Thinning and retreat of Arctic sea ice are consequences of global warming and are causing a major change to the face of our planet, a state-change of the Arctic from an ice-covered cryospheric mass to a partly ice-free cold ocean region. Both the rate of ice retreat and the rate of thinning greatly exceed the predictions of IPCC models, and with thinning faster than retreat the whole ice cover will collapse at some point.
Arctic sea ice has been in slow retreat since the 1950s at 3% per decade, which speeded up to 10.7% per decade from 1996. In Sept 2007 the minimum area reached 4.1 million sq km, a million sq km less than the record set in 2005. Although the area stabilised in 2008-10 the continuing decline in multi-year (MY) ice fraction suggests that the total ice volume has continued to decrease. New model predictions, tuned to match recent changes, predict disappearance of summer sea ice in 20-30 years.
One consequence is that the open water area undergoes radiation-induced warming up to 5°C, which delays the onset of fall freezing and warms the seabed over the shelves, helping to melt offshore permafrost. This in turn allows the release and decomposition of trapped methane hydrates, causing methane plumes with global warming potential.
Our understanding of governing processes has to be based on measurements of ice thickness and area throughout the year, particularly winter. Ice area can be tracked by satellite, but thickness distribution can only be accurately measured by sonar from below. This task has been carried out since 1958 by submarines of the US and British navies, with the most recent UK voyages being in 2004 and 2007. Since 1971 I have been responsible for data gathering and analysis from UK submarines, sailing on most missions myself. I published the first evidence of Arctic ice thinning (15% to 1987), while more recent UK and US data show an enormous 43% decline in thickness from the 1970s to the late 1990s.
It is clear that a link exists between retreat and thinning, in that a thin ice cover breaks up more readily in summer, leading to greater oceanic radiation absorption, enhancing melt rate. A tipping point occurs when the annual thickness cycle has a summer minimum at which the winter ice cover disappears, especially first-year ice. The evidence is that we are approaching this critical point, and in this paper I explore the nature and speed of the relevant changes.
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