Mt. Eyjafjallajökull is wreaking havoc on European travel right now, but what about the continent's environment? That's a lot of CO2 in the air, after all, but you'd be surprised to learn the airplanes it's grounded are much, much worse.
As you can see in the big, colorful graph, the European aviation industry's daily CO2 output completely dominates poor Eyjafjallajökull with a daily tally of 344,109 tons of CO2 per day. Compared to the volcano's 15,000 tons, that's quite the difference.
That said, it would be interesting to see what the ground travel contribution has been in the wake of the eruption. I know of a handful of people who took trains and buses from London to Milan, for example, to escape the ash zone and find a flight at another airport.
In any event, while travelers might malign the volcano for grounding them for days on end, the earth is apparently having a good laugh. Or maybe a belch or some flatulence—whatever human bodily function you happen to personify a volcano with is fine. [Information is Beautiful]