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 by Certain Entities.  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 also report to Ecology.

Natural gas utilities receive no-cost allowances under the Climate Commitment Act's Cap-and-Invest Program (CCA), subject to certain conditions and in accordance with a specified allocation schedule.  To continue receiving no-cost allowances in the CCA, a natural gas utility must provide to Ecology a GHG emissions report also submitted to the United States Environmental Protection Agency (EPA).

Greenhouse Gas Inventory Reporting.  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 state 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 CCA, the Clean Fuels Program, and the 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 inventory 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 report.

In January 2025, Ecology released its 2024 inventory report.  The inventory is developed using a tool and data from 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 CCA, the Clean Fuels Program, 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.

Greenhouse Gas Inventory Reporting.  Ecology and Commerce must post and maintain on Ecology's website, and report to the Governor and the Legislature, the total GHG emissions for the most recent two years for which data are available.  The report must be completed by December 31st of each even-numbered year through 2030, and must be completed by December 31st of each year beginning in 2031.

In addition to the biennial report required in 2026-2030, by December 31, 2027, and December 31, 2029, Ecology and Commerce must post and maintain a summary of the total GHG emissions for the most recent year for which data are available, and the totals in each major source sector.

Greenhouse Gas Reporting by Certain Entities.  Natural gas utilities must submit to Ecology an annual GHG emissions report required under the Washington Clean Air Act (CAA) to receive no-cost allowances in the CCA.

Legislative Intent.  Codified legislative intent in the CAA is amended to specify that the consistent tracking and annual reporting of GHG emissions in a GHG inventory is an important responsibility that allows the Legislature to determine whether state emissions are on a trajectory to achieve statutory emissions reduction limits, or whether new or amended policy interventions are necessary to achieve those limits.