Predicted Increases in Arctic Precipitation Now Observed

National Geographic News: NATIONALGEOGRAPHIC.COM/NEWS
April 25, 2008

Arctic Getting “Wetter” Due to Human-Driven Warming
Mason Inman for National Geographic News

In addition to heating up faster than almost
anywhere else on the planet, the Arctic has
gotten wetter and snowier because of global
warming, according to a new study.

The extra precipitation could freshen ocean water
in the Arctic and North Atlantic, researchers
say, which might disrupt the so-called ocean
conveyor belt, a current that runs through the
Atlantic and carries warm water northward from
the Equator.

The new study is the first to show that changes
in precipitation in the Arctic are in part
human-induced, said study leader Francis Zwiers
of the government agency Environment Canada.

The study also shows that previous computer
models underestimated how much precipitation
would change because of global warming.

Contrary to the simulations, Arctic rain and
snowfall increased by 7 percent over the past 50
years, the study found. In just the Canadian
Arctic, precipitation jumped 11 percent.

“That might not seem very big, but a 10 percent
change is quite a lot” when it comes to
precipitation, Zwiers said.

The discrepancy means that models predicting
future change “may underestimate what’s coming
down the pipeline,” he said.

“If people are using these models for planning,
they should keep in mind that what the models
show may be weaker than what will happen.”

Turning On and Off

For their work, Zwiers and colleagues considered
whether changes in precipitation are due to human
activities or are being caused by natural events
such as volcanic eruptions or changes in the sun.

The team also tested whether natural, chaotic
variability in climate could be to blame.

Using computer models, the researchers could turn
each of these forces on and off to see which
contributed the most to the changes that have
been observed.

“Our conclusion is that, by a long shot, the best
explanation for the change in Arctic
precipitation is that it’s due to human
influence,” Zwiers said.

The ability to switch different forces on and off
is the “most exciting” part of the new study,
said James McClelland of the University of Texas
at Austin. McClelland studies Arctic river flows
but was not involved in the new research.

“If you pull the natural forcing out, it doesn’t
change things much,” McClelland said. “The
largest factor driving the changes we’ve seen
turns out to be humans.”

Increasing greenhouse gas emissions heat up the
air, allowing it to hold more moisture, study
author Zwiers said.

This means that the air blowing up from the
tropics carries more water and thus creates more
rain and snow when it reaches the Arctic.

“One of the effects could be to shift the storm
tracks closer to the Poles,” he said. (Read: “Jet
Stream Shifts May Spur More Powerful Hurricanes”
[April 24, 2008].)

Extra precipitation could also make Arctic
surface waters less salty, which would affect
ocean circulation.

“Fresh water is more buoyant, so it has less tendency to sink,” Zwiers said.

“It’s the sinking of this water [in the North
Atlantic], a process known as deep convection,”
that drives the ocean conveyor belt.

This ocean circulation apparently shut down about
8,200 years ago, triggering a cool spell in North
America and Europe.

Some scientists think that if it shut down again,
it would chill Europe, while others think it
would merely temper the heating there due to
global warming.

But how big of an effect this might be and how
suddenly it might kick in is far from clear.

“There’s a debate about this,” McClelland said.
“Will it be gradual? Or is there a tipping point?
Should we expect sudden changes?”

Rain-on-Snow

Mark Serreze, of the National Snow and Ice Data
Center in Boulder, Colorado, was not involved in
the new study.

He noted that researchers have long expected
Arctic precipitation to increase with global
warming.

“That other projected effects of greenhouse
warming are already being seen in the Arctic is
very clear,” he said.

“We are quickly losing the sea-ice cover, and
permafrost is warming.” But until now, evidence
that precipitation has been on the rise “has been
spotty,” Serreze said.

“We’ve been looking for the precipitation
signal,” he added. With this new study, “it seems
to have arrived.”

Changes in Arctic rain and snow could also
directly harm the people and animals that live in
the far North, according to recent studies.

Jaakko Putkonen, of the University of Washington
in Seattle, has been studying a  phenomenon
called rain-on-snow.

When spurts of rain occur during the cold season,
the rain can refreeze, forming a hard icy shell
over the snow.

Animals such as reindeer and musk oxen can’t
break through the ice to plants below, and they
die of starvation.

“People there are basically living off those
animals-herding them for subsistence and also
because they attract tourism,” Putkonen said.

So changes in precipitation in the Arctic “are a
big deal for these people in the North, who can
least afford it.”

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