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As known to most by now, the Icelandic volcano Eyjafjallajökull, is currently in outbreak.  However, it is likely that this is only a forewarning to the eruption of the much larger neighbour, Katla.

When Katla erupted in 1823, it was preceded by an eruption of Eyjafjallajökull.  The same thing has happened a number of times before 1823, so if we experience a repeat of this, what will the consequences be?

Nobody can tell for sure, but it is pretty certain the smoke, steam and ashes will hinder some percentage of the Sun's light in reaching the Earth's surface.  This will make it colder for some years after the event, despite the volcano's attempt to follow IPCC and counteract with exhalation of huge amount of CO2 (estimated 219 Mt in year 934).

The most extensive eruption from the Katla system (Eldgja Fissure System), was in 934 during the Viking age.  There are scientists who assume that a repeat of this magnitude could be plausible.  However, Katla is monitored both visually and with instruments and does not show signs of increased activity.

The picture shows the volcano Eyjafjallajökull before it erupted.  The outbreak is happening at the plateau in the upper half of the picture, which was covered by ice.
Photo by Ívar Örn Benediktsson.

References:
Eldgja Fissure System, Katla Volcano, Iceland
Vulkanudbrud i Island: Er Katla ved at vågne?