It's complicated to explain so I'll refrain from doing so, instead I'll just tell you the basics (f you'd like detailed explanations just ask).
It's physically impossible for the planet to cool down in the short term unless humans intentionally or unintentionally cause massive global cooling. Cooling can only occur when heat loss exceeds heat gain, the physical composition of the planet means we will continue to gain more heat than is being lost.
The natural cycles that the planet goes through puts us in a long term warming phase, a very slow one but warming all the same. If all humans disappeared off the face of the planet it would continue warming of it's own accord for thousands of years.
The talk about the possibility of the planet cooling seems to originate from the notion that we may be causing another ice age as a consequence of global warming. This notion has no scientific backing and seems to be the product of an over-imaginative media more than anything.
One possible consequnece is that global warming could lead to localised cooling in some parts of the world. This is not global cooling but merely a redistribution of heat. For every bit of cooling there will be an equal amount of warming elsewhere.
This scenario is a little understood possibility that could happen (it's unlikely but possible) . With the melting of the polar ice caps comes the introduction of cold, fresh water into the seas and oceans. This additional burden could interupt the more vulnerable parts of the oceans circulatory system. Most at risk is the Gulf Stream which carries warm water to the shores of northwestern Europe.
If the Gulf Stream were to shut down, diverge, truncate or divert then countries such as Iceland, Ireland, the UK and parts of Scandinavia would cool down by up to 7°C. The rest of the world would be pretty much unaffected whilst over the course of thousands of years glaciers would readvance into the parts of Europe that had cooled significantly.
This is only a possibility and isn't clearly understood. The more time that passes the less chance there is of it happening. Meltwater is already pouring into the North Atlantic and so far it's had no effect on the Gulf Stream.
As I mentioned, if you'd like any of the above points explaining in detail please let me know.