project to be significant if and only if they exceed established thresholds of significance. According to a 2001 survey, however, few agencies have actually developed thresholds of significance. The survey's analysts wondered, "if most agencies are not developing thresholds and publishing them for public review, then what criteria are they using?". In absence of thresholds of significance developed independently by lead agencies, impact assessments apply the significant criteria detailed in Appendix G, Environmental Checklist, of the CEQA Statutes and Guidelines, which is produced by the California Office of Planning and Research (OPR). Lead agencies can also defer to authority agencies that publish their recommended guidelines for the resources they regulate. Examples of this include the California Department of Conservation has threshold tests for assessing impacts to agricultural resources using the LESA model, and the Bay Area Air Quality Management District has published guidelines for air quality impacts. Environmental impact analysis The lead agency must analyze project impacts to 18 different environmental resource factors detailed in Appendix G during their CEQA review. The next section discusses background and setting for each resource factor; and how lead agencies conduct their analysis and the authority for their significance thresholds for these resources. Aesthetics Agriculture and Forestry Resources Air Quality Biological Resources Cultural Resources Energy Geology and Soils Greenhouse Gas Emissions GHG legislative history; rule making; regulated greenhouse gases and their sources; and GHG CEQA responsible agencies are discussed below. The next section describes the analytical approach to determining if a project has significant GHG impacts using guidelines from two Air Quality Management Districts in California. Finally, mitigation measures recommended by air districts and used on various projects are presented for GHG impact analysis. Introduction and setting Greenhouse gas emissions were not required to be analyzed as an impact to the environment when CEQA was originally adopted. Several scientists have since attributed greenhouse gases to climate change and resulting negative impacts to humans and the environment. The Intergovernmental Panel on Climate Change (IPCC) issued a report in 2007 stating that human activities are responsible for increased global temperatures. As a result of these assessments, the California Legislature passed Assembly Bill 32, the Global Warming Solutions Act of 2006 that mandated greenhouse gas emissions be reduced to 1990 levels by 2020. Governor Arnold Schwarzenegger signed Executive Order S-3-05 in June 2005 that declared California is vulnerable to climate change impacts resulting in loss of Sierra