We do not know how long the Iceland volcano will erupt, or if it will have the same impact of the Mt Pinatubo in the Philippines. However, here is a global temperature plot showing the Pinatubo impact. (Click for a larger image)
HadCRUT3 global monthly temperatures illustrating the effect of the June 1991 volcanic eruption of Mount Pinatubo in the Philippines. The temperature effect apparently lasted to the end of 1992, as indicated by the shaded area.
Stay tuned, I will be posting updates as more information on the impact of high latitude large scale eruptions becomes known.
H/T to Climate4You for the graphic.
Update #1(04-15-10, 8:50): Watts Up With That has a discussion of volcano impacts on climate.
Update #2 (04-15-10, 9:15) In November 2009, Joe D’Aleo wrote a paper on Volcanoes and CO2 and Global Temperatures. He wrote this about volcanoes and cooler temperatures, especially in the summer. Do not put away your sweaters yet.
Major volcanoes (those that reach well into the stratosphere) do have more effect on climate than CO2 but the effects are temporary. It is not the ash but the sulfur dioxide which is quickly converted to sulfate aerosols that is responsible for reducing the incoming solar radiation by backscattering it to space. Reductions of 7% were observed after Agung in the 1960s. For this reason, major eruptions of volcanoes cause global cooling. This is well established and non-controversial. The cooling is greatest in summer with a mixed picture in winter depending on whether the volcanoes are low or high latitudes. Tropical volcanoes lead to a stronger polar vortex and positive arctic oscillation in winter which mean less cold in Western Europe and eastern United States. High latitude volcanoes have a tendency for a negative arctic oscillation and colder temperature into mid-latitudes winter and summer (Oman and Robock 2006).
Full paper here, showing the impact of volcanos when combined with an El Nino, La Nina, and PDO.