Volcanoes have been at the forefront of many Europeans’ minds in recent days. Particularly those in Iceland or stranded abroad by planes grounded by the ash cloud from Eyjafjallajökull. But what are the implications of this eruption for climate?
“We need to keep this eruption in perspective,” Caspar Ammann of the US National Center for Atmospheric Research told environmentalresearchweb. “It is – despite all the impact on air traffic – not really very large. The current eruption will certainly not have any recognizable climate effect outside of the immediate domain of the ash cloud.”
In the past volcanic eruptions such as that of Mount Pinatubo in the Philippines in 1991 have had clear impacts on climate. But these events were much larger than Eyjafjallajökull, injecting massive quantities of aerosols and dust high into the stratosphere, the layer of atmosphere that begins roughly 4–12 miles above the Earth. Pinatubo, for example, released about 17 million tons of sulphur dioxide, cooling climate for several years. “This time around [for Eyjafjallajökull] I have not seen any clear injection into the stratosphere,” said Ammann, “the eruption was not intense enough.”
The lowdown
For Eyjafjallajökull to affect climate in its current state, Ammann says that it would have to erupt continuously. But even then it might be difficult to recognize a clear volcanic effect.
Episodes of further ash clouds towards Europe are certainly to be expected. Caspar Ammann
“Ash has two effects,” he explained. “It holds sunlight back from reaching the surface, though the density [from Eyjafjallajökull] appears not that high – at least I have not seen too many images online where people visually identify the ‘ash cloud’ other than a few sunset images. But ash also absorbs sunlight and outgoing longwave radiation, and thus warms up.”
The net outcome depends on the elevation of the ash above the Earth and how long it remains there. Generally the cooling effect wins but if the ash is lower down in the atmosphere and slow to be removed by rain, slight regional warming is possible.
Going higher
So far Eyjafjallajökull has emitted ash and gases mainly into the troposphere, the atmospheric layer next to Earth’s surface. “The life time of any particle [in the troposphere] is of the order of days to maybe a couple of weeks, thus [it has] only local and regional potential,” said Ammann. “But the real climatic effects are when particles can reside much longer. That only happens if they are injected or form in the stratosphere, the layer above where our weather happens.”
Aerosol droplets, particles and gases in the stratosphere can affect climate on a hemispheric or global scale. When it comes to large-scale effects on climate, ash also becomes a much less significant factor as it is relatively heavy and falls to Earth comparatively quickly; sulphur is the real player.
In the stratosphere sulphur-bearing gases such as sulphur dioxide transform into tiny sulphuric acid particles, “like cloud particles, only smaller”, says Ammann. “Because they are small, their lifetime is very long – months to years, depending where they get transported. These particles are very efficient at scattering sunlight, though they warm up a bit as well. But the overall effect is to reduce the light reaching the surface.”
This dry haze, so called because it is high enough up to persist even after thunderstorms, which usually wash out everything in the atmosphere below them, can result in spectacular sunsets. “These sunsets are different to these red sun disks that you have seen in the past days,” said Ammann. “What we are talking about are reflections much higher up in the atmosphere that glow when the sun is long below the horizon, up to 90 minutes past sunset. The purple and red colours are very distinct.”
Clouding the issue
In technical terms, such volcanic sulphate reduces shortwave radiation to the ground through direct radiative forcing. But its influence on clouds is less well understood. “There is an indication that cirrus cloud formation is enhanced because the sinking sulphuric acid particles serve as condensation nucleii,” said Ammann. “That effect is a slight warming, but it doesn't seem to be a big effect – certainly much smaller than the cooling from the stratospheric layer.”
Matters get more complex again when an indirect effect comes into play; as the stratospheric sulfate layer warms up, it influences horizontal temperature gradients. This can enhance westerly winds and nudge atmospheric circulation towards a more zonal flow in winter, leading to climate modification on a regional or even hemispheric scale. “More vigorous westerlies leads to more advection of warmer air, and thus the term ‘winter warming’ has been associated with large volcanic eruptions,” said Ammann. “Because of the stronger winds, cold air that usually forms close to the ground gets mixed and removed.”
Calculating carbon
So far the largest climate effect of Eyjafjallajökull appears to be the grounding of Europe’s airlines. Colin Macpherson of Durham University UK told Channel 4 News he estimates that the volcano was emitting around 150,000 tonnes of carbon dioxide per day for the first three days of its eruption, about 40% of the daily emissions from European aviation.
But if neighbouring Katla were to erupt too, which has happened after Eyjafjallajökull’s last three eruptions, the picture would change. “Katla has a history of large and very large eruptions,” said Ammann. “It is the source of the largest basalt eruption in recorded history; Eldgja [part of the Katla system] in the 1930s was about 50%larger than Laki.” When Iceland’s Laki volcano erupted for eight months in 1783 it released around 122 million tonnes of sulphur dioxide into the upper troposphere and lower stratosphere. This caused a smell of sulphur and a continuous haze in the atmosphere across large swathes of Europe, as well as disruption to climate. In Iceland emissions from Laki killed half the country’s livestock, leading to widespread famine.
Many groups might start modeling efforts to better predict where ash spreads and how fast it is removed. A few people in the aviation business might even sponsor such research. Caspar Ammann
“Speaking speculatively, if Katla were to get triggered, then we have a volcano involved that is historically more powerful, and has a much larger glacier – and thus more water – to access,” said Ammann. “Key for a climatic effect would be that very explosive phases should happen that inject large amounts of sulphur into the stratosphere.” Meltwater from the glacier at Eyjafjallajökull has caused much of the volcano’s explosive activity – water trapped within the erupting lava results in steam explosions. As this water has evaporated the ash creation appears to have slowed.
Model future
Although bad news for airlines, Eyjafjallajökull may provide a happy outcome for modellers of atmospheric particle transport. “I would bet that many groups start modeling efforts to better predict where ash spreads and how fast it is removed,” said Ammann. “I would think there are a few people in the aviation business that might even sponsor such research.”
Ammann himself has been using the indirect effects on climate of past volcanoes to study how the Earth’s atmosphere and circulation respond to external forcing such as manmade emissions. “What we are doing with the increase in greenhouse gases is imposing a radiative forcing to which the circulation is responding,” he explained. “The problem is (a) it’s slow and (b) we have only short instrumental records. Therefore other radiative forcing events, such as volcanic eruptions and solar cycles, could tell us with more statistical rigour what the ‘usual’ response of the circulation is to these forcings. So we can learn from history.”
However, it’s not a straightforward process – researchers must evaluate if these often more short-term responses translate to the longer timescales of greenhouse warming, and if radiative forcing from the sun or volcanoes also causes similar dynamical effects, says Ammann. “In the end, these natural forcings help even on their own to test the ability of models to reproduce climatic response,” he explained. “Models have to pass these tests if we are to believe in their regional forecasts. This is hard because there is a lot happening randomly once you go to the regional level, it’s just like weather. Climate is easy once you look at global average.”
Europeans with air-based travel plans are obviously keen to know the outlook for Eyjafjallajökull. The forecast isn’t great, particularly if the wind conditions that have transported ash south and east from Iceland remain unfavourable. “What else does the past tell us?” said Amman. “I would say that it is not unusual to have this volcano oscillate in activity for some time – weeks to maybe a few months. As more lava is produced and the glacier is reduced around its crater, I would expect the overall ash production to go down somewhat. Nevertheless, episodes of further ash clouds towards Europe are certainly to be expected.”
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