Changes in North American spring

Onset of the growing season in mid-latitudes is a period of rapid transition, which includes heightened interaction between living organisms and the lower atmosphere. Phenological events (seasonal plant and animal activity driven by environmental factors), such as first leaf appearance or flower bloom in plants, can serve as convenient markers to monitor the progression of this yearly shift, and assess longer-term change resulting from climate variations. We examined spring seasons across North America over the 1900-1997 period using modelled and actual lilac phenological data. Regional differences were detected, as well as an average 5-6 day advance toward earlier springs, over a 35-year period from 1959-1993. Driven by seasonally warmer temperatures, this modification agrees with earlier bird nesting times, and corresponds to a comparable advance of spring plant phenology described in Europe. These results also align with trends towards longer growing seasons, reported by recent carbon dioxide and satellite studies. North American spring warming is strongest regionally in the northwest and northeast portions. Meanwhile, slight autumn cooling is apparent in the central USA. Copyright (C) 2000 Royal Meteorological Society.