LONDON Feb 15 Vast uncertainty remains over the
causes of melting Arctic sea ice and when it may disappear
altogether during the summer, which would have consequences for
oil explorers, shipping firms and the fight against climate
The answer will depend on the balance of natural and manmade
Those causes include warmer air and seas as a result of
greenhouse gas emissions, variations in atmospheric circulation,
and a faster southward ice drift down the east coast of
A major study said last month that science had
underestimated the impact of soot, also called black carbon,
adding another complication to an already difficult task of
making a forecast.
The Arctic region accounts for 15 percent of the world's
undiscovered oil and 30 percent of its undiscovered gas, the
U.S. Geological Survey estimates.
Scientists are still unsure about the exact causes of sea
ice melt, and predictions of when the North Pole could have an
ice-free summer vary from 2015 to 2080 or later, with forecasts
centring around 2040.
Oil producers including Russia's Rosneft, Norway's
Statoil, U.S.-based Exxon Mobil and Anglo-Dutch
Shell are preparing to drill in areas of melting sea
ice, despite the technological difficulties and costs.
SEA ICE RETREAT
Last September sea ice reached its lowest level in the
satellite record, which dates back to 1979, a development that
has implications for local native communities and wildlife,
local coastal erosion and possibly northern hemisphere weather.
Observations of the extent of Arctic sea ice made by ships
in earlier decades suggest last year's record would at least
stretch back to a cold period in the mid-nineteenth century
known as the Little Ice Age.
Data show a clear trajectory of ice losses in recent
decades. (See Chart 1)
An acceleration last year past a previous record in 2007
reflects a self-perpetuating process as well as thinning over
Under the albedo effect, dry snow reflects more than 80
percent of solar radiation; bare ice 65 percent; and open water
just 5 percent.
And increasing expanse of open water each summer warms up
faster than ice-covered sea, meaning new ice will be thinner and
more vulnerable the following year.
Chart 2: (slide 17)
There are various underlying causes of the melt.
Rising greenhouse gas emissions drive up air and sea
For example, a discussion paper published online last month,
"North Atlantic warming and declining volume of arctic sea ice",
highlighted the role of water from a warmer Atlantic entering
the Arctic Ocean through the Fram Strait between eastern
Greenland and Svalbard, an island due north of Norway.
A similar inflow of warmer Pacific water through the Bering
Strait has long been identified as an important process that
causes the thinning of ice in the central Arctic.
Last month's study estimated a particularly strong impact
from warm Atlantic water north of Svalbard, where recent
accelerated melting is clearly visible (Chart 2).
"The Atlantic water temperature has been increasing since
the mid-1960s," said four scientists from U.S., Russian and
"Our lower-end conservative estimates indicate that the
recent Atlantic water warming episode could have contributed up
to 150-200 cubic kilometres of sea ice melt per year, which
would constitute about 20 percent of the total 900 cubic
kilometres per year negative trend in sea ice volume since
Another human-derived impact is from soot emissions as a
result of forest fires; household and industrial burning of wood
and coal; and diesel transport fuel.
Such soot, also called black carbon or particulate matter,
contributes more to global warming than previously thought
according to a major study published last month - "Bounding the
role of black carbon in the climate system: A scientific
Black carbon absorbs and scatters sunlight both in the
atmosphere and when deposited on snow and ice, where it darkens
But there are complications. For example, local warming of
the Arctic atmosphere could reduce poleward winds from the
south, which may result in a cooling effect.
The net impact from the combined effects of black carbon is
to warm the Arctic, experts who contributed to the recent report
concluded in an email correspondence.
There are also weather effects that may be natural, partly
natural or entirely due to greenhouse gas emissions.
A particular weather pattern contributed to a big melt six
years ago by sending air towards the central Arctic, according
to researchers from the Netherlands, Sweden and Germany.
"We find that in summer 2007 there was an anomalous
atmospheric flow of warm and humid air into the region that
suffered severe melt," the researchers said in the paper, "Warm
winds from the Pacific caused extensive Arctic sea-ice melt in
In a longer trend over the past 50 years, stronger winds
have caused a faster southward drift of ice through the Fram
Strait along the east coast of Greenland, according to
researchers in Norway.
The winds are a result of air pressure changes and more
intense storms in the Nordic Seas north of Norway. The ice flow
is additionally channelled along the Greenland coast and rides
on the underlying, southward East Greenland Current.
"The dramatic loss of Arctic sea ice in the last decades is
thus only partly caused by increased long-wave radiation related
to ongoing atmospheric CO2 increase," the authors reported in
their 2011 paper, "Recent wind driven high sea ice export in the
Fram Strait contributes to Arctic sea ice decline".
"The ice export has likely been an effective contributor to
Arctic ice loss since the 1960's."
Given that scientists have not yet pinned down the causes,
it is impossible to predict further sea ice decline.
The outlook depends on how much of a role has been played by
natural variability, which could reverse.
The opposite scenario is one of a growing, cumulative impact
from rising greenhouse gas emissions, which combined with a few
freak summers, ice export and the albedo effect could finish off
summer sea ice rather quickly.
"Simulations ... show that after the ice thins to a more
vulnerable state in response to rising greenhouse gas
concentrations, a reinforcing kick from natural variability may
trigger an initial, abrupt ice loss," several seasoned Arctic
ice watchers wrote in the journal, "Eos Transactions American
Geophysical Union" in 2008.
"While natural variability may instead stabilize the ice
cover for the next few years, the long-term outlook is
disturbing. Our view is that a seasonally ice-free Arctic Ocean
might be realized as early as 2030."