Spotlight On: Sea Ice Loss
17/05/2010
The Arctic plays a vital role in the planet’s health: not only do changes to its vulnerable environment act as an early-warning system for potential changes across the Earth, but its sea ice plays a vital role in keeping the polar regions cool and moderating our global climate.
As such, the fate of the Arctic sea ice has been the subject of much research and debate in the last few years. Is the sea ice extent shrinking or expanding? Is it getting thinner or thicker? And how can we even get a clear picture of what’s happening at the farthest reaches of the world? Here we outline how sea ice data is gathered, and what the current consensus is amongst the scientific community.
Measuring Changes in Sea Ice
Starting in 1958, US and British submarines collected upward-looking sonar profiles, primarily for navigation and defense, but this information was consequently converted into estimates of ice thickness. However, the areas covered were driven by military requirements rather than scientific ones, and in the post-Cold War era, the requirement and thus availability of this information has diminished.
As a result, obtaining reliable measurements of changes in Arctic sea ice was difficult, until the satellite era began in the early 1970s. With the launch of NASA’s SMMR satellite in 1978, it became possible to measure sea ice extent – i.e. the area of the ocean covered in at least 15% concentration of ice. Since then, this and other satellites have provided scientists with the average 1979 to 2000 reference data against which all current sea ice extent comparisons are made.
Calculating changes in sea ice thickness has proven more problematic. This requires the overflying spacecraft to gauge the difference between the top of the floe and the top of the water (known as the ‘freeboard’) from which the volume of the ice can then be determined. This was something these early satellites were unable to do accurately from space.
In 2003, NASA launched ICESat (Ice, Cloud and land Elevation Satellite), a benchmark Earth observing system. From 2003 to 2008, the mission provided the data to allow scientists to give their best estimate of ice thickness and volume of the Arctic sea ice cover for this period, but an issue with the onboard laser in 2009 has meant no new data has since been returned.
In the Spring of 2009, the first Catlin Arctic Survey set out to make surface measurements of the thickness of the sea ice cover in the northernmost Beaufort Sea area, near the North Pole. The data, which covered a 440 kilometer transect trekked over a 70-day period, along with existing measurements by submarines, satellites and buoys, will be used to help modelers create a clearer picture of the fate of the sea ice cover.
On the morning of 8th April 2010, the European Space Agency launched CryoSat-2, a satellite with the capability to monitor the changes in the thickness of marine ice floating in the polar oceans. Their first satellite (CryoSat) was lost owing to a launch failure in 2005, but this one has already relayed the first data that will prove critical to scientists' understanding of the role ice plays in the Earth system.
Most recently, the 2010 Catlin Arctic Survey saw an Explorer Team undertake further surface observations and measurements of the sea ice thickness encountered over a 495km expedition across the Arctic Ocean.
Dr Seymour Laxon, a Reader in Climate Physics in the Department of Earth Sciences, University College London, is a member of the CryoSat-2 Science Advisory Group and has also worked with the Catlin Arctic Survey. We asked him for his views on the work being undertaken.
"Whilst radar and laser altimeter measurements have been providing data on sea ice thickness for some time, previous coverage has hampered combining the data with ground measurements. However CryoSat-2 will provide near complete coverage of Arctic sea ice and surface measurements, such as those gathered by the Catlin Arctic Survey, will be key to validating and better understanding the CryoSat-2 data."
Current State of the Sea Ice
We know that the extent of sea ice in the Arctic is decreasing. Thirty years of satellite measurements have shown a continual decline, beyond natural variations, averaging 11.7% a decade in summer and 2.7% a decade in winter.
From 1981 to 2000, multiyear ice (that which survives for one or more summer melt seasons and increases in thickness year on year) made up on average 30% of winter sea ice cover. By March 2009, only 10% of Arctic sea ice was more than two years old.
Surveys of sea-ice thickness undertaken by submarines suggest ice draft (a measurement of ice thickness below the waterline which serves as a close proxy for total ice thickness) may have reduced by about 40% compared with the 1960s and 1970s.
The data from the first Catlin Arctic Survey, taken together with decades of existing measurements by submarines, satellites and buoys, led scientists from the University of Cambridge to suggest there is a significant probability that by around 2020 only 20% of the Arctic Ocean basin will have sea ice cover in the late Summertimes. There is also a significant possibility that, by 2030-40, the white North Pole sea ice will have been transformed into an entirely blue, open ocean in the summers. Of course, this in not the only conclusion drawn within the scientific community, but there is a broad consensus that such ice-free summers are a distinct probability, leading to a state-change within the Arctic Ocean environment.
In addition, the available data does not always represent a continuous downward spiral. For example, the ice extent for April 2010 was the largest for that month in the past decade, reaching near “normal” levels and falling just 310,000 square kilometers (120,000 square miles) below the 1979 to 2000 average.
Whilst this makes for an encouraging headline, a closer look at the area using higher resolution sensors revealed numerous polynyas (areas of open water in the pack ice) in the Bering Sea, and broad areas of more scattered ice cover in the Sea of Okhotsk, Barents Sea, and Hudson Bay. Such conditions usually indicate that ice is about to retreat rapidly, and much of the Arctic coastline showed indications of low-concentration sea ice. This further underpins the importance of getting accurate sea ice thickness measurements, not just those of the sea extent.
There seems little doubt that there is an overall trend towards diminishing ice cover. The challenge for scientists now is to understand the processes driving these changes and to characterize the trends in an effort to understand the impact they may have, not just on local ecosystems, but also across the globe.
The final word goes to Professor Peter Wadhams, Professor of Ocean Physics and Head of the Polar Ocean Physics Group in the Department of Applied Mathematics and Theoretical Physics, University of Cambridge.
“The decline in area and thickness of the Arctic sea ice is one of the most important ways in which the Earth is responding to global warming, with serious implications for the entire planet. Already it is changing the planet's appearance when seen from space; a large blue area now sits near the top of our Earth in summer, when previously all was white. We need to understand in detail what is happening to the Arctic in order to be able to understand and mitigate the overall impacts of climate change upon the Earth.”
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