Observing long-lived longwave contrail forcing

Abstract. Contrail microphysical simulations and climate simulations have indicated that contrail cirrus cause a substantial fraction of aviation's climate impact. While the approximations and parameter selections in these simulations have been well-validated over the past 2 decades, the heat trapping of contrails has not been observed using satellite data beyond a few (3–6) hours. This is because contrails lose their linear shape after a few hours, making them difficult to distinguish from natural cirrus clouds. Here we provide satellite-driven analysis of the longwave component of “long-lived” contrail cirrus forcing (i.e. both linear and non-linear contrail cirrus) over North and South America. We aggregate a dataset of GOES-16 estimated outgoing longwave radiation and advected trace density of flight paths, and apply causal inference to discern the effect of contrails while controlling for radiative and cloud confounders. As a means of validation, we also generate synthetic datasets with known ground truth, and confirm that applying the causal inference method is able to recover the synthetic ground truth. Since this method yields an estimate which has some differences from both “instantaneous radiative forcing” (iRF) and “effective radiative forcing” (ERF) estimates which have been reported in the literature so far, we introduce the new term “observational radiative forcing, 12 h” (oRFH=12). Our analysis estimates the longwave oRFH=12 from contrails over the Americas averaged 46.9 gigajoules per flight kilometer (95 % CI: 35.2 to 58.6 GJ km−1) during April 2019 to April 2020.