Greenhouse Gases.  Gases that trap heat in the Earth's atmosphere are called greenhouse gases (GHGs).  GHGs include carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and several other gases identified by the Department of Ecology (Ecology).  Global warming potential (GWP) is used as a method to compare one GHG's capacity to trap heat in the atmosphere with that of another.  GWP is measured as a function of how much of the gas is concentrated in the atmosphere, how long the gas stays there, and how strongly the particular gas affects global atmospheric temperatures.  Under state law, the GWP of GHGs are measured in terms of their equivalence to the emission of an identical volume of carbon dioxide over a 100-year timeframe.

Greenhouse Gas Reporting.  In Washington, entities emitting more than 10,000 metric tons of carbon dioxide equivalent of total GHG emissions per year in Washington must annually report their emissions to Ecology.  Some of these entities must have their reports verified by a third party.  Certain fuel suppliers and electric power entities with emissions exceeding the 10,000 metric tons of carbon dioxide equivalent threshold must report to Ecology.

Greenhouse Gas Inventory.  The Washington GHG inventory is a historical record of GHG emissions in the state.  It estimates statewide emissions and measures reductions compared to a 1990 baseline.  In 2020 Washington updated its GHG emission limits.  The next requirement applies in 2030, where the Washington must reduce human-caused GHG emissions to 50 million metric tons or 45 percent below 1990 levels.  There are separate, specific GHG emission limits for state agencies.  Ecology administers several programs aimed at reducing GHG emissions, such as the Cap-and-Invest Program, Clean Fuel Standard, and Refrigerant Management Program.   

State law requires Ecology and the Washington State Department of Commerce (Commerce) to report to the Governor and Legislature on the total emissions of GHGs for the preceding two years.  The report must include totals in each major source sector, including emissions associated with leaked gas and GHG emissions from wildfires.  Emissions from wood biomass, ethanol, and biodiesel consumption are excluded from the inventory.

In January 2025, Ecology released its 2024 inventory report.  The inventory is developed using a tool and data from the United States Environmental Protection Agency (EPA), which is current through 2021.  Ecology supplements and replaces default data in EPA's reporting tool for the analysis of electricity consumption, natural gas leakage, and wildfires.  The 2024 inventory report data predates the effective dates of the Climate Commitment Act, Clean Fuel Standard, zero-emission vehicle standards, some fluorinated gas regulations, and clean buildings regulations.  Ecology notes that, while the inventory report will always be retrospective, it is pursuing strategies to decrease the lag between the data contained in each report and the date of publication.

By December 31st of each year, beginning in 2026, Ecology and Commerce must report to the Governor and Legislature the total emissions of GHGs for the preceding year, subject to certain exceptions in existing law.