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Maritime glaciers and continental glaciers

In glaciology we like to distinguish between maritime glaciers and continental glaciers, because the climates that sustain these two kinds of glacier are quite different. The two adjectives make it sound as though the maritime ones ought to be near the sea and the continental ones oughtn’t, but the reality is more complicated and more subtle.

Most glaciers consist of an accumulation zone at high altitude, where they gain mass, and an ablation zone at low altitude, where they lose mass. Roughly in the middle is the equilibrium line, at an altitude (the equilibrium line altitude or ELA) where loss and gain just balance.

From year to year the ELA varies through hundreds of metres, but on average over the decades it changes much more slowly, as the changing climate alters the balance. But what pins the equilibrium line to a particular average altitude? Why 1000 m, say, and not 2000 m, or 0 m?

A short but inaccurate answer is “Temperature”. The hotter it is, the more ice you can melt. A slightly longer but much more accurate answer is “Temperature and snowfall”. The more snow falls, the more heat you need to melt it. So the climate at the slowly-changing ELA is a measure not just of how hot it is, or of how snowy it is, but of how hot and snowy it is.

By definition, you need just the right amount of heat at the ELA to melt just the amount of snow that falls. Observations show that, although the ratio varies quite a bit, you get about 5 mm of melted snow for every positive degree-day, that is, for every degree Celsius sustained above the melting point for 24 hours. The snowier it is, the lower the ELA has to be, because it is warmer at lower altitude.

If the winter snowfall is equivalent to 10,000 mm of meltwater, you need about 2,000 positive degree-days in summer to melt it all. A snowfall of 1,000 mm of meltwater-equivalent requires only 200 positive degree-days, and 100 mm means only 20 positive degree-days. These numbers span very roughly the range of actual snowfall on real glaciers, from coastal Norway and Patagonia near the snowy end to the highest glaciers in Bolivia and Tibet near the dry end.

If your glacier has to keep flowing downhill to find an ELA that is hot enough, it eventually reaches sea level. It becomes a tidewater glacier, and icebergs start falling off. The ocean is doing some of the work (of adjusting the size of the glacier to the climate) that the atmosphere can’t manage by itself.

Mountain ranges are traps for moisture, and potentially for snow, because they force the air to rise, cooling it and condensing out the moisture. What is more, once the wind has negotiated the mountain range it will be a lot drier, so we invariably find that across our glacierized mountain ranges the ELA rises to leeward.

This is the essence of what “maritime” and “continental” mean to glaciologists. We stretch the words so that maritime means “warm and snowy ELA” and continental means “cold and dry ELA”. Some of the most “continental” glaciers are close to shorelines. There are not many maritime glaciers far inland, but the ones in the eastern Himalaya probably qualify, because the monsoon still packs a punch even after blowing over peninsular India.

Like all scientists, I get a lot of scientific papers to review. Between a fifth and a third of the ones I get start with “Glaciers are sensitive indicators of climate change”, and I always cross it out, commenting that it is a boring cliché. The thing about glaciers is that they are insensitive indicators of climate change, because they integrate over temperature, precipitation, winter, summer, altitude, and possibly a significant horizontal extent. In the process they tell us things that no thermometer or rain gauge can.

They tell us, for example, that there is more to climatic change than rising temperatures. Because it had got snowier, the maritime glaciers of coastal Norway, but not the continental ones further inland, were gaining mass until about a decade ago. But more recent measurements show that rising temperature has now overcome this effect. And in the big picture, careful comparisons make it clear that less snowfall is definitely not why nearly all glaciers are losing mass. The message from the glaciers is that ELAs are rising, and rising because it is getting warmer. The reduction of snowfall that would be required to explain rising ELAs is enormous, and far beyond what we observe.

Posted by Graham Cogley on August 23, 2010 3:00 PM | Permalink