Glacial melt in the Himalayas has been increasing over the last 30 years, a new study argues.

The glaciers feed the Indus, Brahmaputra and Ganges rivers which supply water to around 1.4 billion people in Asia.

Potential consequences of changes in the glaciers include unsustainable water supplies from major rivers and geo-hazards such as glacier-lake outbursts and flooding, all which could threaten the livelihoods and well-being of populations in the downstream regions, says the study, led by Yao Tandong, director of the Institute of Tibetan Research at the Chinese Academy of Sciences in Beijing and glaciologist and paleo-climatologist Lonnie Thompson of Ohio State University.

"The majority of the glaciers have been shrinking rapidly across the studied area in the past 30 years," Yao told Nature's Climate Change, the journal that published the study.

A prolonged glacier retreat would increase the volume of water in rivers and also the sediments, which could choke water supply and disrupt agriculture, the study says.

While previous studies of Himalayan glaciers had been based on data over seven years from the Gravity Recovery and Climate Experiment satellite mission known as GRACE, Thompson said it's also important to look at the longer-term picture because climate is generally considered a 30-year average of the weather.

For example, the Naimona'nyi Glacier, which feeds the Indus River, had shrunk by 508 feet during the 30 years of the study, a rate of about 16.4 feet annually.

"We were surprised to find that at 19,849 feet [the height at which the glacier is located] there had been no net accumulation [of ice] since the late 1940s," Thompson told IRIN, the news service of the U.N. Office for the Coordination of Humanitarian Affairs.

Separately, researchers at the Nepal Climate Observatory Pyramid, 16,663 feet high in the Himalayas, have lately focused on how "black carbon" -- fine soot and ash produced by diesel exhausts, thermal power plants, brick kiln smokestacks, and forest fires - may be accelerating the melting of ice and snow.

The U.N. Environment Program and some scientists and international research organizations say that increased black carbon deposits on Himalayan glaciers cause them to absorb more sunlight, thus accelerating glacial and snow melt.

"Although glacier melting is predominantly due to global temperature rise, the deposition of pollutant particles like black carbon can enhance this effect," says Paolo Bonasoni of the Italy's Institute of Atmospheric Sciences and Climate, Inter Press Service reports.