There are several major uncertainties that come into play when considering the increase in greenhouse gases. One of these factors is the impact of the Sunís changing brightness. The increase of greenhouse gases in the atmosphere since the industrial revolution (carbon dioxide CO2, methane CH4, nitrogen oxide N2O, various CFCís) has had roughly the same effect as increasing the energy received from the Sun at the surface of Earth by about one percent. However, there are indications that over time periods of a century the Sun does become brighter or dimmer by 0.5% quite independently of what happens on Earth. In fact, there is evidence that the Sunís changing brightness may have contributed to part of the warming of the last 150 years. (We will learn more about the possible role of the Sunís changing brightness other lessons.)
Another source of uncertainty is the impact of water vapor. Although the physics of the greenhouse effect are relatively simple, the response resulting from an increase in water vapor in the air complicates matters greatly. The true magnitude of global warming depends on the calculated amplification that water vapor will have on the warming, and this can be greatly moderated (or enhanced!) by changes in the formation of clouds, requiring a consideration of changes in the entire water cycle. (The role of water in the climate cycle is expanded upon in another lesson)
When all the other responses of the climate system to greenhouse warming are considered, the problem becomes enormously complex, because right away all kinds of poorly understood dynamics enter the picture: cloud formation, snowfall, precipitation patterns and soil moisture, vegetation response, and ocean circulation. The really big unknowns have to do with the possibility of strong positive feedback from threshold effects: a decrease of the ability of a warming ocean to take up heat and carbon dioxide; methane addition from permafrost regions or from the sea floor; disappearance of snow cover and sea ice (that is, a change in the albedo), and many other such possibilities. Such threshold effects are elusive because we try to suppress them in computer models, to keep the models from going into odd directions. The real Earth may not be so accommodating.