Long-term observations confirm that our climate is now changing at a rapid rate. Over the 20th century, the average annual US temperature has risen by almost 1 F ( .6 C). There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities. The science indicates that the warming in the 21st century will be significantly larger than in the 20th century. Human activities have altered the chemical composition of the atmosphere through the buildup of greenhouse gases - primarily carbon dioxide, methane, and nitrous oxide. …
