Without the sustaining support by the non-condensing greenhouse gases, Earth’s greenhouse effect collapsed as water vapor quickly precipitated from the atmosphere, plunging the model Earth into an icebound state

carbon dioxide is responsible for 80 percent of the radiative forcing that sustains the Earth’s greenhouse effect.

water vapor, although contributing 50 percent of the total greenhouse warming, acts as a feedback process, and as such, cannot by itself uphold the Earth’s greenhouse effect.

Apparently the authors believe that CO2 is a good thing. Nevertheless, this study looks flawed. It doesn’t represent an actual planet Earth.

• 1. CO2 is emitted from volcanoes. It would be impossible to have a CO2 free atmosphere on Earth. In fact, CO2 concentrations were much higher in the past.
• 2. The vast majority of the CO2 caused greenhouse effect occurs from the first few ppm. The effect tails off logarithmically after that. Given 1 and 2, the whole premise of this paper is fatally flawed. It represents a hypothetical situation which could never occur, and is backwards from the evolutionary history of the planet.

• 3. Next problem is that radiative transfer models show that the loss of all CO2 in the tropics causes less than 2% reduction in downwelling longwave radiation at the surface. It doesn’t seem credible that this causes the equator to freeze up.
• 4. The authors claim that all clouds disappear. Did they forget that this also means a lot more more SW radiation reaching the surface? Imagine a body of water in the tropics being warmed by directly overhead sunshine 12 hours a day. Is this water not going to warm, evaporate, and return water vapour to the atmosphere, thus massively increasing the greenhouse effect and warming higher latitudes?
• 5. The claim that water vapour is only responsible for 50% of greenhouse warming is absurd. The number is much higher, particularly when considering that clouds are made of water vapour.

This study is a marketing exercise, not a scientific one. It represents an impossible hypothetical situation, the claims are incorrect, and it tells us nothing about the effect of increases beyond 390 ppm.