The Eyjafjallajökull has been unstable since the end of 2009, but this eruption from the center crater where all the ash is coming from only started on April 14th. It was only active three times in recent history, but the average time of eruptions was nearly a year each time, and every time it was active, the nearby Katla volcano also started erupting. And according to scientists, the Eyjafjallajökull is just a glass of sparkling water compared to the Katla.
Imagine what happens if this continues for some prolonged period of time and the wind changes every few weeks from east to west, sending the dust to both the European and North American continent. It would make air traffic impossible in a large part of the Western world, totally changing life for many.
It wouldn't be the first time that a volcano from Iceland had severe impact on civilization in the northern hemisphere. In 1783 the Laki caused severe ash pollution in Europe, triggered extreme weather conditions and is even said to have contributed to the French revolution in 1789 due to famine as a result of failed harvests.
One good thing though, large volcano eruptions are believed to stop the global warming temporarily.