From Powerline:

The Competitive Enterprise Institute has obtained an EPA study of the "endangerment" to human well-being ostensibly caused by carbon dioxide emissions, together with a set of EPA emails indicating that the study, which concludes that carbon dioxide is not a significant cause of climate change, was suppressed by the EPA for political reasons.

The Powerline blog entry provides links to the CEI correspondence and supporting email messages, and to the suppressed study.