Tradeoffs are a bitch

Treehugger.com (and that should tell you which side they come down on) has an article by Jeff Nield on who's worse for the environment, airplanes or volcanoes:

Admittedly, it's pretty tough to gather reliable data from an erupting volcano, but we're suckers for a graph, and the folks over at Information is Beautiful have been busy crunching numbers to give us the visual above. They've taken a crack, three cracks actually, at graphically comparing the daily CO2 output of the Eyjafjallajökull eruption and the CO2 savings from all the grounded flights. (Yes, we know, it's not that simple; read on for more explanation.)
The authors of the graph, like any good data geeks, have been transparent in the data they've used to create this visualization, but it's really too early to tell if it is accurate. That being said, even if Eyjafjallajökull's emissions were double their estimate, the immediate daily CO2 emissions saved from grounded planes would outstrip the mountain's output.
Breitbart suggest another view. Extrapolated over a year, the emissions would place the volcano 47th to 75th in the world table of emitters on a country-by-country basis, according to a database at the World Resources Institute (WRI), which tracks environment and sustainable development. A 47th ranking would place it above Austria, Belarus, Portugal, Ireland, Finland, Bulgaria, Sweden, Denmark, and Switzerland, according to this list, which is from 2005.
That's a pretty hefty amount but, of course, it's not often that a volcano erupts for an entire year. The report continues with some perspective:

Experts stressed that the volcano contributed just a tiny amount, less than a third of one percentage point, of global emissions of greenhouse gases. Total emissions of six heat-trapping gases in 2005 were more than 36 thousand million tonnes (36 gigatonnes) as measured in CO2, according to the WRI index. "It's not of any significance compared to the anthropogenic [manmade] budget," said Kjetil Toerseth, director of regional and global pollution at the Norwegian Institute for Air Research.

Finally, for those out there giving Eyjafjallajökull props for slashing European emissions, a (possible) further dose of reality:

...carbon is emitted when passengers stranded by air travel use the train, bus, car or ferry as an alternative. And many flights in, to, and from Europe are merely being deferred until the crisis is over.
"Whether the emissions occur now or three weeks from now does not change things fundamentally," said Herve Le Treut, a French climatologist.

Now that flight restrictions have been lifted and air travel has resumed, any dip in emissions caused by Eyjafjallajökull is but a tiny blip on the global warming radar.