By Graham Cogley
In parallel with but, for practical purposes, independently of higher temperatures, we expect the environment to respond to an enhanced greenhouse effect with a more intense hydrological cycle. More evaporation where there is enough water (for example over the ocean) and a lot of evaporation already, and more precipitation where there is already a lot of precipitation. There are some pretty good indications that this is happening, but now a group of oceanographers has found more evidence in a surprising place (surprising to non-oceanographers like me, I suppose).
Kieran Helm and co-authors document just the kind of changes in the distribution of salt in the sea that you would expect if the hydrological cycle had intensified. Between 1970 and 2005 the maximum salinity of the water column, found at a depth of about 100 m, increased. In contrast, the minimum salinity, at about 700 m, decreased.
They analyzed the measurements by projecting them on to isopycnals, surfaces of constant density. The density of seawater increases when you add salt and decreases when you add heat. The payoff for the extra complexity is that heat and salt, added to or withdrawn from the ocean at the surface, are carried into or out of the interior of the ocean along these surfaces, and it is reasonable to interpret changes of salinity observed (strictly, inferred) on isopycnals as being due to changes at the surface.
The water balance of the atmosphere is a sort of zero-sum game. There isn’t room up there to store more than the equivalent of a few tens of millimetres of liquid water. In the big picture, more evaporation means more precipitation, but probably in a different place. Added water vapour stays in the air for long enough, on average, to be carried up to several thousand kilometres by the wind before it condenses and falls back out.
The atmospheric water balance is usually studied in terms of the single quantity P—E, precipitation minus evaporation, which (because I used to be a hydrologist) I will call Q for brevity. If Q is positive, the surface beneath the air column we are studying is getting wetter. If Q is negative, the surface is getting drier. If the air column is over the ocean, and its Q is positive, the ocean beneath, which is already as wet as it can be, is getting fresher (less salty), while if Q is negative the ocean is getting saltier.
The simplest way to make sense of the Helm results is to interpret the 1970-2005 changes in the distribution of salt as due to increases in oceanic Q of 7% in the higher latitudes of the Northern Hemisphere and 16% in the Southern Ocean, with decreases of 3% in the tropics. Each of these changes is subtle but statistically significant. (Another recent analysis, by Paul Durack and Susan Wijffels, suggests that the numbers might be on the large side.)
What has this got to do with glaciers? For one thing, Q is not the whole story. Glaciers that lose mass, as most do nowadays, are freshening the ocean, and sea ice that melts, as at the surface of the Arctic Ocean, is doing the same. But the thing that really interests me from the glaciological angle is the challenge. The hydrologists and now the oceanographers have produced evidence for a more intense hydrological cycle, and by implication a more intense greenhouse effect. Can we glaciologists rise to the same challenge?
A more intense hydrological cycle should make the shape of the snowline more curvaceous, lowering it by increasing snowfall near the equator and in the middle latitudes, and raising it by increasing evaporation in the desert belts. The snowline, remember, is at the altitude at which accumulation of snow is just balanced by losses due to melting and evaporation (actually, sublimation).
So the challenge is to detect snowline change due to the more intense hydrological cycle, against a background of snowline rise due to general warming. My guess is that, although it would be a big job, we might just be able to manage it. It would also be a race against time, because some of the most important glaciers for the purpose are losing mass so fast that they will not be with us much longer. But it would be worth the attempt, because demonstrating a change in the shape of the snowline is different from demonstrating simply that glaciers are losing mass, which in turn is different from demonstrating that the temperature is rising. The more independent but mutually consistent lines of evidence we have, the more confident can we be that we are on the right lines in interpreting what is happening to our world.