The graph below is Figure 1 in Chapter 8 [PDF] of the latest IPCC report. The black line shows temperatures, the yellow lines show results from 14 different models, and the red line is the average of all the simulations. As you can see, the four biggest tropical eruptions over the past century had slight cooling effects. …
– via edf.org
… the chaos caused by the volcano is necessarily coming to an end, though. Andy Hooper, a geologist who did his postdoctoral research at the University of Iceland’s Institute of Earth Sciences, pointed out in a blog post for Reuters on Monday that “it remains a very real possibility that the volcano will continue to erupt on-and-off for months to come, as occurred during the last eruptive period” at Eyjafjallajokull in 1821-23. Mr. Hooper added, “Like 1821-1823, this current eruption is likely to remain small in terms of volume, but in an age of mass aviation, a relatively small amount of erupted ash is having huge consequences.”
Mr. Hooper also noted that while “Eyjafjallajokull is a relatively small volcano and unlikely to erupt the volumes of material that will have a significant impact on climate,” it is also true that “eruptions of Eyjafjallajokull in 1821-1823 and 1612 were followed in short shrift by eruptions of its much larger neighbor, Katla.” An eruption of Katla could make the disruptions caused by Eyjafjallajokull seem trivial.
Ominously, Mr. Hooper also suggested that increased volcanic activity in Iceland might be inevitable if the planet continues to warm:
At the end of the last ice age, the rate of eruption in Iceland was some 30 times higher than historic rates. This is because the reduction in the ice load reduced the pressure in the mantle, leading to decompression melting there.
Since the late 19th century the ice caps in Iceland have been shrinking yet further, due to changing climate. This will lead to additional magma generation, so we should expect more frequent and/or more voluminous eruptions in the future.