Aerosol-cloud-climate cooling overestimated by ship-track data

Glassmeier, Franziska; Hoffmann, Franziska; Johnson, Jill S.; Yamaguchi, Takanobu; Carslaw, K. S.; Feingold, Graham

doi:10.1126/science.abd3980

Cited by 75 publications

(114 citation statements)

References 55 publications

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“…However, the net almost nil effect was composed of large negative CRE under a clean background ( N d‐bg < 50 cm −3 ), which was nearly fully countered by a positive forcing under polluted background ( N d‐bg > 70 cm −3 ). A possible explanation for this positive forcing is the cloud burning effect of the black carbon from ship emissions (Azzara et al., 2015; Glassmeier et al., 2021).…”

Section: Discussionmentioning

confidence: 99%

The Dependence of Ship‐Polluted Marine Cloud Properties and Radiative Forcing on Background Drop Concentrations

Zhu

Mao

et al. 2021

JGR Atmospheres

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Marine low clouds of the busy shipping lane in the southeast Atlantic during the springs of 2003–2015 were analyzed to study the dependence of their properties and radiative forcing on the background cloud drop concentrations (Nd‐bg). The overall average cloud radiative effect within the shipping lane was larger by only −1 Wm−2 compared to the adjacent clouds. However, this near‐zero averaged effect was composed of large negative cloud radiative forcing (CRF) for the cleanest (13%) of the cases with Nd‐bg < 50 cm−3, which was almost neutralized by a positive CRF for the 40% of the cases with Nd‐bg > 70 cm−3. A possible explanation for this positive forcing is the cloud burning effect of the black carbon from ship emissions. The negative forcing was composed of 45% for the cloud fraction (Cf) effect and 55% for the albedo effects, which included a small contribution of the liquid water path (LWP) effect. Positive Cf susceptibility to Nd‐core was at maximum for the lowest Nd‐bg and disappeared near 60 cm−3. The albedo susceptibility to Nd‐core reached its theoretical limit of 1/3 for constant LWP when Nd‐bg <40 cm−3, and diminishes to 0.2 when Nd‐bg = 60 cm−3. These susceptibilities represent cause and effect relationships because the differences of aerosols caused by the ship emissions vary at a much smaller scale than meteorology. Globally, under similar environmental conditions, nearly half of the area has Nd < 50 cm−3, thus possessing the indicated large susceptibility of negative radiative forcing to anthropogenic aerosols.

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Section: Discussionmentioning

confidence: 99%

The Dependence of Ship‐Polluted Marine Cloud Properties and Radiative Forcing on Background Drop Concentrations

Zhu

Mao

et al. 2021

JGR Atmospheres

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“…These ship track locations were only estimated up to 20 h from emission. Some very long-lived ship tracks have radiative impacts lasting several days (Goren and Rosenfeld, 2012), which would lead to significant changes at longer timescales.…”

Section: Potential Radiative Forcingmentioning

confidence: 99%

“…For a large-scale change in cloud amount, the timescales may be even longer, as it requires the switching of cloud regime from open to closed Published by Copernicus Publications on behalf of the European Geosciences Union. (Rosenfeld et al, 2006;Goren and Rosenfeld, 2012). The timescale for this aerosol response is related to the timescale for a switch between open and closed cells and related to the lifetime of individual cells (around 2 h; Wang and Feingold, 2009a).…”

Section: Introductionmentioning

confidence: 99%

“…With a limited spatial extent, they have clearly identifiable polluted and control regions, allowing the impact of the aerosol on the cloud field to be inferred (Durkee et al, 2000b). Ship tracks also vary along their length (Kabatas et al, 2013) and can be tracked over several days, providing evidence of a long-lasting aerosol perturbation to cloud properties (Goren and Rosenfeld, 2012).…”

Section: Introductionmentioning

confidence: 99%

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Observing the timescales of aerosol–cloud interactions in snapshot satellite images

Gryspeerdt

Goren²,

Smith

2021

Atmos. Chem. Phys.

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Abstract. The response of cloud processes to an aerosol perturbation is one of the largest uncertainties in the anthropogenic forcing of the climate. It occurs at a variety of timescales, from the near-instantaneous Twomey effect to the longer timescales required for cloud adjustments. Understanding the temporal evolution of cloud properties following an aerosol perturbation is necessary to interpret the results of so-called “natural experiments” from a known aerosol source such as a ship or industrial site. This work uses reanalysis wind fields and ship emission information matched to observations of ship tracks to measure the timescales of cloud responses to aerosol in instantaneous (or“snapshot”) images taken by polar-orbiting satellites. As in previous studies, the local meteorological environment is shown to have a strong impact on the occurrence and properties of ship tracks, but there is a strong time dependence in their properties. The largest droplet number concentration (Nd) responses are found within 3 h of emission, while cloud adjustments continue to evolve over periods of 10 h or more. Cloud fraction is increased within the early life of ship tracks, with the formation of ship tracks in otherwise clear skies indicating that around 5 %–10 % of clear-sky cases in this region may be aerosol-limited. The liquid water path (LWP) enhancement and the Nd–LWP sensitivity are also time dependent and strong functions of the background cloud and meteorological state. The near-instant response of the LWP within ship tracks may be evidence of a bias in estimates of the LWP response to aerosol derived from natural experiments. These results highlight the importance of temporal development and the background cloud field for quantifying the aerosol impact on clouds, even in situations where the aerosol perturbation is clear.

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“…2 suggests, a significant fraction of the reflected energy likely takes place between 1-3 days after injection. However, studies suggest that cloud adjustments to aerosol may change significantly over timescales of hours to a few days (Wood 2007;Gryspeerdt et al, 2019;Glassmeier et al, 2021), resulting in changes on longer timescales that may, in some cases, offset some of the Twomey brightening. Thus, although the LES simulations here provide some validation of the heuristic model, there is a need for many more simulations to test its sensitivities under the full range of meteorological, background aerosol, and aerosol injection scenarios.…”

Section: Comparison Of Heuristic Model With Large Eddy Simulationsmentioning

confidence: 99%

Assessing the potential efficacy of marine cloud brightening for cooling Earth using a simple heuristic model

Wood¹

2021

Preprint

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Abstract. A simple heuristic model is described to assess the potential for increasing solar reflection by augmenting the aerosol population below marine low clouds, which nominally leads to increased cloud droplet concentration and albedo. The model estimates the collective impact of many point-source particle sprayers, each of which generates a plume of injected particles that affects clouds over a limited area. A widely-used aerosol activation scheme is used to derive cloud droplet concentration as a function of the sub-cloud aerosol size distribution and updraft speed, and a modified version of Twomey's formulation is used to estimate radiative forcing. Plume overlap is accounted for using a Poisson distribution assuming idealized elongated cuboid plumes that have a length driven by aerosol lifetime and wind speed, a width consistent with satellite observations of ship track broadening, and a depth equal to an assumed boundary layer depth. The model is found to perform favorably against estimates of brightening from large eddy simulation studies that explicitly model cloud responses to aerosol injections over a range of conditions. Although the heuristic model does not account for cloud condensate or coverage adjustments to aerosol, in most realistic ambient remote marine conditions these tend to augment the Twomey effect in the large eddy simulations, with the resulting being a modest underprediction of brightening in the heuristic model. The heuristic model is used to evaluate the potential for global radiative forcing from marine cloud brightening as a function of the quantity, size, and lifetime of salt particles injected per sprayer and the number of sprayers deployed. Radiative forcing is sensitive to both the background aerosol size distribution in the marine boundary layer into which particles are injected, and the assumed updraft speed. Given representative values from the literature, radiative forcing sufficient to offset a doubling of carbon dioxide ΔF2xCO2 is possible but would require spraying over 50% or more of the ocean area. This is likely to require at least 104 sprayers to avoid major losses of particles due to near-sprayer coagulation. The optimal dry diameter of injected particles, for a given salt mass injection rate, is 30–60 nm. A major consequence is that the total salt emission rate (50–70 Tg/yr) required to offset F2xCO2 is a factor of five lower than the emissions rates required to generate significant forcing in previous studies with climate models, which have mostly assumed dry diameters for injected particles in excess of 200 nm. With the lower required emissions, the salt mass loading in the marine boundary layer for F2xCO2 is dominated by natural salt aerosol, with injected particles only contributing ~ 10%. When using particle sizes optimized for cloud brightening, the aerosol direct radiative forcing is shown to make a minimal contribution to the overall radiative forcing.

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Aerosol-cloud-climate cooling overestimated by ship-track data

Cited by 75 publications

References 55 publications

The Dependence of Ship‐Polluted Marine Cloud Properties and Radiative Forcing on Background Drop Concentrations

The Dependence of Ship‐Polluted Marine Cloud Properties and Radiative Forcing on Background Drop Concentrations

Observing the timescales of aerosol–cloud interactions in snapshot satellite images

Assessing the potential efficacy of marine cloud brightening for cooling Earth using a simple heuristic model

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