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Autumn is Falling Back

Now that it’s October, leaves are starting to change from summer green to the brilliant reds, oranges and yellows of autumn. As nights get longer and temperatures start to drop with the changing seasons, especially overnight, leaves start to change color. Weather plays a big part in this process.

Both temperature and rainfall help determine the brightness of color in the leaves, and the timing of their change. For example, in places hit hard by drought or excessive heat over the summer, trees are under stress so some of their leaves may drop before they even get a chance to change color. And since there is such yearly variation of temperature and rainfall, the timing of when leaves change color and fall also varies from year-to-year. However, even with that variation, there has been a growing long-term trend.

If you look at average fall temperatures across the nation, it’s clear that while there’s a lot of change from one year to the next, autumn got progressively warmer on average from 1982 through 2008. And if you look at the end of the growing season as marked by the point where satellites see the overall greenness of foliage start to decline, that’s coming later as well. (These years were chosen based on a study by the Seoul National University in Korea.)

This long-term trend towards a later autumn is related to the fact that temperatures overall are getting warmer as the climate changes — a warming that’s consistent with the fact that we’re pumping ever greater amounts of heat-trapping greenhouse gases into the atmosphere.

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