Why Autumn Colors Are So Late

A gray, grim landscape used to greet residents of the Northeastern United States each November, but autumn's riot of red, orange and yellow came late this year. Delayed fall foliage also occurred in Chicago and parts of Europe.

Some say droughts and a warm summer played a role, while others wonder more broadly about global warming. In fact, it's rising levels of carbon dioxide, not the warmer temperatures fueled by the greenhouse gas, that have been delaying the transformation of green leaves, at least in Europe for a few decades, a new study suggests.