Air always contains water or humidity. When it rains the humidity is 100%. So the more humidity there is, the more likely there is to be precipitation. Furthermore, humidity is a relative term reflecting how much (percent) of the maximum water content there is in the air at a given temperature. In the arctic, the air is relatively dry right now. As the temperature (global warming) goes up, the amount of water in the air will increase. As the Arctic warms and sea ice melts, scientists suspect that system feedback cycles may further speed up the warming process. Now, a new study looking specifically at the Arctic water vapor cycle shows how shifting patterns of humidity may bring about changes in the Arctic atmosphere.
The moisture-holding capacity of the atmosphere is temperature dependent. As the atmosphere warms, it holds more water vapor. The loss of sea-ice cover in the Arctic means more moisture is available. "Sea ice usually separates the ocean and atmosphere like a lid," says Mark Serreze, an atmospheric scientist at the University of Colorado at Boulder and lead author of the new study, published in the Journal of Geophysical Research. "Less sea ice means more water is being pumped up into the atmosphere."
Atmospheric warming causes more water vapor to enter the atmosphere through evaporation, which then promotes further warming, is known as the water vapor feedback cycle. Water vapor itself is a greenhouse gas which is somewhat obvious but generally known by the public. "A big part of the warming we expect to see in the 21st century is due to more water vapor in the atmosphere," Serreze says.
To study how the water vapor feedback cycle might be changing, Serreze and colleagues compiled data from the 1950s to the present and incorporated them into a series of six weather models. "Each of the models spells out a slightly different part of the story, but together they all tell the same story: The Arctic is warming and getting wetter," he says.
In addition to using the latest models, the new study also focuses on seasonal changes in water vapor as opposed to annual changes. The biggest increases in water vapor appear to occur in the autumn in areas that have lost sea-ice cover over the summer. "When fall comes along, the sun goes down and big open areas of water release all their stored heat and moisture into the atmosphere," Serreze explains.
"I think everybody expects to see increasing water vapor in the Arctic," says Jennifer Francis, an atmospheric scientist at Rutgers University who was not involved in the new study. In general, she says, feedback cycles in the Arctic seem to be speeding up, including the water vapor feedback cycle.
The new study does highlight one of the shortcomings of Arctic research — the lack of on-the-ground data, Francis says. "Satellites have a hard time seeing close to the surface so we don’t have a great picture of the lowest layer of atmosphere, where most of the changes are happening," she says. "It’s a longstanding problem, something we really need to work on."
Ground-based studies in the Arctic are logistically challenging and often prohibitively expensive, so improvements will likely depend on developing more sophisticated satellite capabilities, she says.
Next, Serreze and colleagues plan to focus their efforts on how increasing water vapor in the Arctic translates into changes in precipitation and cloud cover. "Humidity doesn’t necessarily mean it’s cloudy and it doesn’t necessarily mean you’ll get more rain. These things are certainly linked, but we need a few more pieces of the puzzle to figure out how exactly."
Increased humidity may also mean more clouds and precipitation which may offset some global warming with cooling. The answer is not not clear.
For further information see Arctic Humidity.
Humidity Chart image via Wikipedia.