Arctic sea ice just hit a record low. Here’s why it matters.

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 summer melting season still has about two more weeks to
go.

Current extent of Arctic sea ice. The line shows the
average minimum extent from 1979 to 2010. (NASA Goddard Flight Center)

It’s clear that Arctic sea ice is now shriveling more quickly 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 soon 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,
Arctic sea ice extent has declined roughly 40 percent. 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 small 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 is
disappearing. 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 scientists have revised their forecasts, saying ice-free summers could come
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 bobbing 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 could prove important when it comes to
the vast ice
sheet covering Greenland.

Greenland
ice sheet mass relative to the 2002-2011 average. (Source: Skeptical
Science)

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 could 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 shutdown could happen this century. 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
here, a catastrophic “methane bomb” erupting from either permafrost or icy
ocean clathrates appears unlikely in the near future. A small consolation,
perhaps.

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.