Heating up the Earth

If, on a clear, sunny day you hold up a one-metre-square, flat piece of black cardboard to the sun, the cardboard will rapidly become very hot.

That is because it is catching solar energy at a rate of several hundred Watts (the energy output of several 100-Watt light bulbs).

If you were to try the same experiment from orbit, above the atmosphere, you would intercept over 1,400 Watts. The side of the Earth facing the sun is receiving around 18E16 (18 followed by 16 zeroes) Watts.

Almost 40% of that gets reflected straight back out into space, leaving about 11E16 Watts to be absorbed by the Earth. This warms our planet, and as it warms it radiates energy as infrared. Eventually input and output balance and the temperature stops rising.

However, when we calculate that temperature, we end up with a world with an average temperature way below freezing. There is an additional factor at work — the greenhouse effect.

If we impede the efficiency with which our planet reradiates energy, then the equilibrium temperature will be higher. The greenhouse effect is what makes our planet inhabitable. This raises a serious issue.

Thanks to years of study and with billions of examples to look at, we now have a pretty good idea of how stars work. They produce energy through nuclear fusion in their cores. Over time the cores become loaded with the waste products of energy production, so the star develops a core of material that is producing no energy, with energy production taking place in a skin surrounding that inert core.

This causes the star to brighten slowly during its mature life, the period between its birth and the point where it starts to run out of fuel.

Our sun was born along with the Earth and other bodies in our solar system about 4.5 billion years ago, and since then it has steadily brightened by some 30%. That would suggest that until very recently, the Earth should have been frozen solid.

However, we know that it wasn't.

In the warm, shallow waters of Shark Bay, Australia, and at some other tropical locations around the world, we find strange stony, mushroom-shaped structures. These are stromatolites.

Colonies of bacteria secrete slime. This catches sand and grit, forming a layer. The bacteria work to the surface and make more slime, which catches more sand and grit. The result is a stony structure composed of onion-like layers which fossilizes well.

The fossil stromatolites look exactly like the stony stromatolite structures we find today. The oldest ones found so far are in rocks about 3.5 billion years old, which means life got going on Earth almost as soon as the Earth had cooled after forming.

Another point is that there must have been warm, liquid oceans back then despite the sun being substantially dimmer than it is today. We are confident in our conclusions about the sun, so the explanation comes from another direction.

It turns out that the atmosphere of the young Earth was substantially different from what it is today. It had no oxygen, but lots of methane and other greenhouse gases.

The much more intense greenhouse effect made our planet warm enough for liquid oceans and life - simple plants and bacteria - to get going.

Through photosynthesis they removed greenhouse gases and released oxygen, and intriguingly, they did this at the right rate to compensate for the brightening sun.

Some years ago, scientist James Lovelock proposed the Gaia Hypothesis, named after the Earth Goddess, where, through evolutionary processes, organisms do not just adapt to their environment, providing conditions change slowly enough, they can also change that environment to keep it more suitable to them.

Ironically, our species seems, at least so far, intent on making its environment as unsuitable as possible.

• Jupiter and Saturn lie low in the southern sky after dark.
• The moon will be full on the 13th.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.