What makes a cold planet?Nicely put together and gorgeously illustrated, a set of essays about our knowledge of ice ages (at least as of 2010; things may have moved on since then). I had not realised that the current period of ice ages (since 2.6 million years ago) is actually a rather rare situation in global history; in general the Earth has had a warmer temperature than now, rising to 8°C above today's average at about 55.5 million years ago. The book looks at how continental drift and changing ocean currents have created (and sometimes failed to create) the conditions for an ice age to happen, and also at the impact of ice ages on human prehistory, going quite deep into hominins and Neanderthals. All quite fascinating. You can get it here.
To create an ice age the first thing you need is continents at the poles. Geologists have run simple climate models to demonstrate this idea, showing that if you put all the continents around the Equator - the so-called tropical ring world - the temperature gradient between the poles and the Equator is about 30°C (54°F). This is due to a trick of both the atmosphere and the oceans. The fundamental rule of climate is that hot air rises and cold air drops; this is why in the tropics the land heats up and the air rises, resulting in towering cloud formations developing as the moisture in the air cools and condenses. At the poles it is cold so the opposite happens - the air falls, pushing outwards away from the pole as it hits the ground. So although ice forms at the pole when the sea water freezes, this ice is blown away from the pole towards warmerwater where it melts. This maintains the balance and prevents the temperature of the pole falling below 0°C (32°F). If, however, there is land on the pole or even around the pole, ice can form permanently, and the Equator-pole temperature gradient is much greater - over 65°C (117°F). This is exactly what we have today in the Southern Hemisphere. In contrast, if you consider the Northern Hemisphere, the continents are not actually positioned over the pole, but surround it, so instead of a single huge ice sheet as we have on Antarctica, there is a smaller one on Greenland, and the continents act like a fence keeping all the sea ice in the Arctic Ocean. The Equator-pole temperature gradient of the Northern Hemisphere, therefore, is somewhere between the extremes of an ice-locked Antarctica and a land-free pole: about 50°C (90°F).
This was the shortest book on my unread pile acquired in 2010, after the Doctor Who comic The Flood which I couldn't find. Next on that list (if I still can't find The Flood) is Fair Trade: The Challenges of Transforming Globalization, by Laura Raynolds, Douglas Murray and John Wilkinson.
