July 03, 2007
|Bad News On Climate — Again||Environment Science/Technology|
Scientific papers tend to have a limited audience, but here's one that deserves to be front page news all over the world. James Hansen, head of NASA's Goddard Institute and one of the world's foremost climate scientists, is lead author of a new study that concludes that the recent report of the Intergovernmental Panel on Climate Change (IPCC), alarming as it was, may have been, in George Monbiot's words, "absurdly optimistic."
As we have noted here many times (for example, here), the most ominous global warming scenarios involve positive feedback loops that make GW self-reinforcing. The thawing of Siberian permafrost, for example, releases methane, a potent greenhouse gas, into the atmosphere — which causes further warming, which causes further thawing, and so on. Hansen et al look at the data on past ice ages and warming periods and conclude that such positive feedbacks make the earth's climate far more sensitive to climate "forcings" (i.e., things like greenhouse gases that upset the balance between the energy the Earth receives from the sun and the energy it radiates back out to space) than previously thought.
When major warming has occurred in the past, it has happened quite suddenly — on a scale of centuries or even decades, rather than millenia — because of the accelerating effects of positive feedback loops. A primary source of this abruptness is the sudden shift that occurs when ice changes phase — i.e., when it melts. When ice becomes wet, it suddenly reflects much less energy than before. The change in its reflectivity, or "albedo," means that more energy is absorbed, which causes more melting, in turn causing more of an "albedo flip," etc., etc. Self-reinforcing. Instead of a linear, gradual change, positive feedback creates a nonlinear system where change happens suddenly. Ice sheets melt, sea levels rise, and quickly.
The IPCC warned that sea levels could rise by as much two feet in the coming century as the West Antarctic ice sheet begins to melt. The melting of the entire ice sheet, though, they projected to take millenia. But according to Hansen et al, the IPCC view is inconsistent both with data on past climate changes ("paleoclimate" data) and with current observations. Ice sheets become unstable and break up suddenly. Hansen et al say it's "implausible" under a business-as-usual scenario (i.e., if humanity stays on its current course with respect to emissions) that the West Antarctic ice sheet would even survive the century. They're talking about the possibility of sea level rises of 75 feet or more.
Here's an excerpt from the paper's abstract:
Palaeoclimate data show that the Earth's climate is remarkably sensitive to global forcings. Positive feedbacks predominate. This allows the entire planet to be whipsawed between climate states. One feedback, the 'albedo flip' property of ice/water, provides a powerful trigger mechanism. A climate forcing that 'flips' the albedo of a sufficient portion of an ice sheet can spark a cataclysm. Inertia of ice sheet and ocean provides only moderate delay to ice sheet disintegration and a burst of added global warming. Recent greenhouse gas (GHG) emissions place the Earth perilously close to dramatic climate change that could run out of our control, with great dangers for humans and other creatures. Carbon dioxide (CO2) is the largest human-made climate forcing, but other trace constituents are also important. Only intense simultaneous efforts to slow CO2 emissions and reduce non-CO2 forcings can keep climate within or near the range of the past million years.
"Whipsaw." "Cataclysm." Remarkably dramatic language for an article in the Philosophical Transactions of the Royal Society. But there's a lot at stake. I recommend you go read the paper. The evidence is persuasive, and the conclusions could hardly be more important.