Climate Fact: A Growing Green Season
In Brief: Later leaf change/leaf drop in autumn, particularly in the Southeast, Central and Mid-Atlantic regions of the United States, has contributed to the overall 9.6 day increase in the Northern Hemisphere’s green season since the early 1980s.
In general, plants want to remain green for as long as possible: the longer the period of the year that they can photosynthesize – use the Sun’s energy to turn carbon dioxide and water into the sugars that sustain them – the more they can grow and the more successful they will be. In the spring, the growing season begins when leaves emerge, the timing of which is mainly determined by the plant experiencing a certain number of days/hours with temperatures above a certain threshold. In autumn, the opposite process is true, with leaf change and leaf fall happening once the plant experiences a certain number of days/hours with temperatures below a certain threshold. With warmer temperatures over the past 30-40 years, plants across the Northern Hemisphere, where most of Earth’s land is, have been staying green for longer periods of the year. Analysis of radar data from satellites shows that since 1982 over the Northern Hemisphere, plants have responded to temperature changes with an overall increased growing season of 9.6 days. The biggest increase in length was experienced in Europe, with about equal contributions from earlier spring emergence and later leaf death in the fall. In North America, however, the increase of 9.4 days came almost entirely from an extension of the growing season into the fall, although earlier spring emergence, particularly from 1982 until 1999 in Alaska and the Northeast United States, also played a role. Lengthening of the growing period into the fall was particularly pronounced in parts of the Mid Atlantic, Southeast and Central United States.
Source: Jeong, SJ et al. “Phenology shifts at start vs. end of growing season in temperature vegetation over the Northern Hemisphere for the period 1982-2008.” Global Change Biology 17 (2011): 2385-2399.