A new paper by scientists at CERN, the European Organisation for Nuclear Research, has been enthusiastically seized upon by some lobbyists and bloggers as a vindication of their claims that the cause of global warming is not the burning of fossil fuels and other human activities, but instead are cosmic rays from outer space.

However, a careful examination of the new research shows it is not the 'smoking ray-gun' of climate change denial that many are claiming. To understand why, we must first step back to examine some of the main 'scientific' arguments that self-proclaimed climate change 'sceptics' put forward to try to justify their opposition to the regulation of greenhouse gas emissions from the burning of oil, coal and gas.

Some 'sceptics' accept the indisputable evidence that carbon dioxide is a greenhouse gas and that increasing its concentration in the atmosphere directly causes the Earth to warm, as John Tyndall pointed out more than 150 years ago. They also accept, although often with equivocation, the clear evidence that the average temperature of the Earth's surface has increased by about 0.8 Centigrade degrees since the beginning of the 20th century. Furthermore, they grudgingly concede that greenhouse gas emissions from the burning of fossil fuels and other activities are at least partially responsible for this global warming through the increase in atmospheric concentrations of carbon dioxide of about 40 per cent that has occurred over the past 200 years or so. However, these 'sceptics' simply reject any suggestions that there may be significant further warming in the future, no matter how high greenhouse gas concentrations in the atmosphere grow, usually citing doubts about the accuracy of computer models of the climate used by researchers.

Other 'sceptics' offer a different argument, and while accepting that the Earth is warming, they simply reject the evidence that the unequivocal rise in concentrations of greenhouse gases in the atmosphere is responsible to any significant degree. Instead they appeal to 'natural causes', often referring to periods over the Earth's 4.5 billion-year history when it was much colder or warmer than today. Of course, saying 'it's natural causes' is the equivalent of exclaiming 'it's magic', unless some plausible scientific explanation is also put forward in support. So these 'sceptics' often suggest an extra-terrestrial alternative.

Past periods of extreme warmth or cold in the Earth's history, such as Ice Ages, have been triggered by fluctuations in the Earth's orbit around the Sun, but we know that has not caused the global warming of the past 100 years. However, the amount of energy released by the Sun also varies over time, so some 'sceptics' try to blame it for global warming. The problem with this is that detailed measurements from satellites and other instruments indicate that changes in the energy released by the Sun can only really explain up to 10 per cent of the global warming that has been recorded.

So some have pointed to another ingenious potential explanation for how the Sun might cause global warming in an indirect way, by altering the amount of cosmic rays that enter the Earth's atmosphere from outer space. This theory relies on linking together a number of steps. First we know that the Earth is bombarded by cosmic rays from outer space, and that the Sun can deflect these rays through changes in its magnetic field, which creates a solar wind. We also know that when cosmic rays from outer space enter the Earth's atmosphere, they can create charged particles that might possibly help to produce the 'seeds' from which clouds can grow. And we also know that clouds block out light from the Sun, leading to cooling of the Earth's surface.

The galactic cosmic ray theory puts these steps together, and proposes that the Sun's magnetic field has been gradually increasing in activity over the past century, decreasing the amount of cosmic rays from outer space that enter the Earth's atmosphere, which, in turn, has reduced the cloudiness of the skies across the world, therefore allowing more sunlight to warm the Earth's surface.

The researchers at CERN today report their findings about one step of this theoretical process - they have successfully produced microscopic particles of aerosol in the laboratory by bombarding a mixture of gases similar to that found in the Earth's atmosphere, including sulphuric acid, with the equivalent of cosmic rays.

While these are new and interesting results, they hardly constitute the 'smoking ray-gun' claimed by some 'sceptics'. For instance, they have not been able to show that the microscopic particles that they produced could grow into the droplets that act as the seeds for clouds.

While cosmic ray theory may seem theoretically possible, there are strong reasons to conclude that it cannot really explain global warming. There is a lack of evidence that the amount of cosmic rays reaching the Earth from outer space has really diminished, particularly over the past 50 years, let alone convincing proof that the sky has become less cloudy over that period.

Indeed, a new analysis published earlier this month by two physicists, Professor Terry Sloan and Sir Arnold Wolfendale, indicates that cosmic rays from outer space could only explain up to 8 per cent of the increase in global average temperature over the past century.

So while climate change 'sceptics' may try to claim that CERN's results are a 'smoking ray-gun' from the extra-terrestrial origin of global warming, the vast and overwhelming weight of scientific evidence still points to the burning of fossil fuels and other human activities as the primary cause of climate change.

Bob Ward is policy and communications director at the Grantham Research Institute on Climate Change and the Environment at London School of Economics and Political Science.

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