Climatic queries from Swedish trees.

Climatic queries from Swedish trees

What was the summer temperature in northern Sweden during the year 1139? An international team of climatologists has come up with an answer that raises questions concerning some long-held theories about Earth's climatic history.

Scientists believe that during medieval times, the planet passed through a relatively warm phase, which lasted from 1100 through 1300 in Europe. But tree-ring records now suggest that the northern Scandinavian region, at least, did not experience such a phase.

K.R. Briffa from the University of East Anglia in Norwich, England, with colleagues from Sweden, Germany, Finland and Switzerland, studied a series of living and dead trees whose rings form a continuous record of summer temperatures in northern Scandinavia and Finland for each year since A.D. 500.

"Our reconstruction dispels any notion that summers in northern [Finland and Scandinavia] were consistently warm throughout that period," the researchers write in the Aug. 2 NATURE. Although the second half of the 12th century had warm summers, the first half was cool -- and 1139 had the coldest summer in the whole record, they report.

The tree rings also fail to reflect the unusually long cold phase, called the Little Ice Age, that supposedly gripped Europe from 1550 to 1700 or 1800. Briffa and his co-workers say their findings imply that the climate swings of northern Scandinavia differed from those of the rest of Europe, or that the accepted climate history of Europe needs correction--or both.