Do you owe your existence to a supernova? It is perhaps rather far-fetched but for some scientists not too big a leap. To follow this story we need to step back and question our understanding of climate change. The consensus model is based on the volume of global carbon dioxide; it was drawn down in the Cenozoic priming Earth Systems such that orbital radiation variations could modulate our climate through mechanism such as the thermohaline circulation. The link between orbital radiation and the climate pulse beat during the Quaternary has been well established by deep sea sediment and ice cores. But not everyone agrees with this, after all science is about disagreement and challenge so let take a look.
Wilson Chamber and Cosmic Rays
A Wilson Chamber is a sealed vessel supersaturated with water or alcohol. The passage of atomic particles and cosmic rays is recorded by a trail of water droplets. It represents one of the earliest ways of seeing atomic and perhaps subatomic particles (Fig. 1). It also shows how these particles can cause condensation, essential for cloud formation.
Figure 1: Wilson Cloud chamber.
This piece of old fashioned scientific kit has led some people to suggest that there is link between cloud formation and cosmic rays (Fig. 2).
Figure 2: Cosmic rays and cloud formation. (Source: Svensmark and Calder, 2007).
Cosmic rays consist of a range of atomic and subatomic particles with varying energy fluxes. Gamma ray bursts are common products from starburst when giant stars die (supernova) and new stars are born. The link to cloud formation is not entirely clear but there may well be a link of sorts. Changing the percentage of cloud cover changes the atmospheric albedo (reflectivity) and atmospheric absorption both of which may impact on the surface energy balance.
Cosmic rays entering our atmosphere all the time, if they collide with an atom they can produce a secondary ray in the form of a neutron. If this collides with a nitrogen-14 atom for example the neutron displaces a proton to give the carbon isotope Carbon-14. Carbon-14 is the stuff we use for carbon dating and the proportion in the atmosphere and therefore in living matter is known to vary through time. In fact it varies with the frequency of cosmic rays; more cosmic rays more C-14.
Now cosmic rays are quite bad but we are protected by the Sun’s magnetic field; the weaker the field the greater the C-14 production. This also implies the weaker the field the greater the potential for cloud formation via cosmic rays assuming this link exists. Sunspots come and go on the surface of the Sun with an 11 year cycle (Fig. 3), but the magnitude of this activity varies over longer time scales. It just so happens that one weak period of sunspot activity corresponds to the Little Ice Age (Fig. 4). This coincidence has been known for a while but the mechanism has been unclear until Svensmark (2007) put forward the link between clouds and cosmic rays.
Figure 3: solar sunspot cycle. (Source: David Hathaway, NASA, Marshall Space Flight Center – http://solarscience.msfc.nasa.gov/predict.shtml)
Figure 4: Sunspot activity through time. Note the Maunder minimum which corresponds to the Little Ice Age.
Little Ice Age
The River Thames doesn’t freeze these days, but it did in the past. Bishops were sent for in Alpine villages to exercise demons and evil spirits from advancing glaciers. The European wine harvest failed. Between about 1650 and 1750 AD things were tough. This period with its wealth of documentary evidence is known as the Little Ice Age (Fig. 5). Whether it was a European focused event or more global is still a matter of some debate and may in part reflect the availability of the European historical record. But cold it got.
The cause of this cold event is unknown, some have linked it to a slowdown in the thermohaline conveyor, but the evidence is unclear. Could this event be the result of the sunspot minima? More clouds would increase atmospheric albedo, meaning that less solar radiation would make it through the atmosphere to warm surface.
Figure 5: Little Ice Age in context.
In theory this should be a testable hypothesis. We have records for cosmic rays going back several years and also satellite records of cloud cover. While this would not prove the cause of the Little Ice Age it would give veracity to the mechanism. Figure 5 shows a correlation between cloud cover and a single solar cycle. The correlation with high altitude cloud cover is poor which is surprising since it is high altitude clouds that are most important in controlling the albedo of the atmosphere, but with low altitude clouds it is strong.
The thing about science is that it has to be repeatable. The onus is on the authors of a paper to make sure their data sources and methods are clear, so that in theory they can be repeated. Well many people have tried to repeat the analysis of Svensmark and colleagues using the same and different data and the relationships have not always been replicated (e.g., Sun and Bradley, 2002). For example, Kulmala et al. (2010) could find no evidence of a relationship between the creation of new particles in the atmosphere and cosmic ray activity.
In parallel, theoretical work much of which has been done at CERN (European Organization for Nuclear Research, physicists and engineers) has led to a better understanding of the physics (Kirby, 2007; Kirby et al., 2011). These researchers have greater belief in the potential link, but the test of a hypothesis like this must be against known historical records.
Interestingly, Tsonis et al. (2015) found no link between global temperature during the 20th Century and cosmic rays, but did find a significant link, although modest, between cosmic rays and short-term, year-to-year variability in GT.
The verdict about the link between cosmic rays and cloud remains one of debate and largely unproven. There is a divergence between the physics that can be shown in the laboratory and the actual record. While the consensus is perhaps moving away from cosmic rays it should not be dismissed until proven false. The association between the Maunder Minima and the Little Ice Age is compelling, but is it a case of coincidence or causality?
Figure 7: Correlation between cosmic rays and low cloud cover during a solar cycle. (Source: Carslaw et al., 2002)
Origin of Life and Snowball Earth
Despite the fact that the link between cosmic rays and climate has yet to be proven some researchers continue to build on the hypothesis. In particular they point to a potential link between cosmic rays and the origin of complex life on Earth. This is perhaps even more tenuous, but it is important that scientist keep an open mind about ideas – you never known when the next paradigm shift is going to happen!
The Cambrian Explosion was a massive evolutionary radiation. For much of Earth history there has been little in the way of life apart from bacteria and algae. Slime ball Earth would have been quite an apt description. This very long period in earth history – 3 billion or so years – was vital in transforming the Earth atmosphere, reducing the carbon dioxide content and replacing it with oxygen. This terra forming period created the right circumstances for the evolution of complex life. This really kicked off around 520 million with the Cambrian Explosion.
The causes this radical evolutionary event have been debated, the presence of extensive and flooded continental shelves following the break-up of Rodina may have been important and the availability of Oxygen. It also may be a function of preservation in that the innovation was the development of ‘hard parts’ made of calcium carbonate that could be preserved. Complex multi-cellular life first appeared in the Neoproterozoic with animals known as the Edicarian fauna. The association with Snowball Earth has attracted many geologists attention despite the fact that timings don’t quite work. The Neoproterozoic snowball episodes are constrained to 770 to 635 Ma, approximately 100 Ma years before the Cambrian Explosion (520-488 Ma) when metazoan fauna hugely diversified. By working back via the molecular clock embedded within DNA we can get an approximate date for the origins of ancestral metazoans; that is when the genome was last re-organised into its current form. The data for this event is around 900 Ma. Reconciling these events is difficult because the record is so poor and different elements are often in conflict; in this case the molecular clock and the fossil record.
Figure 8 shows a rather speculative model that links the Cambrian explosion to cosmic rays (Maruyama and Santosh, 2008). The scenario goes something like this. Starbursts between 900 and 600 Ma lead to extensive cosmic radiation within our solar system. This may have led to cloud formation triggering snowball Earth so the argument goes. As Rodina rifted it created seaways with nutrient enriched waters and wide shallow continental shelves idea for the diversification of life. Saturation by cosmic radiation caused the mutation of DNA and the creation of more complex life including sexual reproduction. At around 500 Ma oxygen levels had got to a sufficient level for the life to explode in diversity.
It is important to recognise that is a speculative story – perhaps geological poetry would be a better term – but stories can help scientist to frame question for more rigorous analysis. It does mean that these stories are correct or will be proven correct in time. It is however an interesting hypothesis, even if one excludes the climate link. We know that the founding DNA modern life dates from around 900 Ma; what caused that mutation and re-organisation of the genome? Extra-terrestrial radiation is as good a working hypothesis as any other at this point. So yes you may owe your existence to the explosion of a distance star.
Figure 8: Speculative model linking cosmic rays to the origin of complex life and the Cambrian Explosion. (Sources Maruyama and Santosh, 2008).
Science is about ideas, advancing them, putting forward tests with which to examine them and allowing peer debate. Many ideas are advanced that ultimately are discarded. The link between climate and cosmic rays is increasingly being marginalised, that is not to say however it should not be debated, taught and discussed.
Professor Sandcastle 