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….
“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.
That’s from a Daily Climate piece on this panel discussion at last weeks AGU meeting:
The sensitivity issue is a complicated one, as I’ve discussed. The AGU discussion certainly helps clarify key issues and suggests that some effort is being put into a reconciliation of the different ways of calculating.
Don’t miss the back and forth on whether we are headed toward 25 meters of sea level rise or 70 meters at the end. For some of the science, see Science: CO2 levels haven’t been this high for 15 million years, when it was 5° to 10°F warmer and seas were 75 to 120 feet higher — “We have shown that this dramatic rise in sea level is associated with an increase in CO2 levels of about 100 ppm”.
If you’re interested on Caldeira’s work on a very high sensitivity — 5.5°C to 8°C (10 to 14 F) — click here. It is based on the PETM 55 million years ago and “having strong functioning methane feedbacks.” Thankfully we’ve got nothing to worry about in the Anthropocene (see”NSIDC : Thawing permafrost feedback will turn Arctic from carbon sink to source in the 2020s, releasing 100 billion tons of carbon by 2100).
Here’s more of the Daily Climate piece:
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
dioxideinto 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.
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.”
The time to act was a while ago, but now is much, much better than later.
- A detailed look at climate sensitivity
- Climate Experts Warn Thawing Permafrost Could Cause 2.5 Times the Warming of Deforestation