You might just have the Sun get a little warmer, (some theories still see solar fluctuation as the most likely cause of the cycling in and out we've had since the continents drifted together and made Ice Ages possible) or increased volcanic eruptions causing an increase in greenhouse gases. Maybe the continental configuration theory is wrong and the earth is just returning to its normal tropical state.
One of the theories I've heard most in regard to the present global warming has it triggering an ice age. Greenland ice melts and causes a deluge of freshwater into the North Atlantic. This shuts down the Atlantic circulation which is dependent upon salinity differences remaining the same. The result is that Europe becomes as cold as it ought to be, which is subarctic if you compare it with similar latitudes in NA and Asia.
Here the world is really affected if this happens anytime close to now because the big loser here is Europe, maybe the most world dominating cultural hearth to yet exist. Its already in decline as such, but what will happen if several successive years without summers hit it?
In general I think climactic change can be among the most entertaining AH scenarios possible because the complexity of the causes, coupled with varying theories, means you can turn Florida into the Ra'ub al Khali and get away with it.
One general principle I can think of that applies to these scenarios is that there are winners and losers with everything balancing in the end. Depending on who these are is whether your story has an apocalypse or hardly makes the news. Can anyone think of another?
