 A bill going to the U.S. Senate next week seeking deep cuts in U.S. greenhouse gases by 2050 is a "first step" but not enough to avert damaging climate change, the head of the U.N. Climate Panel said on Friday. Rajendra Pachauri also said that even tougher plans by some other developed nations to rein in emissions were insufficient to head off some projected impacts of global warming, ranging from more heatwaves and droughts to rising seas. The U.S. bill, sponsored by Sen. Joe Lieberman, a Connecticut independent and Sen. John Warner, a Virginia Republican, seeks to cut U.S. emissions by up to 66 percent below current levels by 2050. It will be debated from June 2. "I think it's enough as a first step," Pachauri told reporters during a visit to Oslo. "I wouldn't say it is the final solution one is looking for." He welcomed the effort as far more stringent than a plan outlined last month by President George W. Bush that would let U.S. emissions rise to a 2025 ceiling. The United States and China are the top emitters of greenhouse gases. Bush's plan upset some of his industrial allies because it is far less tough than the U.N.'s Kyoto Protocol under which 37 developed nations have agreed to cut emissions, mainly from burning fossil fuels, by 5 percent below 1990 levels by 2008-12. Several leading scientists in the U.N. panel, which shared the 2007 Nobel Peace Prize with former U.S. Vice President and climate campaigner Al Gore, on Thursday urged far deeper cuts than those now under consideration by major nations. The authors, including British scientist Martin Parry, wrote in the journal Nature that the world had to cut emissions by 80 percent below 1990 levels by 2050 to limit temperature rises to 2 Celsius (3.6 Fahrenheit) above pre-industrial levels. NOT ENOUGH "I would agree with Martin Parry; current efforts are certainly not enough," Pachauri said. A 2.0 Celsius rise is viewed by the European Union and some other nations as a threshold for "dangerous" climate changes. Pachauri said that estimates by the U.N. Climate Panel showed that "if you want to stabilize the increase in temperatures to between 2.0 to 2.4 Celsius we are talking about cuts of 25 to 40 percent by 2020" below 1990 levels. "That is clearly far above what was considered at any stage in the discussions on the Kyoto Protocol," he said. Almost 200 nations including the United States agreed in Bali, Indonesia, in December to work out a new U.N. treaty by the end of 2009 to curb global warming after a first period of the Kyoto Protocol runs out in 2012. U.S. environmentalists are supportive of the Lieberman-Warner bill but want more in the legislation. The business community questions the economic impact, and the politicians who have shepherded it seem gratified that it has managed to get this far -- even though it is unlikely to become law this year.