REDUCED SENSITIVITY OF RECENT TREE-GROWTH TO TEMPERATURE AT HIGH NORTHERN LATITUDES

Tree-ring chronologies that represent annual changes in the density of wood formed during the late summer can provide a proxy for local summ ertime air temperature(1). Here we undertake an examination of large-r egional-scale wood-density/air-temperature relationships using measure ments from hundreds of sites at high latitudes in the Northern Hemisph ere. When averaged over large areas of northern America and Eurasia, t ree-ring density series display a strong coherence with summer tempera ture measurements averaged over the same areas, demonstrating the abil ity of this proxy to portray mean temperature changes over sub-contine nts and even the whole Northern Hemisphere. During the second half of the twentieth century, the decadal-scale trends in wood density and su mmer temperatures have increasingly diverged as wood density has progr essively fallen. The cause of this increasing insensitivity of wood de nsity to temperature changes is not known, but if it is not taken into account in dendroclimatic reconstructions, past temperatures could be overestimated. Moreover, the recent reduction in the response of tree s to air-temperature changes would mean that estimates of future atmos pheric CO2 concentrations, based on carbon-cycle models that are unifo rmly sensitive to high-latitude warming, could be too low.