Another unknown is the high altitude snowfall rates.
"We know almost nothing about precipitation at high altitude and its spatial variation," said Immerzeel. "We installed several gauges at 5,000 meters (16,400 feet), but maintenance and getting accurate long-term records remains a challenge."
Then there is the big difference between the glaciers in the Eastern Himalaya -- which receive moisture from the Indian Monsoon -- and the western Himalayan glaciers the Karakoram.
But climate models have great difficulty simulating the monsoon.
"They can hardly reproduce the past, and the future is an even larger challenge," said Immerzeel. Better high-resolution models validated with ground data are required to solve this, but this is not something that is done overnight.
"We had an acceleration of mass loss since the 90s in the Eastern Himalaya, where the monsoon is the main control," said Rowan. Over the next century they could continue to lose mass, but changes in the monsoon could offset that, she said. "So glacier melt [in the eastern Himalaya] may not be a problem."
On the other hand, glaciers in the Karakoram seem to be gaining mass. Which just goes to show, again, how complicated the Himalaya climate picture is and how poorly understood it is.
Still, while there is still a long way to go, the new assessment is a huge improvement over the 2007 report, which was based on the little information that was then available.
"There were very limited peer reviewed publications and conclusions were drawn on grey literature sources, because of the lack of properly published studies," said Immerzeel. "This has now all changed and a body of literature has developed over the past years on which AR5 (Assessment Report 5) is based."
To move ahead further, one thing is clearly needed: more field work to study what exactly is happening to the glaciers on the world's premiere mountains.