Matters of Faith: Global Warming and Intelligent Design
Global Warming and Intelligent Design share a fundamental characteristic in common, the lack of definitive, proven evidence of the causation which would make them fact rather than theory.
In Intelligent Design you can see the leaps of evolution and the bizarre anomalies in the fossil record and you can say "something must have caused this", but there is no proof of what or who is the cause. You can choose God or aliens or mutation or random unidentified forces as your explanation equally convincingly because no evidence exists to definitively identify the unknown forces driving the evolutionary process. If you believe in a conscious force behind evolution you can only reach that conclusion through faith.
In the same way, Global Warming is a clearly identifiable phenomenon which can be documented through the geological record and contemporary temperature measurements, but it is impossible to point to a specific cause and say "aha, here's THE thing which is causing global warming." Human causation is the popular choice as a cause, but human output of the gasses which cause global warming is substantially less than the cumulative yearly output from natural sources like volcanoes and forest fires and falls well within the normal variations of those gasses from year to year. Natural forces like solar activity and the earth's climate cycle also contribute to global warming and it is impossible to definitively identify one cause as specifically responsible. If your choice is to believe in human causation as the one thing that causes Global Warming, that position can only be reached by the equivalent of a leap of faith...
http://blogcritics.org/archives/2006/07/22/233219.php