Some climate models may need to be "substantially revised" in the light of new research into the airborne particles that seed clouds.
One of the most detailed studies to date of the particles, known as aerosols, has found serious shortcomings in existing descriptions of how they arise in nature. The work suggests that one or more unidentified organic gases – produced either naturally or from human activities – has a significant influence on the Earth's cloud cover.
The research, reported in the journal Nature, has implications for certain predictions about climate change because aerosol particles and the clouds they seed have a cooling effect on the Earth by reflecting radiation from the sun.
Jasper Kirkby, head of the CLOUD (Cosmics Leaving OUtdoor Droplets) experiment at Cern, the particle physics laboratory near Geneva, studied various gas mixtures of sulphuric acid, water and ammonia – the three gases thought to give rise to aerosol particles at the low altitudes where clouds form.
But the experiments produced between ten and a thousand times fewer aerosol particles than are observed in nature, meaning an additional gas or gases must be playing a vital role in the process.
"Some additional vapour or vapours, together with sulphuric acid, is controlling the formation rate of aerosols in the atmosphere and so affecting climate, so it is important to identify these and understand whether their sources are natural or associated with human activities," Kirkby told the Guardian.
"If they come from human activities, it raises the prospect of a new climate impact from humans. Alternatively, if they have a natural origin, we have the potential for a new climate feedback. What is clear is that the treatment of aerosol formation in climate models has to be substantially revised."
Aerosols are tiny liquid or solid particles suspended in the atmosphere. Above a certain size they can become "seeds" for cloud droplets.
Around half of atmospheric aerosols come from the Earth's surface, in the form of dust, sand or sea spray, or as particles from burning biomass or fossil fuels. The rest are produced in the air when vapour particles condense and grow into clusters.
In a second discovery, the researchers found that cosmic rays from the depths of space can increase the formation rates of aerosols by between two and tenfold in some cold regions of the atmosphere.
However, the finding leaves open the question of whether cosmic rays affect Earth's climate in a significant way, because the aerosol particles studied were too small to seed cloud droplets.
In future work, Kirkby's group aims to settle whether or not cosmic rays affect cloud cover and so give a clearer picture of how some variability in the planet's climate might be influenced by the rays and the sun's activity.
The role the sun might play in climate change has long been controversial, with some climate sceptics laying the blame for all global warming at the sun's door. The consensus among climate scientists is that human activity has been responsible for the lion's share of warming since the industrial revolution, through greenhouse gas emissions.
Cosmic rays are high-energy streams of particles that are blasted into space by exploding stars millions of light years away. Some of these rays reach Earth, where they slam into the atmosphere and produce showers of subatomic particles. On average, one cosmic ray passes through a square centimetre of the Earth's surface every minute.
But the intensity of cosmic rays falling on Earth varies. When the sun is more active, the stream of particles it generates, known as the solar wind, creates a stronger magnetic field, which deflects cosmic rays. This means that fewer reach the atmosphere.
"Our work leaves open the possibility that cosmic rays could influence the climate. However, at this stage, there is absolutely no way we can say that they do," said Kirkby.
Philip Stier, who heads the Climate Processes Group at Oxford University, said the study was "an experimental leap forward" but that it was too early to speculate on the implications for climate models or the climate in general. He added that the study would inspire more research in this area.