An unusual pattern of winds drove old Arctic sea ice into a precarious position in the winter of 2020. Now in warming waters, large swathes of the Arctic’s diminishing store of old ice lie at risk of melting. But how did this happen, and why is old ice so important?
When winter’s darkness falls on the Arctic Ocean, temperatures plunge to below -30°C. Existing floating ice thickens as the seawater below freezes, and the ice advances into areas that were previously open water. In summer, when temperatures rise and the sun shines for months at a time, the ice thins and retreats. Each September, scientists wait and watch for the annual minimum coverage of Arctic sea ice – a useful indicator for how fast the region is changing.
Ice that survives the summer melt season endures for another winter. Some ice even survives several summers before finally melting. This happens particularly in the colder regions near the North Pole. Ice that survives the summer is known as perennial ice; battle-scarred from its ordeal, it ends up thicker, rougher and more resilient. It’s an important part of the climate and ecology of the Arctic and it’s disappearing due to global heating.
When sunlight hits the Earth, it’s either reflected or absorbed. Reflected light bounces back into outer space, whereas absorbed light heats the planet. Sea ice covered in snow reflects up to 90% of incoming sunlight, making it a powerful defence against global warming. But as polar sea ice melts due to climate change, sunlight increasingly hits the ocean, where over 90% is absorbed.