Even the smallest glacier is too heavy to weigh, at least by the classical method of lifting your object on to a set of scales and measuring the force with which it deflects a spring, or a known weight on the other arm of the scales. But that is not the only way to weigh something.
What we really want to know about the glacier is its mass balance, that is, the change in its mass over a stated period of time. Various ideas have evolved for measuring the mass balance, but until recently the list did not include what would be the obvious idea – repeated weighing – if the things were not too heavy and unwieldy.
That changed in 2002, when the U.S. National Aeronautics and Space Administration and the German Research Agency for Air and Space Travel launched the GRACE satellite mission. GRACE, short for Gravity Recovery And Climate Experiment, is revolutionizing the measurement of glacier mass balance.
GRACE is actually two satellites in the same orbit, one 200 km behind the other. Each continually measures its separation from the other with radar. Subtle differences in this distance are due to equally subtle differences in gravity as experienced by the two satellites – the two arms of the scales. These differences in the force of gravity are dominantly due, once a long list of corrections has been made, to the distribution (and redistribution) of mass in the solid and liquid Earth beneath the satellites.
Late last year Anthony Arendt and colleagues, and Scott Luthcke and colleagues, showed in impressive detail what GRACE was able to make of four years in the evolution of the mass balance of glaciers in southern Alaska. They were able to resolve changes every ten days, and to show that GRACE can see changes, if they are big enough, within regions as small as 200 km across.
Ten days is much better time resolution than offered by traditional methods, which are expensive, time-consuming, hazardous and sparse. GRACE’s weak points for our purpose are its poor spatial focus and the fact that it needs the signal of mass change to be strong. The Alaskan glaciers were losing mass at an average rate of 20.6 gigatonnes per year, give or take 3.0 gigatonnes. A gigatonne is an awful lot of ice, but here the most interesting number is the error-bar number. If GRACE can resolve changes as small as 3.0 Gt/yr, what are the prospects for the GRACE follow-on mission for which glaciologists and others are already slavering?
Estimates of the global average glacier mass balance involve a lot of interpolation, which is a fancy word for guesswork. For example the number of annual mass balances that have ever been measured by traditional methods in the Karakoram and western Himalaya is exactly four, and they required a good deal of unsatisfactory corner-cutting. My interpolated estimates for this region suggest an annual loss in the neighbourhood of 3.0 Gt/yr. In other words the present GRACE would have a hard time seeing the glaciological signal from these remote and poorly-covered mountains. But if the GRACE follow-on had sharper focus and better sensitivity it would give invaluable answers, and there are several other mountain ranges and high-latitude archipelagos where it would do equally well or better.
Technically, the improvements now being sketched by GRACE specialists will come mainly from switching from radar to laser interferometry for measuring satellite separation, reducing drag on the satellites, and lowering their orbit. They won’t amount to a complete solution of the problem of undersampling of glacier mass balance. There will always be glaciers too small for GRACE to notice, they will continue to contribute a significant proportion of the meltwater flowing into the sea, and we will still need to do small science if we want to understand glacier mass balance. But three cheers for the big science of the GRACE follow-on nevertheless.