Over the past few decades, the sharp decline of summer sea ice in the Arctic has been one of the clearest signs that the Earth's climate is changing rapidly.

So it's time to check in with the sea ice again now that we've reached the end of the 2013 summer melt season. Here's a chart from the Japan Aerospace Exploration Agency (JAXA) showing the extent of sea ice in the Arctic this year (red line) — that is, the area of ocean with at least 15 percent ice:

A few things jump out. The ice extent for 2013 hit a low of 5.099 million square kilometers on Sept. 13. That was the sixth-lowest minimum on record and about 23 percent below the long-term average, according to the National Snow and Ice Data Center.

But there was still a lot more Arctic ice this year than there was in 2012, when the sea ice extent shrunk to just 3.413 million square kilometers, the lowest minimum by far since satellite measurements began. So what happened? Has the Arctic started to recover, as the Daily Mail recently suggested? Not exactly. Here's why.

Short term variation vs. long-term trends

The Met Office in Britain recently pointed out that there are all sorts of reasons why sea ice extent can bounce around from year to year:

-- temperatures naturally vary from one year to the next;
-- the amount of cloud can affect the amount of surface melting;
-- summer storms can also break up ice, which can accelerate the melting process;
-- settled conditions can be more conducive to ice forming;
-- winds may act to spread out the ice or push it together.

Those variables can help explain why sea ice didn't decline in 2013 as much as it did last year: “In 2012 we saw a record low which was likely to have been influenced by a storm which swept through the region in summer, but this year’s weather conditions appear to have been less conducive to ice loss," noted Ann Keen, a sea ice scientist at the Met Office.

Since things can vary a fair bit year to year, the Met Office advises looking at longer-term trends. And those are easy to see. There was less Arctic ice, on average, in the 2000s than there was in the 1990s. And there was less ice, on average, in the 1990s than there was in the 1980s.

Clearly the ice is disappearing. Since 1979, Arctic sea-ice extent has been shrinking by about 4 percent per decade, with summer lows getting about 11 percent smaller each decade. And the volume of Arctic sea ice — which is trickier to measure — also keeps tumbling downward:

Looked at this way, 2013 doesn't really look like a recovery.

So why is the sea ice vanishing?

Scientists think humans are largely to blame for the long-term Arctic melt. A study last year in Environmental Research Letters concluded that between 70 and 95 percent of the Arctic melt since 1979 has been caused by human activity.

More specifically: Man-made global warming has rapidly heated up the Arctic — the region has been warming about twice as fast as the global average. (See here for a good explanation of why.) That melts the ice. What's more, soot and other pollutants from smokestacks in Europe and Asia have traveled up to the Arctic. When those dark particles settle onto the snow and ice, they absorb sunlight and start sizzling.

That means the sea ice is expected to keep declining in the years ahead, though there's a lot of disagreement on exactly how fast it will go. Many climate models don't expect "ice-free" summers in the Arctic for another 30 or 40 years, although some models do indicate that a more rapid collapse is a real possibility.

Yet even if the downward trend is likely to continue, that still leaves room for natural variability each year, thanks to storms, clouds, and other factors. Indeed, some scientists even think we could see more variability as sea ice continues to decline.

“Model simulations of sea ice suggest that as the ice gets thinner you actually get more year to year variability in extent," noted Keen, "because larger areas of the ice are more vulnerable to melting away completely over the summer.”

Why the decline of Arctic sea ice matters

So does it even matter if Arctic sea ice is declining? Apart from being a telltale sign that the Earth is heating up, the ice itself is important for a few reasons.

-- Sea-level rise. This is a bit tricky. The melting of Arctic sea ice, by itself, can't affect global ocean levels. After all, ice was already floating in the water — when it melts, it displaces exactly the same amount of volume.

But there is an indirect effect here: As the sea ice disappears, more and more of the ocean is exposed to sunlight. Since the darker ocean surface absorbs more sunlight than the bright ice, this warms the Arctic region even further. And that, in turn, can help warm and melt the vast ice sheet covering Greenland. That ice really can raise sea levels. And the melting of Greenland's ice sheet appears to be accelerating of late, losing about four times as much mass in 2011 as it did a decade ago.

-- Oil and gas exploration. The melting Arctic sea ice also makes it easier for oil and gas companies to explore northern offshore regions that were once inaccessible. Shell has prepared for oil exploration in the Chukchi Sea off Alaska. In 2011, ExxonMobil signed a $500 million deal with Rosneft to get at some 35.8 billion barrels of oil locked in Russia's once-frozen Kara sea. This is another little-discussed Arctic "feedback" — less ice means more oil and gas which, when burned, will heat the planet further.

-- Extreme weather... maybe? There are also other theories about how vanishing ice in the Arctic could affect weather in Europe and North America by altering the jet stream in specific ways. However, as my colleague Jason Samenow recently discussed at length, this theory is very much in dispute and still an active area of inquiry.

Further reading:

-- Arctic sea ice hit a record low in 2012. Here's why it matters.

-- David Rose got a lot of attention for a recent piece in the Daily Mail claiming that the sea-ice recovery in 2013 was evidence of "global cooling." Over at Skeptical Science, John Mason explains why Rose is wrong about this.

-- As a side note, sea ice on the other side of the planet in Antarctica has actually been growing since 1970. This doesn't come close to offsetting the Arctic decline, as explained here and here, but it is interestnig. Here's some recent research suggesting that wind may be responsible for the strange behavior of Antarctica's sea ice.

-- While I was writing this yesterday Andrew Revkin wrote his own post on the subject, with some excellent charts, videos, and links. This study (pdf) on trends and noise in Arctic sea ice is particularly valuable.