I recieved an email recently asking some questions about Katla and the sender has kindly let me post my replies to him. If anyone has any questions like this please feel free to use the contact form or post a comment!
"About a year ago, Stephen Sackur said that 'when Katla blows, we're all going to suffer'. I'm not sure what he meant by that, but since then I've seen several articles claiming that a Katla eruption can also cause lung conditions and, through the melting of the jokulhlaups or melting the Greenland and polar ice sheet, through depositing an ash deposit over them, affect the salinity of the Atlantic current, changing the climate of the northern hemisphere, and causing crop failures. (http://bit.ly/xLx4sN) I've also read that an eruption could cause acid rain if the acid coating on the ash is removed by rain. (http://bit.ly/w8VDqN)I know the media like to exaggerate, but is it at all likely that we could 'suffer' in any way other than prolonged travel disruption?"
I could certainly write a very long reply to your question but will make it as brief as I possibly can! As with any eruption, people with lung conditions or even lite asthma can be effected by even the smallest amount of voclanic ash in the air (as occured with the eruption of eyjafjallajokull in 2010), unless the eruption were to reach into the stratosphere (which is unlikely), the effect from gases/ash on populations would only likely effect Europe if there were the correct prevailing wind conditions. Any overall effect would likely be small and limited to already underlying lung conditions. Historically, Icelandic volcanism has had an effect on European health after the Laki fissure eruption in 1783, this released a large amount of Fluorine and caused widespread health issues (involving fluorisis - not an expert but can cause bone deformation/growth). However this was of a different eruption type than will likely occur at Katla.
With regard to any other effect, any covering of ash on the Greenland ice sheet would likely be small, if you look at historical eruption sizes these are around VEI 4/5, about the size of St Helens, or more recently Chaitén. Would this change salinity of the North Atlantic Drift - I doubt it, the quantities involved would not be great enough, unless a positively huge eruption occurred (at least VEI 6/7+, which is unlikely in Iceland with this type of eruption). I assume your reasoning or sources reasoning comes from the fact that ice covered in black/grey ash will absorb more light and therefore heat up and melt more quickly.
Can volcanic eruptions cause acid rain, in short yes they can, whether this is prolonged and to what extent depends on the length of the eruption and the distribution of gases/ash in any plume.
With regards to the possible climatic effect I refer you to a comment by someone on a previous post of mine on Katla: http://www.volcano-blog.com/1/post/2011/12/katla-the-volcano-that-everyone-is-waiting-to-blow.html#comments which explains it better than I probably could. More sulphur dioxide in the atmosphere (particularly the stratosphere) can cause temperature regime changes which would affect crops, but again, unlikely on any large or majorly noticeable scale.
In summary, there is always potential for global/regional effects from an eruption of Katla on a global scale, however whilst possible on the whole they are not probable.