Thick ice decline could advance watery Arctic summers

NASA's shipborne ICESCAPE mission cuts a path through multiyear Arctic ice last year. This thicker form of ice is declining fastest, NASA's Joey Comiso has shown in a separate study. Credit: NASA/Kathryn Hansen

The oldest and thickest ice in the Arctic is vanishing the fastest, data studied by NASA scientist Joey Comiso and published last month have shown. “This is alarming since it is usually the thick component that would survive the long summer melt period,” Joey told Simple Climate. “Since the thick component is declining more rapidly, the Arctic summer ice cover is more vulnerable to further decline. Assuming that the surface temperature continues to warm up as it has in the last several decades, this makes it more likely that we will have very little or no sea ice cover in the summer sooner than we previously expected.” And when the Arctic is ice-free in summer, dramatic environmental changes could follow that would speed warming further and limit the supply of fish for food.

Having long studied Arctic sea ice cover, Joey previously showed that 2007’s record smallest summer area was around one quarter smaller than the previous minimum in 2005. That “has been regarded as the event that could trigger an irreversible change in the Arctic sea ice cover”, Joey wrote in this latest research paper in the Journal of Climate. But after that low the area of thickest ice that can survive the summer melts, known as perennial ice, recovered slightly before dipping again this winter.

Intrigued by that recovery, Joey wanted to understand it. He therefore turned to data collected by tools called microwave radiometers that have been flying over the Arctic on satellites since 1979. These can collect information on the ability of different objects to emit microwave energy, or their microwave emissivity. Salt content, or salinity, influences this emissivity. As sea ice is initially around one-third as saline as sea water, microwave emissivity can be used to tell one from the other. It can also separate multiyear ice, which has survived at least two summer melt seasons, from thinner second year ice that has only survived one summer.

Cycling down

“The salinity of sea ice decreases with time as brine pockets within the ice are lost by gravity making them migrate downward,” Joey explained. “This decrease gets accelerated every summer and it takes two summers to become basically freshwater ice. We take advantage of the difference to separate multiyear ice from the thinner ice types.” But the emissivity also depends on when and where measurements are taken, which can complicate sorting the ice types from each other and the sea. To ensure he could do this accurately, Joey used radiometers that could detect three different types of microwave signal and combined the data to create his maps.

In this way Joey showed that the area of perennial ice has shrunk by more than a tenth per decade, which already puts it on course to disappear this century. Multiyear ice cover alone has shrunk faster still. But rather than smoothly following this path, there is a cycle of more rapid shrinkage followed by regrowth. This has happened every 8-9 years since the satellites have been orbiting and “could explain the slight recovery for three years after 2007”, Joey wrote.

Feedback undermines food chain

An image of multi-year sea ice (brighter white) shown over the average total sea ice extent (light blue) during the three winter months ending in January 1980. Credit: NASA's Scientific Visualization Studio, Goddard Space Flight Center.

Joey then compared the patterns of ice loss against other climate conditions. These included Arctic temperatures, again measured by satellite-mounted instruments, and winds and pressure at sea level collected by the US National Oceanic and Atmospheric Administration. Though wind patterns could have been blowing the thicker ice to warmer regions, the changes in ice cover seen were much more closely linked to temperatures. That temperature link is important because ice would normally reflect heat back into space. But as the ice melts, the water that’s left absorbs the heat and warms more, potentially speeding further melting. That could bring an Arctic with no ice in summer even sooner than simply projecting current rates of loss would predict, and many consequences with it.

An image of multi-year sea ice (brighter white) shown over the average total sea ice extent (light blue) during the three winter months ending in January 2012. Credit: NASA's Scientific Visualization Studio, Goddard Space Flight Center.

“A warmer, ice free, Arctic would likely cause a thawing of the permafrost that in turn will release a large amount of the greenhouse gas methane that would worsen the warming problem,” Joey said. “It will also cause a migration of species to the north and the ecology and environment of the region will drastically change. The Arctic Ocean is also part of a dynamic ocean worldwide, whose circulation would be altered by the disappearance of summer ice in the Arctic. As a big component of the climate system, if the physical characteristics of the ocean change, the climate will change. The biology of the ocean will also be affected since plankton blooms have been linked with the spring melt of sea ice. Changing this would affect the marine food chain, and ultimately the supply of fish for world consumption.”