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Sunday, December 7, 2008

Andrew Glikson: Dangerous climate change

[BLOGGER'S NOTE: This post is excerpted from a much larger article -- please see the link below for more information.]

Climate change post-1750 is driven by total radiative forcing tracking toward ca. 3 Watt/m2, near-half the forcing of 6.5±1.5 Watt/m2 associated with the last glacial termination (ca. 14.7–11.7 kyr ago). Given the onset of the Antarctic ice sheet at or below 450 ppm CO2 ca. 34 million years ago (Ma) (late Eocene), and of Arctic Sea ice below 400 ppm ca. 2.8 Ma (mid-Pliocene), the projected consequences of CO2 trajectories toward 550 or 650 ppm through the 21st century, now considered inevitable by governments, likely involve a return to Pliocene conditions (+ 2–3 oC; + 25±12 m sea level rise; permanent El Nino) through likely climate tipping points. Depending on the degree of methane (CH4) release from sediments, permafrost and tropical bogs, developments analogous to the PETM (Paleocene-Eocene Thermal Maximum, 56 Ma; + 6 oC) and attendant mass extinctions may ensue. The Intergovernmental Panel for Climate Change (IPCC) 2007 projections underestimate ice melt and sea level rise parameters. The Garnaut-2008 Report takes limited account of the effects of methane release from permafrost and shallow polar ocean sediments and the synergy of carbon cycle feedbacks and ice melt/water feedbacks, as indicated by the recent climate history of Earth. Assumption of CO2 and climate 'stabilization' and possible reversal on decadal time scale are difficult to reconcile with the sharp transitions between climate states observed in the recent history of the atmosphere. The opening of an ice-free Arctic ocean and slow-down or aborting of the North Atlantic thermohaline circulation lead to a new climate regime in the Northern Hemisphere, possibly similar to events ca. 8.2 kyr ago when ice-melt currents resulted in several degrees cooling and freezing of Europe. According to Anderson and Bows (2008), it may be too late to arrest climate change by reduced carbon emission alone. Humanity needs to fast-track development of techniques for atmospheric CO2 down-draw to levels ca. 350 ppm and below (Hansen et al., 2008).

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