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Administrative Draft Environmental Impact Report Gill Medical Center Project Expressing GHG emissions in CO2e takes the contribution of all GHG emissions to the greenhouse effect and converts them to a single unit equivalent to the effect that would occur if only CO2 were being emitted. Climate change is a global problem. GHGs are global pollutants, unlike criteria air pollutants and toxic air contaminants (TACs), which are pollutants of regional and local concern. Whereas pollutants with localized air quality effects have relatively short atmospheric lifetimes (about one day), GHGs have long atmospheric lifetimes (one to several thousand years). GHGs persist in the atmosphere for long enough time periods to be dispersed around the globe. Although the exact lifetime of any particular GHG molecule is dependent on multiple variables and cannot be pinpointed, it is understood that more CO2 is emitted into the atmosphere than is sequestered by ocean uptake, vegetation, or other forms. Of the total annual human-caused CO2 emissions, approximately 55 percent is sequestered through ocean and land uptakes every year, averaged over the last 50 years, whereas the remaining 45 percent of human-caused CO2 emissions remains stored in the atmosphere (IPCC 2013). Table 4.10-1.Greenhouse Gases Greenhouse Description Gas CO2 Carbon dioxide is a colorless,odorless gas.CO2 is emitted in a number of ways,both naturally and through human activities.The largest source of CO2 emissions globally is the combustion of fossil fuels such as coal,oil,and gas in power plants,automobiles,industrial facilities,and other sources.A number of specialized industrial production processes and product uses such as mineral production,metal production,and the use of petroleum-based products can also lead to CO2 emissions.The atmospheric lifetime of CO2 is variable because it is so readily exchanged in the atmosphere.' CH4 Methane is a colorless,odorless gas and is the major component of natural gas,about 87 percent by volume. It is also formed and released to the atmosphere by biological processes occurring in anaerobic environments.Methane is emitted from a variety of both human-related and natural sources. Human-related sources include fossil fuel production,animal husbandry(intestinal fermentation in livestock and manure management),rice cultivation, biomass burning,and waste management.These activities release significant quantities of CH4 to the atmosphere. Natural sources of CH4 include wetlands,gas hydrates,permafrost,termites,oceans,freshwater bodies, non- wetland soils,and other sources such as wildfires.The atmospheric lifetime of CH4 is about 12 years.2 N20 Nitrous oxide is a clear,colorless gas with a slightly sweet odor.Nitrous oxide is produced by both natural and human-related sources.Primary human-related sources of N20 are agricultural soil management,animal manure management,sewage treatment,mobile and stationary combustion of fossil fuels,adipic acid production,and nitric acid production.N20 is also produced naturally from a wide variety of biological sources in soil and water, particularly microbial action in wet tropical forests.The atmospheric lifetime of N20 is approximately 120 years.3 Sources: 'USEPA 2016a,2 USEPA 2016b,3 USEPA 2016c The quantity of GHGs that it takes to ultimately result in climate change is not precisely known; it is sufficient to say the quantity is enormous, and no single project alone would measurably contribute to a noticeable incremental change in the global average temperature or to global, local, or microclimates. From the standpoint of CEQA, GHG impacts to global climate change are inherently cumulative. Greenhouse Gas and Climate Change 4.10-2 October 2021