Rayons cosmiques, climat et origines de la vie

L’ionisation de l’atmosphère produite par les rayons cosmiques a-t-elle une influence sur la couverture nuageuse ? On le pensait depuis plus d’une décennie, en s’appuyant sur des données issues du cycle solaire 1985–1996. Dans cet article, les auteurs décrivent leurs travaux, qui sont un examen critique de cette thèse. L’analyse de la couverture nuageuse en fonction de la latitude géomagnétique laisse penser qu’il n’y a pas de corrélation causale avec l’intensité des rayons cosmiques atteignant l’atmosphère. L’article évoque également la troublante question du déclenchement de la foudre par les gerbes de rayons cosmiques, et du rôle éventuel de ce phénomène dans l’apparition et l’évolution de la vie sur la Terre.

In his pioneering work on supersaturated vapours, which began in the 1890s, C T R Wilson found that droplets condensed on the ionization trails left by charged particles. This led to many positive advances, among the most significant of which was the use of “cloud chambers" in ushering in the field of elementary particle physics. Just over 50  years ago, Edward Ney at the University of Minnesota suggested that cosmic rays might have an influence on the climate (Ney 1959). He proposed that ions from cosmic rays act as condensation centres for cloud droplets. It is immediately obvious that this is not the phenomenon that Wilson discovered. To make ionization trails visible, cloud chambers need very clean conditions and supersaturation at a level about four times greater than saturation. By contrast, it is rare to find such clean conditions in the atmosphere and supersaturation levels are almost never more than 1% above saturation.

More recently, in 1997 Henrik Svensmark and Eigil Friis-Christensen reported a link between clouds and elementary particles (specifically cosmic rays) that has been used to claim that changes in cosmic-ray intensity cause changes in cloud cover and could affect global warming (Svensmark 2007). In this article we describe work that examines this claim critically. We also touch on the fascinating topic of lightning initiated by cosmic-ray showers and its possible role in the origin and evolution of life.

Cosmic rays and cloud cover

Figure 1 shows the basic evidence used to claim a causal correlation between cosmic-ray intensity (measured by the neutron monitor operated by the University of Chicago at Climax, Colorado) and low cloud cover (at altitudes below 3.2 km as measured by satellites). The data in the figure are for the period 1985 to 2008, which covers two solar cycles. In the 22nd cycle (1985–1996) the correlation between cosmic-ray intensity and low cloud cover was strong and this was the origin of the claim. However, the next (23rd) solar cycle has now passed (1996–2007) and the correlation is much more difficult to see. This suggests that the 1985–1996 observation might have been “accidental" and the effect of something completely different (such as temperature). Nevertheless, we will put this to one side and consider whether the apparent correlation is causal.

A test of the causal hypothesis is to examine the correlation as a function of geomagnetic latitude. The 11-year cosmic-ray variation becomes bigger at higher magnetic latitudes because of the effect of the Earth’s magnetic field. Fewer low-energy cosmic rays enter the Earth’s atmosphere near the magnetic equator than near the poles. This effect is measured by the vertical rigidity cut-off (VRCO) – the minimum rigidity for a primary cosmic ray to reach the Earth’s atmosphere – which is computed from the local value of the planet’s magnetic field. Our analysis looked at the differences between the low cloud cover at solar minima in 1985 and 1996 and that at solar maximum in 1990 at different VRCO (Sloan and Wolfendale 2008). These were then compared with the changes in the cosmic-ray rate as measured from neutron monitors located around the world (figure 2). If the dip in the low cloud cover observed in 1990 was caused by the decrease in ionization from cosmic rays then all of the points in figure 2 would follow the line of the cosmic-ray variation, marked NM. They do not.

Cosmic rays are not the only source of ionization in the atmosphere. We have looked for changes in cloud cover associated with a variety of other sources. The ionization released from nuclear weapon tests in the atmosphere was one example that we examined. At large distances from the test centre, radiation levels are high but other effects of the blast are negligible. For example, measurements showed that the Bravo test (the largest of the US tests), which exploded a 15-megatonne device at Bikini Atoll on 1 March 1954, produced radiation levels of 100 R/h at a distance of 480 km from the explosion. This corresponds to 5 × 107 ion pairs/cm3, i.e. seven orders of magnitude more ionization than that produced by cosmic rays. However, no effects on cloud cover were observed. This shows that the efficiency for conversion of ions to cloud droplets must be low. Similarly, we examined radon concentrations in various parts of the world to see if high-radon regions had more cloud cover than their neighbours with low-radon concentrations. We also examined the ionization released in the Chernobyl disaster in 1986. Again, we did not find any significant effects of ionization on cloud cover.

Recently, Svensmark’s group examined the so-called Forbush decreases in cosmic-ray intensity, which are caused by solar coronal mass ejections. The group found that the six strongest (over the past 20 years) are followed by significant drops in low cloud cover and in other indicators of atmospheric water content. We have examined the evidence in detail and concluded that it is not only statistically weak but that it also needs unphysically long periods (6–9 days) for the change in cosmic-ray flux to manifest itself as changes in cloud cover or the cloud water content.

The correlation between low cloud cover and cosmic rays in figure 1 is presumably therefore not causal because we have found that ionization is not efficient at yielding cloud cover. A more likely cause relates to solar irradiance, not least because the change in energy content of solar irradiance is about 108 times that of cosmic rays. In this context, Mirela Voiculescu of Dunarea de Jos University in Romania and colleagues showed correlations between low cloud cover and either the cosmic-ray rate or the solar irradiance in limited geographical areas (Voiculescu et al. 2006). Such areas cover less than 20% of the area of the globe. A close examination of these geographical areas reveals that only the correlation between the solar irradiance and the cloud cover is seen in both solar cycles. By contrast, any correlation that there is with cosmic rays does not appear in both cycles.

Variation in solar irradiance over the 11-year solar cycle is a much more plausible cause of any correlation with cloud cover than cosmic rays; indeed, Joanna Haigh of Imperial College London has modelled such an effect (Haigh 1996). A comparison of the long-term variation of the global average surface temperature with the long-term solar activity shows that less than 14% of the observed global warming in the past 50 years comes from variations in the solar activity.

This is not to say that ionization has no effect on climate at all. There may well be an interesting effect involving the terrestrial electrical circuit, which seems to be affected by cosmic rays. No doubt the CLOUD experiment underway at CERN will throw further light on this problem and tell us just how much such an effect will be.

Lightning and the origin of life

One fall-out of the work described above has been interest in the role of cosmic rays in a particularly dramatic cloud effect: lightning. Alexander Gurevich and Kirill Zybin of the P N Lebedev Physical Institute in Moscow suggested in 2002 that extensive air showers (EAS) created by cosmic rays play a key role in initiating the leader strokes in lightning. This has been confirmed in more recent observations at the Lebedev Institute’s Tien-Shan Mountain Cosmic Ray Station by Gurevich and colleagues.

This phenomenon has a possible relevance to the origin of life on Earth. The current favourite models for this origin are either on comets in outer space, as Fred Hoyle and Chandra Wickramasinghe suggested, or in the black smokers or alkaline vents that result from volcanic activity in the deep oceans. However, another possibility follows from the famous early experiments of Stanley Miller and Harold Urey, in which they passed a spark through a mixture of liquids (water, methane, ammonia, etc) – the “prebiotic soup". This resulted in the appearance of the basic building blocks of life, such as amino acids, RNA and monomers. One problem, however, was that the available spark energy, from lightning, was thought to be inadequate.

This is where the long-term variability of EAS rates may have relevance. We have shown that there should have been periods during which the EAS rate was higher by orders of magnitude than at present (Erlykin and Wolfendale 2001). Our theory is based on the statistical nature of supernova explosions, which are thought to be the originators of high-energy cosmic rays. Figure 3 shows how, from time to time, periods of high cosmic-ray intensity of tens of thousands of years will occur, as a nearby supernova explodes. This will lead to high lightning rates. One of these, occurring at around 4 Gy before the present (a not unlikely occurrence), could have led to the formation of the building blocks of life via the Miller-Urey mechanism. Life could then have evolved from such a start.

Perhaps less speculatively is the role of NOx (NO + N2O) generated by lightning strokes. It seems that nearly 20% of the contemporary concentration of NOx is produced by lightning. Its rate of production would certainly vary considerably. NOx is poisonous to mammals but promotes growth in plants; thus, an effect on evolution of species, both positive and negative, is likely.

In conclusion, the interaction of cosmic rays with the Earth’s atmosphere is a topic of considerable interest. Although it is unlikely that cosmic rays are a significant contributor to global warming, their contribution to the pool of aerosol-cloud condensation nuclei could be non-negligible; the CLOUD experiment has a big role to play in elucidating the interesting science involved. On a wider canvas it would not be surprising if electrical effects in the atmosphere, initiated by cosmic rays, played a role in the evolution of the Earth’s inhabitants.

• The authors are grateful to the John Taylor Foundation for supporting this work.