State Emission Limits. 
The United States Environmental Protection Agency (EPA) and the Department of Ecology (Ecology) identify carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride as greenhouse gases (GHGs) because of their capacity to trap heat in the Earth's atmosphere.  According to the EPA, the global warming potential (GWP) of each GHG is a function of how much of the gas is concentrated in the atmosphere, how long the gas stays in the atmosphere, and how strongly the particular gas affects global atmospheric temperatures.  Under state law, the GWP of a gas is measured in terms of the equivalence to the emission of an identical volume of carbon dioxide over a 100-year timeframe (carbon dioxide equivalent or CO2e).

Since 2008 state law has established limits on the emission of GHGs in Washington.  Ecology is responsible for monitoring and tracking the state's progress in achieving these emissions limits.  In 2020 additional legislation was enacted to update the statewide emissions limits to the following:

• by 2020, reduce overall emissions of GHGs in the state to 1990 levels, or 90.5 million metric tons of carbon dioxide equivalents (MMT CO2e);
• by 2030, reduce overall emissions of GHGs in the state to 45 percent below 1990 levels, or 50 MMT CO2e;
• by 2040, reduce overall emissions of GHGs in the state to 70 percent below 1990 levels, or 27 MMT CO2e; and
• by 2050, reduce overall emissions of GHGs in the state to 95 percent below 1990 levels, or 5 MMT CO2e, and achieve net-zero GHG emissions. 

The law establishing state emission limits includes various codified legislative findings and statements of intent.  One such legislative finding is that Washington should continue its leadership on climate change policy by creating accountability for achieving the emissions reductions established in the statewide emission limits.

Greenhouse Gas Emission Reporting by Sources of Emissions. 
Under the federal Clean Air Act, GHGs are regulated as an air pollutant and are subject to several air regulations administered by the EPA.  These federal Clean Air Act regulations include a requirement that facilities and fuel suppliers whose associated annual emissions exceed 25,000 metric tons of CO2e report their emissions to the EPA.  At the state level, GHG reporting is regulated by Ecology under the state Clean Air Act.  This state law requires facilities and fuel suppliers whose emissions exceed 10,000 metric tons of CO2e each year to report their annual emissions to Ecology.  As a result of legislation enacted in 2024, Ecology's rules may also require electric power entities to report GHG emissions from all electricity sold, imported, exported, or exchanged in Washington.

State Greenhouse Gas Emissions Inventory.

By December 31 of even-numbered years, Ecology and the Department of Commerce (Commerce) must report to the Governor and the appropriate committees of the Legislature the total emissions of GHGs for the preceding two years, and totals in each major source sector (Greenhouse Gas Inventory).  The most recent Greenhouse Gas Inventory was published in January of 2025, and reported emissions from 1990 through calendar year 2021.  The next Greenhouse Gas Inventory is scheduled to be published by December 1, 2026;  Ecology has stated it will include data through calendar year 2023.  

Ecology and Commerce must annually submit the Greenhouse Gas Inventory for the preceding year, beginning December 31, 2026.

The legislative findings regarding state emission reduction requirements are expanded to state that climate policy leadership and full accountability for achieving emission reductions includes consistent tracking and annual reporting of statewide emissions in the Greenhouse Gas Inventory.