Can Increased Atmospheric CO₂ Levels Trigger a Runaway Greenhouse?

Recent one-dimensional (globally averaged) climate model calculations by Goldblatt et al. (2013) suggest that increased atmospheric CO₂ could conceivably trigger a runaway greenhouse on present Earth if CO₂ concentrations were approximately 100 times higher than they are today. The new prediction runs contrary to previous calculations by Kasting and Ackerman (1986), which indicated that CO₂ increases could not trigger a runaway, even at Venus-like CO₂ concentrations. Goldblatt et al. argued that this different behavior is a consequence of updated absorption coefficients for H₂O that make a runaway more likely. Here, we use a 1-D climate model with similar, up-to-date absorption coefficients, but employ a different methodology, to show that the older result is probably still valid, although our model nearly runs away at ∼12 preindustrial atmospheric levels of CO₂ when we use the most alarmist assumptions possible. However, we argue that Earth's real climate is probably stable given more realistic assumptions, although 3-D climate models will be required to verify this result. Potential CO₂ increases from fossil fuel burning are somewhat smaller than this, 10-fold or less, but such increases could still cause sufficient warming to make much of the planet uninhabitable by humans. Key Words: Planetary atmospheres—Habitability—Carbon dioxide—Environment. Astrobiology 14, 714–731.