Between them, the polar regions make up the majority of what is known to scientists as the cryosphere. The term is unnecessary because “the frozen part of the Earth” is just as clear. It is also wrong because the cryosphere is nowhere near spherical. In some parts of the world it is thousands of metres deep, in others completely missing. Once upon a time, however, the term was exactly right.
It seems that 600-700 million years ago, alien astronomers looking this way would have seen “Snowball Earth”, a world gripped almost entirely by ice. First suggested by Brian Harland of Cambridge University in 1964, Snowball Earth would have looked a lot like Europa, one of the satellites of Jupiter, does today, with a surface dominated by pack ice and sea ice even at the Equator. Ice averaging a kilometre deep would have covered the oceans, thicker nearer the poles and thinner nearer the Equator.
In that era, the Sun gave out about 6 percent less energy than it does today, making the Snowball effect more likely. Snowball Earth happened because of “positive feedback” in the climate. If it gets cooler, it snows more. Snow and ice are white. So the more you have of them, the more solar energy the Earth reflects back into space and the cooler the Earth becomes. Things are becoming especially serious when significant areas of the Earth have snow cover that does not disappear over summer but instead remains from one year to the next. This feedback system is called the albedo effect. As we saw in Chapter 1, the albedo of an object tells you how much of the light that arrives there is reflected away and how much is absorbed. Something perfectly white that reflected away all the energy it received would have an albedo of 1, and a perfectly black object that absorbed all the energy that landed on it would have an albedo of 0. The Earth has an albedo of 0.37 today, but Snowball Earth would have been closer to Venus's present-day albedo of 0.65, which makes it the solar system's whitest planet.
One good question is just how the Earth broke out of snowball status once it had got started. The answer seems to be our old friend the green-house effect.
Most of the surface processes we see around us such as erosion would cease almost totally if the Earth was frozen solid. But nothing stops volcanoes erupting. Over time the greenhouse gases they emitted, mainly carbon dioxide, would have built up. In fact, carbon dioxide would have accumulated even faster with no plants around to absorb it. As the green-house gases built up, the amount of heat that the Earths atmosphere retained would have increased. Eventually, snow and ice would have melted across significant areas. Then the feedback would have run in the opposite direction and things would have got warmer at a rapid pace. The thick ice may have melted in just a few centuries, and sea temperatures could have reached 50°C. There are signs today of the rocks that would have been laid down from the carbon-dioxide-rich atmosphere that caused the melting. Eventually, however, the ability of the land and oceans to absorb carbon would have increased. As its concentration in the atmosphere tell, temperatures would have come down to something nearer to what we regard as normal.