Carbon dioxide constitutes a vanishingly small fraction of our atmosphere, but punches well above its weight in terms of greenhouse warming. So just how potent is it? The geological record provides clues because, over time, Earth has oscillated between greenhouse and icehouse climates. But reconstructing coincident atmospheric CO_2 concentrations is notoriously difficult. Modelling and proxy calculations are starting to converge on a single picture of atmospheric CO_2 during greenhouse episodes, except for one fly in the ointment: estimates derived from the ratio of carbon isotopes in soil-precipitated carbonates are always higher than those derived from any other source.
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