Weather: When volcanic eruptions change the climate

Monday, August 9, 2010

While the Eyjafjallajökull eruption in April is unlikely to have a long-term impact, volcanologists warn that the story may not yet be ended

by HENRY HENGEVELD

This past April, the Eyjafjallajökull volcano in southern Iceland began to erupt, ending a slumber of almost two centuries. Over subsequent weeks, it released an estimated 100,000 tons of ash into the lower atmosphere. The ash plume rose to elevations as high as 9,000 metres, refracting incoming sunlight and providing Europeans with some unusually spectacular sunsets. 

While not large as volcanoes go, the Eyjafjallajökull ash plume also had a major impact on global air traffic – and hence the economy. Europe, directly in the path of the plume for much of the time, was particularly hard hit.

More than 63,000 aircraft flights from and into the region were cancelled within the first week after the eruption began and cancellations continued periodically until mid-May. Those flights that did take off had to take on extra fuel and follow a more circuitous route in order to avoid lingering ash plumes. Estimates of related impacts on the European economy are in the hundreds of millions of dollars.

The Eyjafjallajökull eruption was also significant from a meteorological perspective. That became apparent within a few days after the eruption began, when politicians and airline officials began complaining about the accuracy of the British Meteorological Office's predictions on how winds were affecting the height and path of the ash cloud downwind from the volcano.

Many argued that these forecasts were too pessimistic and hence caused unnecessarily frequent closures of airports across Europe. Some critics were, rather unrealistically, demanding "factual" information on where the plume was located, rather than "probabilistic" forecasts.

However, much of this controversy appears to have been misplaced. First, since the plume's position and height changed from hour to hour, decisions on airport closures needed predictions, not data about the past. Secondly, subsequent data sampling of the ash plume over Great Britain showed multiple layers of ash clouds up to elevations of 6,000 metres, and confirmed that the forecasts were remarkably accurate.

Furthermore, the forecasters quickly responded, their task was to provide the best possible data and predictions to aviation authorities, who then decided on how to minimize risks to travellers.  Aviation authorities, in turn, used two incidences in the 1980s, when aircraft flying through ash clouds over Alaska and Indonesia temporarily lost all engines, as a rationale for zero tolerance for aircraft flying through such clouds.  

The Iceland eruption was also a reminder that volcanoes can be an important factor in climate fluctuations. Historical records provide some startling examples of this. For North
America, the most dramatic historical example of climate impacts due to volcanoes was likely those due to the eruption of Mount Tambora, in south-east Asia, in 1815. Estimated to be the worst eruption of the past millennium, it caused tidal waves that killed more than 70,000 people and injected large volumes of sulphate aerosols into the stratosphere, some 15 to 20 kilometres above the Earth's surface. 

It takes several years for aerosols at this level to settle back to the surface, allowing the aerosols to spread around the entire planet. Because sulphate aerosols are highly reflective to incoming sunlight, they reduce the amount of solar energy reaching to Earth's surface. Hence, their presence cools the Earth's surface.

In the Tambora case, extremely cold temperatures in Europe and North America during the subsequent summer of 1816 – the "year without summer" – brought excessive rain, snow storms and frost in New England in July and August, large-scale crop failures in the western world, famine across much of the Northern hemisphere and subsequent large westward migrations of New England farmers. 

Other notable volcanic events of climatic significance have followed, including that of Krakatoa (Indonesia) in 1883, El Chichón (Mexico) in 1982 and Mt. Pinatubo (Philippines) in 1991. 

However, there are also many explosive volcanic eruptions that have little effect on climate.  One that most of us will remember quite well is the disintegration of the top of Mount St. Helens (Washington State) in 1980. That blast, together with related mudslides and lava flows, killed 53 people in the region and lowered the mountain top by more than 1,300 feet. Yet it had very little effect on global or regional climate.

That is because the dust and ash it generated remained within the troposphere (the lower 10 to 12 kilometres of the atmosphere), where precipitation processes and gravity quickly allowed most of the material to settle to the surface within days and weeks. Very little of the dust reached the stratosphere. 

The good news is that the Eyjafjallajökull eruption earlier this year also released its ash into the troposphere, and hence is unlikely to have a significant long-term effect on weather. However, volcanologists warn that the story may not end there. In the past, an eruption of Eyjafjallajökull has often been accompanied or followed by a much larger eruption of the nearly Laki volcano. 

An eruption of Laki in 1783 spewed out some 14 cubic kilometres of lava and ash and released an estimated 80 million tonnes of sulphate aerosols into the stratosphere over an eight-month period. The following winter, North Hemispheric temperatures are estimated to have dropped by about 1 C below the norm. A quarter of the human population of Iceland perished from starvation. 

Here's hoping we don't get a repeat. BF

Henry Hengeveld is Emeritus Associate, Science Assessment and Integration Branch/ACSD/MSC, Environment Canada.