The invention relates to a device and a method for determining and indicating, on board of an airplane, climate-relevant effects of a contrail produced by the airplane. The method for determining and indicating, on board of an airplane, climate-relevant effects of a contrail produced by the airplane includes determining radiation data of short-wave and long-wave atmospheric radiation directed upwards in the earth atmosphere at location of the airplane, determining an air temperature and air humidity at the location of the airplane, determining a zenith angle of the sun at the location of the airplane, determining a radiative forcing value produced by the contrail based on the determined radiation data, the air temperature and air humidity, and the zenith angle, and indicating at least one of the radiative forcing value and variables derived therefrom.