Cheers fit for a revival meeting swept a hotel ballroom as 1,800 entrepreneurs and experts watched a PowerPoint presentation of the most promising technologies for limiting global warming: solar power, wind, ethanol and other farmed fuels, energy-efficient buildings and fuel-sipping cars. “Houston,” Charles F. Kutscher, chairman of the Solar 2006 conference, concluded in a twist on the line from Apollo 13, “we have a solution.” Hold the applause. 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...