Detection of large-scale cloud microphysical changes and evidence for decreasing cloud brightness within a major shipping corridor after implementation of the International Maritime Organization 2020 fuel sulfur regulations (Preprint)
Geoengineering News
Abstract. New regulations from the International Maritime Organization (IMO) limiting sulfur emissions from the shipping industry are expected to have large benefits in terms of public health but come with an undesired side effect: an acceleration of global warming as the climate-cooling effects of ship pollution on marine clouds is diminished. Previous work has found a substantial decrease in the detection of ship tracks in clouds after the IMO 2020 regulations went into effect but changes in large-scale cloud properties have been more equivocal. Using a statistical technique that estimates counterfactual fields of what large-scale cloud and radiative properties within an isolated shipping corridor in the southeastern Atlantic would have been in the absence of shipping, we confidently detect a reduction in the magnitude of cloud droplet effective radius decreases within the shipping corridor and find evidence for a reduction in the magnitude of cloud brightening as well. The instantaneous radiative forcing due to aerosol–cloud interactions from the IMO 2020 regulations is estimated as O(1 W m-2) within the shipping corridor, lending credence to global estimates of O(0.1 W m-2). In addition to their geophysical significance, our results also provide independent evidence for general compliance with the IMO 2020 regulations.
Stephen Salter
Hi All
What a useful paper!
I am a bit surprised that the instantaneous change is only 1 watt per square metre. The 20 bar grey scale at the bottom of the image below shows that we need at least three bars, 15% change, to detect the direction of a contrast gradient. Ship track images often show much more than this. Is it because somebody has tweaked the gamma range or that we do not see ship tracks that often? Maybe I misunderstand instantaneous.
Stephen
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Adrian Hindes
The Twomey effect estimates are much better constrained for the calculations using Re, but those using Acld show consistent results. The IMO 2020 regulations led to an ~2 W m-2 IRFACI within the shipping corridor during austral spring and an ~0.5 W m-2 IRFACI in the annual mean. Taking the ~70% decline in IRFACI from the austral spring revalues an upper bound and applying it to the -0.1 to -0.6 W m-2 range of forcing due to shipping emissions from global models (Capaldo et al., 1999; Lauer et al., 2007; Peters et al., 2013; Righi et al., 2011; Sofiev et al., 2018), global forcing values of O(0.1 W m-2) and up to +0.4W m-2 due to the IMO 2020 regulations are plausible.
(Re = effective radius, Acld = top of atmosphere albedo with clouds present, IRFACI = instantaneous radiative forcing from aerosol-cloud interactions)
So it sounds like within the shipping corridor itself, it's putatively -2W/m2 instantaneous forcing when ship tracks are present. However, the probability density plot for the estimated Twomey effect in Figure 5 shows quite a spread, particularly for Austral spring. That, along with the uncertainty range for global forcing due to shipping emissions used to extrapolate to the global effect, it sounds like there's a fair bit of uncertainty on the ~1W/m2 estimate.