he degree of the greenhouse effect is dependent primarily on the concentration of greenhouse gases in the planetary atmosphere. The deep and carbon dioxide-rich atmosphere of Venus causes a runaway greenhouse effect with surface temperatures hot enough to melt lead, the atmosphere of Earth creates habitable temperatures, and the thin atmosphere of Mars causes a minimal greenhouse effect.
The use of the term runaway greenhouse effect to describe the effect as it occurs on Venus emphasises the interaction of the greenhouse effect with other processes in feedback cycles. Venus is sufficiently strongly heated by the Sun that water is vaporised and so carbon dioxide is not reabsorbed by the planetary crust. As a result, the greenhouse effect has been progressively intensified by positive feedback. On Earth there is a substantial hydrosphere and biosphere which respond to higher temperatures by recycling atmospheric carbon more quickly (in geologic terms; the timescale for the ocean/biosphere to remove a CO2 perturbation is on the order of several hundred years). The presence of liquid water thus limits the increase in the greenhouse effect through negative feedback. This state of affairs is expected to persist for at least hundreds of millions of years, but, ultimately, the warming of an aging Sun will overwhelm this regulatory effect.
The average surface temperature would be −18°C without a greenhouse effect or 72°C with just the greenhouse effect and no convection, but in reality this temperature is closer to 15°C due to convective flow of heat energy within the atmosphere and partly above much of the thermal IR absorbence of the atmosphere. [2]
Recent measurements of carbon dioxide amounts from Mauna Loa observatory show that CO2 has increased from about 313 ppm (parts per million) in 1960 to about 375 ppm in 2005. The current observed amount of CO2 exceeds the geological record of CO2 maxima (~300 ppm) from ice core data (Hansen, J., Climate Change, 61, 269, 2005). This suggests that the CO2 production rate from increased industrial activity (automobile use and fossil fuel generation) and other human activities such as land-use changes has overwhelmed the normal feedback control mechanisms. Global climate model calculations indicate that the elevated CO2 levels are likely to lead to global warming. There has been an observed global average temperature increase of about 0.5oC since 1960 (Science 308, 1431, 2005). There is still some public controversy about the role of human activities and that of CO2 and other greenhouse gas increases for global warming.