September Arctic Sea Ice

Since at least 1979, Arctic sea ice has generally been on a downward slope, trending 4.5 percent lower per decade overall and 13.7 percent lower per decade during the September summer minimum. In fact, the average September Arctic sea ice extent is down almost 1 million square miles from where it was about 30 years ago. There’s no reason to think this trend will stop anytime soon, as greenhouse gases continue to warm the planet.

At the opposite end of world, September marks the end of winter when the sea ice that surrounds Antarctica swells to its maximum extent. Down there, ice has been growing at a rate of 1.1 percent per decade.

So what’s going on? Isn’t global warming happening in the Antarctic too? It is, of course, but the two regions are so different that you can’t really compare them.

The Arctic Ocean is almost entirely surrounded by land. But the Southern Ocean, which surrounds Antarctica, is wide open. Even with global warming, winter temperatures are so low that ice forms easily and has unlimited room to spread out. In recent years, the winds that make up the South Polar Vortex have grown stronger, spreading sea ice wider and allowing new ice to form in the gaps that result.

Virtually all of Antarctica’s sea ice melts or gets blown away every summer. That’s not possible in the Arctic Ocean. It’s mostly hemmed in by land, so the ice can’t easily be pushed away by the wind. What remains at the end of the summer gets thicker over the following winter as new ice is added to the old. But that multi-year ice is also on the decline, leaving more fragile first-year ice that is easier to melt. In fact, NOAA’s State of the Climate 2013 reported that 58 percent of ice during the March maximum in 1988 was first-year ice, but that number grew to 78 percent by March 2013.

Remember that snow-covered ice is highly reflective, bouncing most of the sunlight that hits it back into space. When the ice melts, it reveals water, which is darker and less reflective. That water then absorbs solar energy and heats up, which leads to even more ice melt. This is the ice-albedo feedback loop that has been established in the Arctic, and is why the Arctic is warming faster than other parts of the planet. This is also why the Arctic Ocean could be completely ice-free for at least part of the summer within a few decades.

It’s important to note that melting sea ice doesn’t cause sea-level rise. The ice is already floating, so the water level doesn’t change when it melts. It’s the same reason your glass of iced tea doesn’t overflow if you let the ice cubes melt. One type of floating ice that can indirectly lead to sea-level rise, however, is an ice shelf — a thick, floating slab of ice attached to a land-based glacier. When the ice shelf melts, as happened to the huge Larsen B shelf off Antarctica in 2002, the glaciers behind it speed up dramatically in their flow into the sea. Ice moving from land to sea does raise sea level, just as dumping more ice into your drink raises the level of liquid. Extra ice flowing into the ocean from both Antarctica and Greenland is a growing factor in rising seas globally, showing that what happens in distant parts of the world can have a big effect here at home.