If the sun was extinguished or there was a permanent worldwide eclipse, how long would it take for us all to freeze to death, and is there anything we could do to stay alive?
鈥� A back-of-the-envelope calculation suggests the whole Earth might freeze solid within 45 days, radiating away its thermal energy according to the Stefan-Boltzmann law, which relates energy loss of a body to its temperature. My calculations assume the vast bulk of the Earth鈥檚 captured solar energy is stored in the oceans, which have an average temperature of 15 掳C down to a depth of 35 metres. Energy carried by water at greater depths doesn鈥檛 count because it would rapidly become isolated from the surface by ice floes.
With its smaller heat capacity, the land would freeze much more quickly than the oceans. Air over relatively warm oceans would rise, pulling in cold air from the continents. This would chill the surface waters and might increase the circulation of water, exposing it to the chilling, perpetual night.
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Interestingly, the volcanic dust thrown up by the eruption of the Tambora volcano in 1815 acted both for and against cooling. The dust blocked out the sun, but it also reduced the escape of thermal radiation from the Earth by dint of a greenhouse effect. Sunlight dimmed by 25 per cent for a while, leading to a dip in global temperatures of 0.7 掳C in 1816. But the fall in temperature was small despite a big reduction in sunlight, suggesting that the Earth might take longer to freeze than 45 days. Indeed, freezing may well be delayed further by the natural greenhouse effect that comes with our atmosphere and the thermal inertia of our oceans.
Nevertheless, Earth would still be able to support a colony of humans. There would still be plenty of energy in the form of fossil and nuclear fuels, and geothermal heat mines. But without plants to replenish our oxygen supply, it would quickly run out, so we would need to build biospheres with artificial light for plant photosynthesis.
Thankfully, switching off the sun is an experiment the Earth will not undergo for another 5 billion years.
Mike Follows, Willenhall, West Midlands, UK
鈥� You can estimate how long it would take us to freeze by extrapolating from the rate of cooling that happens overnight. In areas with clear skies, the temperature can drop to freezing in less than 12 hours. In places with heavy cloud cover, water vapour traps infrared radiation before slowing down the rate at which it radiates away into space, so cooling takes much longer. Here, the temperature drops by perhaps 5 掳C in 12 hours. However, without the thermal energy of the sun constantly evaporating water, this insulating cloud cover would quickly disappear. It is likely that most parts of the Earth鈥檚 surface would be frozen within a few days. The only exception would be near the coastline, where it might take a few weeks because of the amount of heat stored in the oceans.
鈥淚t鈥檚 likely that most parts of the Earth鈥檚 surface would be frozen within a few days鈥�
Could we stop this? Perhaps we could quickly burn the world鈥檚 forests to release large amounts of carbon dioxide to help trap infrared thermal radiation. However, with only fossil fuels, nuclear and geothermal energy left to rely on, we鈥檇 still freeze quickly. And if we didn鈥檛, we鈥檇 soon run out of food and oxygen.
Simon Iveson, UPN Veteran University, Yogyakarta, Indonesia
鈥� The sun can鈥檛 be turned off like a light bulb. It glows because its surface is about 5500 掳C and is heated by the nuclear fusion inside its core which is even hotter 鈥� about 15,000,000 掳C. Even if fusion in the core could be switched off suddenly, the sun would continue to radiate light just as the heating element on an electric stove gives off heat for a time after you switch it off.
Obviously, the sun is bigger and hotter than a stove, so would continue to radiate heat and light for a long time. In addition, the energy produced in the core of the sun takes time to work its way out 鈥� millions of years if you track the energy by following the paths of individual photons. The sun would cool a tiny bit each year, but as the sun cools it would contract, releasing gravitational energy that would heat it and offset some of the cooling. That鈥檚 how stars continue shining. Suffice it to say that it would take many millions of years before our descendants even noticed.
However, if the sun suddenly vanished, the Earth would cool quickly. Unprotected people would start freezing in days, but they could survive much longer if they went down into deep mines, warmed by the Earth鈥檚 heat.
Jeff Hecht, New 杏吧原创 contributing editor, Auburndale, Massachusetts, US
鈥� Even if the sun went out, ecosystems around hydrothermal vents along the Earth鈥檚 mid-ocean ridges would continue to chemosynthesise using geothermal energy for a few thousand million years. So, business as usual for tube worms.
Allan Mann, Alnwick, Northumberland, UK