By Navin Singh Khadka
Environment reporter, BBC News
In the wake of a recent controversy over the retreat of Himalayan glaciers in which the UN's climate science body admitted that it was an error to assert that they would disappear by 2035, water availability has emerged as a key issue with even more uncertainty.
Environment reporter, BBC News
In the wake of a recent controversy over the retreat of Himalayan glaciers in which the UN's climate science body admitted that it was an error to assert that they would disappear by 2035, water availability has emerged as a key issue with even more uncertainty.
Receding Himalayan glaciers grabbed headlines because they feed major rivers in South Asia and some parts of Southeast Asia, which is home to a sizeable proportion of the planet's population.
If the glaciers significantly retreated or even disappeared, it would be an issue of life and death for the many millions of people who depend on these rivers.
But now that the Intergovernmental Panel on Climate Change (IPCC) has said that it was a mistake to say the glaciers will be gone in a matter of decades, does that mean water is not a worrying issue any more?
Many scientists believe it is - even more so given the uncertainty surrounding the future impacts of climate change in a region of not only high population, but one of high population growth.
The broad consensus is that the glaciers themselves are indeed retreating, although the rate of the recession may be debatable.
However, there are other climate-influenced factors that affect river flows, such as changes in precipitation, snowfall and regional temperature.
Uncertain times
"There has been too much focus on glaciers whereas there are other factors like precipitation and snowfall that affect the levels of waters in rivers downstream from the eastern Himalayas," says Mats Eriksson, a senior hydrologist with the Nepal-based International Centre for Integrated Mountain Development (ICIMOD), which has carried out several studies on the glaciers in the Hindu Kush Himalayas.
Below the eastern part of the Himalayas are major rivers like the Ganges and the Bramhaputra, as well as their tributaries.
These are vital lifelines for millions of people in India, Bangladesh, Nepal, Bhutan and Tibet.
A recent study for the World Bank has shown that the volume of water resulting from glacial melt in Nepal makes up less than 5% of the flows of rivers leaving the country and contributing to the Ganges downstream.
"That is, about 95% or more of the river flow is the result of rain and melting seasonal snow," said report co-author Richard Armstrong, a glaciologist from the University of Colorado at Boulder, US.
If that is true, rivers downstream of the eastern Himalayas will hardly be affected, even if the glaciers recede or disappear.
However, would the other contributing factors to the rivers' flow, such as precipitation and snowfall, remain the same in the changing climate?
No, say scientists, but whether that will lead to rise or fall of rivers' levels - and by how much and when - are the questions still waiting to be answered.
"We are seeing some changes in the monsoon," Dr Eriksson said of the seasonal precipitation system that shapes the climate in this part of the region.
"Last year, for example, the monsoon arrived one month late in Nepal and then some places saw 80mm of water in a day during the delayed rainy season.
"But there has been no consistent measurement of precipitation and temperature and there is a lack of proper studies."
'Heat trapping aerosols'
Some scientists believe absorption of solar radiation by aerosols (dust particles and carbon soot) can heat the atmosphere and accelerate regional impacts of global warming, which in turn affect water resources.
William Lau, who heads the atmospheric sciences branch at Nasa's Goddard Flight Center, carried out a study in India last year and found that, as a result of aerosols, regional temperature was rising much faster than expected.
And that, he said, could influence the monsoon systems, resulting in less water availability in the region.
But Dr Armstrong said a warming climate could also mean a stronger monsoon bringing more precipitation that could increase stream flows.
"Having said that, it should be noted that future precipitation patterns predicted by climate models are highly variable and there is a very little regional agreement among the models," he said.
High variability is also an issue with the flow of rivers in the western Himalayas that do not fall within the monsoon regime.
"There is no clear-cut signal as there is a large variation between average annual flows," said Arshad Muhammad Khan, a physicist who heads the Global Change Impact Studies Centre in Pakistan.
"For example, in the Indus River, the maximum flow is twice of that of the minimum."
Unlike the Ganges, rivers like the Indus in the western part of the Himalayas are heavily dependent on glaciers, as this region does not get monsoon rains.
But even here, glacial status is not reported to be uniform.
Rising or falling flows?
Some scientists say increasing temperature has meant that glaciers don't get enough snowfall during winter and therefore river flow during summer is dwindling.
"We have seen the decline in the flow of the Indus, Chenab and Jhelum rivers," says Professor Mohammad Sultan Bhat of Kashmir University, who has conducted field studies with India's flood and irrigation department.
"We have recorded a decrease of 40% in the flow of Jhelum's tributary river… that is fed by the receding Kolahi glacier."
But, Kenneth Hewitt, a glaciologist from Canada who has been doing field studies in Pakistan's Karakoram mountains, told BBC News last October that he had seen at least half a dozen glaciers there advancing since he saw them five years ago.
With glaciers offering such complex pictures, combined with precipitation and temperature patterns becoming increasingly complicated, the region's river systems that depend on all these factors cannot be simpler.
Politics and geography, experts say, have made understanding the situation even more difficult.
"Some countries in the region are not willing to share water-related data because they regard it as confidential," said Dr Eriksson of ICIMOD.
"Since it is difficult to access them, proper studies on water availability remain a major challenge."
