Let Us Be Speculative

The greenhouse effect is obviously demonstrated by our neighboring planets. Venus has an atmosphere of almost pure COj. In addition it is covered by white clouds. Its surface temperature is about 430°C. Thus Hquid water cannot be present on its surface. If present in earHer periods, also then with high contents of carbon dioxide, the radiation would have increased the surface temperature to levels at which water has vaporized. Water vapor also is an effective greenhouse gas, so the temperature increased by the combined action of water and carbon dioxide. Venus is thus a nightmare scenario of the danger with the greenhouse effect. 

The planet Mars has a thin atmosphere, a low greenhouse effect and a low surface temperature, -53°C. In the case of Mars there is almost an equilibrium between radiation in and out. 

The earth is different. Since the very first appearance of life, 3-4 billion years ago, the intensity of the sun has increased by about 30%. Why has the surface temperature not increased more than it really has in spite of that This is thought to be due to the fact that photosynthesis has been developed and lowered the carbon dioxide content of the atmosphere. The influence of the greenhouse effect has decreased and allowed increased radiation out This has preserved the temperature. 

The idea that life itself is capable of adjusting the conditions in favor of its further existence is expressed in the Gaia hypothesis [39.9]. Its originator, JaTnes Lovelock, has also calculated that, without life on earth, the atmosphere would consist of 98% COj and 0% O. Our planet would have a surface temperature of300°C. But then of course there would be no human beings present to talk or write about our planet. 

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