A caveat: This is an ongoing attempt to understand relevant physical aspects of climate modelling. So the Reader should not expect “the truth” but what this Author in his sceptical attempt to “know” considers important. There are no ontologic claims but merely epistemic hints.
Let’s start with a quote from The Editors of Encyclopaedia Britannica “The atmosphere allows most of the visible light from the Sun to pass through and reach Earth’s surface. As Earth’s surface is heated by sunlight, it radiates part of this energy back toward space as infrared radiation. This radiation, unlike visible light, tends to be absorbed by the greenhouse gases in the atmosphere, raising its temperature. The heated atmosphere in turn radiates infrared radiation back toward Earth’s surface. (Despite its name, the greenhouse effect is different from the warming in a greenhouse, where panes of glass transmit visible sunlight but hold heat inside the building by trapping warmed air.)”
Almost everybody (with notable exceptions) agrees upon the hypothesis that the presence of certain gases in the air – in particular, water vapour, carbon dioxide (CO2), methane – contributes to a warming of the surface and the lower atmospheric layers of our planet. This is due to the fact that these gases absorb infrared radiation which is emitted by Earth. The radiation is in turn “fed” by the solar radiation absorbed by Earth. Thereby, from all such gases, water vapour has the largest absolute effect.
Despite this agreement there appears to be a controversy about how (much) exactly anthropogenic increments of the CO2 concentration – for instance, by the burning of fossil fuel and coal et cetera – contribute to the aforementioned warming.
Moreover, there does not even seem to be agreement on the question on how to give physical meaning to the term “global warming” because the notion of temperature on a global scale may be difficult to define. Also, the available data may not support the claims and predictions of “global warming” and its effects.
One of the biggest issues is the quantitative effect of a feedback loop which could potentially amplify anthropogenic global warming (AGW): a very small increase of CO2 in the atmosphere, so goes the hypothesis, might be amplified immensely by allowing the air to “carry” more water vapor.
Water vapor is a powerful infrared radiation absorbent. It might heat up the atmosphere even further, thereby allowing even more water vapor in it; and so on and so forth; until such point when a new balance or equilibrium establishes itself between the radiation energy Earth receives from its sun and the part of this radiation emitted back toward space by Earth (including its atmosphere). Depending on the nonlinear amplification this new equilibrium might be characterized by a much higher, nonlinear, increase of the “surface temperature” (or energy per Earth’s surface area or atmospheric volume at low altitudes) as compared to previous lower CO2 levels.
Thereby, even tiny increases of concentrations of CO2 or methane (e.g., from burning fossile fuel, animal herding and an increase in population) due to anthropogenic factors might result in large changes of temperature. In the long run this might result in large changes of climate.
One of the open issues is the magnitude of this thermic coupling of CO2 and methane to water vapour. Another issue is the possible saturation of absorption of radiation: if almost all radiation has already been absorbed by a certain amount of gases, additional increases of their concentration might have little effects on temperature.
For the time being, to quote Axel Bojanowski, “According to chemical experiments, if the amount of CO2 in the air were to double, the temperature would rise by one degree. According to theory, it is water vapor that dangerously increases the greenhouse effect: Warmer air causes more water to evaporate, which would further heat the air as a kind of steam bell. How strong the effect is is a key question in climate research.”
Other driving factors in the converse direction towards lower temperatures are aerosols, condensed water vapor or clouds which increase the albedo – the amount of reflection of solar radiation due to covering part of Earth by a whitish reflective shield. This radiation energy is not absorbed by Earth but emitted back toward space. This might potentially contribute to much lower temperatures – due to the very same nonlinear mechanisms described above. In the extreme this might yield even to a totally glaciated “snowball Earth“.
So, depending on one’s personal inclinations, preferences and convictions, those hypothetical physical models and mechanisms allow room for varied, converse fears of both global warming as well as global cooling.
But surely science should not be driven by fear but deal with facts.
Unfortunately, presently it is very difficult to quantify the confidence level of the computer models and simulations.
Therefore the public emphasis should be on the subjunctive term “might” associated with those assumptions and conjectures. They cannot be taken as scientific matter of fact but as hypothetical conjectures. Even if all the relevant factors have already been taken into account (this is another conjecture) the plasticity of the respective claims and forecasts is high; ranging from almost nil to an increase of a few degrees in temperature for some areas of our planet.
This Author is convinced that the current status and confidence level of our predictions suggest our further attention, but certainly do not justify drastic measures such any kind of taxation or emissions trading.