Nature 457, 435–441 (2009)
Spring now arrives sooner than it did 50 years ago, say scientists, who suggest that this shift is linked to an increase in atmospheric greenhouse gas concentrations.
Alexander Stine of the University of California at Berkeley and colleagues reconstructed seasonal cycles in the world's temperate zones in the last century, using updated records from the Climate Research Unit at the University of East Anglia, UK. Over land, they found that the onsets of all four seasons have shifted forward by an average of about 1.7 days between 1954 and 2007. During this period, the temperature difference between summer and winter narrowed by about six per cent over land because of warming winters. Both these changes are significantly larger than those observed in the first half of this century from 1900–1953, leading the researchers to infer that human influence, and not natural variability alone, caused the changes.
Warmer winters combined with a premature turn towards spring may be causing earlier spring blooms and bird migrations, effects long reported by biologists. The group's results suggest that these plants and animals are indeed directly responding to a changing climate.