The Group of Eight major powers agreed Tuesday to at least halve global carbon emissions by 2050 in what leaders hailed as a step forward, but developing nations rejected as an "empty slogan". After two days huddled in the Japanese mountain resort of Toyako, leaders of the world's eight most powerful economies also voiced concern about soaring oil and food prices, pledged to speed up aid to Africa and threatened to take further action against Zimbabwe's regime. But the most contentious issue before them was climate change, with US President George W. Bush standing firm on his stance that developing countries must take action before rich nations would budge.