John Yang:

Next Saturday is the autumnal equinox, the beginning of astronomical fall in the Northern Hemisphere. For some though, fall doesn't really begin until the leaves change color. But scientists say climate change is affecting both the timing and intensity of fall foliage.

Earlier I spoke with Bill Keeton, a professor of forest ecology and forestry at the University of Vermont. I asked him to explain why leaves change color in the fall.

Bill Keeton, University of Vermont: It's really a delicate dance between many factors that are driving both the onset of the fall foliage, as well as the intensity of the color that we see. Basically what happens is that the trees sense the onset of winter as the contrast between daytime temperatures and nighttime temperatures increases, and as the length of the day shortens.

And as they sense that onset they begin shutting down, and they try to reabsorb as much of the energy that's stored in leaves as possible, these sugars and carbohydrates that the trees have been producing all summer long.

And as this is happening, the chlorophyll in those leaves, chlorophyll's catalyst for photosynthesis and important for all of life on Earth, it makes vegetation appear green to our eyes. The chlorophyll is beginning to degrade.

And as this happens, it reveals other chemicals that have been in the leaves all summer. So these are the same chemicals that you find in bananas that made them appear yellow and in oranges that make them appear orange.

At the same time this is happening, the trees are trying to buffer those leaves and hang on to them as long as possible, so that they can absorb as much of the energy stored in those leaves as possible. And to do that, they produce another group of chemicals called anthocyanins. These anthocyanins give us the reds and the purples that we find in some species, like maples and oaks, and add a lot of diversity to the color that we see on the landscape.

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