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Glacier melt will contribute to sea level rise - but how much?

Given the importance of glaciers in regulating the water supply for a fair fraction of the Earth's population - given also the - there's possibly no hotter topic in climate science than plotting the future of the world's ice bodies.

As with other aspects of climate change, we're dependent (in the absence of a time machine) on computer models to indicate the range of possible futures.

Unfortunately for policymakers and citizens who need good regional projections on which to base decisions, the range of possibilities can be pretty large.

The latest example of this is a paper on mountain glaciers, , by Canadian-based researchers and .

Overall, all the computer models they used in their study projected that glaciers will shrink in the years up to 2100; but even at the global level, there are big differences.

At the extremes, the Canadian model projects twice the ice loss of Nasa's model, for example.

People don't get their water supplies from global reservoirs, of course, but from their neighbourhood.

And when we get down to these levels, the differences are even more stark.

For many regions, there's a three- or even four-fold difference between the minimum and maximum projected loss.

And for the impacts of glacier melting on global sea level rise, projections for many of the regions run from essentially zero to several centimetres.

Projections of future percentage ice loss (L) and contribution to sea level rise (R) vary markedly from model to model

For context, it's worth emphasising that this is state-of-the-art science.

It uses (generally) the most modern versions of models from leading research institutions.

It uses the most comprehensive dataset available - the , which now contains details of more than 120,000 glaciers around the world.

So this is no tin-pot exercise. :

"...the most comprehensive study of mountain glaciers and small ice caps to date."

Nevertheless, the researchers note:

"Many of these glacierized regions are still facing large uncertainties in the climate projections due to the choice of GCM (global circulation model)."

No-one acknowledges the limitations of computer climate models more readily than modellers themselves, who will frequently bemoan the roughness of the resolution at which they have to work given the tools available.

How fast models' capabilities will increase is anybody's guess - partly because funding for new big science projects is scarce in many nations, partly because there are still big gaps in understanding of how oceans and the atmosphere work, and partly because when it comes to projecting trends such as glacier loss, the path human society takes in terms of economic development is a key factor, and that's certainly a known unknown.

So what should policymakers do?

Invest in extensive water management facilities that can replace glaciers, in the full knowledge that they might not be needed?

Or put off investment until another day, when the likely extent of glacier disappearance becomes more readily predictable from observations, but by which time the solutions available might be more expensive and difficult to implement?

If you take the view that human society never will get to grips with carbon emissions because we are inextricably wedded to cheap energy and fossil fuels - the view that all the coal and gas and oil available will inevitably be burned - then the science suggests the disappearance of glaciers from many regions is a given - it simply becomes a question of "when?"

European glaciers are projected to lose 50-90% of their ice - an acceptable risk?

The point has been made in many fora down the years that climate change policy isn't a question of looking for exact forecasts of impacts and deciding what to do about it.

It's about risk.

Are you willing to accept the risk that the European Alps could lose 90% of their ice by 2100, or New Zealand's ranges 85%?

What about Antarctic islands, or the mountains of the Caucasus?

One of the most succesful tactics that groups opposed to climate action down the years have employed is to cast doubt on the science - often by pointing out the scale of uncertainties in published research.

Campaign groups have on occasion sought to counter this by playing down the uncertainties and claiming that certain climate impacts are certain - claims that can rebound if the science is shown not to be so certain.

However, some observers have maintained that it's the uncertainties that make things really scary - largely because policymaking then becomes a judgement call based on guesstimates and susceptible to influence from all sorts of political and economic forces, rather than a logical response to a quantified threat.

Fresh water is one of the few things none of us can do without.

The latest research confirms that the natural storage systems that supply fresh water to many millions of people are likely to undergo dramatic changes in the next century.

Precisely how dramatic, though, is a question that cannot be answered precisely; and policies will have to be made in the full knowledge of that fact.

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