By Richard Black
Environment correspondent, BBC News website, Vienna
Cosmic rays have their origins in hugely energetic events in space
Research has thrown further doubt on the notion that cosmic rays are a major influence on the Earth's climate.
The idea that modern global warming is due to changes in cloudiness caused by solar influences on cosmic rays is popular with "climate sceptics".
But scientists found changes in cosmic ray flux do not affect cloud formation - the second such report in a month.
Separately, other researchers have found that particles from space may affect temperatures at the poles.
Both pieces of research were presented here at the European Geosciences Union (EGU) meeting.
Cosmic rays, hugely energetic particles coming from space, smash into the top of the Earth's atmosphere, creating a cascade of charged particles lower down.
These particles may help water droplets to coalesce, and so aid the formation of clouds.
The proposed link to climate change is that cosmic rays can be partially blocked by the Sun's solar wind.
When the Sun is forceful, there are fewer cosmic rays arriving in the atmosphere, so fewer clouds form, which has a net heating effect on the Earth.
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If the mechanism has an impact today, several scientists have hypothesised, it should be possible to spot a link between the intensity of cosmic rays and the formation of clouds.
Jon Egill Kristjansson from the University of Oslo is one; and he unveiled his new results at the EGU meeting.
Over the southern Atlantic, Pacific and Indian oceans, where air is much cleaner than in more urbanised regions of the world, particles from ship's chimneys change the properties of clouds in a way that is clearly visible to the Modis instruments (Moderate-resolution Imaging Spectroradiometers) onboard Nasa's Aqua and Terra satellites.
The particles are stimulating the formation of water droplets.
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If cosmic rays play a significant role in cloud formation, Dr Kristjansson reasoned, sudden changes in cosmic ray intensity should also show up, producing increases in cloud cover, changes in the size of droplets, and possibly in the total amount of water carried in the clouds.
"We have short-term changes called 'Forbush decreases', caused by eruptions on the Sun, where cosmic ray flux can decrease dramatically over one or two days and then gradually recover," he told BBC News.
"The cosmic ray signature on clouds, if there is one, should show up here."
He identified 13 Forbush events between 2000 and 2005 and looked for evidence in Modis data of concurrent changes in could properties.
Although some of the events were followed by a decrease in cloud cover or changes in the size of cloud droplets, others preceded an increase in cloud cover, or no change at all.
Overall, the results essentially appeared random; abrupt dips in cosmic ray intensity did not produce any discernible pattern of changes in clouds, either immediately or in the four days following the Forbush decrease.
"This is a careful piece of work by Jon Egill Kristjansson that appears to find no evidence for the reputed link between cosmic rays and clouds," commented Joanna Haigh from Imperial College London, who is also attending the EGU meeting and has also studied possible links between solar variability and modern-day climate change.
Data showed no impact of galactic cosmic ray (GCR) flux on cloudiness
"It's supporting other recent work that also found no relationship," she added, referring to a research paper published two weeks ago by a UK team which, using different sets of data and different means of analysis, also found no discernible influence of cosmic rays on cloud cover.
"I think that as a factor in climate change, it's pretty clear that we don't have any indication at this point that this is important at all," added Dr Kristjansson.
"Whereas global mean temperatures have been rising steadily over the last 30 years, we see that the cosmic ray flux has been steady."
The EGU meeting also saw the first presentation of other research that could perhaps help to explain temperature variations seen between different regions of the Arctic and Antarctic.
Computer models have predicted that energetic particles hitting the top of the atmosphere in polar regions may change temperatures by stimulating the production of nitrous oxides (NOx).
"The energetic particles induce NOx production, and the NOx is then transported down to the stratosphere," explained Annika Seppala, who led the project from the Finnish Meteorological Institute and also works with the British Antarctic Survey.
"NOx destroys ozone in catalytic reaction cycles; and when you change ozone in the stratosphere, that... can then feed down to surface temperatures," she told BBC News.
Periods of intense activity warmed (red) some regions and cooled others (blue)
Dr Seppala's observations appear to bear out the models' predictions, at least in winter in the polar regions.
In periods of relatively intense particle activity, some areas of the Earth's surface in both the Arctic and Antarctic are warmer while others become colder, showing differences of up to 2C or 3C compared to the long-term averages.
In periods of unusually low particle activity, the patterns are reversed.
The mechanism appears to be redistributing heat across the polar regions; there is no evidence for any overall warming or cooling, Dr Seppala added, nor that the scale of the effect has changed over time.
"The results were amazing, and I think it's something significant that we have to take into account," commented Katje Matthes from the Free University of Berlin, who chaired the EGU session which saw the new data presented.
"I think it's rather a local effect," she added, "and I don't think it has a big impact on global temperatures."
The Antarctic picture is particular fascinating. High particle flux places a big red patch, indicating warmth, over the Antarctic Peninsula, an area that is feeling the impacts of climate change faster than most other parts of the planet.
The heating and cooling from this mechanism might be short-term; but scientists studying the loss of ice from this region of Antarctica will surely want to understand whether the short-term natural highs and lows combine with the overall warming trend in a way that speeds melting.
Dr Seppala's team now intends to investigate what happens in the other seasons of the year, which will give a better understanding of the importance of this newly confirmed process.