Dump carbon-absorbing algae into the world’s oceans to pull greenhouse gases out of the air? Inject huge amounts of sulfate aerosols into the stratosphere to block the sun’s warming rays? Not so fast, say scientists and government officials. Meeting at the United Nation Convention on Biodiversity in Bonn, nearly 200 countries have agreed on a moratorium on at least one so-called “geo-engineering” scheme — the proposal to seed the oceans with CO2-absorbing algae. Scientists fear the plan could have unintended consequences, such as making the oceans more acidic. Meanwhile, writing in the journal Science, a team of scientists has given the thumbs-down to another geo-engineering dream: pumping sulfate aerosols into the stratosphere. Some scientists believe that the aerosols would block the sun’s rays and cool the earth, similar to the way massive volcanic eruptions lower the planet’s temperature. But the Science report warns that the release of the sulfate aerosols could have a major unintended consequence: the destruction of stratospheric ozone, which protects the earth from harmful ultraviolet radiation.