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Autumn is on the Move

If it seems like autumn leaves are taking longer to change color, you’re not imagining things. Over the past 25 years, the onset of autumn has shifted throughout the lower 48 states, with leaves now staying on trees about 10 days longer than they did in the early 1980s.

How do we know? Using satellite-based measurements of the Normalized Difference Vegetation Index (NDVI), which gauges leaf cover over wide areas, researchers at the Seoul National University in Korea found that the end of the growing season occurred progressively later over the course of their 26-year study. By noting the time of year changes in color occurred most rapidly, the researchers could track when fall in the lower 48 states started between 1982 and 2008.

 <--IMAGE TO LEFT: Difference in last day of growing season for the Contiguous U.S., 1982-2008. Brown indicates the season arrives later than average, while the green indicates earlier.
Click image to enlarge.
Credit: Climate Central.

As the two graphics show, there is significant variation from year to year, and from region to region. However, by looking at a given region as a whole, and comparing the five-year average between the start and the end of the study, we found that fall in the continental U.S. now arrives 10.5 days later on average — a shift of about a week and a half.

When leaves change color, what scientists call the transition from “active growth to leaf-drop” is based on several natural factors. Chillier temperatures associated with the onset of fall are one, along with the shorter fall days; changes in rainfall from one season to the next play an important role as well (a dry summer means that leaves will die and drop earlier). This is why we don’t see the changes we associate with fall happen at exactly the same time every year – there’s so much natural variation in the weather. However, as this study shows, the average time these changes occur can shift over the course of many years. This long-term trend towards a later autumn is related to the fact that temperatures overall are getting warmer. In other words, there is natural year-to-year variability as well as a long-term trend in the data.

On a global scale, this has implications for the carbon cycle: a shift in the length of the growing season means leaves will stay greener for longer and can continue absorbing atmospheric carbon dioxide for more time during the year. This can affect the timing of when and where CO2 builds up in the atmosphere.

Video: Global Warming Could Delay Color Change in Autumn Leaves:
Video Credit: Scientific American