"The climate is a complex system," said Silvia Ribeiro, an organizer of the petition, in an accompanying statement. "Manipulating climate in one place could have grave environmental, social and economic impacts on countries and peoples that had no say on the issue. Scientists estimate that blasting particles into the stratosphere could alter monsoon and wind patterns and put at risk the food and water sources for two billion people."
The petitioners cited the eruption of the Puyehue volcano in Chile this month. Like Pinatubo, it may have a cooling effect -- but they said it also disrupted air travel for thousands of miles around it.
"If we could experiment with the atmosphere and literally play God, it's very tempting to a scientist," said Richard Odingo, a Kenyan earth scientist who attended a meeting on geoengineering in April. "But I worry."
Nothing is going to happen soon; most scientists agree none of the ideas proposed so far are yet possible. But people don't even know very much about them, which is why Princeton's Oppenheimer says a conference on geoengineering isn't such a bad idea.
"I never want to see it used," he said, "but I think we should be talking about it."
The Associated Press contributed to this report.