.2 meters) above present. This means that even if atmospheric CO2 stabilizes around current levels, the global mean sea level would still likely rise at least that high, if not higher.