The Great Cloud Cover is a planet-wide cloud cover that consumes the entire of Vexillium, theorised to help explain the lower-than-modelled surface temperatures. However, recent work by climatologists suggests the suggested level of cloud cover would not only plunge the planet into a dark, freezing ice-age, but would, perversely, require extremely high temperatures to sustain.
Theoretical origins EditThe theory of the Great Cloud Cover originated in the late 90s, as being necessary to explain climatological assertions of "low" temperatures on the planet's surface. The Armatirion Space Agency later published a graphic of Vexillium from space showing the planet entirely covered in cloud with only whispy gaps over Cruisana. It is not known whether this was a computer-generated simulation of theory or an actual photograph.
Theory debunked Edit
Effect of cloud cover on climate EditClouds are both indicative of a planet's climate and a cause for it. Firstly, cloud is formed from water evaporation. In order for a high level of cloud to exist, as it does at the tropics, there must also be sunlight to evaporate the water into vapour that rises and collects in the upper atmosphere. Secondly, clouds have a high reflectivity (albedo), as evidenced by the fact that they appear coloured white. This reflectivity means that they act as an insulator, reflecting sunlight.
An atmospheric energy balance states that the amount of cloud cover will reflect sunlight and therefore reduce the amount of energy hitting the planetary surface, which warms it, but that the amount of cloud cover is indeed produced BY that same amount of sunlight striking the oceanic surface.
Converse to expectations, a hot, humid surface Edit
Should the amount of sunlight striking the planetary surface be severely restricted by heavy cloud cover, as suggested by the "Great Cloud Cover" Theory, then the amount of energy sustaining that same cloud cover through oceanic evaporation could only be sustained by high surface temperatures on the planet.
This is, in fact, the case on the solar system's second planet (closest to Sol), where the little sunlight that actually reaches the planetary surface is sufficiently intense (by virtue of being so close to the sun) that it maintains a surface temperature in which all surface water is evaporated. Conversely, a planet's surface temperature could be maintained by geothermal activity
Converse to the theory, the Great Cloud Cover-ed Vexillium would be a frigid planet that would quickly thaw when incoming sunlight, and thus planetary surface temperatures, would be entirely insufficient to sustain the high-cloud cover density without extremely high-levels of geothermal input. Were those same levels of geothermal input attained, the atmosphere would consequently be highly sulfrus and unlikely to be habitable by human life.
A modified GCC theory Edit
Critics to such theory debunking retort that it was never the theory's assertion that the planet was "covered", only that Vexillium had a higher level of cloud cover than the theoretical model for "Earth". Yet, according to the atmospheric energy balance, this higher level of cloud cover would have to be sustained by higher surface temperatures, even despite the amount of reflected sunlight.
In other words, the increase in sunlight reflected by the higher level of cloud cover would result in lower proportional energy input to the surface, and thus less cloud. Therefore, for this theory to be sustainable, energy input intensity from Sol, that is, the Watts per square-metre, would need to be higher. This can be achieved either by having the planet closer to the energy source, Sol, or having Sol output a higher level of energy than theoretical-Earth's solar equivalent.
Either way, the modified GCC theory would still necessitate higher surface temperatures and therefore would act against the objectives of the theory.
A "tropics" age Edit
This, of course, may also be explained by suggesting that the planet is, in fact, in the middle of a "Tropics" Age, the inverse to an "Ice Age", where surface temperature and humidity is increased (and thus cloud cover) by a cyclical variations in planetary climate.
Of course, the basic physics that dictate that a certain amount of energy, and thereby temperature, is required to create increased cloud cover (compared to the Earth model) and that the mean surface temperature and humidity would still be higher than on the Earth model, counteracting the goals of the GCC theory.
Vexillium land proportion and its effects on planetary energy input Edit
Despite the apparent setback of the modified GCC theory, Vexillium would have a higher cloud cover, all-things being equal, compared to the Earth climate model by virtue of the planet's higher oceanic proportion.
In a recent paper, Dr Hubert Bottonsworther of the University of Luka, posited that the ratio of land-to-sea on Vexillium is more heavily biased toward sea than on the Earth model, and that, due to oceans' lower reflectivity (albedo) than land, the amount of energy absorbed by the planetary surface would be higher on Vexillium and thus the level of cloud cover (as well as mean surface temperature and humidity) would be slightly higher.
Yet, the paper also suggests that the effect might be substantially negated or minimal due to the majority of Vexillium's land mass being heavily biased toward the equatorial region, whereas on the Earth model it is biased heavily toward the poles. Since sunlight strikes, on average, at the equator of a planet, the effect could be entirely removed.
Alternative Theories Edit
Were the GCC, or a scaled-down version, in fact Vexillium's experience, the planet would be hot, humid and possibly also having a poisonous atmosphere. The low proportion of sunlight reaching the planet surface would be offset by a planet receiving a much higher sunlight concentration, either by a stronger solar size or being much closer to the source, and thereby negating the likelihood of life emerging on the planet.
With the GCC theoretically unlikely, necessitating a Vexillium of very high surface temperatures, contrary to the goal of the GCC theory, the explanation of Vexillium's lower-than-Earth-modelled temperatures remains open. Several intriguing possibilities remain, not least the suggestion Vexillium's own topography would sustain a cooler yet less ice age-ish climate.
Effects of a colder planet Edit
A recently-published paper by Professor Ihava Tioree set about modelling Vexillium in an ice age with the goal of understanding whether this was the cause for the planet's lower temperatures and compare the model's results with experiences. The results were surprisingly more mild than previous modellings performed on the Earth model.
Since Ice Sheet formation is much faster on land than at sea, under the Earth model, the drop in temperature results in the formation are large ice sheets across the northern hemisphere, extending as far south as 50°S, due to the large bodies of land close to the poles. Conversely, the extent of ice sheet formation on Vexillium would be quite minimal since little land exists at the poles.
Ice has a high albedo, or sunlight reflectivity, which means that the more ice, the more sunlight is reflected and thus the lower the temperature can both remain. The Earth model found this, with the ice age extending for millenia before conditions sufficiently changed to allow a thawing.
A secondary consequence is that the lower temperatures means less cloud cover, which means less rainfall, which means higher levels of deserfication. An ice age Earth was found to have less forests, which have a low albedo, and more grasslands, which have a high albedo. Consequently, the planet's vegetation changes also raised the albedo, the reflectivity.
Finally, the model found that a higher proportion of land, which has a higher albedo than the oceans, meant that more sunlight energy overall would be reflected rather than absorbed.
Conversely, Professor Tioree's models of Vexillium found the planet's lack of land at the poles prohibited extensive ice sheet formation, and thus substantial raising of planetary albedo. Any ice age, he concluded, would be mercifully short, and thus Vexillium's temperature variations over tens of thousands of years would be far less dramatic than on Earth, and far more stabilised.
Secondly, he found that the higher proportion of surface water on Vexillium would result in overall higher humidity than Earth. Consequently, Vexillium under the same temperature conditions would be more humid, having higher forest coverage especially at the tropics, and thus raising temperatures further (forests have a lower albedo than plains). Consequently, to have equitable forest coverage and cloud coverage, the planet Earth would need higher temperatures, while Vexillium would be cooler.
Consequently, there would be a minimal increase in Vexillium's albedo, reflectivity of sunlight, due to a lower temperature compared to Earth and thus any momentary change in mean surface temperature would be fleeting at best and not result in wide temperature differences. Furthermore, the larger water proportion would mean a more stabilised temperature.
None of which would explain why Vexillium would be cooler, Professor Tioree concluded, but it would suggest that despite lower temperatures the planet would be still greener than Earth and would not succumb to the same ice age freeze.
Scientific theory suggests climate cloud cover levels and ice sheet coverage of a planet are symptoms of climate rather than causes for it. A high level of cloud will be indicative of a hot planet, whereas lower mean surface temperatures will cause the opposite to occur, including deserfication, a reduction in the forest coverage of the planet's surface. And thus, were a cool planet with a high-level of cloud cover to exist, the low temperature and lack of sunlight would mean less evaporation and thus the cloud cover would quickly disperse.
Vexillium's low mean surface temperature is perhaps better explained as either temporary (a mini-ice age) or the result of lower solar energy input due to being further from (compared to the Earth model) the solar source or having a solar source slightly weaker than modelled. The variance, however, would be tiny compared to the Earth model.