No offense to Pumpkin Spice Lattes and decorative gourds, but the real signs of fall are the leaves—leaves that transform from green to red. And green to orange. And green to yellow. Autumnal romance aside, it's a change that is, of course, chemical in nature.
It goes like this, as the video above, from the American Chemical Society, explains: Trees eat sunlight, essentially, converting solar rays into energy through photosynthesis. (That process results in glucose for the trees and oxygen for Earth's atmosphere—a nice win-win.) Photosynthesis requires chlorofyll, the pigment that accounts for the luscious green color leaves have in the spring and summer months; chlorofyll production, in turn, requires energy.
And trees, like all organisms, are hungry. But trees, like not all organisms, are good planners. As daylight hours lessen, they try to save up as much energy as they can to sustain them through the fall and winter. They essentially go on diets to make sure that they survive the reduction in sunlight that comes with the winter months.
Chlorophyll also contains a structure called porphyrin—and as there's less sunlight available for trees to "eat," the porphyrin begins to break down. Which causes it to break down. And when the breakdown occurs, other pigments —including carotene (the stuff that makes carrots orange) and lycopene (found in tomatoes)—are revealed.
The most vibrant colors, if you're into leaf-peeping, are the result of the sugars trees produce. Those stored sugars help the trees boost their production of anthocyanin, a pigment that provides a brilliant red leaf. That production, in turn, is bolstered by sunlight—so a sunny fall means, generally, a colorful fall. (A less-sunny fall? You'll get more yellows and browns, the results of consistently-produced caratenoids and tannins in the leaves.)
So there you have it: The world reddens in the fall because trees, like so many other creatures, are watching their weight.