Reviews Popular science
Planetary luck? T
here are now more than 770 planets known to orbit other stars in the Milky Way, and estimates suggest there may be as many planets in the universe as there are stars – a staggering 50 sextillion, give or take. This important insight, which has only emerged in the past 15 years, immediately leads to the question: how many of those planets harbour life, or even intelligent life?
The Goldilocks planet: the 4 billion year story of the Earth’s climate
Authors Jan Zalasiewicz and Mark Williams Publisher Oxford
University Press Year 2012 Pages 336 Price £16.99 ISBN 9780199593576
A more anthropocentric version of the same question can be formulated as: just how lucky were we to have a planet that not only turns out to be habitable, but remains habitable for some four billion years, long enough for life to come up with complex cells, then with complex body plans, and finally with complex minds? Is this a one-in- a-million occurrence, or might it be sufficiently unlikely to happen only once per galaxy (400 billion stars on
average), or even only a few times in 50 sextillion tries? Earth is the ‘goldilocks planet’, which turned out just right for life and, importantly, stayed right, unlike one or two of its neighbours. This much is trivial, as we wouldn’t be here if this condition hadn’t been met. This philosophical constraint, the anthropic principle, means that we cannot infer probabilities from the fact that it worked out alright for us. Closer inspection of the history of our planet, from the hellish Hadean to the recent Ice Ages, however, reveals that our climate has been on a wild rollercoaster ride, and survival of our biosphere was by no means a forgone conclusion. We have been extremely lucky on a number of occasions. For instance, when a collision with a Mars-sized object very nearly tore our planet apart, but instead ended up giving us the highly unusual Moon, which helps stabilising our planetary movements, and thus our climate. We were lucky again, when life originated – a process so unlikely and unusual that scientists have so far been unable to replicate it. And then again, several times, when ‘Snowball Earth’ conditions thawed before killing off all life. As mammals, we should also be grateful to whatever wiped out the dinosaurs, because it opened up the field for our ancestors and relatives. One of the most intriguing
streaks of luck is the most recent one, the epoch known as the Holocene. The Pleistocene, a
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relatively cold period defined by the presence of ice caps on both poles – but nothing like earlier snowball events – had wild swings between ice ages and interglacials, and even the interglacials suffered from climate fluctuations that look very scary on graphs. In marked contrast to this preceding epoch, the Holocene not only offered a benign climate for the past 11,600 years, it also provided an amazing level of climate stability not seen since the globally warm periods, hundreds of millions of years ago.
One may wonder whether the rise of agriculture was a symptom of this stable climate rather than a hallmark of human inventiveness. In the previous rollercoaster climates, human populations may well have tried cultivating crops, only to starve as soon as the climate performed one of its violent mood swings. Maybe agriculture could have happened at an earlier or a later time, but what sealed its success was the surprising climate stability that
occurred during the Holocene. Considering this, it is all the more
frustrating that we, as a civilisation, are now overturning the climate conditions that have served us so well for the past 11 millennia. Global carbon dioxide concentrations are already at a level last seen in the Pliocene, an epoch when the Earth had only one ice cap. Pliocene style temperatures are bound to follow. Geologists Zalasiewicz and
Williams describe the history of our climate and its continuous interaction with both biology and geology in a very engaging and readable way. Necessarily, the timescale is logarithmic rather than linear, as the information available on past climate starts from virtually zero and increases the closer we get to the present. In just 100 pages, we zip through more than 4bn years to arrive at the start of the Oligocene, the point some 33m years ago, when the Antarctic ice cap formed. From this point onwards,
Earth’s climate history is reasonably accessible to investigation, not just from sediments but also from ice cores, and a multitude of clever isotope measurements. With the onset of written records, we also get reports of observations, and in the last couple of centuries, we can add measurements of an increasing range of parameters to the mix.
46 Chemistry&Industry • November 2012
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