WASHINGTON - Democrats introduced ambitious climate-change legislation Wednesday in the Senate with the goal of reducin greenhouse-gas emissions by 20 percent by 2020 from 2005 levels, a larger reduction than the 17-percent approved in June by the House.

The bill introduced by Democratic Senators John Kerry and Barbara Boxer outlines a plan to curb toxic emissions by creating a market for companies to buy and sell pollution permits.

President Barack Obama, who has made reducing global warming and promoting clean energy top priorities for his administration, has asked Congress to pass a bill before a major United Nations meeting in December in Copenhagen, where governments are expected to thrash out a new global treaty to combat climate change.

In June, the House of Representatives narrowly approved a climate-change bill that for the first time would force US companies to limit greenhouse-gas emissions blamed for global warming.

While the 219-212 vote marked a major victory for the Obama administration, the legislation divided politicians, businesses and climate groups.

The centrepiece of the legislation is a so-called cap-and-trade system, which already exists in Europe and creates a market for reducing the carbon-dioxide emissions that cause global warming. Companies would get pollution allowances that can be traded between dirtier and cleaner firms.

The United States, which together with China emits about half of the world’s greenhouse gases, has long faced pressure from other governments to more aggressively reduce its carbon footprint.