Radiative balance refers to the equilibrium between the amount of energy from the sun that the Earth receives and the amount of energy that the Earth radiates back into space. When this balance is maintained, the Earth’s surface temperature remains relatively stable. However, changes in the upper troposphere and stratosphere (UTS) at altitudes of 10 to 50km in greenhouse gases like water vapor or water in the condensed form such as cirrus clouds (ice clouds) can disrupt this balance, causing an energy imbalance and leading to a warming or cooling of the Earth’s surface. A quantitative understanding is crucial because small changes in the radiative balance can cause significant changes in the Earth’s climate system.

Water vapor is the most significant natural greenhouse gas in the atmosphere, and even though its concentrations are very low in the stratosphere, it contributes significantly to the climate feedback. It is transported from the troposphere into the stratosphere via small to large scale mixing and transport processes and is further distributed via the Brewer-Dobson Circulation. During transport, the water vapor content can change further due to the formation of cirrus clouds in particularly cold areas. Cirrus clouds absorb the Earth’s long-wave thermal radiation, causing an overall warming of the Earth. However, the radiation properties depend on the properties of the ice particles, the altitude and the thickness of the cirrus cloud and can even reverse the climatic influence from warming to cooling. The uncertainties in variations and trends in stratospheric water vapor and upper tropospheric cirrus clouds are a significant source of uncertainty in decadal climate variability and future predictions.