Monday, May 17, 2010

Global Cooling and the Rise of Humans

Here is an interesting bit from a post on Nigel Calder's blog Calder's Updates:
The CO2 folk are flummoxed. In the current issue of Science (14 May) William F. Ruddiman of the University of Virginia wrings his hands over the mismatch between unchanging carbon dioxide levels and the drastically cooling climate over the past 20 million years. “Major glaciations began in the Northern Hemisphere around 2.75 million years ago, after a long prior interval of climatic cooling,” Ruddiman says, “… but our understanding of the driving forces behind the cooling remains incomplete.”

For Henrik Svensmark and me, an explanation for that big freeze of 2.75 million years ago is the “jewel in the crown” of climate history, because of its importance for the subsequent origin of the first human beings.


Cosmic scenarios told how the Earth’s climate and our history as living creatures marched to the drums of stellar explosions, with the chorus of cosmic rays casting their spells over our ancestors’ DNA. The ‘jewel in the crown’, as it was called in Chapter 7, would be a cosmic-ray explanation for the cooling 2.75 million years ago that provoked the loss of African forests and the rise of tool-making and meat-eating bipeds.

… The team at the Technological University of Munich, who discovered the traces of a nearby supernova in material from the Pacific floor, helpfully reverted to their original date for the event, around 2.8 million years ago. That age was what first inspired the Munich team to propose a possible link between cosmic rays, the cooling climate and human evolution. They volunteered the suggestion back in 2004 … before this book was even conceived.

Secondly, it turned out that the dating of individual events might not, after all, matter very much for tracing the general climatic connections at that time. In 2007 Svensmark thought again about the predicament of the Sun and Earth during their present cruise among the explosive stars of Gould’s Belt. As mentioned in Chapter 7, the stellar explosions have blown a Local Bubble of hot, thin gas that contrasts with cooler and denser interstellar gas beyond it.

The shell of the Local Bubble contains shock waves and strong magnetic fields, like a gigantic version of those at the edge of the Sun’s own protective bubble, the heliosphere. As a result the shell tends to repel cosmic rays arriving from the Galaxy beyond. But it also turns back many of the cosmic rays generated by any supernova occurring locally, when they try to escape into the wider cosmos. So the Bubble is a bottle of cosmic rays. It is a chilly place for our planet to be, almost regardless of exactly when or where individual stars have blown up.

With about half a dozen giant stars dying explosively every million years, Svensmark estimated that the intensity of bottled-up cosmic rays is generally higher by perhaps 20 per cent, than in the surrounding region of the Galaxy. What matters most for the Earth’s climate, in this interpretation, is the timetable of the Local Bubble’s origin and growth, and how and when the Sun and Earth first encountered it. By making simple assumptions on those points, Svensmark found he could match the history of the Earth’s cosmic experiences over the past 5 million years to the climatic record surprisingly well.

By bottling up cosmic rays from nearby exploding stars, the Local Bubble may be the chief governor of the Earth’s present climate. (Based on a NASA illustration)

A warm spell lasting from 4.5 to 4 million years ago seemed to signal exactly when the Sun and Earth ran through the swelling shell of the Bubble. There the cosmic rays would have been fewer than either outside or inside the shell. Once inside, the Earth felt the intensifying bombardment by cosmic rays originating within the Local Bubble. The fastest cooling occurred around 2.75 million years ago in this reckoning, exactly when ice was spreading in the North Atlantic and Africa began to dry out, setting the stage for human evolution. In this perspective the Munich supernova seems to have reinforced a general trend.

The rate of cooling then slowed down, by Svensmark’s calculation, still in keeping with the geological evidence, as the climate came into equilibrium with the bottled-up cosmic radiation. That seems to be the situation our planet is in now, with the long-term icehouse conditions getting no worse. The Local Bubble has evolved into a chimney, releasing hot gas into the Galaxy’s halo, and as a result the cosmic-ray count may decline in the future, and the icehouse climate relent a little.

‘The good agreement with the climate history came out almost too easily’, Svensmark remarked. After giving a seminar on the subject, he left the local cosmic theatre to re-examine the grander scenery of the Galaxy at large.
Go read the original to see the many illustrations and get the links to subsidiary material.

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