Ice Sheet Disintegration

Gravity satellite data of Greenland and Antarctic ice sheet mass loss. The monthly data (navy blue curves with plus signs) read from Velicogna, I. Geophys. Res. Lett., 36, L19503. (Also in PDF.)

The figure above shows the rate of mass loss from the Greenland and Antarctic ice sheets, based on a recent publication of gravity satellite data. The rate of mass loss from Greenland has increased during the past 2-3 years, as it has from Antarctica. This is one of the most important geophysical measurements being made, so it is important to get a follow-on gravity satellite into space. A planned European gravity satellite is not sufficiently capable to yield accurate ice sheet mass change, and a planned NASA follow-on gravity mission is low on NASA’s priority list.

Sea Level

Sea level change is an important climate diagnostic. However, interpretation of the moderate rate of sea level rise of the past century is complicated by the fact that several competing processes contribute to net sea level change. The most important processes are melting of land ice (the large ice sheets on Greenland and Antarctica and mountain glaciers) and thermal expansion of ocean water as the ocean warms. Storage of water behind dams and depletion of groundwater aquifers also affect sea level.

Over the past 5000 years the average rate of sea level rise was much less than 1 meter per millennium (1 mm/year), as shown by geologic markers on coastlines. In the past century tide gauges at coastal cities reveal a larger rate of sea level rise. The second chart below shows that sea level rose about 2 mm/year during 1920-2000.

Satellites began to measure sea level precisely in 1993. Since then sea level has been rising about 3.2 mm/year. This rate is equal to just over one foot per century. The impact of such sea level change is substantial, yet the concern is that far greater sea level rise will occur this century if the major ice sheets disintegrate.

Global sea level change measured by TOPEX satellite (1992-2001), Jason-1 (2002-mid 2008) and Jason-2 (mid 2008-present). Data through September 10, 2011. Data source: University of Colorado at Boulder: Sea level change. Figure also available in PDF. (Last modified: 2012/02/14)

Sea level change for 1870-2001, based on tide gauge measurements, from Church J.A. and White N.J. “A 20th century acceleration in global sea-level rise” Geophys. Res. Lett. 2006; 33: L01602. University of Colorado data are shifted to have the same mean for 1993-2001 as Church and White. The trends were computed for 1870-1920, 1920-1975, 1975-2001 for Church and White data, and 1993 – September 10, 2011 for University of Colorado data. Figure also available in PDF. (Last modified: 2012/02/14)

Greenland and Antarctica have begun to melt faster in the past few years, as shown on Ice Sheet Disintegration page, yet sea level rise slowed slightly in the past few years. The reason seems to be that ocean heat storage decreased in the past five years reducing thermal expansion. Reduced heat storage may be related in part to solar minimum radiation. Ocean heat uptake will surely resume, so acceleration of sea level rise in the next few years may occur. We will discuss the interconnections of climate forcings, global temperature, ocean heat storage, ice melt, and sea level in a paper now in preparation.