This is a slide from a talk given by David Bader of Lawrence Livermore on behalf of Brian Soden of the University of Miami. It shows the four main feedback mechanisms that are believed to play a role in climate change.
They are the temperature and water content of the atmosphere; ice and snow cover; and cloud cover. Physicists are fairly certain that the first three are have a positive-feedback effect — that is they tend to increase the rate of global warming — but they are not so sure about clouds.
In particular, the effect of stratocumulus clouds on climate has been very difficult to understand. The problem is that these puffy clouds are very thin and turbulent, making it hard to understand the physics of how they participate in the transfer of radiation into and out of the atmosphere.
According to Bader “uncertainty of cloud feedback is the primary cause of uncertainties in climate models”. That sounds like a challenge to the physics community.