A fluid such as air or water that is heated will expand, and expanding fluid will cool,
See: http://www.chem.ufl.edu/~itl/4411L_f00/gamma/gamma.html
If the Earth's oceans and atmosphere is heated to more than the average temperature, then the said fluids will expand to a greater than average volume.
If the expanding atmospheric molecules have a velocity that exceeds the escape velocity of the planet, then the molecules have the ability to disperse into space.
If the atmospheric molecules escape the planet, then the adiabatic expansion and cooling of gasses will take effect and the planet's atmosphere will become colder.

So, what is heating the Earth's atmosphere?
Perhaps the greenhouse effect is true, perhaps the sun's energy has increased, or perhaps something else that people have been doing in the past seven decades is responsible, or perhaps all of those effect are combining to give us the observed results.

I suspect the following.
Number of operating nuclear reactors in the world= 436, generating 374,411 MW(e)
http://en.wikipedia.org/wiki/Nuclear_power_by_country

a 1000MW(e) nuclear power station – produces approximately 30 tonnes of high level solid packed waster per year
http://nuclearinfo.net/Nuclearpower/WebHomeWasteFromNuclearPower

30 times 374,411 = 11,232,330 tonnes of high level solid packed waster per year.
If that amount has been accumulating for ten years, then there is about 112,323,300 tonnes of the waste material in storage.

Heat generated by radioactivity, see
http://geoinfo.nmt.edu/resources/uranium/basics.html

According to this site, one gram of uranium will give off about the same amount of energy as burning three tonnes of coal.

If there is 112,323,300 tonnes of highly radioactive uranium around and one gram of it gives off an equivalent energy of three tonnes of burning coal, then 3 times 112,323,300 = 336,969,900 that equals the energy equivalent of burning coal that is given out by each gram of the radioactive material.
There is million grams in a ton, therefore we would need to burn 112,323,300,000,000 times 3 or 336,969,900,000,000 tonnes of coal to get the same amount of energy that has been released by the stored nuclear waste material during the estimated time of ten years.

I'm still wondering if such amount of extra energy expelled into the environment can affect the global warming and subsequent cooling effect.