[1] A few tree ring studies indicate recent growth declines at northern latitudes. The precise causes are not well understood. Here we identify a temperature threshold for decline in a tree ring record from a well-established temperature-sensitive site at elevational tree line in northwestern Canada. The positive ring width/temperature relationship has weakened such that a pre-1965 linear model systematically overpredicts tree ring widths from 1965 to 1999. A nonlinear model shows an inverted U-shaped relationship between this chronology and summer temperatures, with an optimal July– August average temperature of 11.3°C based on a nearby station. This optimal value has been consistently exceeded since the 1960s, and the concurrent decline demonstrates that even at tree line, trees can be negatively affected when temperatures warm beyond a physiological threshold. If warming continues without significant gains in effective precipitation, the large-scale greening of recent decades could be replaced by large-scale browning. Such browning could slow or reverse carbon uptake by northern

Introduction [1] In their paper, Lanfredi et al. [2003] raise several issues about our analysis of normalized difference vegetation index (NDVI) data for areas north of 30#N between 1981 and 1999. As we describe below, their comments represent a basic misunderstanding of the data, a misrepresentation of our work, or an anticipation of our recently published results. Here we address these issues in order in which they are given by Lanfredi et al. 2. Areas Around Water Bodies Characterized by a Large Negative Trend [2] In section 2.1 of Lanfredi et al. [2003] the authors confuse boundaries with NDVI values. This issue cannot be easily detected in the printed figures and does not affect our results. Of the 13,349 grids (0.25 0.25 degrees) shown in Plates 1 and 2 of Zhou et al. [2001] between 25#N and 75#N, only 10 show 25% decline in NDVI. This confusion partially results from the way that the two figures were plotted as contour maps. However, it can be visually exaggerated if they

In this first issue of 2009 [Volume 21], we are pleased to have another installment in our periodic Perspectives on EOS series. In this series, we’ve been presenting a variety of personal experiences on the history of the Earth Observing System Program. Our objective with this series is twofold: (1) to share the fascinating story of the “difficult journey of a good idea ” as the vision of a series of satellites that would study our home planet evolved and eventually became reality; and (2) to provide a historical perspective that helps inform those involved in developing future Earth science missions (such as those now in the planning stages initiated in response to the

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