Could aerosol emissions be used for regional heat wave mitigation?

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 ef5 fect 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 strato10 sphere. 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 μg m−2 s−1 over the region, temperature de15 creases 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 ef20 fects 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 25 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 re30 gional 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. Correspondence to: J. David Neelin ([email protected]) 35