| Abbreviation | Definition |
|---|---|
| CIEH | Chartered Institution of Environmental Health |
| CIER | Center for Integrative Environmental Research |
| CIER | Centre for Indigenous Environmental Resources |
| CIESIN | Center For International Earth Science Information Network |
| CII | Carbon Intensity Indicator |
| CIMS | Chesapeake Information Management System |
| CIRE | Center for Integrated Research on the Environment |
| CIRM | Critical Incident Response Model |
| CISM | Critical Incident Stress Management |
| CITES | Convention on International Trade in Endangered Species of Wild Fauna and Flora |
| CKD | Cement Kiln Dust |
| CLEA | Contaminated Land Exposure Assessment |
| CLEAN | Citizens League for Environmental Action Now |
| CLEAN | Community Labor Environmental Action Network |
| CLEAN | Community Led Environment Action Network |
| CLEAN | Comprehensive Local Environmental Action Network |
| CLEB | Committee for Laboratory and Environmental Biosafety |
| CLIMA | The Penn State Center for Climate Risk Managemen |
| CLIVAR | Climate Variability and Predictability |
| CLOCA | Central Lake Ontario Conservation Authority [Ontario, Canada] |