Averaged across the Earth’s surface, the temperature is close to 15 °C. In the absence of reflectors on one hand and greenhouse gases on the other, its temperature would be only ~5 °C. Important to our discussions is that clouds, ice and other reflectors reduce the Earth’s temperature by about 25 °C and H2O vapour, CO2 and other greenhouse gases increase it by about 35 °C (see Fig. 1.1). As both reflectivity and greenhouse strength have varied over the course of geologic time, so has the Earth’s surface temperature.

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Before delving into the evidence documenting these...

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