This would be a good time to review the USCCB's 2001 statement on this issue: Global Climate Change: A Plea for Dialogue, Prudence, and the Common Good.
For a more recent February 2007 statement by the USCCB on climate change, see: U.S. Bishops Call for Moral Focus on Global Climate Change; New Report Demands Urgent Attention
The USCCB currently is engaged in a Climate Change Justice and Health Initiative. See the website for additional information and resources for parishes.