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The snowline and the climate

If the climate were to change, you would expect the snowline altitude to change. It does, and we can show that it has in recent centuries, but we can also turn the proposition around. The snowline makes a very good tool with which to think about the climate. Here, again, is a graph of the global snowline.

A global approximation of the climatic snowlineA global approximation of the climatic snowline. South Pole on the left, North Pole on the right. Each little square is at an altitude which is the average of many “mid-altitudes”, each of which is the average of one glacier’s minimum and maximum altitude.

It isn’t just that the snowline makes sense of the glaciologist’s definition of “maritime” and “continental”. The temperature at the snowline varies in the graph from purple (very cold and continental, on the ice cap covering Illimani and on other peaks above 6 km in the Bolivian Andes) to dark red (very warm and maritime, in the northern mid-latitudes).

Why is the “line” fat in the northern mid-latitudes? Obviously there are a good many glaciers to sample there, but it is not obvious until we colour the little squares that the fatness is because regional climates vary in continentality. In southern Alaska, for example, the shoreline runs crudely east-west, continentality increases inland, and the snowline actually rises towards the pole.

Putting aside regional variations, why doesn’t the global snowline define a neat triangle, highest at the equator and lowest at the poles? The answer lies in the so-called general circulation of the atmosphere. The snowline dips in the tropics, between 30°S and 30°N, because that is the region through which the Inter-Tropical Convergence Zone travels as it follows the Sun. Here the airflow derived from subsidence over the desert belts of each hemisphere converges on the ITCZ. The subsidence implies warming of the air and therefore reduction of its relative humidity, which is why the desert belts are desert belts. Glaciologically, the subsidence means that you don’t need much heat to melt what little snow accumulates, so the snowline (strictly, the equilibrium line) is very cold and therefore very high. Between the desert belts, convergence at the ITCZ forces the air to rise and cool, provoking snowfall. The extra snow requires more heat, and a lower and therefore warmer equilibrium line, than in the desert belts.

I wonder if I can convince you that in the mid-latitudes of each hemisphere the snowline is concave up? It is a subtle but physically genuine depression of the equilibrium line, and as at the ITCZ it is due to convergence and thus to lifting and cooling of air. Again, the cooling provokes more snowfall and in turn a lowering of the equilibrium line. This time the converging airmasses are flowing poleward from the desert belts and equatorward from the poles.

Did you notice the asymmetry of the hemispheres? Anywhere poleward of the tropics, the snowline is hundreds of metres or more lower in the southern hemisphere than at the equivalent latitude in the northern hemisphere. It reaches sea level at about 60—65°S, but where we run out of land at 84°N it is still a few hundred metres above sea level.

The temperature at the surface of the Antarctic Ice Sheet is about 25°C colder than at the surface of the Arctic Ocean. Something like 18°C worth of the difference is simply because the ice sheet is about 3 km above sea level. The remainder, and the depression of the snowline throughout the southern extra-tropics relative to the north, are due to the chilling effect of the ice sheet on the general circulation.

Finally, a question that always makes my head spin. What would the altitude of the snowline be if there were no mountain range? There would be no orographic moisture trap, and no glaciers of course. If we knew the temperature of the snowline, we could go to the atmospheric temperature records and find where the snowline would be if there were land. But, first, by supposition there isn’t any land. Second, we know that if there were it would draw the snowline down to meet it, that being why glaciers start out maritime at the coast and become more continental the further inland you go. Third, that means that if there were land the temperature would be different from what it is in the free atmosphere, which returns us to where we started from but with the realization that we ought not to have started from there.

So I can’t produce an answer to the question. But I can see that the snowline teaches us a lot about the climate, including the proposition that the general circulation, the temperature and the topography are all mixed up in it together.


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