Depends how you define "disaster", I suppose

This is, of course, a very fair and reasonable blog, and although it has long decided that the world ought to be working hard to urgently limit CO2 reductions, it's not above pointing out some of the confusions and exaggerations which occur on my side of the argument.

Hence, regular readers might recall that in January this year, I noted that papers talking about the effect of loss of glaciers in the Himalayas on water flow in Indian and Chinese rivers seemed to be using some pretty confusing figures which were hard to reconcile.

Well, my confusion was justified, it seems. Nature reports on a new paper which tries to put the issue of glaciers and water flow in that area more into perspective. It starts with the heading "Global warming impact on Asia's rivers overblown", which appears to be part of environmental journalists new campaign not to be caught exaggerating again, even when the thing they are reporting on is quite seriously bad. Here are some extracts:

Although global warming is expected to shrink glaciers in the Himalayas and other high mountains in Central Asia, the declining ice will have less overall impact on the region's water supplies than previously believed, a study concludes.

It's an important finding, says Richard Armstrong, a climatologist at the US National Snow and Ice Data Center in Boulder, Colorado, who notes that the Intergovernmental Panel on Climate Change had previously predicted dire restrictions on water supplies in Asia. "There clearly were some misunderstandings," he says.

Yes, well, OK, but while the current study did find that the importance of glacier meltwater is not so important for some rivers, it's still pretty important for others:

The researchers behind the latest study began by calculating the importance of meltwater in the overall hydrology of five rivers: the Indus, the Ganges and the Brahmaputra in India, Pakistan and Bangladesh, and the Yellow River and the Yangtze in China¹. The authors found that meltwater is most important to the Indus, with a contribution roughly 1.5 times that from lowland rains. In the Brahmaputra, meltwater flow is equivalent to only one-quarter of the volume supplied by lowland rainfall, and, in the other rivers, it forms no more than one-tenth of the input.

Furthermore, the study found that in the Indus and Ganges basins, glacial ice contributes only about 40% of the total meltwater, with the rest coming from seasonal snows. In the other three rivers its contribution is even lower.

OK, so with some model's predicted changes to rainfall/snow, what effect might AGW have?:

Climate change will therefore have two effects, Immerzeel says. One will be to reduce the contribution of glaciers to total run-off. The other will be to change weather patterns, including rain and snowfall. Combining these and looking at averages from five climate models, Immerzeel and colleagues concluded that the change in upstream water inputs will range from a decrease of 19.6% for the Brahmaputra to a 9.5% increase for the Yellow River. The latter, he notes, is due to increased winter rains. "The Yellow River depends only marginally on meltwater," he says, "and, on average, the models project an increase in winter precipitation in the Yellow River basin."

What this means, Armstrong says, is that river flows are dominated by seasonal rains. "The glaciers are tiny, compared with the monsoon," he says.

All sounding relatively comforting, kind of, until you hit the next paragraph:

Nevertheless, the study concludes that climate change will reduce water supplies enough that by 2050, declines in irrigation water are likely to reduce the number of people the region's agriculture can support by about 60 million — 4.5% of the region's present population.

So the previous over-estimation of how many in India may be badly effected by AGW within 40 years is downgraded to a mere three times as many as the population of Australia.

How very comforting for them.

Of course, precise predictions of changes to rainfall is one of the rubberiest areas of climate science at the moment, but still, it would seem a fair bet that one of the worst hit areas from human induced climate change will be the relatively helpless poorer people in parts of Asia.