China's envoy to global negotiations on climate change expressed optimism on Wednesday that a new agreement to reduce greenhouse gases would be reached this year, and he said that his nation's efforts to curb carbon pollution already had produced results that he called 'second to none.' But the envoy, Yu Qingtai, also underscored China's opposition to placing a ceiling on its emissions of greenhouse gases, a step that some experts have called crucial to efforts to slow global warming. China passed the United States a few years ago as the world's largest emitter of carbon dioxide, the main greenhouse gas produced by human activity, although it remains well behind when pollutants are measured on a per-person basis. Together, China and the United States account for about 40 percent of pollutants linked to climate change. In a briefing at China's foreign ministry, Yu presented a list of Chinese achievements in limiting its carbon emissions, including claims that Beijing is the world's foremost user of nonpolluting hydropower and solar power, and fourth in wind power. By 2020, he said, 15 percent of China's energy will come from renewable sources. Yu also said the nation had made impressive strides in energy efficiency, reducing the amount of energy used per unit of gross domestic product by a tenth since 2005. The government has said it will achieve a total reduction of 20 percent by 2020, a goal Yu called 'a binding target.' Yu said he was hopeful for a comprehensive agreement to reduce greenhouse gases at a December meeting in Copenhagen, where nations will try to craft a successor to the protocol reached at a 1997 U.N. meeting in Kyoto, Japan. Part of that optimism, he said, stemmed from the fact that global climate problems are so fearsome that 'we cannot afford to fail.' 'Not a single country in the world will be able to stay out of trouble,' he said. 'Not a single country can say that it can keep safe and intact from global warming. So the only way out is cooperation -- global cooperation.' Still, he repeated China's longstanding opposition to placing an absolute limit on its own emissions of greenhouse gases. Limiting China's development would hamstring efforts to raise its living standards closer to a level that the developed world -- which Yu noted is largely responsible for climate change problems -- has enjoyed for decades. Yu devoted much of his briefing to calls for the world's rich nations to take concrete steps toward reducing their pollutants instead of issuing what he suggested were empty pledges. China has proposed that the developed world commit to cutting its greenhouse gas emissions by 40 percent by 2020. 'The developed countries, in realizing their industrialization, have discharged a large amount of greenhouse gases in the course of one or two centuries,' he said. 'The cumulative emissions by the developed countries have caused global warming. Who should take the historical responsibilities?' Yu also said that talks with the Obama administration on climate change issues had been 'quite fruitful' and that China saw great potential for further progress in bilateral negotiations. 