I think you are mixing weather and climate. Climate is long term average of weather (directly from definition of word "climate"). Weather is very hard to predict 2 weeks ahead. Climate is stuff like "average rainfall in march in past 50 years". Some processes affecting climate may also be chaotic, but they are slow (by definition, otherwise they would be part of weather), and so I don't think 500 years is beyond speculation at all. Glacial periods have lately been around 100000 years, so 500 years to future we are not going to be much more or less glacial than we are now.