What would happen to Earth's atmospheric temperature if the amount of incoming shortwave radiation from the Sun decreased and was less than the longwave radiation lost to space?

The Earth, to remain thermally stable over time, must be able to evacuate, on average, all the energy received in the form of radiation. Otherwise, its temperature would rise to 800 000 ° C over the next billion years [1].

Of the solar energy that arrives on Earth, in the form of shortwave radiation, almost 30% [2] is reflected back to space by the surface and the atmosphere (see albedo), reaching the surface on average about 240 W / m² The energy that manages to reach the earth's surface is returned to space in the form of infrared radiation. However, greenhouse gases such as water vapor and carbon dioxide cause the bulk of this infrared radiation to be emitted into space from about 5 km of altitude [3], causing heating of the lower part of the the atmosphere we know as the greenhouse effect. The net flow of energy entering and leaving the climate system is called the terrestrial energy balance [4] or, alternatively, radiative balance [5].