The Boston GlobeAmid concerns the planet is warming, the market for clean, green technology is beginning to show signs of overheating, too. "I call it the global warming bubble,' said Bob Metcalfe, who knows something about bubbles. An inventor of the ethernet and founder of Marlborough, Massachussets networking company 3Com Corporation, the stock of which plummeted when the tech bubble burst in 2000, Metcalfe now serves as chief executive of Cambridge biofuel start-up Green Fuel Technologies Corporation. But while the flurry and investment that comes with a bubble creates a froth that inevitably results in some big losers, Metcalfe and others argue it also creates an environment where disruptive ideas can emerge and change the world. "What bubbles do is they're an accelerator in technological investments to be made; they cause the status quo to be questioned,' Metcalfe said. "The trick of course...is to have a chair when the music stops.' With 930 energy start-ups worldwide counted in Lux Research's Cleantech Report last year, and corporate spending on clean-tech research and development up 10% from 2005 to 2006, to $22 billion, there can be no doubt that many idealistic Companies promising to change the world will flop. But Ted Sullivan, senior analyst at Lux Research, said the influx of money and the interest in renewable energies isn't creating one big unsustainable bubble, but rather "boomlets'