Climate change sceptics have relied on two arguments to make their case. Firstly, some say the climate is not, in fact, changing at all; the the warming itself is a myth. It is hard to make this case these days as the figures are so persuasive. The second, much stronger, argument states that even if the world is getting warmer, it is the Sun, not us, to blame.
Understandably, then, sceptics are getting pretty excited by a paper published in Nature magazine this week in which scientists at CERN, the nuclear research laboratory near Geneva, have shown that radiation from space can affect the way clouds form in our atmosphere – and in turn - perhaps - affecting temperatures on the ground (you can read the CERN press release on the paper here; the Nature paper itself is behind a firewall).
Back in the late 1990s, when the science of climate change was far from settled, I found the argument that changes in solar activity could account for most - if not all - of the temperature rise we saw in the 20th Century extremely persuasive. The ‘manic-Sun hypothesis’, championed by a pair of Danish physicists called Henrik Svensmark and Eigil Friis-Christensen posited that it was changes in ‘space weather’ rather than human activity that were largely to blame for climate change. The hypothesis has been flagged up in this country by the science writer Nigel Calder.
The hypothesis centres on the role of cosmic rays. These are extremely energetic sub-atomic particles (mostly protons) thought to be generated by exploding stars or colliding black holes in galaxies millions or billions of light years away. Several hundred cosmic rays pass through your body every minute at nearly the speed of light.
But the intensity of this cosmic radiation varies. When the Sun is particularly active – lots of sunspots and solar flares – it generates a powerful solar ‘wind’ that buffets the cosmic rays away. When the Sun is quiet, this shielding effect is reduced.
How could this affect our weather? The hypothesis says that cosmic rays may play a key role in cloud formation, by ionising (inducing an electric charge) tiny natural particles of dust or other ‘aerosols’ in the atmosphere, such as minute sulphuric acid droplets. It is thought that such ionised droplets act as ‘seeds’ for clouds to form, allowing water vapour to condense and form a droplet of liquid.
So, more cosmic rays equals more clouds equals more sunlight reflected into space equals lower temperatures. And conversely, a ‘manic sun’ means fewer cosmic rays, fewer clouds and higher temperatures. Furthermore, Svensmark and Friis-Christensen noticed a correlation between solar and weather, particularly temperature. They and their supporters claim that for much of the 20th century the Sun was unusually active, cosmic ray intensity unusually low and hence global cloud cover significantly thinner than average. Voila! Global warming explained. And humanity is in the clear.
It is a good theory and, as I said, initially persuasive enough to tilt me firmly into the sceptical camp. But some hard evidence – beyond statistical correlations – would be nice. Enter Jasper Kirkby, a British physicist at CERN. When I first spoke to him, in 1999, he outlined a simple experiment that could test the cosmic ray theory in a lab. Simply build a box, fill it with a mixture of gases and aerosols to duplicate the Earth’s atmosphere, and then bombard this box with artificially-produced ‘cosmic rays’ from one of CERN's particle accelerators. Then see what happens.
The problem is, Dr Kirkby couldn’t get funding. ‘It’ll only cost about £100,000” he told me at the time. This, to me (and many others) seemed bizarre: here was someone who had come up with an experiment which could, possibly, prove or disprove the whole anthropogenic climate change thesis at a stroke and no one was prepared to spend the peanuts it would cost to do so.
The sceptics have had a field day, seeing this as a sinister attempt to keep the global warming bandwagon rolling. Why pay someone do go and pour water on the greatest scare story in environmental history and kick all those IPCC climate researchers out of their jobs? Of course it is (probably) not so simple. I am not entirely clear why it has taken the CLOUD experiment to be carried out, but now it has and the results are interesting. However, they are for from conclusive in terms of deciding what role, if any, the Sun plays in global warming.
Firstly, the ionised droplets produced in the experiment were too small to produce reflective cloud droplets. Secondly, there is the issue of what layer of the atmosphere the experiment simulated. Cloud formulation in the lower troposphere (the bit closest to the surface) works differently to cloud formation at higher altitudes. One of Dr Kirkby’s findings shows that aerosol particles we see in the atmosphere cannot be accounted for by the accepted models – meaning a big chunk of atmospheric science – and hence climate science – may need a rethink. Possibly one or more gases, produced by humans, may be adding to the cloud-forming ‘mix’ in a way hitherto unanticipated. This may be good news, bad news or wholly neutral. We do not yet know.
So where does this leave the sceptics? Where I would agree with them is that experiments like this reinforce just how complex is the physics and the chemistry of the atmosphere. Of course we knew that, but the CLOUD (Cosmics leaving OUtdoor Droplets) data add an extra layer of uncertainty to the equations which means that we can, perhaps, be a little less certain than we were.
But the manic sun hypothesis, although initially convincing, is hardly watertight. The correlation between solar activity and cloud cover is only true at low altitudes not high (the opposite of what one would expect as cosmic rays are absorbed by the atmosphere and much stronger at the higher you go).
Commenting on the Nature paper, Prof Arnold Wolfendale, Professor of Physics at Durham University has pointed out several other holes in the cosmic ray idea, notably the fact that we would expect to see cigar-shaped clouds in the high atmosphere following cosmic ray tracks - “we don’t” - and, most importantly, the fact that comic ray intensity has remained pretty constant since the 1960s – the period during which global warming has been most marked.
I am no longer a sceptic about climate change. I accept it is happening (the evidence on the ground is too strong to ignore) and the link with rising CO2 levels seems to be pretty incontrovertible. The manic sun hypothesis is plausible, but as time goes on it is beginning to look like it might explain SOME but by no means the bulk of the warming we have seen.
That said, I agree with those who claim here has been something of a concerted campaign to write the Sun out of the climate change picture – a campaign that is misguided and scientifically incorrect. The Sun is the driver of ALL climatic phenomena. On a banal level, switch the Sun off and the atmosphere freezes and we have no climate at all.
We know our star’s output varies – not by much but by enough to certainly make at least some difference here on the ground. Lastly we know that massive climate changes have occurred in Earth’s past – most recently the great glaciations which took place just a few thousand years ago – and that these must have been driven by wholly natural forces.
One suspects that, as ever, the truth lies somewhere in between. Humans are probably responsible for the bulk of recent warming (and hence it is incumbent upon us to do something about this) but we cannot ignore natural variation, particularly variation in the Sun. More experiments are needed and more variables may yet be discovered - interstellar dust may play a role in long-term climate variations – and the role of manmade pollutants such as sulphur dioxide and other chemicals is still not entirely clear. Hats off to Jasper Kirkby for doing the experiment – an experiment that should have really been done a decade ago.