After decades of rancorous debate, only a handful of the most doctrinaire die-hards still dispute the idea that human activity is heating up the planet. all the signs seem to point that way: storms have become more intense and weather patterns more erratic; the past decade has been by far the hottest on record; and the rise in temperature has been greatest in polar regions and around cities. These facts dovetail ominously well with the theory that carbon dioxide (CO2), released by burning coal, oil and gasoline for heat, electricity and transportation, is trapping excess energy from the sun. Global warming is realÑand will probably get worse.

The only way to slow it down, almost every scientist agrees, is to restructure the way we produce energy. Such stopgap measures as insulation, carpooling and energy-efficient light bulbs are all useful ways to begin curbing the burning of carbon-rich fossil fuels. But in the long run, as the world's population continues to increase and living standards rise, these measures will not be enough.

That's why experiments now going on in laboratories around the world are so important. At a research center outside Stuttgart, Germany, engineers at DaimlerChrysler have created a high-performance car whose tail pipe emits nothing but water vapor. In a giant wind tunnel at NASA's Ames Research Center in California, engineers are set to analyze air turbulence in order to make superefficient wind-power turbines. In Japan scientists are perfecting paper-thin solar cells that will be cheap to produce and could turn every house into its own electricity supplier. These ventures, along with many others, are beginning to draw the outlines of a world in which energy use keeps rising and, though fossil fuels remain an important power source, CO2 levels in the atmosphere actually begin to drop.

Cars like the necar4, housed in a lab near Stuttgart, could help make that happen. This experimental vehicle, being jointly developed by Ford, DaimlerChrysler and Canada's Ballard Power Systems, gets its energy from hydrogenÑthe most abundant fuel in the entire universe. Hydrogen, unlike fossil fuels, contains no carbon atoms and thus generates zero carbon dioxide.

However, it could produce some pollution, since burning hydrogen taints the atmosphere by rearranging air molecules to form nitrogen oxides and ozone. But necar4 doesn't burn hydrogen. Instead, it uses an onboard fuel cell, developed by Ballard, to let hydrogen combine slowly with oxygen at moderate temperatures. What comes out is plain H2O and electricity.

Fuel cells were invented in the 1800s and adopted by NASA for generating clean power in space in the 1960s. Only in the past decade have they been made small enough to fit inside a car. The necar4, based on a Mercedes-Benz A-class compact sedan, accommodates five people plus luggage, reaches speeds of 90 m.p.h. (145 km/h) and goes about 280 miles (450 km) between fill-ups. "It's comparable," says Ferdinand Panik, head of DaimlerChrysler's Fuel Cell Project, "to the impact the microchip had on computer technology."

That impact may be felt very soon. The first major field test of fuel-cell vehicles will take place this year in California. And by 2004, DaimlerChrysler and FordÑas well as GM, Toyota and other companiesÑexpect to be selling fuel-cell cars directly to consumers.

Ideally, the hydrogen would be produced sustainably with renewable electricity from the sun or wind. But even under the most optimistic predictions for improvements in renewable technology, the electricity required to split H2O into H and O would be prohibitively expensive. So the first large-scale plants will probably wrest hydrogen from old-fashioned fossil fuels.

That's extraordinarily good news for China, whose gigantic size, rapid industrialization and huge domestic coal reserves threaten to pump cataclysmic amounts of CO2 into the air over the next century. While scaling fuel cells down to fit inside cars and trucks has been a challenge, scaling them up or linking them together to run factories and power plants should be no problem at all.