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Sea Level Rise and Ice Melt

  • Sea Level Rise

    • One of the most-known predictions by the IPCC (Intergovernmental Panel on Climate Change) is the potential rise in sea level as the global average temperature increases.

      The most recent report from the IPCC (Fifth Assessment Report, 2014, IPCC = Intergovernmental Panel on Climate Change) puts the expected rise in global temperature by 2050 at between 2 and 3°C relative to the start of the Industrial Revolution (when fossil fuels were started to be used en masse). The prediction in the IPCC report is for a 59cm rise in sea level.

      The precision of the prediction belies the fact that there is a great deal of uncertainty involved. This is due to unknowns such as the rate of ice melt. Jim Hansen, of NASA/GISS, has a far more dramatic expectation: a sea level rise of 25m at 3°C temperature increase, and in a much shorter timeframe. This estimate is based on studies of ice melt in the polar regions, which may not occur linearly with temperature rise, but 'flip' states, suddenly changing from solid ice to water, and causing considerable sea level rise in short time.

      But this does not address the question of what will happen if the temperature is so high, all of Earth's ice melts? The average depth of the world's oceans is 3800m. 2% of global water is ice and snow, so if this enters the oceans, it will add 2% to the water mass. A simple calculation shows that 2& of 3800m is 76m.

      But that is not the end of the story. There will also be thermal expansion, since the oceans will also be warmer. Water is at its densest at 4°C. At higher temperatures, there will be an expansion of the volume of the sea, bringing the rise to over 80m. Since the majority of large human cities are situated on coasts, a sea rise of this magnitude would be devastating to human civilisation, no matter how protracted the occurrence.

      Greenland Ice Loss

      Greenland lost 1500 cubic kilometers of ice between 2000 and 2008, and caused 17% of the world's sea level rise.

      The GRACE satellite measures the ice sheet by detecting changes in its gravitational force. This measurement can be compared to models of differences in annual snowfall and snowmelt, to reveal net ice loss. The good correlation of the two data sets allows a estimate of 0.75mm sea level rise per year due to Greenland's ice entering the sea, compared to a global sea level rise of the same period of 3mm average per year.

      Research groups: Ted Scambos of the US National Snow and Ice Data Center in Boulder, Colorado. Michiel van den Broeke of Utrecht University, Netherlands. [Science, DOI:10.1126/science.1178176]

      Uneven distribution of meltwater

      As the West Antarctic ice sheet melts, it releases pressure on the land mass underneath, causing it to rise and displace sea water. In addition, the loss of mass at the poles will cause the Earth's spin axis to change, with increasing centrifugal forces causing a disproportionate amount of the new water to be pushed up at the mid-latitudes. Locations like the US east coast will receive the brunt of the extra sea level rise.

      Researchers: Jerry Mitrovica, University of Toronto, Canada [Science, DOI:10.1126/science.1166510].

  • Climate change indicators

    • Climate change indicators are changes to the Earth's surface conditions, natural cycles and biospherical dynamics which are considered to be occurring at a rate which is more easily explained by human causes rather than any natural process.

      Global surface temperatures are increasing at an increasing rate. Since 1920, the increase of the mean surface temperature of the Earth has been 0.78°C. The trend between 1956-2005 of 0.13°C per decade is linear, and is at least twice that of the preceding 100 years.

      In 2007, the IPCC reported that eleven of the twelve warmest years on record (i.e. since 1850) occurred between 1994-2006. (IPCC AR4 - 4th Assessment Report - 2007a)

      The annual melting rate of glaciers has doubled since 2000 compared to 1980-2000.

      Arctic ice has been melting, and this is accelerating: 3% per decade 1979-1996, 11% 1996-2006.