WASHINGTON (AFP) — China and the United States must lead the world in responding to climate change to have an effect on the global threat it poses, two leading US senators warned.

Senator John Kerry, who chairs the Senate Foreign Relations Committee, said he was encouraged by his recent discussions in Beijing with senior Chinese officials, business leaders and scientists.But at a hearing with China and climate change experts, he cautioned: "Our words and our actions will set the tone.

Washington and Beijing will inevitably lead by example."With the world's largest economy, the United States has historically been the biggest emitter of the greenhouse gases that are blamed for climate change. Recently however it has been overtaken by China.

Kerry said that together the two countries were responsible for nearly half the planet's carbon emissions."Either we will create the necessary momentum to finally galvanize a global response, or else we risk a global catastrophe," he said.

"My message to the Chinese was simple: America understands that we have an obligation to lead. But you need to understand that, politically speaking, America will not enter into a global treaty without a meaningful commitment from China to be part of the solution," he said.Kerry was referring to a UN conference on the climate in December in Copenhagen that aims to produce an international agreement to cap and reduce carbon emissions.

The committee's ranking Republican, Senator Dick Lugar, said China's responses to climate change to date have been "complex and contradictory.""The American domestic debate on the issue will be profoundly influenced by perceptions of China?s willingness to set aside doctrinaire positions and agree to verifiable steps to limit greenhouse gas emissions," he warned.

A bill that passed key committee in the House of Representatives in May aims to reduce greenhouse gas emission by 17 percent by 2020 from 2005 levels."China and other developing countries do not need to take the same actions that developed nations are taking -- but they do need to take significant national actions that they commit to internationally, that they quantify and that are ambitious enough to be broadly consistent with the lessons of science," US climate change envoy Todd Stern said Wednesday before traveling to Beijing.

Kerry said the discussions he had with Chinese officials during a visit to Beijing last week were "enormously encouraging.""Chinese decision-makers insisted to me repeatedly that China now grasps the urgency of this problem," he said."People who a few short years ago weren?t even willing to entertain this discussion are now unequivocal: China is eager to embrace low-carbon development pathways and is ready to be a 'positive, constructive' player in negotiations going forward," he said.

Ken Lieberthal, a China expert at the Brookings Institution, told the committee that Washington and Beijing should work to develop a major clean energy partnership."Achieving this partnership will provide new momentum for the Copenhagen efforts," he said.But he said China "won't accept caps at this point as it does not see how it can actually cap emission growth in the face of ongoing urbanization and of the demand."