Cheers swept a hotel ballroom as 1,800 entrepreneurs and experts watched a presentation of promising technologies for solar power, wind, ethanol and other farmed fuels, energy-efficient buildings and fuel-sipping cars. There was talk of "a solution" to global warming.

But for all the convention's talk of a solution to the problem of global warming, it will be an immense challenge to limit emissions of carbon dioxide, which traps heat, and it will take a long while. "We've got a $12 trillion capital investment in the world energy economy and a turnover time of 30 to 40 years," said John Holdren, a physicist and climate expert at Harvard University and president of the American Association for the Advancement of Science. "If you want it to look different in 30 or 40 years, you'd better start now."

The problem is aggravated in a world likely to add 2.5 billion people by midcentury, experts say. Most of those people will live in countries like China and India, which are just beginning to enjoy an electrified, air-conditioned mobile society. Yet 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. Japan is the only economic power that increased research spending in recent decades, according to the International Energy Agency.

In the private sector, studies show that energy companies stint on 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 nonpolluting energy sources and systems became far more aggressive, the world would probably face dangerous warming and international strife as nations compete for increasingly inadequate resources. "We cannot come close to stabilizing temperatures" unless humans, by the end of the century, stop adding more carbon dioxide to the atmosphere than it can absorb, said W. David Montgomery of Charles River Associates, a consulting group.

"And that will be an economic impossibility without a major R&D investment."

A sustained push is needed not just to refine, test and deploy known low-carbon technologies, but also to find "energy technologies that don't have a name yet," said James Edmonds, a chief scientist at the Joint Global Change Research Institute of the University of Maryland and the Energy Department.

At the same time, many energy experts and economists agree on another daunting point: To make any resulting "alternative" energy options the new norm will require attaching a significant cost to the carbon emissions from coal, oil and gas. "A price incentive stirs people to look at a thousand different things," said Henry Jacoby, a climate and energy expert at the Massachusetts Institute of Technology. Environmental campaigners, focused on promptly establishing binding limits on emissions of heat-trapping gases, have tended to play down the need for big investments seeking energy breakthroughs.

Many energy experts said this stance is counterproductive because there was no way, given global growth in energy demand, that existing technology could avert a doubling or more of atmospheric concentrations of carbon dioxide in this century. Other researchers say that any technology quest should 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. Without big reductions in emissions, the midrange projections of most scenarios envision a rise of about 4 degrees Fahrenheit (2.2 degrees Celsius) in this century, four times the warming in the last 100 years.

That could, among other effects, produce a disruptive mix of intensified flooding and withering droughts in the world's prime agricultural regions.

Nicholas Stern, the chief of Britain's economic service and author of the new government report on climate options, has summarized the cumulative nature of the threat succinctly: "The sting is in the tail."

Carbon dioxide accumulates in the atmosphere as long as emissions exceed the rate at which the gas is naturally removed from the atmosphere by oceans and plants. It is hard to address the problem promptly, because the technologies producing the emissions evolve so slowly. A typical new coal-fired power plant, one of the largest sources of emissions, is expected to operate for many decades. About one large coal-burning plant is being commissioned a week, mostly in China.

Many experts say this means the only way to affordably speed the transition to low-emission energy is with advances in technology at all stages of maturity.

Examples include:

* Substantially improving the efficiency and cost of solar panels;

* Conducting full-scale tests of systems for capturing carbon dioxide from power plants and pumping it underground;

* Seeking efficient ways to generate fuels from crops;

* Finding new ways to store vast amounts of energy harvested intermittently from the wind and sun.

Carbon dioxide levels will stabilize only if each generation persists in developing and deploying alternatives to unfettered fossil-fuel emissions, said Robert Socolow, a physicist and co- director of a Princeton "carbon mitigation initiative," created with $20 million from BP and Ford Motor. 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, Holdren wrote in the MIT 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.

Another area requiring immediate intensified work, Holdren and other experts say, is large-scale demonstration of systems for capturing carbon dioxide from coal burning before too many old-style plants are built. The components for capturing carbon dioxide and disposing of it underground are already in use, particularly in oil fields, where pressurized carbon dioxide is used to drive the last dregs of oil from the ground. In this area, said David Keith, an energy expert at the University of Calgary, "We just need to build the damn things on a billion-dollar scale."

The next challenge will no doubt fall to the next generation of engineers and entrepreneurs, and the one after that. Those innovators will not have much to build on without a great increase in the investment that now goes to basic research.