Long time Target of 2*C Now Seen as "prescription for disaster"
New perils seen to even modest warming
Letting global temperatures rise even 2ºC could trigger catastrophic sea-level rise and major social upheaval
The Daily Climate, Dec. 6, 2011
SAN FRANCISCO – Amounts of warming previously thought to be safe may instead trigger widespread melting of the world's ice sheets and other catastrophic impacts, scientists said Tuesday.
Accelerating melting on the world's ice sheets and other new observations have scientists concluding that even a two-degree Celsius rise in temperatures – a benchmark long seen as safe in global climate talks and other emissions reductions scenarios – could lead to an 80-foot rise in sea levels.
"The dangerous level of global warming is less than what we thought a few years ago," said James Hansen, director of NASA Goddard Institute for Space Studies. "It was natural to think that a few degrees wasn't so bad.... (But) a target of two degrees is actually a prescription for long-term disaster."
Antarctica and Greenland are losing ice at a surprising clip, Hansen said, and methane hydrates – a potent source of greenhouse gas frozen beneath the seas – are starting to bubble up.
The key question for climatologists: How sensitive is the climate to increasing amounts of fossil fuel emissions. Last year humanity pumped almost 10 billion tons of carbon dioxide into the atmosphere, a half-billion tons more than 2009 and the largest jump in any year since the Industrial Revolution, according to the Global Carbon Project.
"There's evidence that climate sensitivity may be quite a bit higher than what the models are suggesting," said Ken Caldeira, a senior scientist at Stanford University's Carnegie Institution for Science.
Caldeira and Hansen spoke at the American Geophysical Union meeting in San Francisco, one of the world's largest gatherings of geophysical researchers.
The problem, those researchers said, is the "hang time" for carbon dioxide in the atmosphere. Even if greenhouse gas emissions stopped tomorrow, "climatically important" amounts of carbon dioxide and other compounds emitted today would continue to influence the atmosphere for thousands of years, Caldeira said.
That kind of pressure, or "forcing," on the atmosphere could be devastating, he cautioned.
About 55 million years ago a tremendous amount of methane was released into the atmosphere over a period of about 1 million years, and the planet heated by five degrees to eight degrees Celsius, or 10 degrees to 14 degrees Fahrenheit. The result was an ice-free planet with sea levels 230 feet higher than they are today.
In the eons since, carbon dioxide levels dropped and the ice reformed. But humanity's emissions have the potential to send the globe back to those conditions, Caldeira and Hansen said.
"If you doubled CO2, which practically all governments assume we're going to do, that would eventually get us to the ice-free state," Hansen said.
Scientists don't expect that ice to melt quickly. Assuming the current accelerated melting continues on the world's ice sheets and glaciers, various climate models predict the ocean would rise between 1.5 feet and 2.3 feet by century's end, said Tad Pfeffer, a glaciologist with the University of Colorado.
But the ice melted with atmospheric carbon dioxide levels at about 1,000 parts per million, Caldeira said. And he suspects that even 750 ppm, or about double today's levels, could send the globe spiraling toward an ice-free state. Current emissions trends suggest the globe could reach that by the end of the century.
"We can't double CO2," Hansen added. "We would be sending our climate back to a state we haven't adjusted to as a species."