Arctic Getting "Wetter" Due to Human-Driven Warming

Mason Inman
for National Geographic News

April 25, 2008

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 mysterious 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."