Posted March 27, 2008
Jim Hansen, NASA’s top climate scientist, put global warming on the policy agenda with his 1988 Congressional testimony that he was 99% confident that a long term global warming trend was underway and that heat-trapping gases were probably to blame. Many scientists at the time thought Hansen had gone a bit too far and it took about a decade for consensus scientific assessments to catch up with Hansen’s foresight. Almost two decades later Hansen has compiled a remarkable track record of publishing testable hypotheses that have been born out by subsequent data.
Now Hansen is upping the ante again with his conclusion that current concentrations of carbon dioxide in the atmosphere (about 385 ppm) are already in dangerous territory and that we should be aiming to lower concentrations back to 350 ppm, rather than aiming to stabilize them at about 450 ppm, as I and many other advocates of climate protection have previously called for. Many will again feel that Hansen has gone too far, but given his track record Hansen’s latest conclusions must be taken very seriously. And few would argue that we wouldn’t all be better off had we responded more forcefully to Hansen’s first warning.
Hansen lays out the scientific case for a 350 ppm target in a carefully reasoned draft paper available on his Columbia University web site. Using evidence from Earth’s long-term climate history he shows that when all climate feedbacks are considered, particularly those associated with changes in ice sheets, the sensitivity of global temperatures to increases in heat-trapping gases is twice as large as predicted by climate models that include only fast feedbacks. The implication is that the longer CO2 concentrations remain above 350 ppm the greater our risk of passing the point of no return where the feedbacks take over and propel us to a “different planet,” far outside the range of natural variability seen during the entire history of the human species.
This would be very depressing except that Hansen tells us there “is no reason to be so glum.” The key to getting out of the danger zone is to phase out emissions from the use of coal as rapidly as possible—by 2030 according to Hansen. Hansen says this does not necessarily mean phasing out the use of coal; it means that continued reliance on coal requires use of technology to put the carbon it contains back underground. And the first rule of getting out of a hole is to stop digging, which means it’s simply folly to build new conventional coal plants that don’t capture their CO2. If emissions from coal are phased out by 2030 Hansen finds that most, but not all, conventional oil and gas reserves could still be used and still allow us to return to the 350 ppm target. So it’s a good idea to keep pristine areas off limits to development and unconventional fuels, such as tar sands, shale oil, and liquid coal, should be off the table.
Twenty years ago Jim Hansen went out on a limb to warn us that pollution-driven global warming was underway. We mostly ignored him to our everlasting regret. He is out on a limb again. If we ignore him this time we may find that we are all on the limb with him and that it is being sawn off.
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