"Our result represents another argument against the hypothesis that cosmic rays represent an alternative, natural way of explaining a significant part of the undoubted global warming," team member Arnold Wolfendale of Durham University told environmentalresearchweb. "The argument against a direct solar origin for warming is equally strong."

Despite unambiguous experimental evidence, such theories have received widespread support from certain media trying to downplay man's influence on the climate.

Wolfendale stated, however, that small variations in surface temperature do seem to follow variations in the solar cycle.

Some scientists have suggested that cosmic rays might be associated with changes in the weather and climate. For example, the low amount of cloud cover observed during certain solar cycles correlates well with the decrease in cosmic rays reaching Earth. Fewer cosmic rays in the atmosphere leads to less ionisation of particles, which reduces cloud cover and in turn allows more sunlight to warm Earth.

The team came to its conclusions by analysing published data on cosmic-ray rates over the past 50 years, together with data on sunspots and the solar radiation over the same period, to look at the relation between these factors.

"The likelihood of our being wrong is very small," insisted Wolfendale. "The chance of the natural cosmic-ray or solar irradiance explanation being responsible for more than 14% of the observed warming is quite negligible." According to assumptions made in the work, the effect of varying solar activity – either by direct solar warming or changing cosmic-ray rates – must be less than 0.07 °C since 1956.

The results also confirm that international policies to reduce greenhouse-gas emissions are on the right lines, added Wolfendale. And they highlight the importance of carefully observing natural phenomena over long periods of time.

Wolfendale said that his team will continue to look at possible natural causes of global warming. "This problem is so important that every aspect must be studied," he insisted. In particular, the researchers will study whether cosmic rays cause any clouds at all via ionisation in the atmosphere.

"The biggest shortcoming in climate models so far seems to be how to deal with clouds and this is why we are concentrating on them," he said.

The work was published in Environmental Research Letters.