More than a dozen private companies around the world are looking to profit from extreme measures to combat global warming " filling the sky with sunlight-blocking particles, brightening clouds or changing the chemistry of the oceans. The problem is that nobody knows how to control the unintended consequences. Some scientists who've studied and modeled the complexity of Earth's oceans and atmosphere say any "geoengineering" scheme big enough to affect the climate could put people at risk of dramatic changes in the weather, crop failures, damage to the ozone layer, international conflict and other irreversible problems. Environmental lawyer David Bookbinder is more afraid of geoengineering than he is of climate change. "The consequences of geoengineering could happen a lot faster and with much less warning," he said. He said the world lacks the legal or regulatory framework to ensure no single government or private entity takes a risky initiative. At the same time, "there's a clamor for tech solutions." Mark Z. Jacobson, an atmospheric modeler ... said we've already seen the results of several natural experiments. Some forms of air pollution have been cooling the planet by about 1 degree C, but that same pollution also kills millions of people from respiratory illnesses. In 1815, the eruption of Tambora injected so many particles into the atmosphere that 1816 was dubbed "the year without a summer." People died from crop failure and famine.