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[预告]05.25 Satellite Remote Sensing and Climate Studies in the Arctic

  主讲人:William Perrie 教授

Tom Yulsman| February 8, 2018 5:39 pm177

  主持人:陈卓奇 副教授

Arctic Sea Ice Just Set Another Record Low—In Winter

  时 间:5月25日(周五)14:00-16:00

And what’s happening in the New Arctic isnot staying there

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I shot thisiPhone photo of Arctic sea ice in the Davis Strait between
Greenland and Baffin Islandwhile flyingfrom Icelandto Denver on Jan. 30,

  1. (Photo: ©Tom Yulsman)

Another month, yet another record low forArctic sea ice extent in a
warming world.

January’saverage ice extent in the Arctic was 525,000 square miles below
the 1981-to-2010 average, making it the lowest January extent in the
satellite record. This is an astonishingly large loss of ice —
equivalent to 80 percent of Alaska.

But what happened in January was equally, if notmore significant, for
its timing. Ithappened when the Arctic was grippedby frigid, polar
weather.

Record lows in the Arcticonce occurred mostly in September — at the end
of summer when relatively warm temperatures naturally cause the frozen
lid of sea ice to shrink to an annual minimum extent. With human-caused
warming added on top of relatively mild summertemperatures,
recordmelt-backs in summer perhaps arenot sosurprising.

But now, dramatic reductions in sea ice are occurring more and more
often during the cold season.

“Now we are seeing winter really get into the act as well,” says NSIDC
director Mark Serreze. “The shrinking Arctic sea ice cover is no longer
something that just stands out in summer.”

Thisshift to record lows in winter,scientists say, is yet another
indication that human activities have already transformed the region
into what they’re calling “the new Arctic.”

No surprises here: average January #Arcticsea ice extent was the
lowest in the satellite record…

Data from@NSIDCat

— Zack Labe February 2, 2018

Why should those of us who don’t live in the Arctic care about what’s
happening up there?

Péter Szijjártó, Minister of Foreign Affairs and Trade for Hungary —
obviously not an Arctic nation — answered that question in this way,
during an address to theArctic Frontiers conferencein Norway last month:
“Whatever happens here in the Arctic has a direct and immediate impact
on the rest of the world, and especially Europe.”

I don’t know about “immediate,” but there are myriadways that changes in
the Arctic are affecting the rest of the world — for example, changes to
fisheries as fish stocks move north, and possible (but still unproven)
effectson weatherfar to the south.

Shrivelingsea ice also has turnedthe region into something of a new
frontier. Many nations are eyeing the Arctic’s opening sea routes, its
strategic position between Eurasia and North America, and its
potentially hugereserves of oil and gas, as well as other resources.

This, in turn, is having geopolitical consequences. Among them: Russian
military moves that some analysts believe are designedto bring down an
“ice curtain” in the region — intended todeny other nations access to
large swaths of the Arctic.

As sea ice growth last monthwas lagging far behind normal and heading
for its record low, politicians andscientistsattending the Arctic
Frontiers conference were discussing the ramifications of thenew Arctic.
Among the scientists was Ingrid H. Onarheim,a researcher atthe
University of Bergen and Bjerknes Centre for Climate Research.

She began anoverview talk about Arctic sea ice by putting the trendinto
a long-term context: “The recent sea ice loss is unprecedented,” she
said, at least during the last 160 or so years.

Modern satellite monitoring of sea ice could not by itself reveal that
insight, because it dates back only to 1979. So fora longer term
perspective,researchers fromNSIDC and elsewhereturned to noveldata
sources. These includedwhaling ship logs,sea ice chartsfrom the Danish
Meteorological Institute, compilations by U.S. Navy oceanographers,
observations from aircraft, and other sources. All of this disparate
informationhad to be digitized and then synthesized to be compatible
with one another.

The resulting database, going all the way back to 1850, showsthat at
least since then “we’ve never had as little ice as we have now,”
Onarheim told her colleagues at Arctic Frontiers.

Here’s what that looks like in graphic form:

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Source:“A database for depicting Arctic sea ice variations back to
1850,” John E. Walsh et al, Geographical Review, 11 July 2016

Before getting into what these maps show,I shouldpoint out that there is
a typo in the oneon the left. It should be 1850-1900.

With that correction in mind, let’s turn to the details. The
left-handmap above shows the concentration of sea ice during September
of1854. Thenew databaseshows thatthis month had the smallest ice extent
duringthe entire 1850-1900period.

The other maps in the triptych show what sea ice looked like for the
lowest September in each of those respective periods.

The take-away message from the triptych is pretty clear. As theauthors
of the paper describing the new long-term database wrote:

It is apparent that the recent September minimum of 2012 is far less
than the minima of the two earlier . . . periods. This comparison
indicates that the summer ice minima of the past decade have no
precedents in earlier decades back to 1850.

But as last month’s record low ice extent shows, significant sea ice
losses are no longer mostly confined to the warmer months. I think you
can see that pretty well in this graphicshowing how Arctic sea ice fared
during each of the 1,956months between1850 and2013:

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The status of Arctic sea ice month-by-month (vertical axis) and
year-by-year (horizontal axis). Blues indicate ice extent that’s below
the long-term mean. Reds indicate the opposite. (Source: Cooperative
Institute for Research in Environmental Sciences, after a figure by
Julienne Stroeve, National Snow and Ice Data Center)

The first thing that jumps out atme is the dominanceof blue, meaning
lower than average sea ice, starting around 1975. It’s most pronounced
during July, August and September, the warm months. But recently,
deeperblues have been spreading out into the cold season months
ofNovember through April.

“We are losing sea ice in all seasons now,” Onarheimsaid, echoing the
NSIDC’s Mark Serreze. “The changes in ice are spreading from the summer
to winter season.”

There is no doubt as to what’s behind the accelerating decline of the
sea ice at the top of the world: warming fromhumankind’s emissions of
carbon dioxide and other greenhouse gases. There is also no doubt that
the Arctic is actually ground zero for climate change — it’swarming
twice as fast as the globe as a whole.

But the Arctic is a very big place, and the regional patterns are just
as important as the overall trend.

No region has been affected more than the Barents Sea, located north of
Norway and Russia, and the nearby waters just north of the archipelago
of Svalbard. And as it turns out, thestrongesteffect in this part of the
Arctichas been — you guessed it — during winter, according to Onarheim.

Scientists are working hard to explainwhy this is so, and research
suggests that the answer can be summed upthis way: In this region, the
Arctic isexperiencing what some call “Atlantification.”

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The Gulf Stream, carrying warm Atlantic water, moves north along the
Norwegian coastanddivides into two main branches, one on either side
side of the island archipelago of Svalbard. In the Arctic Ocean, this
Atlantic water becomesdenser as it cools and therefore sinks. After
circulating, the now cold water leaves the Arctic Ocean, mainly through
the Fram Strait between Svalbard and Greenland.(Illustration: Audun
Igesund, Norwegian Polar Institute).

Scientists have known for 100 years that warm Atlantic water rides north
on the back of the Gulf Stream, and that extensions of that massive
current take it all the way up into the Barents Sea and over the top of
Svalbard. (See the map above.) Research has shownthat increasingly
warmAtlantic Ocean water carried on thesecurrents — about 1 degree C
warming since 1979 — isinhibitingsea ice from forming, even as winter
air temperatures continue to plunge well below freezing.

Research by Onarheim and her colleaguesshowsthat warming Atlantic waters
arriving north of Svalbard on the currents are having several specific
impacts. To start with, theypush underthe sea ice that does form,
inhibiting further growth and even causing it to melt from underneath.
This leaves the ice thinner and less extensive than it otherwise would
be.

With less floatingice forming a cap on the sea during winter, the
relatively warm sea water is able to give upmore heat to the atmosphere.
And that helps explain a nearly 7degree C increase in mean air
temperature north of Svalbard in winter, according to Onarheim’s
research.

As air temperatures naturally warm in the spring, the thinner ice can
melt out faster. That leaves the water exposed to sunlight for longer
periods. So it absorbs more energy and heats up— inhibiting the
formation of ice when cold air temperatures return with a vengeance in
the fall. And that means still less sea ice in the winter months.

图片 5

Here is how energy has been accumulating within Earth’s climate system,
thanks to humankind’s emissions of greenhouse gases. As the graph shows,
most of the energy has been been absorbed by the oceans. The rest has
gone to melting ice, warming continental land masses and the atmosphere.
(Source: Intergovernmental Panel on Climate Change Fifth Assessment
Report)

The oceans make up 70 percent of our planet’s surface.And water is
particularlyadept at soaking up heat. So much so, in fact,that Earth’s
oceans have absorbed more than 90 percent of the heat that has
accumulated in theplanet’s climate system due to our emissions of
greenhouse gases.

As we’ve now seen, some of that heat has a tendency to come out —
nowhere more readily than in the Arctic, where the frigid atmosphere is
practically begging to absorb heat, and where the Gulf Stream’s
northernmost extensions have been only too happy to oblige.

Onarheim’s overarching take-away message during her Arctic Frontiers
talk was this: Computer modeling of the climate system suggests that
unless we significantly rein in CO2 emissions soon, Arctic waters could
be sea-ice free during summer by about mid-century. Sea ice would still
form in winter. But the models also predict that continuing warming
would lead to ice-free Arctic watersin winterbetween 2061 and 2088.
(The rather large range represents the possible impact of natural
climate processes.)

In other words, the models are saying that in just a little more than
forty years, Arctic waters could be ice freeyear ’round. That would
give us a radically new Arctic — and a verydifferent planetthan the one
we live on today.

One lastthing: Sofar, Arctic sea ice has been disappearing more quickly
than the models have predicted. So we may not have to wait four decades
for completely ice-free Arctic waters.

“If we want to keep the ice cover, we have to reduce the CO2emissions,”
Onarheim says. “The faster we emit the CO2, the faster we will lose the
sea ice.”

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