Climate change queries (2) - arctic sea ice and Himalayan glaciers
With global warming, how come there is recently a rise in sea ice coverage in the Arctic ?
Although the Arctic summer sea ice extents in 2008 and 2009 were greater than that of the record low in September 2007 (Figure 1), the sea ice extents were still well below the long-term average and the decreasing trend in the last few decades is significant. According to the U.S. National Snow and Ice Data Center, the sea ice extent for 2009 was the 3rd lowest and the past five years have seen the five lowest ice extents since the start of records in 1979.
Figure 1 Average monthly arctic sea ice extent - summer (September) 1979 - 2009
(Source : U.S. National Snow and Ice Data Center)
What about the melting glaciers in the Himalayas ?
IPCC recently admitted that the rate of retreat and the estimated disappearance date for Himalayan glaciers (in the Working Group II contribution of the IPCC Assessment Report in 2007) were poorly substantiated. The mistake was however pointed out by scientists themselves. Evidence still shows that Himalayan glaciers and other glaciers around the world are melting and that global warming is real and very likely man-made.
Figure 2 Mean annual mass balance [Note] of reference glaciers from
1980 to 2008 with positive values in blue and negative values in red.
(Source : World Glacier Monitoring Service : Preliminary glacier mass balance
data 2007/2008 http://www.geo.unizh.ch/wgms/mbb/sum08.html)
The latest global results of glacier melts are presented by the Switzerland-based World Glacier Monitoring Service (WGMS) in Figure 2. WGMS pointed out that one century of systematic observation clearly reveal a general shrinking of mountain glaciers at a global scale.
The melting of the glaciers and ice caps (excluding Greenland and Antarctica) is expected to add about 10 to 25 cm to sea level by 2100. The other main contributor to global sea level rise is the thermal expansion of ocean water. Even if all emissions (e.g. electricity generation, industrial activities, and transportation) stop tomorrow, global temperatures are expected to rise about 0.6 oC because of the latent effects of carbon dioxide already emitted into the atmosphere. This would have a knock-on effect on ocean water, i.e. ocean water will warm up and expand further. Depending on the emission scenarios, a recent study indicated that thermal expansion of ocean water alone would raise the sea level by about 20 to 50 cm by 2100. This is far greater than that brought about by glacier and ice cap melts.
How sure are the conclusions of IPCC ?
IPCC pointed out in 2007 that the climate change we are witnessing is 'very likely' a result of anthropogenic (i.e. human) activities. By 'very likely' it meant a certainty of 90%, not 100%. Though less than perfect, this is in fact consistent with the history of science development.
So what do all these lead us to ? We need to be careful about interpreting short-term changes. They have to be assessed against the long-term trend. Do not confuse the weather, which is short-term, with the climate, which is long-term and has to be based on at least 30 years of record. Last, don't be blinded by a single mistake in the IPCC 2007 report. So far, only the rising level of carbon dioxide in the atmosphere can explain such observed phenomena as global warming, melting of glaciers and ice caps, and the rising sea level.
Mark F. Meier, Mark B. Dyurgerov, Ursula K. Rick, Shad O'Neel, W. Tad Pfeffer, Robert S. Anderson, Suzanne P. Anderson, Andrey F. Glazovsky, 2007 : Glaciers Dominate Eustatic Sea-Level Rise in the 21st Century. Science, 24 August 2007: Vol. 317. no. 5841, pp. 1064 - 1067
Susan Solomona, Gian-Kasper Plattnerb, Reto Knuttic, and Pierre Friedlingsteind, 2009: Irreversible climate change due to carbon dioxide emissions. Proceedings of the National Academy of Sciences, February 10, 2009, vol. 106, no. 6, pp1704 -1709.
Note : "Glacier Mass Balance" is determined by the difference between the amount of snow and ice accumulated in winter and the amount of snow and ice removed by melting in summer. The unit is in millimetre of water equivalent (mm w.e.). A positive value of the mass balance implies the glacier has increased in volume while a negative value indicates the glacier volume has decreased.