TOYAKO, Japan (AFP) - Leaders of the world's eight top industrial powers wrapped up a summit Wednesday with pledges to act on soaring oil and food prices, but failed to bridge deep differences with poor nations on how to fight climate change. US President George W. Bush hailed his last G8 summit, at which rich nations agreed to at least halve global greenhouse gas emissions by 2050, as "very productive" before he left the resort venue in the mountains of northern Japan. "I'm pleased to report that we've had significant success," Bush said. Emerging nations invited to attend a special summit on climate change however declined to back the G8's much-touted carbon emissions goals, saying they amounted to empty rhetoric. Leaders grappling with climate change and spiralling prices also made time to address the crisis in Zimbabwe, where President Robert Mugabe won a violence-marred election from which his chief rival dropped out. The summit "made it clear we would impose new sanctions against an illegitimate regime that has blood on its hands," British Prime Minister Gordon Brown said, rallying world support for UN sanctions on Harare. The G8 powers -- Britain, Canada, France, Germany, Italy, Japan, Russia and the United States -- account for two-thirds of the world's gross domestic product. Their leaders said in a joint statement that while global growth had "moderated," they remained positive on the future. They called for efforts to bring down oil prices, which have jumped five-fold since 2003, as well as the soaring cost of food which has set off riots in parts of the developing world. "There's a need to improve transparency on the oil market," Japanese Prime Minister Yasuo Fukuda told a news conference. G8 leaders also called on all countries to end export restrictions on food, Fukuda said. The summit was dominated by discussions on global warming amid growing concern that rising temperatures caused by carbon emissions are threatening entire species of plants and animals. The rich countries ' club agreed Tuesday on the need for a global emissions cut of at least 50 percent by 2050, a step praised by G8 leaders as a step forward after years of hesitation by Bush. But developing countries slammed the statement as too weak, tussling with rich nations at a special expanded summit on Wednesday bringing together the G8 with eight other leaders including Chinese President Hu Jintao and Indian Prime Minister Manmohan Singh. The deadlock between rich and developing nations has held up talks on reaching a new climate treaty by the end of 2009 in Copenhagen -- a goal set in December at a UN-backed conference in Bali. "Climate change is one of the great global challenges of our time," the 16 leaders said in a statement.