The climate impact of water vapour emissions from aviation

Aviation is a unique source of anthropogenic emissions: the only regular source above the atmospheric boundary layer. Av iation currently accounts for almost 5% of total anthropogenic climate forcing, depending on the metric used. This perce ntage is expected to increase as a result of increasing passenger traffic, and decreases in the contributions from other sectors. Around 50% of the total forcing due to aviation is from the emissions of non-CO2 gases and particles, and the formation of contrails, yet many of these components have low levels of scientific understanding.

Aviation water vapour emissions have not recently been investigated in detail, but mitigation strategies need to be judg ed on their total effect. This is difficult to do if there are large uncertainties associated with certain emissions. Wa ter vapour emissions have often been assumed to be negligible. However, there is a large uncertainty associated with the best estimate of the radiative forcing. Hence, it is important to reduce this uncertainty, despite the apparent smallne ss of the best estimate.