Gov. Arnold Schwarzenegger (search) is proposing to combat global warming (search) by setting voluntary goals for reducing emissions of greenhouse gases in the nation's most populous state.

Schwarzenegger wants to roll emissions of greenhouse gases (search) — which include carbon dioxide, methane and nitrous oxide — back to 2000 levels by 2010, to 1990 levels by 2020 and to 80 percent below 1990 levels by 2050.

The Republican planned to announce an executive order establishing the goals and a "Climate Action Team," led by California's environmental protection secretary, at Wednesday's opening of the five-day U.N. World Environment Day Conference (search).

At least 70 mayors from cities such as London, Rio de Janeiro and Shanghai were expected to attend the conference trading ideas on renewable energy, recycling, public transportation, parks and clean air and water.

Although the governor's goals are voluntary, they could have an impact since California has about one eighth of the U.S. population. If it were a nation, it would have the world's eighth-largest economy.

In September, California became the first state to adopt regulations to curb greenhouse gas emissions from automobiles. The Bush administration has joined automakers in challenging those standards in court.

Schwarzenegger has made other efforts to reduce greenhouse gases. His "hydrogen highways" program encourages the installation of enough hydrogen fueling stations to enable the use of zero-polluting vehicles across the state, and his "million solar roofs" program would subsidize the installation of solar power equipment in homes.