They also find that the lake's surface area peaked in 2004, directly contradicting other papers saying that Imja has been expanding at an accelerating rate. "Some previous studies have less observational foundation," Koji Fujita, leader of the team, told environmentalresearchweb. "Baseless rumours have been published and reported by the media, so I tried to publish our observational fact."
Fujita and colleagues from Nagoya University assessed how Imja – which only began to form in the 1960s – had changed since a previous survey in 1992. Together with Japan's National Research Institute for Earth Science and Disaster Prevention and Nepal's Ministry of Environment, Science and Technology, they interpreted a combination of satellite imaging through to 2008, a direct bathymetric survey conducted in 2002 and a GPS survey conducted in 2007.
From the satellite data the researchers found that the lake had covered 0.93 sq. km in 2004, and then shrunk by 3.5% the following year. By 2008 this had expanded again to 0.92 sq. km. Fujita admits that the team was initially surprised by what they saw. "I thought that constant expansion was one of the most significant features of the Himalayan glacial lakes," he said.
However, comparing how their bathymetric surveys changed between 1992 and 2002 showed a clear expansion of the lake bottom adjacent to the glacier. Combining this depth survey with their surface area measurements Fujita and his colleagues found that the volume of the lake had increased from 28 million cubic m in 1992 to 35.8 millon cubic m in March 2002. They claim that this is the first time a volume change in a Himalayan lake has been calculated.
The team's surveys also showed that the moraine damming the lake is lowering at rates ranging from 0.06 to 1.03 m per year for the period from 2001 to 2007. This is a slower lowering than found in a previous study, which the scientists say indicates the moraine is stabilising, although there remains a risk of overflow. Fujita says that their work demonstrates that remote satellite imaging can help to monitor threats posed by glacial melt, but intermittent direct surveys will remain necessary.
"Change in the damming moraine is rather more important than lake area for risk monitoring," he pointed out. "I think Imja glacial lake is in a safer condition now. Other glacial lakes should be monitored more intensively than Imja even if they are at the head of a watershed where fewer people are living."