human impact on the earth is undeniable. the concentration of carbon dioxide in the atmosphere today is higher than it was at any time during the last 650,000 years. greenhouse gases are released through the combustion of fossil fuels to power our cars, heat our houses and create the electricity we use everyday. the trouble is, even if carbon-dioxide emissions were to cease immediately, the extra gases already added to the atmosphere would linger for centuries, causing sea levels to rise and changing other characteristics of the earth for hundreds of years. earth’s clock does not run backwards, so we cannot undo the damage we have already wrought. take a stance and make a change now, while there is still time to stop runaway global warming. for more information, visit www.thebulletin.org.