Walsh, J. E., Fetterer, F., Stewart, J. S. and Chapman, W. L. (2016) A database for depicting Arctic sea ice variations back to 1850. Geographical Review, doi:10.1111/j.1931-0846.2016.12195.x
For example, below is one of the aerial survey maps from the Arctic and Antarctic Research Institute in Russia. The different colour shading indicates the coverage of sea ice.
You can see another source of data below, this time one of the maps from the Danish Meteorological Institute. These are remarkable for their information value and because they represent a cooperative international effort to report ice conditions in a systematic way that was sustained over decades. The red symbols and terms in the legend (see close-up below the map) indicate the sea ice extent. “Tight pack-ice”, for example, indicates ice of 70% to 90% concentration of ice over the sea.
The result of all this work is our new dataset, “Gridded Monthly Sea Ice Extent and Concentration, 1850 Onwards”. This has a much more realistic representation of the year-to-year fluctuations in sea ice, as well as the long-term trend.
The charts below illustrate the new record. The blue line shows Arctic sea ice extent in March, at the end of the winter when the ice is at its annual maximum. The red line shows sea ice cover in September, at the end of the summer when ice extent shrinks to its yearly minimum.
Most fundamentally of all, the new dataset allows us to answer the three questions we posed at the beginning of this article.
First, there is no point in the past 150 years where sea ice extent is as small as it has been in recent years. Second, the rate of sea ice retreat in recent years is also unprecedented in the historical record. And, third, the natural fluctuations in sea ice over multiple decades are generally smaller than the year-to-year variability.