Like tree rings, layers of growth in a long-lived Arctic alga may preserve a temperature record of past climate. Specimens from the Canadian Arctic indicate that sea-ice cover has shrunk drastically in the past 150 years — to the lowest levels in the 646 years of the algal record.

Satellite records of the Arctic's shrinking sea-ice cover date back only to the late 1970s. Jochen Halfar of the University of Toronto at Mississauga, Canada, and his colleagues have found a new palaeoclimate proxy in the coralline marine alga Clathromorphum compactum (pictured). It can live for hundreds of years and builds a fresh layer of crust each year.

The thickness of each layer, and the ratio of magnesium to calcium within it, are linked to water temperature and the amount of sunlight the organism receives. The discovery suggests a new way to calculate how much polar sea ice existed hundreds of years ago.

Credit: NICK CALOYIANIS

Proc. Natl Acad. Sci. USA http://doi.org/p6g (2013)