The positive feedback of water vapour has been the basic feature of General Circulation Models (GCMs), which approximately doubles the warming impacts of any other climate drivers. Some published scientific papers have shown that simple climate models without this feature can simulate the temperatures of the 2000s very well. On the other hand, the observed humidity observations revealed that it varies, but not according to the water feedback theory. There is a need for an optional method for calculating the warming impacts of water vapour. In this study, the radiative forcing (RF) value of water vapour for different atmospheric water amounts has been calculated by applying the line-by-line (LBL) method. A simple climate model by the author has been modified by implementing this dependency in the same way as for the other greenhouse (GH) gases. This model has been used for the simulations of absolute yearly temperature and humidity changes, as well as for decadal-long changes by applying CERES (Clouds and the Earth’s Radiant Energy System) observations. These simulations reveal that humidity increases are strongly related to the primary energy changes of the absorbed solar radiation (ASR). The yearly temperature variations of the hemispheres show that water vapour increase has about a 14 % temperature impact and not about 100 % as assumed by the water feedback theory. This water vapour RF effect explains good results in simulating the high temperatures of the 2000s. The recent rapid warming during the 2000s is mainly caused by ASR variations, and this new calculation method can be applied in temperature simulations.
