As you will probably know the greenhouse effect is essentially the trapping of extra solar radiation by 'greenhouse gases'. We actually need some greenhouse gas otherwise the average temperature on earth would be perpetually below zero. Unfortunately, in the last 200 years or so the activities of man have pumped a lot of extra greenhouse gas into the atmosphere and extra warming is now taking place.
By increasing the atmosphere’s ability to absorb infra-red energy, our GHG emissions are disturbing the way the climate maintains the balance between incoming and outgoing energy. A doubling of the concentration of greenhouse gases (predicted in the next 100 years) would, if nothing else changed, reduce the rate at which the planet can shed energy into space by about 2 percent. This energy can't just accumulate - the climate will somehow have to adjust to get rid of the energy - and while 2 percent doesn’t sound much, across the entire earth it is equivalent to trapping the energy content of about 3 million tons of oil every 10 minutes.