The only people who say they are certain are the proponents of 2012, not scientists. We show how they cannot be certain based on the all important pieces of evidence we have at hand - that is to say, the evidence we have does not agree with the proponents claims.
For example, pole reversals are known to happen and expected to happen in the future, but no amount of evidence that has ever been found has pointed to it occurring in the span of two years - even in an average human lifetime you'll have a challenge waiting for something to happen, and even then, in the case of Geomagnetic reversal, you wouldn't know much about it anyway, besides compasses pointing in another direction.
With objects in space, we are finding new things all the time and as soon as possible we work out what their future is in relation to ours, and once again, nothing has been found with a certainty of slamming into us in 2012, of any size and shape that proponents want to mention.
The LHC has gone through multiple checks and tests - it wouldn't have been built had it not be shown to work, but it is only one of many particle accelerators around the world: the second highest energy particle accelerator ever built (the LHC has since become the highest) is the Tevatron, located in the middle of America. That's caused no panic, it seems hardly anyone has heard about it, but the principles of it and the LHC are the same, the LHC just happens to be bigger and better.
In other words, we do not say with certainty that nothing will happen (Earthquakes, for example, are pretty much guaranteed to happen because the Earth is still a geologically active planet), but we do say with certainty that the evidence does not agree with the likes of Geryl and Rand, and that what they say is so far removed from science that it's the claims are usually impossible in the first place.
Hopefully that's cleared it up a bit.