The Earth’s poles serve as critical “thermostats” for the rest of the planet, with major effects on global temperatures due to their high albedo, polar wind vortexes and cold ocean currents.
Since the 1980s mean temperatures at the Earth’s poles have risen faster, by a large factor, than rises in mid and low latitudes, due to the feedback effects which combine decreased albedo of melting ice sheets, and absorption of infrared and warming of exposed water, which in turn further re-melt ice.
In 2008, surface temperature anomalies in the Arctic and northern Siberia, the Antarctic Peninsula and parts of west Antarctica have risen by near-3.5 degrees C higher than the base period of 1951-1980, while global mean in 2008 was 0.44 degrees higher than the base period.
In Antarctica new satellite and surface-based analysis indicates surface temperatures rose by 0.12 degrees C per decade between 1957-2006, namely 0.5 degrees C in the last half century, with maximum rise in west Antarctica of 0.17 degrees C per decade.
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Recent studies which correlate atmospheric CO2 levels with the appearance and disappearance of the polar ice sheets, define thresholds at levels no higher than about 500 parts per million CO2 — within the range of the Garnaut report’s alternative targets of 450-550 ppm.
This suggests that, depending on ice melt lag effects, at the current CO2 growth rate exceeding 2 ppm/year (for 2005-2009) (at Mouna Loa), by mid-century CO2 levels and temperatures will rise to levels at which not much ice will be left in he northern hemisphere, and much less ice left in Antarctica.
At a current CO2 rise rates of ~2.2 ppm/year, CO2 levels by mid-century would reach 470-480 ppm (current 387 ppm, plus ~90 ppm to 2050), even without considering CO2-rise due to feedback effects. This corresponds broadly to ~0.5 degrees C rise.
Such a rise, combined with carbon cycle and ice melt/water feedback effects, which are hardly specified by the IPCC AR4 2007 report, will exceed upper CO2 and temperature thresholds defined for the Greenland and Antarctica ice sheet melt conditions.
With consequent tens of metres-scale sea level rise, in contrast to IPCC projections of 0.59 metres by year 2100. The IPCC report specifically excludes “future rapid dynamical changes in ice flow” [Table SPM-1]. It thus takes little account of the evidence that temperature rises in the range of about 2 to 4 degrees C (the median levels of 21st century scenarios (Fig. SPM-5) take the Earth to a greenhouse climate state before the ice sheets formed some 34 million years ago (the end of the Eocene).
Which precedes the flourishing of large mammals, including humans, on Earth.
There is little evidence the IPCC reports, and thereby reviews such as the Stern and Garnaut reports which are partly or largely based on the IPCC, have taken the full implications of 450-550 ppm targets for the terrestrial environment and survival of civilisation into account.