After the Observer magazine cover story about our Arctic campaign, there was a flurry of interest around the statistic that we've lost 75% of the Arctic sea ice since 1979. Here's Peter Wadhams, Professor of Ocean Physics at Cambridge University, on why he thinks the 75% figure is correct.
When it is mentioned that the volume of sea ice in the Arctic in summer has declined by 75% over 30 years this figure is often disbelieved or ignored. Yet it is a simple matter of observational fact. The area of sea ice in September - the month of greatest retreat - has declined by 40% since the 1970s (from 7.0 to 4.2m sq km) – this is easily seen from satellite data, which has given continuous coverage over that period.
The mean thickness of sea ice declined by more than 43% from the 1970s to the late 1990s – this comes from submarine transects of the Arctic carried out by US and British scientists. This together with the area decline gives a 66% loss of volume, even if we assume that thinning stopped in 2000.
The Pan Arctic Ice Ocean Model Assimilation System (Piomas) project of the University of Washington has used a model to extrapolate further thickness loss to the present day, and has concluded that by 2011 the volume loss reached 75%. The rate of decline has accelerated and it is not unreasonable to expect that the summer Arctic will become ice-free within a very small length of time, probably before 2020 and possibly as early as 2015 or 2016.
Why is this important? For many reasons, but one of the reasons - and a cause of real fear among climatologists - is the triggering of positive feedback mechanisms which will accelerate global warming.
One mechanism is ice-albedo feedback, or how much of the sun's energy is reflected back into space: sea ice reflects about 80% of the solar radiation falling on it, while open water reflects 10%, a massive difference. The loss of several million square km of highly reflective ice will measurably decrease the average albedo of the planet, speeding up the rate of warming.
Another mechanism is more subtle but possibly more dangerous: as the summer sea ice retreats from the wide, shallow continental shelves of the Arctic the ocean warms up (temperatures of up to 6-7C were measured last year from satellites), and thus the seabed does too. This melts offshore permafrost and releases trapped methane, which is now seen to be emerging in huge plumes in summer. Its level in the atmosphere has started to rise, and as it is a very powerful greenhouse gas this will cause an immediate and serious acceleration of warming.
Incidentally, during the past few days the rate of decline of sea ice area in the Arctic has increased and is now taking us below the 2007 record low for this time of year. If this trend continues, 2012 may be the summer in which Arctic ice begins its final plunge towards complete disappearance.