This study determined the state of the practice and emerging trends in developing greenhouse gas (GHG) thresholds of significance for use under the California Environmental Policy Act (CEQA). To describe the adopted, proposed or considered approaches for developing thresholds of significance by air districts and the thought processes behind these decisions, information was obtained through surveys and phone interviews from twelve employees of air districts in California, State agencies, and consultants that are considered experts on this issue. The results of this study include a comparison matrix of the approaches of the three air districts that have adopted or proposed GHG thresholds of significance, and identification of the common themes from air districts’ responses that do not have adopted or proposed thresholds. While the development of GHG thresholds of significance is an evolving practice, emerging trends in this practice were identified. These trends include providing flexibility in options, an emphasis on programmatic approaches and a preference of including bright-line thresholds and efficiency thresholds. Furthermore, this paper concludes that while the three air districts that have proposed or adopted thresholds have laid the groundwork for other districts, some of the approaches or the thresholds themselves may not be easily transferable to other districts based on regional differences. This research provides insight into how GHG emissions might be addressed in CEQA documents throughout the State.