labs_title

Caldeira Lab Research:Climate Intervention ('Geoengineering')

Geoengineering as an optimization problem

George A. Ban-Weiss and Ken Caldeira

In this study we use an approach commonly used in engineering called optimization to investigate what distribution of stratospheric sulfate aerosols would be needed to minimize various metrics of climate change. Using a global climate model, we find that the optimal latitudinally varying aerosol distributions diminished the rms zonal mean land temperature change from a doubling of CO2 by 94% and the rms zonal mean land precipitation minus evaporation change by 74%. However, if the goal is to minimize changes in surface temperatures, many latitude bands become overly dry in the model. If the goal is to minimize changes in the hydrological cycle, many latitude bands have residual warming.


Ban-Weiss, G. A., & K. Caldeira. 2010. Geoengineering as an optimization problem. ERL, doi:10.1088/1748-9326/5/3/034009

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure: Climate model results relative to the low-CO2 climate. The left side shows results when temperature differences are minimized. The right side shows results when precipitation minus evaporation (PminusE) is minimized. The upper panel shows results for temperature and the bottom panel shows results for precipitation minus evaporation. In the model, geoengineering reduces the amount of change in both temperature and precipitation minus evaporation caused by high CO2 concentrations. Using a parabolic distribution of aerosols (more aerosols in the polar regions) slightly improves the cancellation of temperature changes but slightly degrades the ability to reverse changes in precipitation minus evaporation.

Abstract

There is increasing evidence that Earth’s climate is currently warming, primarily due to emissions of greenhouse gases from human activities, and Earth has been projected to continue warming throughout this century. Scientists have begun to investigate the potential for geoengineering options for reducing surface temperatures and whether such options could possibly contribute to environmental risk reduction. One proposed method involves deliberately increasing aerosol loading in the stratosphere to scatter additional sunlight to space. Previous modeling studies have attempted to predict the climate consequences of hypothetical aerosol additions to the stratosphere. These studies have shown that this method could potentially reduce surface temperatures, but could not recreate a low-CO2 climate in a high-CO2 world. In this study, we attempt to determine the latitudinal distribution of stratospheric aerosols that would most closely achieve a low-CO2 climate despite high CO2 levels. Using the NCAR CAM3.1 general circulation model, we find that having a stratospheric aerosol loading in polar regions higher than that in tropical regions leads to a temperature distribution that is more similar to the low-CO2 climate than that yielded by a globally uniform loading. However, such polar weighting of stratospheric sulfate tends to degrade the degree to which the hydrological cycle is restored, and thus does not markedly contribute to improved recovery of a low-CO2 climate. In the model, the optimal latitudinally varying aerosol distributions diminished the rms zonal mean land temperature change from a doubling of CO2 by 94% and the rms zonal mean land precipitation minus evaporation change by 74%. It is important to note that this idealized study represents a first attempt at optimizing the engineering of climate using a general circulation model; uncertainties are high and not all processes that are important in reality are modeled.

 

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