20 03 2008

from the blog of Joseph Romm, author of Hell and High Water

Hansen (et al) must read: Get back to 350 ppm or risk an ice-free planet


Also, if I am reading Hansen et al correctly (and Lord knows I may not be), then I think he may be mostly right for a different reason than he thinks, which is to say, I think the carbon-cycle feedbacks (including the tundra melting and sink saturation) act as the equivalent of the amplifiers that he models (”loss of Greenland and Antarctic ice and spread of vegetation over the vast high-latitude land area in the Northern Hemisphere” — I will come back to that vegatation issue in a future post). In other words, if you get near 450 ppm and stay there for any length of time, you will shoot up to 700 to 1000 ppm, which certainly gets you an ice-free planet. Or perhaps the simplest way to put this — the IPCC is right when it says:

Climate-carbon cycle coupling is expected to add carbon dioxide to the atmosphere as the climate system warms, but the magnitude of this feedback is uncertain. This increases the uncertainty in the trajectory of carbon dioxide emissions required to achieve a particular stabilisation level of atmospheric carbon dioxide concentration. Based on current understanding of climate carbon cycle feedback, model studies suggest that to stabilise at 450 ppm carbon dioxide, could require that cumulative emissions over the 21st century be reduced from an average of approximately 670 [630 to 710] GtC to approximately 490 [375 to 600] GtC. Similarly, to stabilise at 1000 ppm this feedback could require that cumulative emissions be reduced from a model average of approximately 1415 [1340 to 1490] GtC to approximately 1100 [980 to 1250] GtC.

We’re at 8 GtC/yr and rising 3% annually. We need to average below 5 GtC/yr — and maybe considerably less — for the whole century to avert catastrophe. We need to be near zero or below by 2100.




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