The sun

March 30 2007

The biosphere filters the radiation that impinges on it. It admits the life-giving rays and repels those that are lethal. But this beneficent play of radiation on the biosphere from external sources will continue to be beneficent only so long as the filter is not put out of action and so long as these sources of radiation remain unvarying; and our sun, like every other in the stellar cosmos, is undergoing change all the time.

Unknown to anyone, one of the filters was being put out of action as Toynbee was writing this c 1973: chemical compounds developed in 1928 for use in refrigerators, called chlorofluorocarbons, had built up in the atmosphere and were destroying the ozone which filters out ultraviolet radiation. A large, seasonal hole in the ozone layer was found over Antarctica in 1981. Arctic lacunae were discovered later. Cfcs have since been banned, but it will take decades for the ozone to return even to its 1981 levels.

Another kind of filter was being added. Heat-trapping carbon dioxide and other greenhouse gases were accumulating in the atmosphere as a result of Man’s large-scale burning of the biosphere’s ex-organic constituents, fossil fuels.

It is conceivable that, at some future date, some of these cosmic changes, either in our sun or (sic) in other stars, might so alter the incidence of the radiation received by our biosphere as to make what is now a biosphere uninhabitable, and, if and when our biosphere comes to be threatened with this disaster, it seems improbable that mankind’s material power will be great enough to be able to counteract a deadly change in the play of cosmic forces.

The sun’s luminosity is increasing. The Earth will cease to be habitable long before the sun enters its Red Giant phase in 4-5 billion years. (Single-cell organisms appeared on the Earth about 4 billion years ago, soon after the Earth was formed, and when the universe was 10 billion years old, so we are about half way, at least, through the story of life on Earth.)

The sun will then expand in size to reach the current position of the Earth’s orbit, but a loss of mass earlier in that phase may have caused the Earth’s orbit to move further out, preventing it from being engulfed. However, most of its atmosphere will have been lost by then and its water will have been boiled away.

Mankind and Mother Earth, OUP, 1976, posthumous

2 Responses to “The sun”

  1. davidderrick Says:

    It seems that the Earth may cease to be inhabitable in as little as 1 billion years.


  2. […] for us, is the – biosphere shall remain habitable until it is made uninhabitable eventually by cosmic forces beyond human […]


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