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Gases trapped in polar ice provide our most direct record of the changes in greenhouse gas levels during the past 150,000 years. The best documented trace-gas records are for [CO.sub.2] and [CH.sub.4]. The measurements corresponding to the industrial period document the recent changes in growth rate. The variability observed over the last 1000 years constrains the possible feedbacks of a climate change on the trace gases under similar conditions as exist today. Changes in the levels of greenhouse gases during the glacial-interglacial cycle overall paralleled, at least at high southern latitudes, changes in temperature; this relation suggests that greenhouse gases play an important role as an amplifier of the initial orbital forcing of Earth's climate and also helps to assess the feedbacks on the biogeochemical cycles in a climate system in which the components are changing at different rates., Man has caused large increases in atmospheric concentrations of [CO.sub.2], [CH.sub.4], and [N.sub.2]O since 1800, and of chlorofluorocarbons (CFCs) since 1950. We urgently need to understand how sensitive Earth's climate [...]