In a general sense, the “greenhouse effect” is a natural phenomenon where sunlight passes through the atmosphere and warms the Earth’s surface. Infrared energy is then radiated by the Earth; some of which goes back to space. The heating effect is crucial to permitting the existence of life as we know it on Earth. Water vapor (esp. in the form of clouds) plays the major role in this effect. As temperatures increase, the air can hold more water vapor which can perhaps more than double the effect of radiative forcing.
The crude idea in the common popular presentation of the greenhouse effect is that the atmosphere is transparent to sunlight apart from the very significant reflectivity of both clouds and the surface, which heats the Earth’s surface. The surface offsets that heating by radiating in the infrared. The infrared radiation increases with increasing surface temperature, and the temperature adjusts until balance is achieved. If the atmosphere were also transparent to infrared radiation, the infrared radiation produced by an average surface temperature of minus eighteen degrees centigrade would balance the incoming solar radiation (less that amount reflected back to space by clouds). The atmosphere is not transparent in the infrared, however. So the Earth must heat up somewhat more to deliver the same flux of infrared radiation to space. That is what is called the greenhouse effect.
However, the greenhouse gas/global warming issue has to do with the ramifications of the increasing levels of carbon dioxide, methane, and other man-made greenhouse gases in the atmosphere which can severely exacerbate the normal level global warming to where it presents a significant problem. In a major study of the effect of greenhouse gases on global temperatures, V. Ramanathan et al. estimate that CO2 emissions contribute to 66% of global warming over the period 1880-1980.
Water vapor is just the beginning of the issue. Clouds can either exacerbate or mitigate the greenhouse effect depending of their altitude, density, location, etc., etc. In fact, some scientists believe that water vapor could contribute to drying of the air at high altitudes and increased intensity of convection. This is an active research topic. I think that the complexities of the Earth’s biosphere plus the human element is what distinguishes the situation of Earth from that of Mars in this case. Clearly, water vapor plays a significant role, but the topic is a little more complex than that implied in your post.
Related posts:
- Greenhouse Effect in A Greenhouse
- The Greenhouse Effect: Is Venus’ fate our own?
- Debate about the Greenhouse Effect
- Thoughts on the “Greenhouse Effect is Good” Theory
- GreenHouse Effect Solutions
In response to the sentence of the post;
“In a major study of the effect of greenhouse gases on global temperatures, V. Ramanathan et al. estimate that CO2 emissions contribute to 66% of global warming over the period 1880-1980. ”
The warming since 1880 is in large part from the end of the “Little Ice Age”.
In the Midevil warming period the earth was actually warmer than it is now. Then the Little Ice Age dropped world temps significantly. Now we are going back to a warmer period.
Do not forget that we have actually been cooling since 1998 as well.