NASA’s Latest Hit: Ice Show from Space

If you don’t know what causes the seasons, you’re not alone: a mini-documentary made in the 1980’s showed that lots of Harvard grads don’t, either. For the record, the reason is that Earth’s spin axis is slightly tilted. In the months surrounding June, the Northern Hemisphere leans toward the Sun. There’s more sunlight, days are longer, and the north experiences summer. Down below the equator, there’s less sunlight and less heat, so it’s winter. In the months surrounding December, it’s vice versa.

OK, lecture’s out. Now you get to watch a new video from NASA that shows one important effect of the waxing and waning of the seasons. It shows satellite views of Earth over both the North and South Poles, side-by-side, demonstrating how sea ice expands in summer and melts back in winter, see-sawing from one pole to the other as summer and winter alternate.

It looks as though the changes balance out pretty evenly — but from a climate perspective, they don’t. In the south, the ice sheet covering the Antarctic continent — nearly 2 miles thick in places — remains intact even through the summer. The bright, white surface reflects sunlight back into space, so summer temperatures never rise all that high.

In the Arctic, by contrast, the ice right at the pole is all sitting in the ocean. It melts back a lot more. The relatively dark water underneath absorbs the Sun’s heat, driving Arctic temperatures higher than they’d normally be, in a process known as Arctic Amplification. It’s just one of many climate feedbacks that help speed global warming.

Also worth noting, with some concern: the amount of Arctic sea ice in September, the annual low point, has been much smaller in recent years.