One of the (many) unknowns in the climate change 'debate" is the issue of the temperature structure of the atmosphere and its response to changes in composition and insolation.

Opponents of solar-driven climate change argue that the very small changes in solar output are inadequate to explain the small, but important changes observed in terrestrial temperatures. A way to understand this may come in understanding the vertical and horizontal structure of the atmosphere and its temperature, rather than dealing with the atmosphere as an amorphous blanket.

We see changes of temperature and atmospheric structure on other planets - principally Mars where the recurring dust storms act to increase atmospheric opacity and absorption of incoming solar energy, and so heat the upper atmosphere at the expense of the surface, which cools and therefore stops the convective activity (wind) which originally stirred-up the dust. This is why dust storms on Mars are not permanent.

In a somewhat similar way, changes in the temperature structure of our own atmopshere, due to varying absorption of different wavelengths can cause the surface to warm while the upper atmosphere cools. There may also be polar vs equatorial changes.

And not just temperature is involved - the article referenced by Podrock suggests that changes to the water saturation of the upper atmosphere are important in allowing high-altitude cloud formation.

Now, what is the "root cause" of all this - well, that's the $64 Trillion question...