The Arctic Ocean's vast, frozen expanse of ice is rapidly vanishing. On Monday, scientists at the National Snow and Ice Data Center announced that the extent of sea ice in the Arctic had reached its lowest level since satellite measurements began, breaking the previous record in 2007. That's particularly striking because the 2012 summer melting season still has about two more weeks to go.

It's clear that Arctic sea ice is declining over time, although there are ups and downs each year. And scientists say the melt has been driven by both global warming and other pollutants that humans have put into the atmosphere.

So why does the disappearing sea ice actually matter? Partly it's a sign of how quickly we're heating the planet. Yet the vanishing sea ice can also have its own side effects, from warming up the Arctic further to unlocking once-frozen areas of the north for oil and gas exploration. Below is a rundown of what we know about Arctic sea ice and why it's worth watching.

1) The amount of Arctic sea ice is shrinking each year — and will eventually disappear altogether in the summer months if the planet keeps warming. Since the 1980s, agencies around the world have deployed satellites to measure the extent of Arctic sea ice (this is the amount of ocean area that's at least 15 percent ice). This chart from the Japan Aerospace Exploration Agency (JAXA) shows a marked trend over time:

(Source: JAXA)

Over the past three decades, the summer Arctic sea ice extent has declined roughly 40 percent, and the ice has lost significant volume, according to data from the Polar Science Center. Mark Serreze, director of the National Snow and Ice Data Center, told the Guardian last year: "The extent is going down, but it is also thinning... There will be ups and downs, but we are on track to see an ice-free summer by 2030."

2) Humans are largely to blame for the Arctic melt. A new study in this month's Environmental Research Letters concludes that between 70 and 95 percent of the Arctic melt since 1979 has been caused by human activity. 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.) 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.

Natural variability still plays a role, however. This year, a large storm in August may have helped break up the sea ice and caused it to melt even more quickly. But NSIDC scientists say the long-term warming trend was the main driver — the slushy ice has become even more vulnerable to weather outbursts.

3) In the past, scientists have underestimated the pace at which Arctic sea ice would disappear. In 2007, the Intergovernmental Panel on Climate Change (IPCC) figured we wouldn't see ice-free summers in the Arctic until the end of the century or so. But later observations suggested that sea-ice extent is shrinking far more quickly than the IPCC had forecast.

It appears that earlier climate models underestimated certain "feedback" effects. As Arctic sea ice melts, 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 region even further. What's more, a recent study in the Journal of Geophysical Letters found that IPCC models had low-balled the rate at which melted ice drifts off, further accelerating the collapse. That explains why some scientists have revised their forecasts, saying ice-free summers could come 40 or 50 years ahead of schedule.

4) Melting Arctic sea ice won't, by itself, raise global ocean levels. But a warmer Arctic will cause Greenland's ice sheet to melt — and that matters. Ice that's floating in the ocean can't raise sea levels when it melts, because the ice was already displacing its own volume. But as the exposed ocean absorbs more sunlight, the region will keep heating up. And that's important when it comes to the vast ice sheet covering Greenland.

Greenland's ice sheet is 1.9 miles thick and contains enough ice to raise global sea levels by about 25 feet (7.5 meters) all told. Back in 2007, the IPCC consensus was that Greenland's ice sheet would remain fairly stable this century and wouldn't contribute much to sea level rise. But more recent evidence suggests that this prediction is out of date. The melting of Greenland's ice sheet appears to be accelerating of late, losing about four times as much mass last year as it did a decade ago. That's partly due to warmer air. And it's partly driven by rising ocean temperatures, as warmer water chews away at the edges of the ice sheet.

As a result, a recent study by the U.S. Jet Propulsion Laboratory predicted that sea levels are on pace to rise at least a foot by 2050, and possibly three feet by century's end. (Longer-term forecasts depend on how rapidly Antarctica's own massive ice sheets deteriorate.)

5) The changing Arctic could lead to more extreme summers and winters in the United States and Europe. It's no shock that global warming will make summers even hotter. But could it also make winters colder? Perhaps. For that, we can thank the Arctic. As Jennifer Francis, a climate scientist at Rutgers, has been exploring, the amplified warming in the Arctic might well be contributing to extreme weather.

Why is that? First, the west-to-east jet stream appears to be slowing down, which allows weather patterns to persist in certain areas for longer. This could help account for the onslaught of snowstorms in the United States and Europe in 2009 and 2010, as well as prolonged heat waves like the one that hit Moscow in 2010. Arctic amplification can also increase the "waviness" of the jet stream surrounding the polar region. That could allow more frequent blasts of cold Arctic air to escape down into North America or Europe, leading to frigid winters.

6) Other doomsday scenarios — a shutdown in ocean circulation, or vast methane releases from permafrost — don't appear imminent, though they're worth watching. Scientists are keeping a wary eye on two Arctic developments. First, there's the possibility that a flood of cold water from melting glaciers and icebergs could eventually disrupt the "overturning circulation" in the Atlantic Ocean that keeps temperatures in Europe mild. There's evidence this has happened several times in the past 60,000 years. Yet no climate model has suggested that a total shutdown could happen this century, even if some show a slowdown. Western Europe appears safe for now.

More eye-opening is the chance that the vast permafrost in Siberia and elsewhere will keep melting, allowing vast quantities of methane to seep into the atmosphere, which would, in turn, heat the planet even further. A survey (pdf) of 41 scientists published in Nature last year estimated that, at current melt rates, methane from terrestrial permafrost could eventually contribute 2.5 times as much to climate change as deforestation does now. That's very significant. On the other hand, as ocean chemist David Archer explains, a catastrophic "methane bomb" erupting from either permafrost or icy ocean clathrates appears unlikely in the near future.

7) A warmer Arctic will allow oil and gas companies to produce even more fossil fuels. The melting Arctic sea ice makes it easier for oil and gas companies to explore northern offshore regions that were once inaccessible. As my colleague Juliet Eilperin just reported, Shell is sending a drill ship to the Chukchi Sea off Alaska as it prepares for oil exploration in the region. Last year, 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.

8) The Arctic hasn't yet reached the "point of no return," but the world would have to cut emissions very quickly to stabilize the sea ice. One 2010 study in Nature found that it was still possible to halt the shrinking of Arctic sea ice. A more recent study in Science, looking at 10,000 years of Arctic melt, also concluded that we're not yet at a "tipping point," where the collapse in sea ice becomes inevitable. "The good news," said Sven Funder, a co-author of the Science study, "is that even with a reduction to less than 50% of the current amount of sea ice the ice will not reach a point of no return."

And yet, as climate blogger Joe Romm explained after the Nature study came out, we may have reached a practical point of no return. The Nature authors estimated that the world would have to cut greenhouse-gas emissions by 60 percent rapidly and keep cutting thereafter to avert the inexorable collapse of summer sea-ice. That's unlikely. A (shrinking) number of climate scientists still think the world can cut emissions significantly and avert a 2°C rise in temperature. Yet the prognosis for Arctic sea ice looks a bit grimmer.