One of the defining characteristics of climate change is poorly appreciated by most people: the higher temperatures and other effects induced by increasing levels of carbon dioxide in the atmosphere will persist for a very long time. Scientists have long realized that carbon dioxide emitted during the burning of fossil fuels tends to linger in the atmosphere for extended periods, even for centuries. Over the last few years, researchers have calculated that some of the resulting changes to the earth’s climate, including increased temperature, are more persistent still: even if emissions are abruptly ended and carbon dioxide levels gradually drop, the temperature will stubbornly remain elevated for a thousand years or more. The earth’s thermostat is essentially being turned up and there are no readily foreseeable ways to turn it back down; even risky geoengineering schemes would at best offset the higher temperatures only temporarily.