Short, local sea salt spraying cooled a simulated 2019 Northeast Pacific marine heatwave

Why it matters

Marine heatwaves can be forecast months ahead and have hurt fisheries and ecosystems. This study tests, in a model only, whether a brief and local intervention could reduce the harm without wide side effects. The authors say the results justify more study, not deployment.

What they did

The team used a global climate model forecast ensemble (CESM2 SMYLE) started on May 1, 2019 to predict the “Blob 2.0” heatwave. They added a steady surface emission of sea salt aerosol in a Gulf of Alaska box (40−50 °N, 155−135 °W) in 20-member ensembles. They tested a lower and a five-times-higher emission rate, and versions extended through April 2020. They compared each with the unmodified control ensemble, using significance tests against internal variability.

Key findings

• The lower emission rate cooled summer sea surface temperature in the zone by 0.79 °C on average, fully offsetting the forecast heatwave and slightly overcorrecting it.
• Surface oxygen losses tied to the heatwave were also reduced. Oxygen fell below the mixed layer, probably because less sunlight cut photosynthesis.
• Cooling came mainly from brighter clouds, with a summer shortwave reduction of −28.6 Wm−2 at the surface.
• In every ensemble, the local effect vanished by November 2019, even with stronger or longer emissions, because winter limits the cloud effect and deepens the mixed layer.
• Summer remote effects were mostly within internal variability, but some cooling of about 1 °C persisted south of the zone and over North America in winter.

Limitations

• Results come from one climate model, which is among the most sensitive to sea salt aerosol, and from one event. Other models may respond differently.
• Effects beyond the simulation period, such as lagged teleconnections to the tropics, could not be ruled out. The forecast ensemble mean was also well below the observed heatwave.
• This is a preprint that has not been peer reviewed. Cost and feasibility were only roughly sketched.

Glossary

• Marine cloud brightening: Spraying sea salt particles into low marine clouds so they reflect more sunlight.
• Marine heatwave: A long spell of unusually warm sea surface temperature, here above the local 90th percentile.
• Twomey effect: More particles make more, smaller cloud droplets, which makes clouds brighter.
• Signal-to-noise ratio: The size of a difference between ensembles compared with the spread from natural variability.