Abstract
Abstract. The COVID-19 pandemic caused significant economic disruption in 2020 and severely impacted air traffic. We use a state-of-the-art Earth system model and ensembles of tightly constrained simulations to evaluate the effect of the reductions in aviation traffic on contrail radiative forcing and climate in 2020. In the absence of any COVID-19-pandemic-caused reductions, the model simulates a contrail effective radiative forcing (ERF) of 62 ± 59 mW m−2 (2 standard deviations). The contrail ERF has complex spatial and seasonal patterns that combine the offsetting effect of shortwave (solar) cooling and longwave (infrared) heating from contrails and contrail cirrus. Cooling is larger in June–August due to the preponderance of aviation in the Northern Hemisphere, while warming occurs throughout the year. The spatial and seasonal forcing variations also map onto surface temperature variations. The net land surface temperature change due to contrails in a normal year is estimated at 0.13 ± 0.04 K (2 standard deviations), with some regions warming as much as 0.7 K. The effect of COVID-19 reductions in flight traffic decreased contrails. The unique timing of such reductions, which were maximum in Northern Hemisphere spring and summer when the largest contrail cooling occurs, means that cooling due to fewer contrails in boreal spring and fall was offset by warming due to fewer contrails in boreal summer to give no significant annual averaged ERF from contrail changes in 2020. Despite no net significant global ERF, because of the spatial and seasonal timing of contrail ERF, some land regions would have cooled slightly (minimum −0.2 K) but significantly from contrail changes in 2020. The implications for future climate impacts of contrails are discussed.
Highlights
The COVID-19-reduced aviation water vapor emissions (COVID)-19 pandemic lockdown caused lots of economic disruption in 2020
We focus on the differences between ensembles with and without aviation or COVID-19-affected aviation for key climate parameters
We will refer to the COVID-19-affected aviation simulations in the figures as “COVID”
Summary
The COVID-19 pandemic lockdown caused lots of economic disruption in 2020. The reduction in greenhouse gases (GHGs) and pollution (Le Quéré et al, 2020) likely impacted global temperatures (Forster et al, 2020). As recently reviewed by Lee et al (2021), global aviation warms the planet through both CO2 and non-CO2 contributions. The resulting clouds scatter solar (shortwave – SW) radiation back to space, cooling the planet. Integrating over space and time, the net effect of contrails is to warm the planet (Lee et al, 2021) through a balance of the longwave (warming) and shortwave (cooling). A. Gettelman et al.: The climate impact of COVID-19-induced contrail changes in contrails due to COVID-19 would be expected to cool the planet. This study will document the updated version of the contrail model that is publicly available as part of the Community Earth System Model, version 2.2, and its contrail effective radiative forcing (ERF) for a “normal” aviation in year 2020 with no pandemic reductions.
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