TOYAKO, Japan, (AFP) - The Group of Eight major powers agreed Tuesday to at least halve global carbon emissions by 2050, in what leaders hailed as a breakthrough but environmentalists called toothless. After a night of tough negotiations in the Japanese mountain resort of Toyako, leaders of the world's eight most powerful economies agreed to toughen language on the most contentious issue among them. US President George W Bush had previously rejected any statement beyond saying that the major economies would "seriously consider" cuts in the greenhouse gas emissions blamed for heating up the planet. The leaders of the G-8 - Britain, Canada, France, Germany, Italy, Japan, Russia and the United States - said they shared a "vision" of reducing emissions by at least 50 percent by 2050. But in a nod to Bush, the G-8 leaders also called on major developing nations to join them in cutting emissions, something the US leader has insisted is necessary. "In our view, and in the view of the leaders in the room, this represents substantial progress from last year," said Dan Price, Bush's assistant for international economic affairs. The G-8 nations also said they would each set their own interim targets for curbing greenhouse gas emissions for a still unspecified amount of time after the Kyoto Protocol's obligations expire in 2012. "We acknowledge our leadership role and each of us will implement ambitious economy-wide mid-term goals in order to achieve absolute emissions reductions," the joint statement said. Meanwhile, leading scientists were critical of the G-8's stance on global warming, saying its pledge was too vague and too distant to brake the oncoming juggernaut of climate change. Experts acknowledged the Group of Eight's usefulness in setting a goal of at least halving worldwide greenhouse gas emissions by 2050. But, they complained, the G-8 statement did not mention a base year by which this 50-percent cut would be compared. Nor - more importantly - did it identify what cuts would be made in the next decade, a period critical for determining whether the fight against climate change will succeed or fail. "It is a pretence that they understand the problem and plan to take needed actions," said James Hansen, one of the major figures in the history of climate science. Rajendra Pachauri, chairman of the UN's Nobel-winning panel of climate scientists, said the Toyako statement lacked some "very vital details," especially over the plans for the medium term. "The sooner we start reducing emissions, the greater the likelihood of avoiding some of the more serious impacts and temperature increases that are going to take place a decade or two down the road," he said from Delhi. Pachauri said that, by factoring in the 20th-century increase in temperature and CO2 pollution already pumped out this century, emissions would have to peak by 2015 to limit overall warming to 2.0-2.4 degrees Celsius (3.6-4.3 degrees Fahrenheit). By postponing the cap until, say, the 2030s or the 2040s, Earth's temperature would stay higher and for longer, he said.