Richard Black, BBC News 21 Apr 10;
Watching the enormous plumes of dust and ash rising from Eyjafjallajokull, it is hard to imagine that this almost week-long eruption would not have any effect on weather and climate.
But that is the likelihood; that the impact on Britons, Europeans and the citizens of the wider world will be limited to cancelled flights, with no other effects on the skies.
Volcanoes produce tiny particles - aerosols - which have a net cooling effect on the world because they reflect solar energy back into space.
They also produce carbon dioxide, a greenhouse gas.
Historically, the cooling has outweighed the warming. The 1991 eruption of Mount Pinatubo in The Philippines lowered global temperatures by about 0.4-0.5C - but Eyjafjallajokull, dramatic as it looks, is simply not in that league.
"Icelandic scientists have made a first estimate of the volume of material ejected, and it's about 140 million cubic metres," says Mike Burton from Italy's National Institute of Geophysics and Vulcanology.
"That's a lot in five days; but Pinatubo ejected 10 cubic kilometres - that's 100 times as much.
"So this is not the big climate changing eruption that some people seem to think it is."
As well as the sheer volume of aerosols, the other factor influencing the size of its climatic impact is the altitude they attain.
If material reaches the stratosphere, it can remain aloft for several years; but if it stays in the troposphere, the lowest layer, it tends to come back to Earth in days or weeks.
"At the moment, the eruption cloud reaches around 22,000 feet (7km)," says Anja Schmidt from the School of Earth and Environment at the UK's Leeds University.
"That's high enough to affect aviation but is unlikely to be high enough to have a strong effect on the climate system."
Low carbon life
Dr Burton's team has spent more than a decade refining methods for measuring the gas output from volcanoes, and made a trip to Iceland in early April, before the Eyjafjallajoekull eruption began but after the earlier, less vigorous spell of activity at nearby Fimmvorduhals.
They found Fimmvorduhals was producing about 20-25,000 tonnes of CO2 each day.
Based on the relative size of the volcanoes, he estimates that Eyjafjallajoekull could have emitted about 10 times that amount per day at its peak.
But that lasted for less than a week; things now appear to be much quieter.
And even over that peak period, its daily CO2 output was only about one-thousandth of that produced by the sum total of humanity's fossil fuel burning, deforestation, agriculture and everything else.
In fact, the extra CO2 produced from the volcano is probably less than the volume "saved" by having Europe's aeroplanes grounded.
But any precise comparison of those two effects will depend on the eventual duration of the grounding as compared with the eventual duration and intensity of the eruption.
The last Eyjafjallajokull eruption lasted for two years, and it is possible that this one will do the same; whether it does or not is anyone's guess at present.
"But the thing to realise is that there are already a number of volcanoes around the world, including Etna and Popocatepetl, that are continually outgassing CO2 now," says Dr Burton.
"The amount of CO2 output still pales into insignificance beside human emissions."
The Italian team is planning another trip to Iceland as soon as travel conditions allow, to get more precise measurements of gas emissions from Eyjafjallajokull.
Weather whys
Ash in the sky, but no aeroplanes: a recipe, you might think, for a change in the weather.
When US authorities banned flying following 9/11, the temperature difference between night and day over the continental US increased by at least 1C.
Jet contrails were effectively acting as cirrus clouds, researchers concluded - reflecting solar energy in the day, acting as a blanket by night.
But nothing of that kind has been observed following the Eyjafjallajokull eruption - or indeed any other impact on weather, according to UK Met Office scientist Derrick Ryall.
"Given the size of the eruption, we wouldn't expect any impact, except perhaps around Iceland itself," he says.
"If it goes on for a few months, someone will certainly be keeping an eye on it but it would be hard to ascertain - you'd need some pretty sophisticated analysis."
Dramatic though the pictures from Eyjafjallajokull have been, the likelihood is that history will not rank it as a volcano that shook the world - not a Pinatubo, not a Krakatoa, and definitely not a Toba - the eruption some 70,000 years ago that apparently brought on a six-year global freeze.
