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Human Contribution to the Lengthening of the Growing Season during 1950–99

Carries out an optimal detection analysis to assess the significance of increases in the growing season length during 1950–99, and to measure the anthropogenic component of the change
Finds the signal is detectable, both on global and continental scales, and human influence needs to be accounted for if it is to be fully explained
Finds the change in the growing season length is asymmetric and largely due to the earlier onset of spring, rather than the later ending of autumn
Finds that the growing season length, based on exceedence of local temperature thresholds, has a rate of increase of about 1.5 days decade−1 over the observation area
Finds that local variations also allow for negative trends in parts of North America
The analysis suggests that the signal can be attributed to the anthropogenic forcings that have acted on the climate system and no other forcings are necessary to describe the change
Model projections predict that under future climate change the later ending of autumn will also contribute significantly to the lengthening of the growing season, which will increase in the twenty-first century by more than a month
Concludes that such major changes in seasonality will affect physical and biological systems in several ways, leading to important environmental and socioeconomic consequences and adaptation challenges

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