In the ECHAM5/MPI-OM model a collapse of the Atlantic thermohaline circulation results in a global surface cooling of 0.72 K. The mechanisms that are responsible for this cooling are investigated. Additional experiments were performed with a one-dimensional radiative convective model in which anomalies from the climate model were prescribed. Fast atmospheric feedbacks are essential to maintain and strengthen the global surface cooling caused by a THC collapse. Reduced downward long wave radiation exceeds the decreased upward long wave radiation. This decreased downward long wave radiation is caused by reduced water vapor content rather than by ice-albedo feedbacks. Also, the decrease in water vapor is much stronger than suggested by the water vapor feedback expected from the simulated albedo change. The large decrease in water vapor is the main feedback. On the regional scale, changes in cloud water and cloud radiative forcing further modify the surface cooling.