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Ash Cloud Closes UK Airports. What are the chances, eh?

The equinox has just passed, marking a year since the Earth enacted its revenge on civilized Europe and ‘The Unpronounceable Volcano’ spewed ash across European airspace. The ash cloud suspended flights for six days across Europe. The volcano challenged our organised travel-dependent ways, entertained a frenzied-media, and drained the pockets of the UK airport operators, BAA, of an estimated £28 million.
As the hordes of disgruntled holiday-would-bes sorrowfully returned from their unexpected camping trip in the terminal, they might well have uttered miserably, “What are the chances, eh?”

Frankly, they are quite high. I’m not just talking from a geologist’s perspective, where the planets whizz around the sun, the continents glide across the surface of the Earth, and volcanoes pop up continuously like bubbles in a simmering stew. A fairly simple analysis of the frequency of Icelandic eruptions and the wind patterns across the Atlantic shows that we should expect ash clouds disrupting UK airports every few decades.
Iceland is a volcanically active country; a volcanologist’s play-ground, if you like. Since the turn of the century, we have seen eruptions from Hekla (2000), Grímsvötn (2004) and Eyjafjallajökull (2010). Right now, three of the 26 active volcanoes in Iceland are categorised as “restless” or have eruption warnings.

Notable Icelandic eruptions of the last century include the Gjálp (1996) and Grímsvötn (1998) eruptions which melted through the Vatnajökull icecap and caused a large flood (called a jökulhlaup); Eldfell/Heimæy (1973), where the townsfolk famously sprayed seawater on the lava flow in an attempt to divert it; and Surtsey (1963-67) when a new island was born out of the north Atlantic.

Can we put a number on the frequency of eruptions? Historically Icelanders are extremely literate (even now the country has the highest literacy rate of any nation in the world, at 100%). Icelandic writings since the earliest settlers in 930 A.D. provide detailed accounts of ‘Earth fires’. Field studies can link accounts of ‘Earth fires’ to volcanic deposits such as lavas or ash which confirm the events. By this method, we find about 200 eruptions in the last 11 centuries. Of those, around 155 were explosive (ash producing), and ash grains from at least 16 of them have been found in soils and lakes across northern Europe. Ash refers to particles less than 0.1 millimetre in size composed of exploded rock fragments and tiny shards of rapidly cooled molten rock called volcanic glass.

A similar eruption rate is found by studying lava and ash layers deposited since the last glaciation approximately 10,000 years ago. After this period, eruptions can be counted by looking at lava flows and ash layers, unless they have been affected (either masked or eroded) by glaciation. Carbon dating is then used to estimate the age of these deposits. So far, around 2,400 individual eruptions have been identified.
So both historic documentation and volcanic deposits from a 10,000 year  archive suggest that there are eruptions in Iceland around once every five years, and three-quarters of those produce ash.

At least 80 air-borne planes have unexpectedly en-countered volcanic ash in the past 15 years. Ash can damage aircraft in a variety of ways, depending on the concentration of ash and time spent in the ash cloud. The hot ash particles can either coat or erode the engine parts, making the engine less efficient or causing it to cut out, and can corrode the electronics in the plane. Additionally, the tiny shards can scratch the pilots’ windows and block the ventilation system. Indeed, ash has caused in-flight loss of jet engine power in seven of the 80 reported ash encounters.
To understand the risk that these eruptions present to UK airports, we need to consider how often ash from an eruption will be blown over to the UK. A sophisticated computer software, NAME III (Numerical Atmospheric dispersion Modeling Environment), has been developed for this purpose. NAME models atmospheric dispersal and predicts where ash grains are blown and dispersed by the wind. The London Volcanic Ash Advisory Centre use NAME software to identify regions where aircraft might encounter ash.

Scientists at the UK Met Office simulated 17,000 eruptions of the Hekla volcano, one every three hours from the 1st Jan 2003 to the 31st Dec 2008, using official weather data. In each simulation, the ash erupted in a 12 kilometre high plume, at a rate equivalent to 2,400 tonnes per second for three hours, then tracked over the following four days. An area was deemed ‘affected’ if the predicted concentration exceeded a certain threshold (equivalent to around 0.05 milligrams of ash per cubic metre). Aircraft typically fly between 9 – 12 kilometre high and so will be affected by a plume at this altitude.

The results of each eruption were compiled to create probability maps that show the likelihood of an area being affected by ash during different time periods. The probability distribution does not reflect the ash cloud shape since it represents an average of many scenarios.
Most of Europe above 50°N has at least a 20% probability of being affected within four days. The results show the track of an “average” plume, which reaches the North Atlantic (after ~24 hrs), then Scotland and Scandinavia (~48 hrs), followed by Western Europe and the Baltic sea (~72 hrs). After approximately 96 hours, it finally reaches Eastern Europe and Russia.

The Met Office researchers went on to plot the probability of 20 different European airports being affected by ash in the four days following an eruption. As we might expect, Scotland and Scandinavia come off worst. Overall, UK airports have a 1 in 3 chance of being affected by an eruption similar to that of Hekla in 1947.

Perhaps the Eyjafjalljökull disruptions were down to bad luck after all? Well, we can combine what we have learned from these studies to estimate the probability of an eruption affecting UK airports in any given year.

However, first, there are a number of important factors to consider.

Not all eruptions are equal. Icelandic eruptions can vary in size, duration and the amount of ash produced. All these factors will affect the risk of ash interruptions in the UK.

The Met Office study was carried out before the Eyjafjalljökull eruption, when the advice for aircraft was to avoid all ash. The ‘affected area’ threshold was 0.5 milligrams per cubic meter. This was good advice; the United States Geological Survey recently found that around 20% of reported aircraft-ash encounters since 1953 had resulted in engine damage or failure. However, since 2010, a new threshold of 2 milligrams per cubic metre has been introduced. This is higher than the threshold used in the study, so it is less likely to be exceeded and, as such, the probability of airport closure is overestimated.

These factors add uncertainty to any calculations, which currently only provide a rough estimate. In any given year, the probability of a UK airport being affected by ash from an Icelandic eruption is calculated as:

P = [chance of eruption] x [proportion of eruptions that produce ash] x [probability of ash reaching the UK]
P = (1/5) * (3/4) * (1/3) = 1/20

Taking into account the uncertainty added by the factors above, it is fair to say that we can expect airport closures around every 20 years. This may be a simplistic calculation, but the take-home message is that this is not just a once-in-a-millennium, once-per-century even, or even once-in-a-lifetime event. This is something we should anticipate and consequently be planning for. Perhaps we should all plan a low-cost back-up holiday in the UK as a ‘just-in-case’.

Regardless, given the effects of volcanic ash on climate, health hazard and the global industry, there is a need to further understand the factors affecting ash generation and dispersal. When Iceland strikes again, we might be able to keep the chaos to a safe minimum.

STOP PRESS: Since this article was written, there has been an eruption at another Icelandic volcano, Grímsvötn. The eruption was much more powerful than both the eruptions in the Met Office simulation and the Eyjafjallajökull 2010 eruption. Despite this, disruption to air travel was limited to short closures of some Scottish and Icelandic airports. The smaller impact was due to the weather, as the wind took much of the ash away from us, and the new rules for aviation, which allowed airports such as Heathrow to remain open, despite model simulations that suggested small quantities of ash (below the new threshold) were present across the whole UK.

John Stevenson is a post-doc in the School of Geosciences


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