California's largest industrial facilities will be legally required to disclose emissions annually from 2009 after a new rule was approved yesterday by the state's Air Resources Board.

Approximately 800 sites will be affected by the new disclosure laws, including power stations, cement plants, oil refineries and other industrial sources that emit over 25,000 tonnes of CO2 a year. Such facilities are estimated to account for 94 per cent of greenhouse gas emissions from industrial and commercial businesses in California.

Facilities fitting this criterion will be required to start tracking their emissions in 2008 and report them in 2009. The Air Resources Board claims this phase-in period will enable businesses to train employees in carbon data collection and facilitate an effective reporting system before reports become subject to third-party verification from 2010.

Paul Dickinson, chief executive of the Carbon Disclosure Project (CDP), which aims to encourage wider corporate reporting of greenhouse gas emissions, warned that the Californian law could soon be emulated in other states and countries. "We expect other countries to follow suit," he said "Climate change is a serious threat to the economy so it is very likely that legislation will be passed to make disclosure compulsory. But it is very difficult to know exactly when that will happen."

The CDP advises firms to adopt carbon reporting best practices now as the best means of preparing for future corporate reporting legislation. "We recommend the GHG Protocol, as the clearest, most complete and practical reporting methodology," added Dickinson.

A spokeswoman for the Global Reporting Initiative (GRI), a lobby group that offers best practice guidelines for carbon reporting, said that firms affected by the new legisl ation would have to be careful to ensure they use standardised methodologies for measuring carbon emissions.

"All relevant entities will have to use the same metrics for reporting on the same issues," she said. "As this leads to relevant, comparable data that can drive better performance and can help policy makers assess whether their strategies are working."

Separately, California also passed new rules that set the state the legal target of ensuring annual emissions in 2020 are cut to 1990 levels.