While the effect of volcanic eruptions on the global mean surface temperature is cooling, there are circumstances where the effect is not only smaller than the mean, but warming was observed. For example, in the first winter following a number of different volcanic eruptions, the Northern Hemisphere and Eurasia on average warmed, in contrast to northern Africa and southwestern Asia, which cooled. Robock and Mao propose that the warming is due to changes in the winter circulation pattern, associated with an enhanced polar vortex, which lowers the extent of normal winter cooling. 
A small volcanic eruption has started near Bardarbunga volcano, according to the Icelandic Met Office. All air traffic is now prohibited in a large radius around the volcano. The Met Office has upgraded its alert level to red. A 25 km (16 mi) long dike has formed beneath the surface.
Authorities say that an evacuation program has been set in motion, but there are currently not enough information to decide whether Kelduhverfi and Oxarfjordur, on the north coast, will be evacutaed. A number of tourists are in the area.
This story, by the Icelandic National Broadcasting Service (RUV), was updated on 23 August 2014, at 14.59 GMT.
Updates in English will be posted at: ruv.is/volcano
The Icelandic Met Office says a small subglacial eruption started today beneath the Dyngjujokull outlet glacier, near the Bardarbunga caldera. Scientists aboard a surveillance airplane above the glacier see no signs of an eruption yet. All flight traffic has however been banned near the volcano.
Kristin Jonsdottir, geophysicist at the Icelandic Met Office says that this morning, increased seismic activity and increased tremor was observed around the Bardarbunga caldera, especially in a 25 km. long dyke intrusion north and east of the caldera, near the edge of the Dyngjujokull outlet glacier.
Shortly after 2 PM GMT, the Met Office declared an aviation alert for a large area around Bardarbunga and said a small eruption was believed to have started under the glacier. No signs of glacial flood has been observed and scientists in a surveillance airplane above the glacier see no sign on the surface of the glacier. Even so, the measurements indicated a small eruption under the glacier and it is now believed that it was small enough, not to cause significant melting of glacial ice. A larger eruption can not be ruled out, according to the Met Office.
The dyke intrusion has been forming over the last few days. It is now believed to be around 25 km. long, and about 0,2 - 0,3 cubic kilometers of magma is thought to have entered the intrusion from a magma chamber beneath the Bardarbunga caldera.
At this stage measurements taken are based on a small event. The Jökulsárgljúfur canyon has been closed and evacuation of tourists in that area and around Dettifoss waterfall has started. The situation at this stage does not call for evacuation of habitants in Kelduhverfi, Öxarfjördur and Núpasveit. People in those areas are encouraged to watch news closely and have their mobiles switched on at all times.
23rd August 2014 17:08 - status report
Overall assessment from the joint daily status report 230814 of the Icelandic Met Office and the University of Iceland, Institute of Earth Sciences:
The aviation color code has been raised to "red" as the data is currently interpreted as a subglacial eruption. Both the thickness of the ice at the possible contact point (100-400 m) and the volume of lava in possible contact with ice are highly uncertain. It could be 0-20 hours before lava reaches the surface of the ice. It is also possible that the lava will not break through the ice, and the eruption could remain subglacial.
Meanwhile, the attention on Bárðar’s bulge has prompted a discussion of the relationship between volcanoes, melting glaciers, and climate change. As a glacier recedes, its enormous mass is removed from the land. Relieved of that load, the land rebounds slightly and the air pressure underground is reduced, enabling more magma to accumulate; eventually, some of that magma will rise and erupt through the Earth’s surface. In other words, global warming could alter the shape of the planet. “If you deglaciate Iceland, volcanism in Iceland should increase,” Jerry Mitrovica, a professor of geophysics at Harvard, said. “We’re moving the hand from the door and allowing the door to swing open.”
What is the best-case scenario?
That the dyke intrusion stalls in the crust and cools, and the eruption at its tip ceases.
What is the worst-case scenario?
Unfortunately there is more than one. The first is that an eruption might start at the Bárðarbunga volcano itself, and there is a remote possibility that this could be a large explosive eruption producing an ash cloud. Fortunately because of the ash cloud produced during the Grímsvötn eruption of 2011 we have a fair idea of what this might look like and how best to minimise disruption to air travel. A reassuring fact is that lessons learned and changes made as a result of the Eyjafjallajökull 2010 eruption meant that despite erupting twice as much ash, disruption due to the Grímsvötn 2011 ash cloud was much less.
The second is that the dyke intrusion continues to the north-east and triggers an eruption at the Askja volcano. Askja last erupted in 1961, but its most notorious eruption was in 1875 when an explosive rhyolite eruption produced an ash cloud that spread over northern Europe. Rhyolite is a “sticky” magma type that fragments more easily into ash, hence has a higher potential to produce ash clouds that cause disruption to air travel.
However, we are unlikely to have a repeat of the 1875 eruption because there is probably not much rhyolite magma left, plus there is now a deep crater lake covering the 1875 eruption site. There remains the possibility that some ash could be produced if a dyke intrusion mixed with the remaining rhyolite magma and triggered an explosive eruption.
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