The Albedo Effect, 16 July
This morning, the glacier is shimmering. It’s a stunning addition to an already stunning view. But it’s nothing to do with the ice itself. Oddly, there would be no shimmer if I were standing on an ice pinnacle, looking sideways along the glacier. This optical effect is a beautiful illustration of a really important piece of glacier physics.
Checking out some Swiss cheese holes
Up here in base camp, I’m perched on a rock with a small but enthusiastic cloud of mosquitoes in attendance. It’s really warm, which isn’t a surprise – it’s midsummer and there is 24 hour daylight here. An almost uninterrupted stream of energy is arriving from the sun. Of course it’s hot.
But I’m looking right at a vast expanse of ice. And this ice has been basking in sunlight for months. If I put an ice cube out in the garden on a similar day in the UK, it would be converted into a small puddle very quickly. The ice I’m looking at has been basking in direct sunlight for a long time. Why hasn’t it melted?
A clue is to be found several miles up the glacier. A few days ago we arrived at a filming site by helicopter and as soon as I stepped out on to the ice I saw that the surface was pock-marked with beautiful little holes. It looked like Swiss cheese. The holes were generally rounded, about 5-10 cm in diameter and they went down 30 cm or so. Each one had a different shape, and the sides were absolutely vertical. If you looked directly downwards, you could see that the bottom of each hole was black.
The Swiss cheese holes are due to something called the albedo effect.
When light falls on a surface, two things can happen. The light (and the energy it carries) can be absorbed by the surface, or it can be reflected or scattered back upwards. Generally, it’s some mixture of the two. If the light is absorbed, that energy must go somewhere so the surface heats up a bit. If it’s reflected, there’s no heating.
The black stuff that I could see at the bottom of the holes is volcanic dust. It’s blown across the landscape and has fallen into small hollows on the ice surface. It’s black because it’s absorbing all the light that falls on it. The white ice is very reflective. Glaciers are like giant mirrors. So the black dust absorbs sunlight, heats up and melts the ice directly beneath it, while the surrounding white ice is unchanged. The meltwater seeps away and the black dust falls down into the hole that’s left. And then it heats up more, melts more of the ice directly beneath it and so the dust slowly digs itself a hole.
The glacier isn’t melting quickly under the bright sunshine because the sunlight never really reaches it. It’s bounced straight back from the surface as soon as it arrives. The Earth’s ice caps are massive energy reflectors, sending the sun’s energy back out into space.
Perched on my rock here, I’m looking out across our campsite. All the ground between me and the glacier is either bare black rock or dark plants. It’s absorbing the sun’s energy like crazy and so it’s heating up. Then the ground heats the air just above it, causing a heat haze – the swirling currents of warm air just above the surface are distorting my view of the ice and making the glacier shimmer.
The shimmer occurs because the dark ground absorbs energy from the sun, and the glacier stays unmelted because it doesn’t. This simple difference is really important for the heat budget of the planet. If the icecaps shrink, we’ll get more energy overall from the sun. So the fact that I can sit comfortably right next to a glacier while only wearing t-shirt and shorts tells us something very important about the influence of ice on our planet. I love a view with a nice bit of physics thrown in. If only the mosquitoes would go away…