Positive feedback can work in either direction, however, and ice ages can end rapidly. Within no more than a century the ice sheets can be in full retreat, with temperatures rising rapidly everywhere. This rapid warming is due largely to ice-albedo positive feedback. Transposed to modern times, this is the albedo effect that climate scientists worry about most.
It could happen, for example, if the area covered by sea ice in winter were to decrease by a large amount. This would have no effect on sea level, because the ice is already part of the sea, but it would reduce the albedo and the sea would absorb more warmth. This would raise the temperature of the air over the sea.
When the plans were first made to exploit oil reserves on the North Slope of Alaska, it was obvious that the oil would need to be transported across the state to an ice-free port. A pipeline was the most practical way to achieve this, but there were fears of the adverse effects such a pipeline might have. Below the surface of the terrain the Trans-Alaska Pipeline would cross, there is a layer of permafrost—permanently frozen ground. Oil passing through the pipeline would be warm—the oil would not flow if it froze—and the warm pipeline might melt the permafrost. This would have three adverse effects. The first was industrial. As the ice melted, the ground above the permafrost would become soft and it would no longer support the weight of the pipeline. There was a risk the pipeline might fracture. The second risk was ecological. In summer, the layer above the permafrost—the active layer—thaws. This allows plants to grow. If the upper part of the permafrost were to melt, however, the active layer would deepen, turning the ground into a quagmire of mud. Vegetation would still colonize it, but animals, such as the caribou that feed in the area, would have difficulty. The third effect was climatological. Melting permafrost might release large amounts of greenhouse gases. The problem was solved by carrying the pipeline on supports, so it is held well clear of the ground—and migrating caribou are able to pass beneath it. Construction took a little more than three years, and the design has succeeded. Since the pipeline opened in June 1977, the permafrost has not melted.
These were genuine fears. Some people also feared that a fracture of the pipeline might spill oil over a large area. If this were to happen in winter, when the ground is covered by snow, a white surface would turn black, drastically reducing its albedo—perhaps from 0.90 to 0.10. The black oil would absorb solar radiation, melt the snow beneath it, and the warmth might extend downward until it melted the permafrost. This fear was unfounded. If the pipeline did fracture, spreading black oil across a snow-covered surface, the next snowstorm would bury the oil, restoring the high albedo.
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