In an article titled “Scientific reticence and sea level rise” Hansen et al., 2007, stressing the non-linearity of ice melt processes, state: “as a physicist, I find it almost inconceivable that business-as-usual climate change would not yield a sea level change of the order of meters on the century timescale”.
Most recently Hansen and Sato, citing Velicogna, 1009 [Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE, Geophys. Res. Lett., 36, L19503, doi:10.1029/2009GL040222.], point to Gravity satellite evidence for a doubling time of about 10 years of Greenland and Antarctic ice melt rates (Figure 3), implying the possibility of multi-meter sea level rise this century.
In a new article Velicogna and Rignot, from mass balance calculations and GRACE gravity satellite measurements, report the Greenland and Antarctic ice sheets are losing mass at an accelerated rate, losing a combined mass of 475 billion ton ice per year on average and becoming the dominant contributors to sea level rise. This is consistent with Hansen and Sato’s view of the non- linearity of ice melt dynamic processes reinforced by ice melt feedback processes, rendering multi-meter sea level rise this century possible. The extreme rate of greenhouse gas forcing, rising at ~2 ppm CO2/year, leads potentially to tipping points such as high-rate methane release from permafrost and Arctic sediments, and the potential collapse of the North Atlantic Thermohaline Stream.
Doubts are often raised among climate scientists whether the Greenland and West Antarctic ice sheets can survive the 1.8 – 4.0 C temperature rises projected by the IPCC-2007 , leading to meters-scale sea level rise.
The increase in frequency of extreme weather events manifests a rise in the atmosphere/ocean energy level. Global temperature rise, ice melt and sea level rise lag behind the rise of GHG forcing of about ~2 Watt/m2  (Figure 4). The precise time scale of this lag remains unknown.
The Earth climate is entering uncharted territory.
Australian National University
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