February 2010

Sea level rise: a new Task Group formed

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Sea level is one of the major impacts of anthropogenic climate change: it is higher now and is rising much more rapidly than any time in the past 3000 years.  Sea level will continue to rise for many centuries even after global temperatures are stabilized as it takes that long for the ocean and ice sheets to fully respond to a warmer climate.

During the last 3 000 years, the mean sea level has been below values observed at present (Figure 1a). The rate of sea-level rise increased considerably from the nineteenth to the twentieth century (Figure 1b), and now satellite measurements show sea level rising by 3.4 millimetres (mm) per year from 1993 to 2008, almost twice the average rate for the twentieth century. 

Plot of global sea level over the last 3,000 years   graphic
Click on image for enlargement
 

Figure 1: (a) Sea-level changes from 1000 B.C. to A.D. 2000 inferred from a range of sources (CSIRO, http://www.cmar.csiro.au/sealevel); (b) global mean sea level from 1870 to 2008, based on in-situ measurements with one standard deviation error estimates, is in red (Church and White, 2006), and the Topex/Poseidon/Jason-1 and Jason-2 satellite altimeter global mean sea level, based on the same standard processing from 1993 to 2008, is in black. 

One of key scientific achievements since Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) is that the estimates of various factors contributing to the global sea-level rise have started to sum up to a total that matches the observed values over recent decades with unprecedented accuracy, especially from 2003 to date. At the same time, results of the ongoing research and recent observations point to two factors suggesting that the IPCC AR4 conclusions about the rate of future change of the global mean sea level may be on the conservative side.

The first factor is widespread evidence of the increased melting of glaciers and ice-caps since the mid-1990s. New estimates show that the mass loss of glaciers and ice caps now contribute about 1.2 mm per year to global sea-level rise.

The second reason is the relatively fast dynamic response of the Greenland and Antarctic ice sheets to global warming that has been observed recently. In the case of Greenland, the pattern of ice sheet change is one of thinning in coastal areas, primarily in the south along fast-moving outlet glaciers.  Passive microwave satellite measurements of the Greenland ice sheet melt area show its expansion since 1979 (Figure 2).

graphic   Figure 2 — The total melt area of the Greenland ice sheet increased by 30 per cent between 1979 and 2008 based on passive microwave satellite data, with the most extreme melt in 2007. In general, 33 to 55 per cent of the total mass loss from the Greenland ice sheet is caused by surface melt and runoff. For 2007, the area experiencing melt was around 50 per cent of the total ice sheet area. The low melt year in 1992 was caused by the volcanic aerosols from Mount Pinatubo, which resulted in a short-lived global cooling (Steffen et al. 2008).
Click on image for enlargement    

Antarctica is also losing ice mass at an increasing rate, mostly from the West Antarctic ice sheet due to increased ice flow, and this contributes to sea-level rise at a rate nearly equal to that of Greenland. Narrow fast-moving ice streams in East Antarctica are now also contributing to the mass loss of the ice continent.

The IPCC AR4 estimated a sea-level rise for 2090-2099 relative to 1980-1999 to be between 18 and 59 centimetres (cm). However, the data from coastal tide gauges and the satellite estimates show that the current rate of sea-level rise corresponds to the upper limit of model projections given in the IPCC Assessments in 2001 and 2007 (Figure 3). Simple statistical models, based on relations between observed sea levels and temperatures, predict sea-level rise for 2100 of up to 1.4 m above the 1990 value.

graphic   Figure 3 — Observed and projected (starting for the year 1990) sea level: red line – coastal stations, blue line – altimetric data, dashed black lines and the grey area and various dashed lines within it – range of model based sea-level rise projections from (Rahmstorf et al, 2007).
Click on image for enlargement  

Significant progress has been achieved by science in quantifying and explaining the observed sea-level change in recent decades. The main challenge for climate science is now to address the uncertainties in the projection of the future sea-level changes due to recently discovered processes in the Greenland and Antarctic ice sheets.

In view of the clear need for improved information on sea-level rise by many stakeholders around the world, the World Climate Research Programme (WCRP) proposed to establish a Sea-level Task Group during its last Joint Scientific Committee meeting (JSC-30, Maryland US, http://wcrp.essic.umd.edu/JSC2009/index.html). The Group would be one element of the implementation of the WCRP Strategic Framework and would be co-sponsored by the Intergovernmental Oceanographic Commission (25th Session of IOC Assembly http://www.ioc-unesco.org/index.php?option=com_oe&task=viewEventRecord&eventID=188) .

The IOC/WCRP Task Group on Sea-Level Variability has been tasked to further improve our ability to assess periodically, explain, predict global and regional sea level, and all environmental factors related to it, and to use this information for informed decision-making. The Task Group is lead by an Executive Committee that will meet for the first time in Bern, Switzerland, in March 2010 in order to plan activities with the wide scientific community.

References

Church, J.A., and N. J. White, 2006. A 20th century acceleration in global sea-level rise. Geophysical Research Letters, 33:L01602, doi:10.1029/2005GL024826.

Commonwealth Scientific and Industrial Research Organization (CSIRO) Marine and Atmospheric Research (CMAR). Sea Level Rise. Understanding the past –Improving projections for the future. http://www.cmar.csiro.au/sealevel/index.html.

Rahmstorf, S., A. Cazenave, J.A. Church, J.E. Hansen, R. Keeling, D.E. Parker and R.C.J. Somerville, 2007.  Recent climate observations compared to projections. Science, 316 (5825):709, doi: 10.1126/science.1136843. 

Steffen, K., P.U. Clark, J.G. Cogley, D. Holland, S. Marshall, E. Rignot and R. Thomas, 2008. Rapid changes in glaciers and ice sheets and their impacts on sea level. In Abrupt Climate Change: A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research, Reston, Virginia, United States Geological Survey. pp. 60– 142.

 
 

 

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