(On a related note, the greater transparency would have led to increased temperatures, as more sunlight reached the surface of earth. So, one might conjecture that this atmospheric change was related to the end of the theorized "Snowball Earth" period. However, I am not convinced these changes happened together. The Snowball Earth period appears to have been much earlier in time than when the atmosphere became transparent.)
- Big & Warm Thunderclouds Increasing with Temperature Rises
http://www.livescience.com/20445-pollution-creates-big-warm-thunderclouds.html
"Pollution particles in the air strengthen thunderstorm clouds, causing their anvil-shaped tops to spread out high in the atmosphere and capture heat — especially at night."Though the article did not mention what kinds of pollution might be causing bigger thunderstorm clouds, but carbon dioxide is a considered a major pollutant. Thus, some 250 Ma, when carbon dioxide levels were approximately 15 times higher, you would naturally expect much more cloud cover, it seems.
- Earth's Clouds are Getting Lower as CO2 Levels Rise
http://phys.org/news/2012-02-enasa-satellite-earth-clouds.html
"Earth's clouds got a little lower -- about one percent on average -- during the first decade of this century, finds a new NASA-funded university study based on NASA satellite data. ...In this case, the point I want to make is that the effects of increased CO2 levels on clouds are not well understood yet. If CO2 levels were raised 15 times higher, would there be a blanket of low-lying clouds around the earth? Would the temperature higher in the atmosphere be greatly increased and the temperature at the surface of the earth be decreased, causing the surface of the earth to freeze over? Those are just some questions to be answered as scientists learn more. We don't really know.
A consistent reduction in cloud height would allow Earth to cool to space more efficiently, reducing the surface temperature of the planet and potentially slowing the effects of global warming."
During the Snowball Earth period, I don't think we can assume that there were relatively few clouds. The freezing temperatures might have been partly because of too much cloud cover. Unfortunately, we don't have any direct way to determine the amount of cloud cover that existed in the distant past.
- Climate Forecasting: A Break in the Clouds
http://www.scientificamerican.com/article.cfm?id=climate-forecasting-a-break-in-the-clouds
"Together, those clouds and the fine particles, which are known as aerosols, do more than just obscure the planet's surface. By reflecting, absorbing and emitting radiation, they have a major role in setting Earth's temperature and have proved maddeningly difficult to simulate in atmospheric models. For decades, they have been the biggest sources of uncertainty in forecasts of future climate.We see from this article from May 2012 that understanding how clouds affect temperatures is also not well understood. So, the bottom line here is that increased CO2 levels may increase temperatures at lower levels, but who really knows what might happen if you increased CO2 levels by a factor of 10 or 15? Would there be a great deal more clouds? How would those clouds affect temperatures?
But researchers say they are beginning to turn a corner in simulating clouds and aerosols. In recent months, climate scientists have started rolling out initial results from the newest generation of models, which represent atmospheric chemistry and microphysics in much more sophisticated ways than previous incarnations. These models allow clouds and aerosols to evolve as they interact with each other and respond to factors such as temperature, relative humidity and air currents. ..."
