Earth’s changing orbit shows up in tree ring data

Earth’s changing orbit shows up in tree ring data. Under the gravitational influence of Jupiter and Saturn, the eccentricity of Earth’s orbit and the tilt and precession of its rotation axis slowly fluctuate. Those changes affect how much solar radiation reaches a given geographical location and are responsible for Earth’s ice ages. According to a new study, they are also responsible for a more recent phenomenon: the cooling of Scandinavia from 138 BC to AD 1900 at a steady and significant rate of 0.31° per 1000 years. To reach that finding, Jan Esper of Johannes Gutenberg University Mainz in Germany and his collaborators assembled a record of tree rings from the trunks of young and long-dead Scots pines at 17 sites in northern Finland and Sweden. Thanks to the sites’ stability and the availability of buried and submerged trunks, the record is unprecedented in its continuity and consistency. Orbital calculations indicate that summer insolation at northern latitudes has indeed been declining steadily for the past 2000 years, consistent with the long-term cooling that Esper and his collaborators inferred from their tree rings. However, the cooling trend is absent from other, less homogeneous tree-ring records that have been used to reconstruct northern Europe’s past climate. If the Scandinavian tree rings embody the climate’s true behavior, then summer temperatures during Roman times and the Medieval Warm Period were a few tenths of a degree higher than previously estimated. Yet even though summer insolation continues to fall in Scandinavia, the temperature trend manifested by the region’s trees after 1900 is upward. (J. Esper et al., Nat. Clim. Change, in press.)