text: To the extent that human greenhouse-gas emissions are causing dangerous changes in the Earth's climate, the best ways to mitigate those changes can only be determined by the same relentless application of science as we demand for the understanding of climate change itself. That means transparently defining measures of human welfarethe values side of the policy equationand then carefully assessing not only the risks to human welfare of burning fossil fuels but also the benefitsas well the risks and benefits of geoengineering and of restricting people's access to fossil-fuel energy.