Madhav Khandekar has provided us with another informative guest post.
Madhav Khandekar is a former research scientist from Environment Canada and is presently on the Editorial Board of the international Journal Natural Hazards (Kluwer Netherlands). Khandekar was an Expert Reviewer for the IPCC 2007 climate change documents and his latest contribution to sea level rise is a Chapter (Global warming, glacier melt and future sea level rise) in the Book “Global Warming” published by Sciyo Publishers (Sciyo.com) October 2010.
Global warming, glacier melt and sea level rise: need for more realistic future estimates by Madhav Khandekar
There is now a heightened interest on the possibility of rapid melting of world-wide glaciers and ice caps ( e.g., Greenland and Antarctic ice caps) as a result of ongoing warming which could lead to escalated sea level rise in the ‘near future’. Sea Level Rise (SLR) is an important climate change parameter which is being intensely discussed at present in the context of human-induced global warming and the climate change debate. Many newspaper articles as well as science magazine articles often refer to world-wide glacier melts and the possibility of sea level rise of 3 to 7 ft (1 to 2m) over the next fifty to one hundred years.
Several recent observational field studies using sophisticated remote sensing technology have provided estimates of “ice sheet mass balance” of Greenland and Antarctic ice caps; these estimates suggest significant acceleration of ice mass loss in recent years (Rignot et al, 2011 Geophysical Research Letters) and conclude that future SLR will be dominated by melting of the two ice caps. Papers published in the last ten years using atmosphere-ocean general circulation models suggest thermal expansion as the largest contributor to future SLR, which is estimated to be in the range of 20 to 37 cm over next 100 years.
A recent publication (Vermeer & Rahmstrorf, 2009 Proc. National Academy of Sciences) obtains values of 1m and higher for SLR over the next one hundred years using an empirical model which links future warming of the earth’s surface to increased sea levels. These studies plus recent Hollywood movies like An Inconvenient Truth showing big ice-shelves breaking off and sliding down into the cold Arctic Ocean have created a perception that “ice caps and glaciers are indeed melting rapidly causing sea level to rise dramatically”.
How rapidly is the sea level rising at present? Let us look at some latest studies and numbers:
Some basic facts on sea level rise:
1. It is now generally accepted that global sea level increased by about 120 m as a result of de-glaciation that followed the LGM (Last Glacial Maximum) about 21000 y BP ( years Before Present). By about 5000-6000 y BP the melting of high-latitude ice mass was more less complete after which sea level rise was small and globally-averaged SLR over the last 1000 years and prior to the twentieth century has been estimated to be just about 0.2mm/year.
2. The 20th century SLR has been most intriguing and has sparked a large number of studies (see, for example, Khandekar 2009, Energy & Environment). The total SLR during the 20th century is now estimated to be about 1.6 to 2.0 mm/yr.
3. GIA (Glacial Isostatic Adjustment): This refers to the gradual springing back of the earth’s crust in response to the removal of snow loads since the LGM, especially in the region of Gulf of Bothnia (also refereed to as Fennoscandia) where the ice was as much as several km thick during the LGM and where relative SLR is still falling at the rate of about 5-10 mm/yr.
4. Recent satellite data using TOPEX/Poseidon satellite altimeter obtains a value of SLR about 2.8mm/yr and possibly higher.
5. The IPCC (Intergovernmental Panel on Climate Change) in its latest (2007) document has estimated SLR to be between 14 and 41 cm (mean value 29 cm) under the A1B (greenhouse gas) scenario in which earth’s mean temperature is expected to rise between 2.3C and 4.1C over next 100 years. The IPCC 2007 projects SLR due to thermal expansion (steric component) as about 23 cm while the contribution due to melting of glaciers and ice cap (eustatic component) is estimated as about 6 cm over next 100 years
Several recent papers have provided SLR numbers which need to be examined carefully in the context of present debate on global warming and sea level rise. A paper by Holgate (2007, Geophysical Research Letters) analyzed nine long and nearly continuous sea level records over one hundred years ( 1903-2003) and obtained a mean value of SLR as 1.74mm/yr, with higher values in the earlier part of the 20th century compared to the latter part.
A comprehensive paper by Prof (emeritus) Carl Wunsch and co-workers ( J of Climate December 2007) generate over 100 million data points using a 23-layer general circulation ocean model which include different types of data ( salinity, sea surface temperature, satellite altimetry, Argo float profiles etc) and obtain an estimate of SLR as 1.6mm/yr for the period 1993-2004. A more recent paper by Wenzel & Schroter (Journal of Geophysical Research-Oceans 2010) analyzes tide gauge records over a period 1900-2006 and obtains a mean value of 1.56mm/yr with NO statistically significant acceleration in sea level rise. The latest paper by Houston & Dean (Journal of Coastal Research 2011) carefully analyzes 57 tide gauge records each with a record length of 80 years which include 25 gauges with data from 1930-2010. This study finds no acceleration in sea level rise, but instead a small average deceleration of -0.0014 and -0.0123 mm/yr2. These latest findings appear to contradict the general perception that sea level rise is escalating at present.
Recent observations and studies of breaking of ice shelves and ice sheet mass losses must be carefully assessed in the context of Arctic climatology which is now identified as being linked to low frequency atmosphere/ocean oscillation with a period of 60-80 years. An excellent temperature dataset for the entire Arctic basin has been prepared by Dr Igor Polyakov (University of Alaska) for the period 1860-2005.
This dataset shows clearly that the Arctic was at its warmest in 1935 and 1936 and the present temperature in the Arctic is about the same as it was in the mid-1930s. Further, the Arctic witnessed significant icecap and glacier melting during the 1920s and 1930s as evidenced by the following commentary “The Arctic sea is warming up, icebergs are growing scarcer, great masses of ice have been replaced by moraines of earth and stones, at many points well-known glaciers have entirely disappeared (US Weather Bureau 1922)”.
Also, the temperature history of Greenland shows that the 1920s and 1930s were the two warmest decades over Greenland, in a long dataset from 1880 to 2007. These observations and the US weather Bureau report strongly suggest that the Arctic witnessed significant ice melt and icecap mass loss during the 1920s and 1930s, however, no detailed quantitative calculations (of icecap mass loss) were possible then due to lack of adequate remote sensing technology.
An estimate of sea level rise can be made by observing that from 1940 to 2010, global sea level has risen by about 13-14 cm. Of this rise, the steric (thermal) component of SLR can be estimated at about 6 cm while the eustatic (melt part) contribution is about 8 cm. If these estimates are used to extrapolate SLR to 2100, we obtain a maximum of 12 cm of SLR due to the eustatic (melting) contribution, while another 8 cm or so due to steric (thermal expansion) contribution.
In summary, the estimate of over 1m and higher rise in sea level by 2100 (in next 90 years) seems unrealistic, when analyzed in the context of present sea level rise which is just about 1.5mm to 2.0 mm per year with almost NO component of acceleration. For the global sea level to rise by over 1m in the next 90 years would require acceleration (in sea level rise) of up to 0.28mm/yr2, which is almost two orders of magnitude larger than present. This seems highly unlikely at present given the fact that the earth’s climate has not warmed in the last ten years and further that the earth’s mean temperature seems to be declining at present.
There is a definite need to obtain more realistic estimates of future SLR than what are available at present.
Acknowledgements: I am grateful to Prof Roger Pielke sr for inviting me to write this commentary.