Results from an experiment built to study how clouds form suggests that our knowledge of this subject may need to be revised, Nature journal reports.
Tiny particles (aerosols) form the basis of the "seeds" from which clouds grow.
These seeds form when sulphuric acid and ammonia molecules cluster together - and cosmic rays may help this happen.
But these ingredients create only a tiny fraction of the cloud seeds formed in the atmosphere.
The result surprised Dr Jasper Kirkby who led the research. He told BBC News: "We've shown sulphuric acid and ammonia can't account for nucleation (the very early stages of cloud seed formation) observed in the lower atmosphere.
"We've found that this can only account for a tenth to a thousandth of the rate that's observed. So it's clear from this first set of measurements that our present treatment of aerosols in climate models needs to be revised quite a lot."
Professor Mike Lockwood of Reading University, UK, concurs: "Something else, as yet unknown, is helping enhance the nucleation rates there. Depending on its source, this could even be unexpected additional (human-caused) climate forcing or feedback effect (on the climate)," he explained.
The aim of the study is to create a better understanding of how clouds form and in particular the role of cosmic rays. Dr Kirkby said that the work will lead to better computer models of how the Earth's climate is influenced by clouds.
"Even though aerosols and clouds are very important (in climate modelling) the basic numbers haven't been measured properly and we're doing that," he said.
The so-called Cloud experiment is based at the European Organization for Nuclear Research (Cern), just outside Geneva. It consists of a large stainless steel chamber filled with highly purified air into which scientists can infuse trace amounts of the vapours they believe to be involved in the formation of aerosols that can grow to become cloud seeds.
A beam of particles from one of Cern's particle accelerators provides a controllable source of artificial cosmic rays.
Clouds play an important part in determining global temperatures as they reflect a proportion of the Sun's heat back into space. However, the formation of the aerosols that seed clouds is not well understood and is a source of uncertainty in climate models.
In particular, researchers want to understand the precise role played by cosmic rays. These are charged sub-atomic particles that hit the Earth's atmosphere from space. These create more charged particles - which may enhance the formation of cloud seeds.
The first results from the Cloud experiment at Cern show that cosmic rays cause a ten-fold increase in the formation rate of nanometre-sized aerosol particles. However, Dr Kirkby stressed that these particles are still far too small to seed clouds and so it is premature to conclude that cosmic rays have a significant influence on climate.
The number of cosmic rays that hit the Earth is reduced when the Sun's activity is high. It has been proposed that reduced cosmic rays may lead to reduced cloud formation, causing global temperatures to rise.
Some climate change "sceptics" claim that this process, rather than the burning of fossil fuels, can explain much of the Earth's recent rise in temperature.
Climate scientists point out that there is evidence to show that the sustained rise in global temperatures over the past 15 years cannot be explained by cosmic ray activity. They also point to a vast body of research pointing to rising carbon dioxide (CO2) levels to be the cause. According to Professor Lockwood, it is very unlikely that variations in cosmic rays have played a significant role in recent warming.
"The result that will get climate change sceptics excited is that they have found that through the influence of sulphuric acid, ionisation can enhance the rate of water droplet growth. Does this mean that cosmic rays can produce cloud? No," he told BBC News.
Professor Lockwood says that the air-induced aerosols only grew to about 2 nanometres. To influence incoming or outgoing radiation to Earth, droplets must be of the order of 100 nanometres (nm). The growth rates would be really slow from 2 to 100nm because there simply is not enough sulphuric acid in the atmosphere.
"There are a great many arguments as to why the cosmic ray cloud effect is not a major driver of climate change and these results do not yet impinge on those arguments," he said.
Nevertheless, it seems that air ions generated by cosmic rays can helping cloud formation get started. Neither the role of aerosols or the effects of cosmic rays are well understood and this limits the ability of computer models to predict how the Earth's climate will change.
The Cloud experiment is aiming to settle these questions.
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