Massive outbursts of CO2 and other greenhouse gases have acidified the oceans in the geologic past, but equilibrium returned as the oceans stored away excess CO2 in minerals on the seafloor. This time nature may be slow to heal. "Our emissions are huge compared with natural fluxes," Caldeira says. "If you could stop emissions and wait 10,000 years, natural processes would probably take care of most of it." These days we're simply dishing it out faster than the oceans can mop it up.