Two years ago, Arctic sea ice shrank to cover the smallest area ever recorded — only 1.32 million square miles were measured on Sept. 16, 2012, more than a million square miles less than what would be considered "normal" at summer's end.
This year, Arctic sea ice most likely has not hit its low point yet, which usually happens near the end of September — before winter makes the sea ice start growing again. Even so, it doesn't look like there will be another record low, a fact that some websites have seized upon to claim that Arctic sea ice is recovering.
But not hitting a record low doesn't mean that the amount of sea ice is climbing again. We started measuring Arctic sea ice using satellites in 1979, and from one year to another, the amount of ice varies. Sometimes, it's higher than the year before, but the overall trend is still clear: The amount of Arctic sea ice is going down.
On Sept. 13, 2013, the quantity of sea ice was at its lowest point for the year: 1.97 million square miles. This year, on Aug. 17, almost a month before the average low point, sea ice was measured at 2.36 million square miles. The National Snow and Ice Data Center projects that 2014's Arctic sea ice minimum will fall between 1.9 and 2.1 million square miles. Those numbers are all lower than the average minimum amount of ice measured between 1981 and 2010, 2.4 million square miles.
The NASA chart below puts this year and the two years prior in some historical context. It shows the amount of sea ice the Arctic is projected to lose this year, in square kilometers (the "extent"), based on data models following the patterns of different years.
So if this year follows 2012 trends (dark green line), there would be much more ice loss than if we followed 1980 trends (light green line). You can also see that the dotted purple line representing the average ice loss in 2007-2013 shows a much steeper dip than the dotted green line representing the 1981-2010 average.
Projections for 2014 still show the minimum amount of ice as higher than in 2012, but remember: 2012 was a record low, and there are multiple reasons for that.
Why 2012 Was A Special Case
For one, 2012 was the continuation of a downward trend. There's been some variation from year to year, but overall, the amount of sea ice measured at the low point is going down and has been going down since we started monitoring sea ice by satellite in 1979.
That clear downward trend is shown by the below chart, which compares the minimum sea ice each year to the average low between 1981 and 2010, which is represented by the "0" on the y-axis. The amount of sea ice at the low point hops up and down each year, but the overall trend — represented by the dotted line — is down, down, down.
You can see the amount of ice at the low point in 2013, for example, was higher than during the record low in 2012 but still far below average — and lower than in any individual year before the early 2000s.
In addition, as Phil Plait at Slate explains, another 2012 event had a significant impact that year.
The longest river in Canada, the Mackenzie River, feeds into the Beaufort Sea in the Arctic. Usually, water from the river slowly flows into the sea, which means that warm river water steadily cools in the Arctic. But in 2012, water was mostly trapped behind a barrier of sea ice.
Sometime between June and July, the ice dam broke. Researchers found that the river volume was particularly high that year — with the largest outflows from the river ever recorded, meaning that a crazy quantity of warm water had been stopped up.
Here's a thermal image of the area before the dam broke, which we first spotted at Wired:
And here's what happened after:
A rush of warm water flooded into the sea, raising temperatures in some places by as much as 15 degrees Celsius. That meant that in 2012 alone, the normal melting of the sea ice was significantly accelerated by a one-time event: the rapid flooding of the relatively warm Mackenzie River into the cold Arctic.
Plait writes that this event caused "a huge increase in thermal energy; by my rough calculations it would far, far exceed the energy released by a nuclear weapon. No wonder that ice melted."
Why Arctic Sea Ice Matters
Decreasing amounts of Arctic sea ice make the planet significantly more vulnerable to climate change, which is why this downward trend is bad news.
Arctic sea ice helps moderate the climate all around the world. A big, thick, ice cap is bright white, and in the Arctic, that ice reflects 80% of the sunlight that hits it in summer and sends it back into space. Smaller levels of ice, as with the current trend, mean that there's more dark ocean to absorb heat from the sun, gradually making the Arctic — and eventually, the planet — warmer over time.
Additionally, recent research has shown that the snow cover on Arctic sea ice has thinned 30 to 50% over the past 50 years, which may further hasten the Arctic meltdown, as that snow helps insulate the sea ice.
What About Antarctica?
Meanwhile, the amount of sea ice around Antarctica, the icy continent at the opposite pole from the Arctic, is at record highs. This seems like it could be good news, but "seems" is the operative word here. Antarctic sea ice doesn't work the same way Arctic sea ice does, and the amount of ice at the South Pole doesn't do nearly as much to regulate rising temperatures.
Arctic sea ice does more to protect against climate change than Antarctic sea ice because of simple geography. As the National Snow and Ice Data Center explains, the Arctic is a sea surrounded by land. That land traps the shrinking amounts of Arctic ice while the summer sun is strong, so it helps reflect summer light.
However, since Antarctica is a continent surrounded by water, summer sea ice there tends to float north where it melts. This means that there's less thick ice there to reflect the sun's rays away during the hot summer months, when it would make the biggest difference — which is why Arctic sea ice is considered more important.
Even so, the fact that sea ice in Antarctica is not shrinking still throws people off, but there are other explanations for this.
Many factors contribute to Antarctic conditions, but a relevant one is that the winds that circle Antarctica are responsible for creating much of the ice surrounding that continent — something has been described as a "permanent polar vortex." Those winds have actually grown stronger in recent years.
More importantly, the growth in Antarctic sea ice is more than offset by the loss of Antarctic land ice, which is a very real problem that is already happening.
A new study shows that as Antarctica gains sea ice, it is simultaneously losing a net 125 cubic kilometres of land ice a year — not quite as much as Greenland, which is losing 375 cubic kilometres a year, but still a huge amount.
Over time, it's clear that Antarctic land mass is melting, and Arctic sea ice is melting. The small gains in Antarctic sea ice don't begin to make up for those massive losses.