One can hardly imagine a more interesting time for Arctic sea ice, or a more challenging time if one wishes to predict the minimal extent of the sea ice at the end of this year’s melting season (the Arctic melting season generally ends around 20 September). Sea ice in 2007 reached a minimum of 4.2 million km2 on 21 September, an extent 40% below the average for the past 28 years and so low that it surprised all observers and called into question many of the assumptions we might use to estimate 2008 conditions.

We now understand that an unusual weather pattern of warm winds and clear skies played a large role in 2007 melting, and we know that we started 2008 with an unusually large amount of new (first year) ice. An international group of researchers has, for the first time and starting from May, produced, shared, and compared monthly estimates of the 2008 minimum. Those groups, from 15 or more institutions, use recent, in some cases daily, satellite, ship and buoy data, climate models, weather models, and historical data in comparisons, correlations, extrapolations and estimations – you can follow their work in very interesting detail at the SEARCH Sea Ice Outlook page.

In May, those researchers produced estimates for sea ice extent in September 2008 ranging from 3 to 5.5 million km2, some higher and some lower than the 2007 value. As the groups assimilated and considered additional data in June and July, their monthly revised estimates have stayed in the range 3 to 5.5 million km2 but a larger portion of the estimates have converged in the range 4.5 to 5.5. Almost every estimate, especially in the most recent reports in July, highlights the large amount of first-year ice and the potential for rapid, above-average melting of that ice during a normal or slightly warm few weeks in August and September.

The most recent satellite data shows a rapid melting of ice, with a decline of 2 million km2 during August. What will happen in the remaining three to four weeks?

Follow these developments on the NSIDC, SEARCH, and ESA websites. Download two simple data sets, of the recent estimates for September 2008 from many groups and of the 28-year time history of Arctic sea ice September values to plot your own time series or distributions.