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Nonlinear climate response to regional brightening of tropical marine stratocumulus

Key Findings

  • Seeding clouds in three tropical marine regions cools earth's globally-averaged surface temperature by 0.5 degrees Celsius, which is roughly in line with prior modeling studies.
  • Cloud seeding over the southeastern equatorial Pacific enhances the climatological east-west sea surface temperature gradient in the equatorial Pacific, producing a "La Nina-like" climate response.
  • Climate responds non-linearly to cloud seeding in different equatorial regions, with globally-averaged temperature being much more sensitive to cloud seeding over the southeastern Pacific than either the south Atlantic or northeastern Pacific
  • Direct scattering of sunlight by the injected aerosols – neglected in prior climate response studies of cloud brightening – accounts for over half of cloud seeding's effect on earth's radiation balance.

Spencer Hill and Yi Ming. Journal: Geophysical Research Letters. DOI: 10.1029/2012GL052064

Summary

To combat global warming, there have been suggestions to increase the albedo of (i.e. brighten) low-level marine clouds by deliberately injecting them with aerosols. Though such cloud seeding could mitigate global-mean temperature rise through the aerosol indirect effects, the full climate response to this geoengineering scheme is poorly understood. For example, one prior simulation of cloud seeding exhibited catastrophic rainfall decrease over the Amazon, while another showed moderate rainfall increase there.

In this paper, we describe results from a simulation of cloud seeding using the GFDL AM2.1 atmospheric model coupled to a mixed-layer ocean model. Using a version of the model that incorporates the aerosol indirect effects, we increase by five times the sea salt concentrations in the boundary layer over three marine tropical regions featuring persistent low level clouds.

This research sheds light on aerosol-cloud interactions and how they can affect the atmospheric circulation, and it may help guide future work on geoengineering.

The radiative flux perturbation,in Watts per square meter. The three black boxes indicate the three regions where clouds were modified. The strong negative values within them indicate that much more solar radiation is now being reflected away to space.


The change in surface temperature due to cloud seeding, in degrees Celsius. The stronger cooling in the eastern equatorial Pacific than western induces La Nina-like conditions. This explains the temperature dipole (i.e. warming right next to cooling) in the North Pacific and Alaska/Canada. This dipole is a fingerprint of the Pacific-North America (PNA) oscillation, a large-scale climate variability mode that is strongly tied to El Nino/La Nina. In other words, seeding clouds in the SE equatorial Pacific induces a La Nina-like response, which then drives this PNA pattern.


The change in the precipitation rate, in millimeters of precipitation per day. The tropical changes resemble observed anomalies in La Nina events.