Spring is Not Arriving Earlier in North America
White, Michael A., de Beurs, K.M., Kidan, K., Inouye, D.W., Richardson, A.D., Jensen, O.P., O'Keefe, J., Zhang, G., Nemani, R.R., van Leeuwen, W.J.D., Brown, J.F., de Wit, A., Schaepman, M., Lin, X., Dettinger, M., Bailey, A.S., Kimball, J. Schwartz, M.D., Baldocchi, D.D., Lee, J.T., and Lauenroth, W.K. 2009. Intercomparison, interpretation, and assessment of spring phenology in North America estimated from remote sensing for 1982-2006. Global Change Biology 15: 2335-2359.
White et al.'s study covers the period of most rapid global warming (1982-2006); and a unique aspect of it is their use of 16,000 ground-based phenological measurements from across North America, as well as data on snow melt and hydrology.
The authors found that the ten methods of extracting a start-of-spring (SOS) signal from the satellite data differed widely, yielding a range of 60 days around the 10-model mean estimate. The methods also tended to be biased to detecting SOS earlier than ground-based phenology data indicated, and to have weak correlations with the ground data, perhaps because there were difficulties classifying pixels in some cases, particularly for desert, semi-tropical and Mediterranean (California) zones in which the arrival of "spring" is not a well-defined phenomenon. In the end, the twenty-one researchers say they "found no evidence for time trends in spring arrival from ground- or model-based data," and that "using an ensemble estimate from two methods that were more closely related to ground observations than other methods, SOS trends could be detected for only 12% of North America," with 7% of them showing a trend to earlier spring and 5% exhibiting a trend toward later spring.
In conclusion, the wide range of results obtained by most past and current methods of detecting the start of spring appear to have led to premature claims of the season's increasingly earlier occurrence over the latter part of the 20th century. And that finding, in turn, raises significant concerns about the validity of near-surface air temperature measurements that suggest the world has warmed significantly over the past several decades.
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