evidence: Global warming is the rise in average global surface temperature caused primarily by the build-up of human-produced greenhouses gases, mostly carbon dioxide and methane, which trap heat in the lower levels of the atmosphere. [...] Science suggests that to mitigate the human contribution to global warming, we should reduce carbon dioxide and other greenhouse gas emissions. Because some additional warming is inevitable—even if we achieve significant greenhouse gas reductions quickly—we should make plans to adapt to coming climate change. If we are unable to control emissions and/or adapt to unavoidable changes quickly enough, a carefully selected geoengineering strategy could conceivably provide an emergency stopgap to slow global warming. As yet, however, several of the strategies being discussed are very risky and unproven. [...] This pattern (statisticians call it a "right-skewed distribution") suggests that if carbon dioxide concentrations double, the probability of very large increases in temperature is greater than the probability of very small increases.