Could aerosol emissions be used for regional heat wave mitigation?

D. N. Bernstein, J. D. Neelin, Q. B. Li, and D. Chen
Atmos. Chem. Phys., submitted, 7/2012.
Preprint Paper (6.1 MB)
Supplemental Material (PDF 427 KB).

Abstract Geoengineering applications by injection of sulfate aerosols into the stratosphere are under consideration as a measure of last resort to counter global warming. Here adaptation to a potential regional-scale application to offset the impacts of heat waves is critically examined. The effect of regional-scale sulfate aerosol emission over California in each of two days of the July 2006 heat wave using the Weather Research Forecast model with fully coupled chemistry is used to quantify potential reductions in surface temperature as a function of emission rates in the lower stratosphere. Over the range considered, afternoon temperature reductions scale almost linearly with emissions. Local meteorological factors yield geographical differences in surface air temperature sensitivity. For emission rates of approximately 30 g m-2 s-1 over the region, temperature decreases of around 7 C result during the middle part of the day over the Central Valley, one of the hardest hit by the heatwave. Regions more ventilated with oceanic air such as Los Angeles have slightly smaller reductions. The length of the hottest part of the day is also reduced. Advection effects on the aerosol cloud must be more carefully forecast for smaller emission regions. Verification of the impacts could be done via measurements of differences in reflected and surface downward shortwave. Such regional geoengineering applications with specific near-term target effects but smaller cost and side effects could potentially provide a means of testing larger scale applications. However, design trade-offs differ from global applications and the size of the required emissions and the necessity of emission close to the target region raise substantial concerns. The evaluation of this regional scale application is thus consistent with global model evaluations emphasizing that mitigation via reduction of fossil fuels remains preferable to considering geoengineering with sulfate aerosols.

Note: The present study is motivated by the argument that it is useful for groups whose primary research focus lies elsewhere to contribute to evaluation of potential geoengineering proposals, especially where tools developed for other purposes can contribute at low cost to particular aspects of understanding the issues involved. In this case a setup of the the Weather Research Forecast model with fully coupled chemistry (WRF-Chem) is used to to provide a model-based evaluation of a potential regional application of geoengineering. We regard this as providing initial information that could be used in assessment of such regional applications, about which we share deep reservations.

Citation Bernstein, D. N., J. D. Neelin, Q. B. Li, and D. Chen, 2012: ould aerosol emissions be used for regional heat wave mitigation? Atmos. Chem. Phys., submitted, 7/2012.

Acknowledgments. We thank A. Robock for discussion and J. E. Meyerson for graphical assistance. We also gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model used in this publication. This study was supported by National Science Foundation Grant 580 AGS-1102838.