Bangalore: Melting ice from Himalyan glaciers and other global ice sheets has contributed more to the rise in the global sea level over the past 80 years than was previously estimated, increasing the need for an effective global emission control regime.
The findings are being released in Friday’s issue of journal Science. A G20 meet on climate kicks off in Tokyo on Friday.
In their article, researchers from the National Central University in Taiwan report that the contribution of ice melt is higher than previously thought because earlier calculations have left out the contribution of water reservoirs that, it turns out, have been responsible for a drop in sea level by 30mm over the past 50 years.
Dokriani glaciers in Garhwal Himalayas are receding at the rate of 1.5-2m every year (Photo by: DP Dobhal, Wadia Institute of Himalayan Geology, Dehradun)
India’s glaciers are melting fast and the Intergovernmental Panel on Climate Change (IPCC), the scientific body meant to study climate change, warned in 2007 that if steps were not taken to check this, there was a likelihood of water shortage in rivers (when needed) and flooding of coastal regions.
“The concept is simple,” says the lead researcher from the National Central University of Taiwan team, Benjamin F. Chao. “We know that the global sea level (GSL) has been rising at 1.8mm/year over the past century, accelerating in the last decade. We also know that people have been building reservoirs impounding large quantity of water behind artificial dams, but we don’t quite know how much negative impact these dams have had on GSL rise.”
Having raised this question back in 1991, Chao and several others have come up with various estimates of the rise in sea level. “Those estimates were very crude as there was no compiled data, but this is the most accurate and believable to date,” claims Chao of the 30mm number. His group has “reconstructed” the water impoundment history by assembling a comprehensive tally of the world’s reservoirs constructed since 1900 and studied their impact on the rise in GSL.
“This is an important finding as it opens up the much-debated field of climate change for further research,” says V.P. Dimri, director of the National Geophysical Research Institute in Hyderabad. For instance, this research only touches upon, but has not investigated in detail, the impact of sediments brought by big rivers, adds Dimri. “It’s now essential to study what sediments do to the sea level rise. It can even neutralize the negative impact of reservoirs,” he says.
The total rise in sea level over the past century is due mostly to ocean water expanding in volume as it warms up, and ice melt from mountain glaciers and Greenland and Antarctic ice sheets owing largely to global warming. But, even after summing up all the natural causes, scientists are still short of explaining fully the GSL rise that they observe.
A 2007 report by IPCC said that the issue of GSL rise had “not yet been closed satisfactorily”, and researchers say the impact of man-made reservoirs on GSL makes the situation even more difficult to explain. However, Chao is convinced that with his team’s report, as far as uncertainties in climate change are concerned, “it’s one more down.”
Though Indian glaciers are among the least studied anywhere, recent studies show they are melting at an alarming rate. According to D.P. Dobhal, a glaciologist from the Wadia Institute of Himalayan Geology in Dehradun, who has been studying Chorabari and Dokriani glaciers in the Garhwal Himalayas since 2003, they are receding at the rate of 1.5-2 metres every year.
“What is worrying is that the glaciers are losing density... largely due to global warming; the snow is melting faster than it can accumulate as ice, thereby thinning the glacier,” says Dobhal.
Still, the new finding does not mean building more artificial reservoirs could check the problem. In fact, a 2007 report from Brazil’s National Institute for Space Research showed that India’s large dams are responsible for about one-fifth of the country’s global warming, emitting large amounts of greenhouse gases.
Globally, there’s a slowdown in the construction of large dams, according to a World Commission on Dams report dated 2000, but the situation in India is different. “It is true that the pace of dams achieving completion in India reached its peak of 1,263 in the decade of 1971-1980 and slowed down to 347 dams in 1991-2000, but in the new millennium, the pace seems to have picked up again,” says Himanshu Thakkar, convenor of activist group South Asia Network on Dams, Rivers and People, in New Delhi.
According to the latest data from the Central Water Commission, India has 4,050 completed large dams (those above 15 metres from the lowest foundation level) and 475 such dams are under construction.
Chao’s research, however, is not about dams and reservoirs, but the causes of the rise in GSL. This serves as an unmistakable indication of the real problem behind the global climate change, says Chao.
“That is what makes this research intriguing.”