Earth’s Climate Enters Uncharted Waters

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”.

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Figure 3. Greenland (a) and Antarctic (b) mass change deduced from gravitational field measurements by Velicogna (2009), as read by M.Sato from Velicongna’s 2009 graph, and the derivative of these curves, i.e., annual changes of the Greenland (c) and Antarctic (d) ice sheet masses. From Hansen and Sato

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 [2], leading to meters-scale sea level rise.

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Figure 4. Sea level vs CO2 plot, showing glacial and interglacial data points from Greenland and Antarctic ice cores, position of Earth atmosphere and sea levels during the pre-industrial period, shift of the Earth atmosphere and sea level since the 18th century and position of the mid-Pliocene (3 m.y.) CO2 and sea level values

andrew sliksonThe 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 [2] (Figure 4). The precise time scale of this lag remains unknown.

The Earth climate is entering uncharted territory.

Andrew Glikson
Australian National University


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