2.75 million years ago

Climate Change – News and Comments and Updating The Chilling Stars

Why the big freeze 2.75 million years ago?

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.Here’s a picture from The Chilling Stars of one of the earliest known stone tools, which were made less than 200,000 years after the big freeze began.

But if you want CO₂ to be the big driver of climate change, as Ruddiman evidently does, the commonly used data are disobedient. Here are his graphs.

(A) Oxygen-18 index of deep-ocean temperature and ice volume (after Miller & Fairbanks1987 and Zachos et al. 2001). (B) Estimates of past CO2 concentrations from alkenones (after Pagani et al. 2005). Source: W.F. Ruddiman, Science, 14 May 2010, p.839.

Ruddiman thinks that the data must be wrong. He suggests pushing the CO₂ up a little, 20 to 10 million years ago (using boron/calcium ratios) and finding a decline between 5 and 2 million years ago in new alkenone data. He concludes, “Geochemists still have work to do in refining the CO₂ proxies.”

But that would be wasted effort if CO₂ were not the driver. In The Chilling Stars Henrik Svensmark and I tell a completely different story about what was happening 2.75 million years ago. In Postscript 2008 we relate how the Sun and Earth, wandering through the Galaxy, blundered into a region of space packed with extra cosmic rays – just what was required to chill the world by making more low clouds, in accordance with the Svensmark hypothesis.

Here’s what we wrote.

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.

24. 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.

To update this part of The Chilling Stars, perhaps all we need to do is to reproduce Ruddiman’s diagram shown at the outset. To be generous we could include the rather small adjustments he proposes. Even then, it will fit very nicely with the subtitle of the Postscript 2008 in which the passage appears: Carbon dioxide is feeble.

Note added 24 May 2010. In The Chilling Stars we make much of the radioactive iron-60 atoms deposited on the Earth by a nearby supernova. A puzzle is to work out which of the nearby clusters of young, exploding stars may have been responsible. We rashly suggested the best-known of all the clusters, the Pleiades or Seven Sisters, but now we realise that the last of their massive, explosive siblings blew up 100 million years ago.

Another candidate is the Lower Centaurus-Crux sub-group of the Scorpius-Centaurus OB association, but that seemed too far away – more than 300 light-years away 2-3 million years ago. The suggested limit for the source of the iron-60 was 120 light-years, if the radioactive atoms were to survive their journey to the Earth, so we could only suggest a possible outlier of the cluster.

But in an interesting development the half-life of iron-60 has been drastically revised, to make them more durable. A report by a German-Swiss team (see the Rugel reference) increases the half-life from 1.49 to 2.62 million years. The authors comment:

Furthermore, a supernova deposit of ⁶⁰Fe on Earth, assumed some 2 to 3 Myr ago, should now yield a lower value at that time because of the longer half-life. Hence a more distant source must be assumed.

This makes the Lower Centaurus-Crux sub-group a more convincing candidate.

Henrik Svensmark and I are grateful to Dr Rainer Facius for drawing this to our attention.

References

W. F. Ruddiman, “A Paleoclimatic Enigma?” Science, Vol. 328, pp. 838-9, 2010

H. Svensmark & N. Calder, The Chilling Stars, Icon 2008, tools ill. p. 188, text extract pp. 233-6, iron-60 story pp. 180-197

G. Rugel et al., “New Measurement of the ⁶⁰Fe Half-Life”, Physical Review Letters, Vol. 103, pp. 072502 1-4, 2009