Tipping Point?

Nearly all coal emissions need to stop by 2050 to avoid accelerating ice loss that could lead to 4 feet of sea level rise this century, scientists say.

The world needs to eliminate nearly all carbon dioxide emissions from coal burning by 2050 to avoid pushing Antarctica's ice sheets past a tipping point that could cause a major surge in sea level rise, new research shows.

If CO2 emissions from fossil fuels continue at their present pace, many Pacific islands and millions of people along low-lying shores like the U.S. Gulf Coast and the Bay of Bengal could be swamped by 1.3 meters (more than 4 feet) of sea level rise before the end of this century, an international team of scientists found in a new study published Thursday in the journal Environmental Research Letters.

The researchers said their work supports evidence that global warming of more than 1.9 degrees Celsius could push parts of the West Antarctic Ice Sheet past a melting threshold that would rapidly increase the pace of sea level rise.

"What we are increasingly seeing is that we have been on the conservative side in estimating sea level rise," said study co-author Carl-Friedrich Schleussner, a climate physicist at Climate Analytics, a climate science and policy institute.  "Parts of the West Antarctic Ice Sheet appear to already be in substantial decline.  If that continues, it's not a matter of how much, but how fast sea level will rise."

Keeping within a strict carbon budget, including steep and rapid reductions of CO2 from fossil fuels, might limit this century's sea level rise to just half a meter, or 1.6 feet—still a hazard for many low-lying communities, but not as catastrophic elsewhere.

The new estimate of the worst-case scenario is more alarming than the consensus view of the Intergovernmental Panel on Climate Change (IPCC) in its 2014 assessment, but closer to the increases anticipated by more recent research, Schleussner said.  That's largely because the new study incorporates recent science showing the risk of more severe melting of Antarctic ice under future warming, even if the Paris climate agreement is somewhat successful in keeping warming moderately under control.

The study also uses a newer, more sophisticated approach to assessing the various pathways the world could take as governments struggle to keep global warming to less than 2 degrees Celsius above pre-industrial times.  Which pathways policy makers follow would influence how much and how fast the planet warms.

What 2 Degrees Could Mean for the Ice Sheets
Key to the new projections is a March 2016 study in the journal Nature suggesting that even the large ice sheets of Antarctica could be susceptible to large-scale collapse at a warming threshold of about 2 degrees Celsius.

At that point, giant ice cliffs at the ocean's edge could start to crumble from above and below.  From the surface meltwater will pour down into deep cracks and pry the ice apart; from below, tongues of warmer ocean water will free the floating shelves from rocky anchors, speeding the flow of the ice sheets into the sea.

Data from University of Massachusetts-Amherst climate scientist Rob DeConto, lead author of the 2016 Nature study, shows that the Antarctic ice sheets and glaciers get more sensitive to higher levels of warming, Schleussner said.  "Antarctica is the main driver of the risk of really high sea level rise, so we really need to understand what's happening there." 

The West Antarctic Ice Sheet and the Thwaites Glacier appear especially vulnerable to the 1.9 degree Celsius tipping point, said Ted Scambos, a polar ice expert with the National Snow and Ice Data Center.  The Thwaites Glacier has accelerated.  Other glaciers in the area are getting thinner, and their floating extensions, the ice shelves, are thawing from beneath and losing hold of their fixed anchor points.

"It looks like it (Thwaites Glacier) could express this kind of rapid retreat in the coming few centuries.  The physics are certainly plausible," said Scambos, who was not involved in the new study.  Other scientists have found deep gouges on the Antarctic sea floor caused by icebergs during ancient times of temperature change, signs that are consistent with the theory of rapid ice shelf disintegration, he said.

If summer temperatures in that region warm by about 2 degrees Celsius, it would lead to a drastic increase in the amount of water in the snowpack on the surface of the Thwaites Glacier.  The water eventually pools in reservoirs that can then take a toll on the ice.

"Those lakes are like sledgehammers.  They crack the glacier open," Scambos said.  He said an international effort is planned to explore the Thwaites ice field system from its where it begins all the way out to where it meets the Amundsen Sea.

A Case for Quickly Cutting Emissions
The melting ice cliffs at the edge of the ocean become yet another self-reinforcing climate feedback, said co-author Matthias Mengel, an ice sheet modeler with the Potsdam Institute for Climate Impacts Research.

"What is directly behind the ice cliff is an even higher ice cliff, and it will also break off.  If an ice front is too high, it can't carry its own weight.  To us, this new approach was really kind of a bomb.  It changed our idea completely about how Antarctica could change in the future," he said.  "This means a big change in the numbers of the amount of sea level rise we could get from Antarctica.  It's a new twist for ice loss, but we also have to wait for results to come in.  This is new science, and it's evolving."

Right now, the thick coastal ice shelves are still frozen in place to rocks on the sea floor, holding back the land-based ice sheets, but if the ice shelves start to disappear, the scientists expect the ice flow from the interior will accelerate, he said.

Read more at Tipping Point?