This paper examines voluntary disclosures about greenhouse gas emissions by the US S&P 500 firms to the Carbon Disclosure Project (CDP). Trends in three disclosures (answering the questionnaire, disclosing emissions and disclosing accounting methodology for emissions) are examined from 2006 to 2008. The frequencies of all three disclosures increased over this period. The finding that many firms answer the questionnaire, but do not disclose their emission amounts or how they account for them, is consistent with a prediction from the legitimacy theory literature that firms will disclose the minimum to avoid scrutiny. Disclosure patterns are routine since previous disclosures are the most significant variable in explaining subsequent ones. The research contributes to the understanding of emission disclosures, in particular, and voluntary disclosures, in general, by highlighting the importance of considering previous disclosures in understanding subsequent ones.