Halocarbons like CFCs and their replacements, HCFCs and HFCs, are greenhouse gases that contribute to global warming. As CFC production decreases, emissions of their substitutes are expected to rise. A model shows that halocarbons will cause 4-10% of total greenhouse warming by 2100. But if substitutes grow unchecked, halocarbon production could increase eight-fold, doubling their impact on warming.