James Hansen at Geophysics Meeting: 2 Degrees of Warming a Recipe for Disaster
This week is the annual meeting of the American Geophysical Union, the largest conference of scientists and which covers a broad range of topics related to Earth and planetary sciences.
Among the presenters on Tuesday was a group led by Dr. James Hansen, discussing our current knowledge on what can be learnt from paleoclimatology wrt. the Earth systems response to atmospheric changes in CO2.
Given this week is also the high level meeting of the UNFCCC Conference of Parties in Durban, Dr. Hansen discussed the often used 2 degrees of warming threshold target used by government negotiators:
The target set by nations in global warming talks won’t prevent the devastating effects of global warming, according to climate scientist James Hansen, director of NASA’s Goddard Institute for Space Studies.
The history of ancient climate changes, which occurred over millions of years in the planet’s history as it moved in and out of ice ages, offers the best insight into how humans’ greenhouse gas emissions will alter the planet, Hansen said here today (Dec. 6) at the annual American Geophysical Union (AGU) meeting. And his research suggests the climate is more sensitive to greenhouse gas emissions than had been suspected.
“What the paleoclimate record tells us is that the dangerous level of global warming is less than what we thought a few years ago,” Hansen said. “The target that has been talked about in international negotiations for 2 degrees of warming is actually a prescription for long-term disaster.”
However, signs of changes that will exacerbate the situation, such as the loss of ice sheets that will raise sea level and change how much sunlight is reflected off the planet’s surface, are already appearing, according to Hansen.
Two degrees of warming will lead to an ice-free Arctic and sea-level rise in the tens of meters, Hansen told LiveScience. “We can’t say how long that will take, [but]it’s clear it’s a different planet.”
Some fast-feedback effects show up within decades, and some of these show up only when other parts of the system, particularly the oceans, which warm slowly, catch up with atmospheric warming. This can take centuries.
There are also slow-feedback effects that are expected to amplify global warming, particularly, the melting of ice sheets. The darker ground beneath the ice and the meltwater that pools on top of it absorbs more sunlight, warming the planet even more.
It’s difficult to project how long the effects will take to set in, because, in the history of Earth’s climate, the level of carbon dioxide in the atmosphere has never risen as quickly as it is now.
Because there is evidence that ice sheets are losing mass, the planet is already facing these powerful feedbacks, making the goal of reducing greenhouse gas emissions to a point that prevents warming of 3.6 degrees F (2 degrees C) insufficient, said Hansen and his colleagues […]
The PowerPoint presentation is available here, though the slides may not be self explanatory to those who are not familiar with the topic. However, back in July Dr. Hansen and his team wrote up a webpage describing some of the work for the general audience: Earth’s Climate History: Implications for Tomorrow
and the formally written paper is here.
The key observation is found in the abstract:
Paleoclimate data help us assess climate sensitivity and potential human-made climate effects. We conclude that Earth in the warmest interglacial periods of the past million years was less than 1°C warmer than in the Holocene. [Ed. - Holocene is the current period, starting about 10,000 years ago.] Polar warmth in these interglacials and in the Pliocene does not imply that a substantial cushion remains between today’s climate and dangerous warming, but rather that Earth is poised to experience strong amplifying polar feedbacks in response to moderate global warming. Thus goals to limit human-made warming to 2°C are not sufficient — they are prescriptions for disaster. Ice sheet disintegration is nonlinear, spurred by amplifying feedbacks. We suggest that ice sheet mass loss, if warming continues unabated, will be characterized better by a doubling time for mass loss rate than by a linear trend. Satellite gravity data, though too brief to be conclusive, are consistent with a doubling time of 10 years or less, implying the possibility of multi-meter sea level rise this century. Observed accelerating ice sheet mass loss supports our conclusion that Earth’s temperature now exceeds the mean Holocene value. Rapid reduction of fossil fuel emissions is required for humanity to succeed in preserving a planet resembling the one on which civilization developed.
The Tuesday presentation went farther in that Dr. Rohling (Southhampton Univ, UK) makes the claim that:
In natural context, the ‘equilibrium’ sea level for current anthropogenic forcing is 25 ±3m higher than today.
Now, that doesn’t mean that the sea level will rise 25 meters right away, but he is saying that the Earth system will try to reach equilibrium, based on past records, and given today’s human-caused forcings of the climate, the end result (as far as sea level changes are concerned) of which would be 25 meters.
The point Hansen et. al. are trying to make to the larger audience is that not only are the changes humans are making to the atmosphere occurring more quickly, by several orders of magnitude, than any natural change in the past, but that the paleoclimate records indicate that the resultant changes will be quite significant indeed, even at only a globally averaged 2C surface temperature increase.
Updated 11 Dec 2011:
The press conference at the AGU that Dr. Hansen gave, along with his colleagues, on this topic has been put up at Youtube:
It may be a bit technical for some. The intended audience was for those who are not climate specialists but who nevertheless work in Earth science.