Many scientists claim that we are long past the due date in getting back to the Ice Age. I also encountered so many people who say that the global warming can be a good countermeasure against the planet's tendency to cool itself. Of course, I do not think this argument is worth countering. I do not think either that there is a serious scientist who puts forward this kind of unsophisticated statement.
However, I am reading Bill Bryson's A Short History of Nearly Everything, and I encountered a wonderful line about the above-mentioned comment, which goes like this: "When you are confronted with a fluctuating and unpredictable climate, the last thing you want to do is to conduct a vast unsupervised experiment on it."
Of course, there are other, more scientific arguments, for example, that a rising temperature might induce an ice age by making more clouds with expedited evaporation. However, I think the argument above is more than sufficient to counter such a naive speculation.
To sum it up, we human race do not understand the delicate eco-system. However, it is much better (and safer) to contain our efforts to reducing our impact on the planet, than to tinker around it.