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No 51 - December 2006

Solar global warming

In his book Gaia - A New Look at Life on Earth, James Lovelock observed that, over the last 3.5 billion years, the Sun's output has increased by 25 per cent (1). Despite this, Earth has maintained a fairly constant temperature of between 10 to 20 °C. It is surprising facts like this that underpin his Gaia hypothesis - somehow the Gaia system has managed to counteract increasing heat from the Sun.

Lovelock told me that few astronomers writing about stellar evolution consider the dull middle ages of stars. Consequently, few mention this solar warming. An exception was Nigel Henbest, who observed that 'the Sun is gradually brightening, and is now shining about half as brightly again as it was in the early history of the solar system'. He also noted that, despite this, Earth's average temperature has remained 'between the boiling point and freezing point of water'. His explanation for this was that the amount of CO2 in the atmosphere has 'depleted' (2). Henbest was writing about the time of the publication of Lovelock's Gaia book, and so probably did not realize that he was describing a Gaia activity.


But why is the Sun getting hotter? Lovelock told me that it was because, like the Earth, the Sun suffers its own global warming from a greenhouse gas. As hydrogen is fused in the core of the Sun, helium is produced. While most of this helium accumulates at the centre of the Sun, some escapes to act like a blanket and causes the core region to get hotter, so increasing the rate of the fusion reaction. However, according to Kasting, the accumulation of helium at the core causes the latter to contract and heat up, thereby making the nuclear fusion reactions proceed faster (3).

The gradual increase in the Sun's luminosity (S) is usually calculated from Gough's formula:

S = S0 / [1 + 0.4 t/t0]

Where S0 = present luminosity; t0 = 4.6; and t = time in billion years before present (4). The result is as shown in the table, where the present S is taken as 1.000.

This shows that solar luminosity has increased by 40 per cent since the Earth formed and by 30 per cent since the beginning of life (~3.5 billion years BP). The data also show that the rate of solar warming has been increasing by about 3.7 per cent per half-billion years.

The fact that the Sun was fainter in the distant past led to what is called 'The faint young Sun problem/paradox', the problem being to explain how, with a faint Sun, the Archean climates of Earth and Mars were so mild, or even warmer than today. The answer appears to be that a higher concentration of greenhouse gases (carbon dioxide and methane) was responsible. Subsequently, with the rise of oxygen about 2.3 billion years ago, a decrease in these greenhouse gases led to the paleoproterozoic glaciations, including 'Snowball Earth', when almost the whole planet was a sheet of ice. Glaciation increases Earth's albedo, reflecting more heat back into space. It also lowers sea level, so providing more land area for plants, which can absorb more CO2.

According to Lovelock, the Sun's heat was ideal for life about 2 billion years ago. Since then, although it has become progressively too hot, Earth has maintained an equable climate. Because plants produced the oxygen that led to glaciation, it can be argued that this was Gaia at work. However it seems to have overdone the cooling, nearly extinguishing all life in a planetary glaciation.

Does this mean that the present warming of Earth is partly due to solar warming? Anthropogenic warming is only about a century old, over which time the increase in solar luminosity has been only about 0.000008 per cent. Nevertheless variation in the Earth's orbit and perhaps minor changes in the solar output, on a shorter timescale than the gradual warming, may be partly responsible.

It does mean that we are frustrating Gaia's attempt to cool the planet in the face of increasing insolation. Left alone, Gaia was probably determined to bring about increased glaciation. Whatever we do about global warming, we should take note that the Sun is going to continue increasing its output. In an earlier article (5), I explained how a solar shield is one of the few ways available to cool the planet. Such a shield will be needed eventually to counter the Sun's own global warming. Left alone, Gaia would eventually have to shift to a hotter world.

Notes and references:

  1. I refer to the OUP ed. of 2000, but it was first published in 1979. Unfortunately, Lovelock went on to claim that the Sun's output was 30 % less 3.5 billion years ago. When I pointed out this error (it was 20 % less), he was gracious enough to acknowledge his mistake and told me that I was the first person to spot the mistake in the 27 years since first publication. I noticed similar errors in his latest book (The Revenge of Gaia). When I pointed them out, he was amazed and explained that 'six otherwise good scientists have already reviewed the book without noticing these errors'. I have since pointed the need for further corrections on the matter of the relative strength of the Sun's output.
  2. The Exploding Universe, 1979
  3. James F. Kasting (2005): 'Methane and climate during the Precambrian era', Precambrian Research, 137, 119-129
  4. D.O. Gough (1981): 'Solar interior structure and luminosity variations', Solar Physics, 74, 21-34
  5. Steuart Campbell (2005): 'Cooling the Earth', ASE Journal, 48, 3-6

Steuart Campbell


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Solar global warming

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SAW 2006

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