The climatic effects of rising CO2 levels will become increasingly significantin the future. Consumption of estimated reserves of fossil fuels in the next two or three centuries will add far more CO2 to the atmosphere than has been emitted to date unless ambitious conservation efforts or technological innovations reduce the influx. Atmospheric CO2 concentrations will increase to levels at least twice and possibly four times the highest amounts measured in the last 800,000 years spanned by ice-core records. This warming will overwhelm natural variations in climate and cause climatic and environmental changes unprecedented in human experience. Atmospheric CO2 levels will eventually decrease as the ocean slowly absorbs much of the excess carbon, but this process will acidify the oceans. After the main pulse of fossil-fuel emissions has passed, atmosphericCO2 values will decrease back toward preindustrial levels, but enough CO2 willremain in the atmosphere to prevent future glaciations for tens of thousands of years.