Objection: Clouds are a large negative feedback that will stop any drastic warming. The climate models don’t even take cloud effects into account.
Answer: All of the atmospheric global climate models used for the kind of climate projections synthesized by the IPCC take the effects of clouds into account. You can read a discussion about cloud processes and feedbacks in the IPCC TAR.
It is true, however, that clouds are one of the largest sources of uncertainty in the GCMs. They are complicated to model because they have both positive feedbacks, preventing surface heat from escaping back into space, and negative feedbacks, reflecting incoming sunlight before it can reach the surface. The precise balance of these opposing effects depends on time of day, time of year, altitude, size of the water droplets and/or ice particles, latitude, current air temperature, and size and shape.
On top of that, different types of clouds will interact, amplifying or mitigating one another’s effects as they coexist in different layers of the atmosphere. There are also latent heat considerations — water vapor condenses during cloud formation and precipitation events, and water droplets evaporate when clouds dissipate.
The ultimate contribution of clouds to global temperature trends is highly uncertain, but according to the best estimates is likely to be positive over the coming century. There is no indication anywhere that any kind of cloud processes will stop greenhouse-gas-driven warming, and this includes observations of the past as well as modeling experiments.