(For more G8 summit stories, click on [G7/G8]) (Adds food statement, Japanese official comments)
* G8 says world must act on climate change through UN
* G8 sees downside economic risks, serious financial strains
* Record oil prices, high food prices need to be tackled
* Africa aid commitments confirmed
By Tabassum Zakaria and David Clarke
TOYAKO, Japan, July 8 (Reuters) - G8 nations, papering over deep differences, said on Tuesday they would work toward a target of at least halving global greenhouse gas emissions by 2050 but emphasised they would not be able to do it alone.
In a communique released during a summit in northern Japan, the Group of Eight leaders also agreed that they would need to set interim goals on the way to a "shared vision" for 2050 although they gave no numerical targets.
Mention of mid-term goals was an advance from last year when the G8 agreed at Heiligendamnn, Germany, only to "seriously consider" a goal of halving emissions by mid-century.
But calling on countries involved in U.N. negotiations on climate change to also "consider and adopt" the 2050 goal satisfies the United States, which has said it cannot agree to binding targets unless big polluters such as China and India rein in their emissions too.
Dan Price, assistant to U.S. President George W. Bush for international economic affairs, said the statement reflected that "the G8 alone cannot effectively address climate change, cannot effectively achieve this goal, but that contributions from all major economies are required".
Critics outside the rich nations' club slammed the deal, although many had already predicted that chances for bold steps were slim until a new U.S. president takes office next January.
"The G8 are responsible for 62 percent of the carbon dioxide accumulated in the Earth's atmosphere, which makes them the main culprit of climate change and the biggest part of the problem," environmental group WWF said in a statement.
"WWF finds it pathetic that they still duck their historic responsibility."
Five big emerging economies including China, India and South Africa also panned the G8 and called on rich nations to slash their greenhouse gas emissions by 80-95 percent below 1990 levels by 2050, and make cuts of 25-40 percent by 2020.
South African Environment Minister Marthinus van Schalkwyk said he feared this year's communique was a step backward.
"While the statement may appear as a movement forward, we are concerned that it may, in effect, be a regression from what is required to make a meaningful contribution to meeting the challenges of climate change," van Schalkwyk said.
The Group of Five, which also includes Brazil and Mexico, met in the northern city of Sapporo before joining talks with the G8 on Wednesday.
The European Union and Japan had been pressing for this year's summit to go beyond just "considering" the 2050 goal, and Brussels had wanted clear interim targets as well.
British Prime Minister Gordon Brown praised the agreement.
"There has been major progress on the climate change agenda beyond what people thought possible a few months ago," he told reporters. "For the first time the G8 has said we will adopt at least a 50 percent reduction in carbon emissions by 2050 as part of a worldwide agreement that we hope to get in Copenhagen."
Summit host Japan also said the deal represented "significant progress" from last year and would boost momentum for the U.N.-led talks, although not all Tokyo's hopes had been met.
"Are we disappointed? Not really, because we have been able to achieve consensus among the G8," said Koji Tsuruoka, director general for global issues at Japan's foreign ministry.
The U.N.-led talks aim to create a new framework for when the Kyoto Protocol expires in 2012 and are set to conclude in Copenhagen in December 2009.
The G8 comprises Japan, Britain, Canada, Germany, France, Italy, Russia and the United States.
Global warming ties into other big themes such as soaring food and fuel prices discussed at the three-day summit at a plush mountain-top hotel on the northern Japanese island of Hokkaido, where 21,000 police have been mobilised.
In another statement, the leaders expressed strong concern about sky-high food and oil prices, which they said posed risks for a global economy under serious financial strain.
The G8 said the steep rise in global food prices also threatened food security and called for countries with sufficient food stocks to make available a part of their surplus for countries in need.
The leaders agreed to bring major oil producers and consumers together in a new forum to discuss energy security. One diplomat said it would also be a venue to talk about output and prices.
The price of food and of oil, which hit a record high of $145.85 a barrel last week, is taking a particularly heavy toll on the world's poor.
A World Bank study issued last week said up to 105 million more people could drop below the poverty line due to the leap in food prices, including 30 million in Africa.
The G8 leaders reaffirmed aid targets pledged at their Gleneagles summit in 2005, when they vowed to raise annual levels of aid by $50 billion by 2010, $25 billion of that for Africa.
Aid workers and NGOs had expressed concern about the pledge, saying donor countries might fail to meet their promises, which are not legally binding and are hard to track in actual spending.
The summit has become a magnet for protesters and although Japan has been effective at cracking down on any demonstrations -- helped by the remote location of the summit -- a few thousand have managed to hold protests several km (miles) away.
A group of demonstrators marched to the sound of music and drums on Tuesday, holding signs saying "Smash the G8 summit" and "Free G8 political prisoners".
Tomoyuki Sueoka, a 25-year-old graduate student, said: "G8 nations do not have the right to decide the policies of the world. This is not democratic. They talk about poverty and food shortages but they are simply talking about business." (Additional reporting by Linda Sieg, Yoko Nishikawa, David Fogarty, Alan Wheatley, Chikafumi Hodo, William Schomberg, Vivek Prakash, Chisa Fujioka, Yoko Kubota, Alan Wheatley, Lucy Hornby, Gernot Heller, David Ljunggren; Writing by Linda Sieg; Editing by Sonya Hepinstall and Rodney Joyce)