Arctic sea ice hit a record low this winter. Here's why it matters.

As the planet gets hotter, the Arctic Ocean's vast expanse of floating sea ice keeps shrinking.

In March 2015, scientists at the National Snow and Ice Data Center announced another unsettling new milestone. Arctic sea ice, made of frozen seawater floating in the ocean, usually expands in the cold winter months, reaching a "maximum" around February or March. But this year, the winter maximum appears to be the lowest on record:

(National Snow and Ice Data Center)

As the NSIDC explains, it can be tricky to pinpoint an exact winter maximum, since sea ice can sometimes surge late in the winter under particular conditions. "However," the agency notes, "it now appears unlikely that there could be sufficient growth to surpass the extent reached on February 25."

In other words, this is likely as much Arctic sea ice as we're getting this year — and it's less ice than we usually get. This video from NASA shows the growth of ice during the winter of 2014-15 — and the difference from previous years:

(NASA)

The big question: Will we see another record minimum this summer?

Once the sea ice hits its winter maximum, it will start melting over the spring and summer months. Scientists will watch to see how much ice remains when it hits a "minimum" this summer — that's the more interesting datapoint.

Back in August 2012, Arctic sea ice extent hit its lowest level ever recorded, but then rebounded a bit in the summers of 2013 and 2014. Could we see another record low this summer? Walt Meier, a sea-ice scientist at NASA, cautions that record low winter maximums don't always mean record low summer minimums. A lot depends on whether this summer is especially hot or cool in the Arctic. We'll see.

More broadly, Arctic sea ice has been declining sharply over time, though there are ups and downs from year to year. Scientists say the melt is driven by global warming as well as by other pollutants humans put into the atmosphere. And this vanishing sea ice has sweeping consequences — from unlocking once-frozen areas for oil and gas exploration to potentially mucking with weather patterns in North America and Europe.

1) Arctic sea ice has been declining sharply over time

Since the early 1980s, scientists have used satellites to measure the extent of Arctic sea ice (that is, the amount of ocean area that's at least 15 percent ice). This chart from NSIDC shows the decline over time:

(NSIDC)

Satellite observations began in 1979, and they show a decline in Arctic sea ice extent of about 3 to 4 percent per decade. There's also a less-reliable record extending before 1979, pieced together from old ice charts and other data.

The decline is especially pronounced in the summer

The decline in sea ice has been especially pronounced during the summer months, where Arctic sea ice extent has declined roughly 40 percent over the past three decades, and the ice has lost significant volume, according to data from the Polar Science Center.

If this decline continues, we're likely to see a year when the Arctic is virtually ice-free in late summer. There's still a lot of disagreement, though, on when that might occur. The Intergovernmental Panel on Climate Change simply says an ice-free Arctic in September is "likely" before 2050 under high global-warming scenarios.

2) Humans are to blame for much of the Arctic melt

The Mitchell Power Station, a coal-fired power plant built along the Monongahela River, 20 miles southwest of Pittsburgh, on September 24, 2013, in New Eagle, Pennsylvania. (Jeff Swensen/Getty Images)

How much? In 2012, one study in Environmental Research Letters argued that between 70 and 95 percent of the Arctic melt since 1979 has been caused by human activity. Exact numbers aside, it's clear that human influence has played a big role in the Arctic.

That includes a couple different things: Global warming has drastically heated up the region, with the Arctic warming about twice as fast as the rest of the world. (Here's a great explanation of why.) On top of that, soot and other pollutants from factories and power plants in Europe and Asia travel up to the Arctic. When those dark particles settle onto snow and ice, they absorb sunlight and start sizzling. It all leads to less ice.

That said, natural variability still plays a significant role. In 2012, for example, a large storm in August helped break up the slushy sea ice and cause it to melt even more rapidly. That was one reason why we saw a record low minimum in 2012 but then a slight rebound the following years. That's why scientists expect sea-ice extent to be erratic for years to come — though the long-term trend will be down as Earth keeps heating up.

3) In the past, scientists underestimated Arctic melting

Ship among the icebergs that have broken off the Sermeq Kujalleq ice sheet, Ilulissat, Qaasuitsup, Greenland. (DeAgostini/Getty Images)

Back 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. But later observations suggested that sea-ice extent was shrinking far faster than the IPCC had projected.

Why? Earlier climate models seem to have underestimated certain "feedback" effects. As Arctic sea ice melts, more and more ocean is exposed to sunlight. Since the darker ocean surface absorbs more sunlight than the bright ice, this warms the region further.

More recent climate models seem to capture this process better. A study published in Geophysical Research Letters in 2012 found the newest simulations seem to mirror satellite observations more closely. That said, the authors noted that Arctic ice is still melting a bit faster than the average model predicts — and there's a fair bit of uncertainty about when we'll see ice-free summers.

4) Melting Arctic sea ice won't raise sea levels. But a melting Greenland will.

The number of days of surface melting in June and July 2014 exceeded the 1981-2010 average across most of the ice sheet (Figs. 3.1b and 3.1c), particularly on the western margin. (NOAA)

Frozen seawater that's floating in the ocean can't raise sea levels when it melts, because that ice was already displacing its own weight. (You can check this by putting ice cubes in a glass of water and letting them melt.) So melting Arctic sea ice won't, on its own, flood our coastal cities.

But there's a catch: As the newly exposed Arctic Ocean waters start absorbing more sunlight, the broader region will keep heating up. And that's important when it comes to the vast ice sheet covering Greenland. Greenland's freshwater ice is sitting on land, so when it melts and flows into the ocean, that does raise sea levels.

Greenland's ice sheet is currently 1.9 miles thick and contains enough ice to raise global sea levels by about 25 feet in all. And that ice sheet is indeed shrinkng:

Greenland's ice sheet is melting

Monthly mass anomalies (in Gigatonnes, Gt.) for the Greenland ice sheet since April 2002 estimated from GRACE measurements. The anomalies are expressed as departures from the 2002-2014 mean value for each month. For reference, orange asterisks denote June values (or May for those years when June is missing). (NOAA)

Indeed, the melting of Greenland's ice sheet appears to have accelerated over the past decade, with one 2014 study suggesting it's now losing 243 gigatons of ice per year. That's partly due to warmer Arctic air in the summer. And it's partly driven by rising ocean temperatures, as warmer water chews away at the edges of the ice sheet.

As a result, the IPCC now projects that global average sea levels are on pace to rise at least 1 foot by 2050, and possibly 3 feet or more by century's end. Greenland's melting ice alone could be responsible for up to 5 inches of rise. (Other contributions will come from Antarctica's melting ice sheets, melting glaciers, and the fact that seawater expands when it heats up.)

5) A warmer Arctic will open up new opportunities for oil, gas, and shipping companies

Melting Arctic sea ice isn't bad news for everyone. It could, in theory, make it easier for oil and gas companies to explore polar regions that were once inaccessible.

The Northwest passage could open up to more open-water ships

Back in 2012, for instance, Shell sent a drill ship to the Chukchi Sea off Alaska to prepare for oil exploration in the newly thawed region. That operation was a disaster — in part because the Arctic is still an extremely difficult environment to operate in.

Still, if the ice keeps disappearing, more and more companies could explore the Arctic for oil and gas. It probably won't happen immediately, though: that oil is expensive to extract, and current low oil prices may put a damper on Arctic dreams in the near future.

Alternatively, the melting Arctic could also open up new shipping routes during the summer months. One 2013 study by UCLA geographer Laurence Smith found that open-water vessels may be able to, in theory, cross the Northwest Passage and North Sea Route regularly in the summer by 2050 without icebreakers (blue). And icebreakers (red) may be able to ram through the North Pole:

(Smith et al. 2013)

There's also the possibility that a melting Arctic could lead to fresh tensions among the nations that border it — the United States, Russia, Canada, and so forth. In theory, there's an Arctic Council that's supposed to settle various disputes that are likely to arise as the ocean opens up. (The United States will soon take the two-year rotating chair.) But it remains to be seen how this all unfolds.

6) Scientists are debating whether a changing Arctic might mess with our weather

The Arctic region is warming rapidly. And at least a few scientists think this could cause the jet stream to slow down and weaken and meander all over the place more often.

The jet stream swirls around the Northern Hemisphere, created by the convergence of cold air masses descending from the Arctic and rising warm air from the tropics. Scientists are investigating how declining sea ice might be changing the path and speed of the jet stream and leading to strange weather patterns further south. Image credit: NASA’s Goddard Space Flight Center (NSIDC)

That could have a couple of unpredictable effects on weather in the United States or Europe. It might storms or heat waves to linger in one place for longer periods of time. Or it might allow bigger blasts of frigid Arctic air to travel down to the United States during the winter.

But there's a crucial caveat here: This is a very new area of research, and there's still a lot of fierce debate over the link between Arctic warming and extreme weather. Jennifer Francis of Rutgers sketched out one possible link here. Back in 2013, Elizabeth Barnes of Colorado State disputed the link (and Francis responded). The NSIDC has more commentary here.

7) It's unclear when the Arctic might reach a "point of no return"

One 2010 study in Nature found that it was still possible to halt the shrinking of Arctic sea ice. A 2011 study in Science, looking at 10,000 years of Arctic melt, also concluded that we're not yet at a "tipping point," at which the collapse in sea ice becomes inevitable.

That said, it won't be easy to halt the decline. 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 looking increasingly difficult.

8) On the other side of the world, Antarctic sea ice is growing

Whenever the topic of Arctic sea ice comes up, climate skeptics often point out that Antarctic sea ice is growing. This is true. It's also worth putting in context.

There are two types of ice in Antarctica. First, there's sea ice, which is the ice floating in the ocean around the continent. For reasons that are still unclear, the extent of Antarctic sea ice has indeed been growing in recent years. This increase is less drastic than the long-term decline of summer sea ice up north in the Arctic, but it's real nonetheless:

Arctic sea ice extent underwent a strong decline from 1979 to 2012, but Antarctic sea ice underwent a slight increase, although some regions of the Antarctic experienced strong declining trends in sea ice extent. Thick lines indicate 12-month running means, and thin lines indicate monthly anomalies. (National Snow and Ice Data Center, University of Colorado, Boulder.)

But that's not the only thing going on down in Antarctica. There's also land ice. This is the snow and ice that sits on top of land in large ice sheets.

Land ice is more relevant to humans, since when that ice melts and drips into the ocean, it pushes up global sea levels. (There's enough ice in West Antarctica alone to raise the ocean 10 to 13 feet.) And current estimates suggest that Antarctica is losing land ice.

(IMBIE 2012)

The Arctic and Antarctica are behaving in different ways, but global warming is still with us — and the effects are quite drastic.

WATCH: 'A visual tour of the world's CO2 emissions'

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