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Fig. 7.23 The runaway greenhouse effect. This graph shows the relationship between the solar flux received at the top of the atmosphere (relative to the flux currently received by the Earth: So) and the surface temperature Ts. When Ts exceeds about 550 K, the infrared flux emitted by the planet reaches a threshold. This threshold corresponds to an orbital distance of 0.84 AU from the present-day Sun, and also corresponds to the inner boundary of the habitable zone. Beyond this threshold, any increase, even small, in the irradiation leads to a runaway greenhouse effect, which evaporates the whole reserves of water and takes the surface to temperatures above 1500 K. The flux received by present-day Venus is indicated, as is the flux 4 x 109 years ago (early Venus), the flux that corresponds to rapid escape of H, and the temperature/flux zone where liquid water can exist at the surface. The connection between the flux and distance from the Sun is given on the right-hand vertical axis. Note the break in the temperature axis between 600 and 1400 K (After Kasting, 1988)

200 400 600 1400

Surface temperature

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