By Deborah Zabarenko, Environment Correspondent

UNITED NATIONS, July 31 (Reuters) - The first U.N. special session on climate change focused on the world's rich countries on Tuesday, as policy-makers urged long-standing polluters to shoulder much of the burden for cutting greenhouse gases.

British economist Nicholas Stern said poor and developing countries also need to participate in a "global deal" to curb the human-made emissions that swaddle the planet like a blanket.

Stern, author of a path-breaking report last year on the economic consequences of climate change, said the global target for reducing greenhouse gases -- notably the carbon dioxide released by coal-fired electric plants and petroleum-powered vehicles -- should be a cut 50 percent by 2050.

"Because of reasons of past responsibility and better access to resources, the rich countries should take much bigger objectives than that 50 percent," he said. "They should be looking for around 75 percent cuts."

That responsibility could extend to financing cuts in emissions in other countries, said Stern, formerly head of the British government's economic service and now at the London School of Economics.

British Prime Minister Gordon Brown sounded a similar note in earlier remarks at the United Nations.

"We know that the gains from global prosperity have been disproportionately enjoyed by the people in industrialized countries and that the consequences of climate change will be disproportionately felt by the poorest who are least responsible for it -- making the issue of climate change one of justice as much as economic development," Brown said.

'HIGH ON RHETORIC ... LOW ON REAL ACTION'

"The rich world has to reduce emissions far more drastically than it has done so to date," said Sunita Narain, director of India's Center for Science and Environment. "The political leadership is very high on rhetoric but very low on real action when it comes to delivering the goods on climate change."

Global climate change has been blamed for droughts, floods, rising seas and more intense storms, and these cannot be explained by natural climate variability, John Holdren, an environmental scientist at Harvard University, told the gathering.

The United States, one of the world's biggest emitters of greenhouse gases, made no statement at Tuesday's sessions, and has repeatedly rejected firm targets for cutting greenhouse gas emissions, maintaining this would hurt the U.S. economy.

Instead, Washington has called for voluntary rather than mandatory emissions cuts.

President George W. Bush agreed with other leaders of the Group of Eight major industrialized nations in June to make "substantial" but unspecified reductions in climate-warming emissions and to negotiate a new global climate pact that would extend and broaden the Kyoto Protocol beyond 2012.

The two-day climate meeting at the United Nations, which concludes on Wednesday, is the first of its kind in substance and in style. The gathering is carbon-neutral, with all emissions from air travel and the operation of the U.N. Headquarters building in New York being offset by investment in a biomass fuel project in Kenya.

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