By Laurie J. Schmidt
For much of the year, the cold waters of the Arctic Ocean are covered in layers of frozen seawater. As winter takes hold the sea ice spreads, reaching its peak in March, and then melts during summer to reach its yearly minimum in September.
During the past three decades, the amount of sea ice covering the Arctic Ocean at the end of summer has diminished by about 40 percent, according to Walt Meier, a research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. And big changes happening below the sea surface are less visible, but just as important.
The ice is thinning dramatically—on average, about 50 percent thinner than it was in the 1980s—and the thinner ice is making the climate system more fragile
It's not just about temperature
Scientists now agree, almost unanimously, that increased greenhouse gases are causing global temperatures to rise. But why is warming in the Arctic outpacing the rest of Earth? Satellite measurements from the Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua spacecraft are helping scientists answer this question by unraveling a climate phenomenon known as a feedback.
In climate terminology, a feedback is a process by which a change in one process triggers a shift in another one—like a domino effect. A positive feedback amplifies changes in the climate, with the potential to more than double the amount of warming triggered by carbon dioxide alone.
One of the main drivers of warming in the Arctic is surface albedo, a measure of reflectivity. A surface covered in snow and ice has a high albedo, meaning that it reflects much of the Sun’s radiation back into space. Without snow cover, most of the heat is absorbed, resulting in a warmer surface.
“When sea ice is lost, the sea goes from being very reflective to highly absorbent almost instantly—as soon as the ice goes away,” said Eric Fetzer, AIRS project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “All you need to do is make things a little bit less reflective, and things get warmer, and when things get warmer, they obviously melt.”
It seems like a simple formula: less ice equals warmer temperatures. But the story gets much more complicated.
When things get flipped
If you’ve ever driven to the top of a mountain, you know that air temperature decreases as altitude increases: i.e., the higher you go, the colder it gets. On a clear day this temperature difference typically translates to about 10 degrees Celsius for every 1 kilometer of elevation gain, or about 3–5 degrees Fahrenheit for every 1,000 feet. Under certain conditions, however, Earth’s surface actually becomes colder than the layers above it—a scenario known as a temperature inversion.
Temperature inversions are a key component of the Arctic atmosphere’s vertical structure, and their strength and frequency is directly impacted by surface albedo: as the surface loses heat, the ground becomes colder than the air above it.
“The mere fact that inversions are a ubiquitous feature of the Arctic atmosphere during all seasons justifies why we need to pay special attention to it,” said Abhay Devasthale, a research scientist at the Swedish Meteorological and Hydrological Institute who uses AIRS data to study inversions. “Any change in the Arctic climate system will influence these inversions, which will, in turn, impact other processes in the region.”
Water vapor intensifies the effect
These other regional processes include water vapor, another key player in global climate. As a significant greenhouse gas, water vapor fuels about 90 percent of Earth’s greenhouse effect. And as Arctic sea ice shrinks and temperatures rise, the amount of water vapor in the air increases. “So, you have carbon dioxide causing warming, the warming creating more water vapor, and the water vapor acting as a greenhouse gas,” said Fetzer. “About two-thirds of the warming we’re experiencing right now isn’t just from carbon dioxide—it’s from water vapor.”
More water vapor in the air also triggers changes in the hydrologic cycle. “In the Arctic area, snow stays on the ground for months before spring comes and it melts,” said Hengchun Ye, climate researcher and Geosciences and Environment department chair at California State University, Los Angeles . “It’s a reservoir for the water, but now this reservoir is shrinking because temperature and evaporation are increasing.”
Ye and her colleagues analyzed data from 547 ground weather stations in the Arctic and found that, during the period from 1966 to 2000, warmer air temperatures over northern Eurasia not only delayed the first snowfall, but also shortened the snowfall season by 6.2 days for each degree of temperature increase. “The general consensus is that in winter, precipitation has been increasing in the Arctic,” said Ye. “But the geographical area that is permanently covered by snow in the winter season has actually been shrinking.”
The findings corresponded with AIRS data for the same time period, which means climate researchers like Ye now have access to reliable water vapor information for the “gaps” not covered by weather stations. “By providing data from across the entire Arctic region, AIRS water vapor data are like a missing link in understanding these processes,” she said.
Predicting the future
While climate models are an invaluable tool in predicting future climate change, the lack of observations related to feedback mechanisms like temperature inversions and water vapor has resulted in high uncertainties. “If we have to place high confidence in future climate change estimates from models, it is crucial that these models accurately simulate major phenomena such as inversions,” said Devasthale.
“The main thing that is little understood is that the natural climate system amplifies the carbon dioxide that humans are adding to the atmosphere,” said Fetzer. “AIRS will help improve climate models by making sure they’re describing the atmosphere correctly—things like temperature inversion, water vapor, and cloud structure govern how much heat comes and goes from the surface to the atmosphere, and that’s what really determines warming.”