As any gardener knows, in the natural world timing is everything. The study of the timing of recurring biological events is known as phenology. By systematizing the evidence, a phenologist hopes to see trends and relationships between, say, the time a tree's leaves appear, the time the moth that feeds on the leaves hatches, and the time the bird that feeds on the moth makes its migration. For animals as well as for plants, accurate timing is key to survival.

In the late eighteenth century, European scientists began taking phenological data seriously. Later, as concerns rose that the world's climate was changing, scientists began to look at their records of animal and plant phenology and wondered whether these had been changing relative to rising global temperatures. While the studies are fairly new, the results all show that global warming has been resetting many biological clocks.

Mark D. Schwartz of the University of Wisconsin has found that between 1959 and 1993, the first-leaf date for lilacs had advanced 5.4 days and that the first bloom appeared 4.2 days sooner. The cherry trees in Washington, D.C. now bloom as much as a week earlier than they did 30 years ago. These don't appear to be isolated deviations. Studies of more than 1,500 species worldwide show that on average frogs mate, birds nest, and trees bud more than a week earlier than they did 50 years ago.