Researchers are planning an experiment this year to pipe liquid carbon dioxide into the Pacific Ocean at a depth of at least 1,000 feet to find out whether the ocean could serve as a massive storage bin for CO2, a greenhouse gas. Scientists say that putting the CO2 under high pressure as a liquid could keep it below the surface for several centuries, giving humans leeway to continue using fossil fuels until cleaner, more efficient technologies are developed. So far, environmentalists oppose the CO2 storage plan, which they have labeled “carbon dumping,” saying it could harm marine life and encourage even greater greenhouse gas emissions.