Over the next 250 years, calculates Eric J. Chaisson in a recent paper, the earth's population will start generating so much of its own heat - chiefly wasted from energy use - that it will warm the earth even without a rise in greenhouse gases. The only way to avoid it, he says, is to rethink how we generate energy.His paper examines the planet's growing pool of waste heat, a widespread phenomenon that nonetheless has been little studied as a cause of climate change. Nearly everything that uses or generates energy - chiefly power plants, but also cars, snowblowers, computers, and light bulbs - squanders some energy as wasted heat. And the larger and more energy-hungry the human population grows, the more waste heat remains in our atmosphere."What this means for humans is that this is the ultimate limit to growth," said Dennis Bushnell, the chief scientist at NASA's Langley Research Center, who urged Chaisson to publish his idea. "As we produce more kilowatts, we have to produce more waste heat."Chaisson's prediction suggests we need to change our energy policy - not just by keeping emissions low, but by shifting toward power sources that don't add new heat to the earth's system.The culprits in the waste-heat problem are not only dirty fossil fuels like coal and oil, but also some "clean" power sources like nuclear and geothermal energy, which still add to the problem by pumping new heat into the atmosphere. The only way to stop waste heat-induced global warming, in Chaisson's view, is to rely on energy that already reaches the earth's surface: sunlight, and the wind and the waves that it powers.