… For all the enthusiasm about alternatives to coal and oil, the challenge of limiting emissions of carbon dioxide, which traps heat, will be immense in a world likely to add 2.5 billion people by midcentury…

The challenge is all the more daunting because research into energy technologies by both government and industry has not been rising, but rather falling.

In the United States, annual federal spending for all energy research and development — not just the research aimed at climate-friendly technologies — is less than half what it was a quarter-century ago. It has sunk to $3 billion a year in the current budget from an inflation-adjusted peak of $7.7 billion in 1979, according to several different studies.

…President Bush has sought an increase to $4.2 billion for 2007, but that would still be a small fraction of what most climate and energy experts say would be needed.

Federal spending on medical research, by contrast, has nearly quadrupled, to $28 billion annually, since 1979. Military research has increased 260 percent, and at more than $75 billion a year is 20 times the amount spent on energy research.

Internationally, government energy research trends are little different from those in the United States. Japan is the only economic power that increased research spending in recent decades, with growth focused on efficiency and solar technology, according to the International Energy Agency.

In the private sector, studies show that energy companies have a long tradition of eschewing long-term technology quests because of the lack of short-term payoffs.

Still, more than four dozen scientists, economists, engineers and entrepreneurs interviewed by The New York Times said that unless the search for abundant non-polluting energy sources and systems became far more aggressive, the world would probably face dangerous warming and international strife as nations with growing energy demands compete for increasingly inadequate resources.

Most of these experts also say existing energy alternatives and improvements in energy efficiency are simply not enough.

…Other researchers say the chances of success are so low, unless something breaks the societal impasse, that any technology quest should also include work on increasing the resilience to climate extremes — through actions like developing more drought-tolerant crops — as well as last-ditch climate fixes, like testing ways to block some incoming sunlight to counter warming.

…The most immediate gains could come simply by increasing energy efficiency. If efficiency gains in transportation, buildings, power transmission and other areas were doubled from the longstanding rate of 1 percent per year to 2 percent, Dr. Holdren wrote in the M.I.T. journal Innovations earlier this year, that could hold the amount of new nonpolluting energy required by 2100 to the amount derived from fossil fuels in 2000 —a huge challenge, but not impossible.

…No matter what happens in the next decade or so, many experts say, the second and probably hardest phase of stabilizing the level of carbon dioxide will fall to the generation of engineers and entrepreneurs now in diapers, and the one after that. And those innovators will not have much to build on without greatly increased investment now in basic research.

…But all of the small-scale experimentation will never move into the energy marketplace without a much bigger push not only for research and development, but for the lesser-known steps known as demonstration and deployment.

In this arena, there is a vital role for government spending, many experts agree, particularly on “enabling technologies” — innovations that would never be pursued by private industry because they mainly amount to a public good, not a potential source of profit, said Christopher Green, an economist at McGill University.

Examples include refining ways to securely handle radioactive waste from nuclear reactors; testing repositories for carbon dioxide captured at power plants; and, perhaps more important, improving the electricity grid so that it can manage large flows from intermittent sources like windmills and solar panels.

“Without storage possibilities on a large scale,” Mr. Green said, “solar and wind will be relegated to niche status.”

While private investors and entrepreneurs are jumping into alternative energy projects, they cannot be counted on to solve such problems, economists say, because even the most aggressive venture capitalists want a big payback within five years.

Many scientists say the only real long-term prospect for significantly substituting for fossil fuels is a breakthrough in harvesting solar power.

…The benefits of an intensified energy quest would go far beyond cutting the risks of dangerous climate change, said Roger H. Bezdek, an economist at Management Information Systems, a consulting group.

The world economy, he said, is facing two simultaneous energy challenges beyond global warming: the end of relatively cheap and easy oil, and the explosive demand for fuel in developing countries.

Advanced research should be diversified like an investment portfolio, he said. “The big payoff comes from a small number of very large winners,” he said. “Unfortunately, we cannot pick the winners in advance.”

…“We’re good at rushing in with white hats,” said Bobi Garrett, associate director of planning and technology management at the National Renewable Energy Laboratory. “This is not a problem where you can do that.”