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With 2007 being the International Polar Year, all eyes are on the Arctic. Scientists already know that the Far North is where the effects of global warming have their earliest incidence. Two recent papers by CNRS researchers are improving our understanding of how climate change works in these northern areas, both on the ice sheets of Greenland, and in the atmosphere above.


© MODIS Rapid Response Project at NASA/GSFC

MODIS Satellite image (July 2004) showing a thin layer of smoke above the Arctic Ocean emanating from Canadian boreal forest fires (red dots).

Made up of 2.85 million cubic kilometers of frozen water, the massive ice fields of Greenland make the island a particularly interesting place to study climate change. Hubert Gallée of the LGGE1 recently re-examined three decades of satellite data of the island's surface, with the aim of developing a more accurate model to predict how it will be affected by global warming. According to Gallée, the increase in melting of the frozen mass is twice as fast as previously believed.2

Gallée's new climate model combines on-the-ground observations of temperature, wind speed, and precipitation, with microwave satellite imagery. The algorithm that drives his model was also designed to incorporate the observed effects of snow precipitation and snowmelt on the ice pack. During testing, his model proved to be very accurate when compared with data from automatic weather stations and a Swiss observation camp, with the two sets of data in close agreement with each other.

“But when we compared the satellite observations with the model predictions, we found that they significantly differed,” says Gallée. If the satellites were giving different readings, the researchers needed to find out why. Surprisingly, the answer lay in the clouds.

Greenland has had increasing rainfall over the time period studied, meaning more days when the ice would be vulnerable to melting. Though microwave satellites can detect  a small percentage of the liquid water present in the first meters of the snow pack, it cannot read anything during actual rainfall. The cloud cover had simply interfered with the satellite's ability to detect this effect, and Gallée's model managed to correct the shortfall.

While a doubling of ice-melt may seem alarming, Gallée cautions that it isn't as bad as it sounds, as the amounts are still relatively small. Though the ice is disappearing, at current rates it will still be around for quite some time. But by no means is he calling for inaction. “This is a process that will take a few thousand years–we have time,” he adds, “but once it starts, it cannot stop, and it is starting now.”

Atmospheric chemist Kathy Law3 has a shorter-term proposal that might help slow down the warming of the Northern Hemisphere. In a recent article for Science,4 Law argues that gases other than the usual suspects have an important–and often overlooked–effect on warming in the Arctic. “We decided to focus on trace gases and aerosols emitted at mid-latitudes [Europe, North America, and parts of Asia] that can be transported to the Arctic,” she explains. These airborne pollutants play a part in increasing surface temperatures, especially in the North, even if they do not remain in the atmosphere very long. Studies indicate that tropospheric ozone, for instance, contributed as much as 25 percent to Arctic surface temperature increases in winter over the last century, by acting as an efficient greenhouse gas. Aerosols are more complex. Some, like sulphate aerosols, reflect some of the sun's energy back out into space, thus contributing to cooling. On the other hand, other aerosols like black carbon (soot) absorb solar radiation. They can also settle on the snow and ice of the North, thus decreasing the surface albedo (reflectivity) and increasing energy absorption. This leads to increased surface temperatures and possibly enhanced snow/ice melt. “A decrease in albedo could actually speed up warming,” warns Law.

She points out that many of these city-choking pollutants are already submitted to regulation, though chiefly for health, rather than for reasons that pertain to climate change. Emissions of most of these have leveled off or even decreased in much of North America and Europe, though they are now increasing in Asia.

Since these gases do not linger in the atmosphere nearly as long as carbon dioxide, reducing or eliminating these emissions even further could have a more immediate effect on reducing Arctic warming. Results would happen in years, compared to the decades it would take for CO2. “That makes it attractive to policy makers... It's not the answer to everything, but you might see a positive impact in the Arctic more quickly,” Law concludes.


Mark Reynolds

Notes :

1. Laboratoire de glaciologie et géophysique de l'environnement (CNRS / Université Grenoble 1).
2. X. Feitweiss et al., “The 1979-2005 Greenland ice sheet melt extent from passive microwave data using an improved version of the melt retrieval XPGR algorithm,” Geophysical Research Letters. 34: L05502. 2007.
3. Service d'aéronomie (CNRS / Institut Pierre Simon Laplace / Université Pierre et Marie Curie / Université Versailles St. Quentin).
4. Kathy S. Law and Andreas Stohl, “Arctic Air Pollution: origins and impacts.” Science. (“International Polar Year” special section) 315: 1537-40. 2007

Contacts :

> Hubert Gallée
LGGE, Grenoble.
> Kathy S. Law
Service d'Aéronomie, Paris.


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