What is causing Arctic sea ice decline?

The Arctic Ocean has lost more than 30 percent of its summer ice cover in the last thirty years. Scientists have long thought that climate change is to blame, but a new study provides more evidence for that idea.

Credit: Patrick Kelley, U.S. Coast Guard
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Readers sometimes ask us, “What are the reasons behind Arctic sea ice decline?” In summer months, ice extent has declined by more than 30 percent since the start of satellite observations in 1979. But is climate change really the culprit, or could other factors be contributing?

Scientists have long suspected that the decline in summer sea ice was too strong to be caused by natural variations, such as weather patterns that cause fast changes in ice extent. Climate model simulations have shown that sea ice will decline as the Arctic gets warmer. But Arctic ice has declined faster than models predicted, raising the possibility that massive sea ice loss was caused at least in part by natural variations in weather.

This month, researchers at the Max Planck Institute for Meteorology in Germany released a new study examining all the possible drivers for the observed sea ice retreat. Study lead Dirk Notz said, “There has always been the chance that we simply observed a freak encounter of unusual natural variability with very low ice cover.”

Looking for a smoking gun

Notz said, “The whole study was based on ruling out one possible cause after the other.” The team looked at each possible cause of ice loss, using statistical analyses to determine whether a particular cause could explain the ice decline. The scientists first considered natural variability, or the effect of short-term and seasonal weather conditions and winds. While variable winds and weather clearly play a role in how much ice melts each summer, they found that the amount of variability was far too low to explain the intensity of the decline.

The researchers then looked at the possibility that ice loss was leading to more ice loss, in a vicious cycle known as a positive feedback loop. As ice declines, the increased areas of open water absorb more heat, which can then melt more ice. This is one reason that researchers are concerned about declining sea ice. However, the new study concluded that such a feedback cannot explain the amount of ice loss that has already occurred. Notz said, “After each year with an extreme ice loss, the ice cover always recovered somewhat. Self acceleration is not a major player in the Arctic at the moment.”

Notz and colleagues then looked at other possible drivers for ice decline, ranging from solar radiation and cosmic rays, to volcanic eruptions, wind patterns, and oceanic heat transport. For all of these potential causes, their analysis showed that none of these were correlated with ice extent.

“In the end, only the increase in CO2 remained on our list of possible drivers,” Notz said, “We find a clear, physically plausible correlation of increasing CO2 and decreasing sea-ice cover.” While other research has previously shown a connection between sea ice decline and global warming, most of those studies focused on climate models. The new study provides an independent confirmation that sea ice is in fact declining because of human-caused climate change.

References

Notz, D., and J. Marotzke. 2012.  Observations reveal external driver for Arctic sea-ice retreat, Geophys. Res. Lett., 39, L08502, doi:10.1029/2012GL051094.

Are icebreakers changing the climate?

On July 20, 2011, the U.S. Coast Guard Cutter Healy traveled through a break in the sea ice and melt ponds in the Arctic Ocean, during the NASA Impacts of Climate on Ecosystems and Chemistry of the Arctic Pacific Environment (ICESCAPE) mission, a field survey aimed at understanding the ecology of the Arctic Ocean.
Credit: NASA/Kathryn Hansen
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In summer months, icebreaking ships head north into the Arctic Ocean, tearing through the sea ice and leaving trails of open water in their wakes. Readers occasionally write in to ask us whether the trails left by these ships contribute to the melting of sea ice. Continue reading »

The Arctic sea ice maximum

NSIDC scientists announced today that the Arctic sea ice cover has likely reached its maximum extent, marking the beginning of the melt season. (For details, see Arctic Sea Ice News & Analysis). What is the sea ice maximum and why does it matter?

The maximum ice extent marks the beginning of the melt season for Arctic sea ice. Leads, long cracks in the ice, begin to open up and the ice cover starts to melt as sunlight brings warmth to the Arctic.
Credit: Angelika Renner

What is the maximum and when does it happen?
Arctic sea ice melts and regrows in an annual cycle, freezing throughout the winter months and melting in the spring and summer. The ice cover generally reaches its maximum extent sometime in late February or March. After that, ice melts through the summer, hitting a low point in early or mid-September. NSIDC scientist Walt Meier said, “The maximum marks the point when the Arctic shifts from a freezing period into the summer melting period.” Continue reading »

Modeling the Arctic climate

In February, polar climate researchers gathered at the National Center for Atmospheric Research in Boulder, Colorado to discuss the newest updates to models of the Earth’s climate system. The researchers are working together to create better models of the Arctic and Antarctic climates, which will feed into larger models of the whole Earth that help scientists understand how climate will change in the future. What goes into a climate model, and what can scientists learn from models that they cannot learn from observations?

This image shows several aspects of climate, including sea ice, surface winds, and sea surface temperature. This image came from data simulated from NCAR's Community Earth System Model.

Credit: ©UCAR. Image courtesy Gary Strand, NCAR
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What’s in a model?
Computer climate models are based on scientists’ understanding of Earth’s climate. The models use mathematical relationships to try to quantify the relationships between parts of the climate system. If you tweak one factor in climate, how does the simulated climate system respond? Models that bring many factors together help scientists learn how the climate system works, and let them run simulations on Earth’s climate. They also allow scientists to assess how climate may be affected by present and future changes in greenhouse gases and solar forcing, and how much of a role natural variability plays. Continue reading »

The Arctic Oscillation, winter storms, and sea ice

The Arctic Oscillation (AO) is a large-scale climate pattern that influences weather throughout the Northern Hemisphere. It alternates between a positive phase (left) and a negative phase (right). In its positive phase the AO tends to bring warmer weather to the middle latitudes, while in its negative phase, winter storms are more common in the Eastern United States and Europe. Credit: J. Wallace, University of Washington.

Last year, many scientists blamed the winter storms that blasted the Northeastern United States and Europe on the negative mode of a weather pattern called the Arctic Oscillation. This winter, the Arctic Oscillation started out in the opposite mode, which scientists connect to the warmer-than-average temperatures and unusually low snowfall over much of the U.S. The swings of the Arctic Oscillation also help control how sea ice moves in the Arctic Ocean, which is of great interest to climate scientists. Readers often write in to ask us about this powerful but mysterious climate phenomenon, and how it affects weather where they live. What is the Arctic Oscillation, and how does it affect Arctic sea ice and the rest of the Northern Hemisphere? Continue reading »