Investigations of the climate system response to climate engineering in a hierarchy of models
McCusker, Kelly E.
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Global warming due to anthropogenic emissions of greenhouse gases is causing negative impacts on diverse ecological and human systems around the globe, and these impacts are projected to worsen as climate continues to warm. In the absence of meaningful greenhouse gas emissions reductions, new strategies have been proposed to engineer the climate, with the aim of preventing further warming and avoiding associated climate impacts. We investigate one such strategy here, falling under the umbrella of `solar radiation management', in which sulfate aerosols are injected into the stratosphere. We use a global climate model with a coupled mixed-layer depth ocean and with a fully-coupled ocean general circulation model to simulate the stabilization of climate by balancing increasing carbon dioxide with increasing stratospheric sulfate concentrations. We evaluate whether or not severe climate impacts, such as melting Arctic sea ice, tropical crop failure, or destabilization of the West Antarctic ice sheet, could be avoided. We find that while tropical climate emergencies might be avoided by use of stratospheric aerosol injections, avoiding polar emergencies cannot be guaranteed due to large residual climate changes in those regions, which are in part due to residual atmospheric circulation anomalies. We also find that the inclusion of a fully-coupled ocean is important for determining the regional climate response because of its dynamical feedbacks. The efficacy of stratospheric sulfate aerosol injections, and solar radiation management more generally, depends on its ability to be maintained indefinitely, without interruption from a variety of possible sources, such as technological failure, a breakdown in global cooperation, lack of funding, or negative unintended consequences. We next consider the scenario in which stratospheric sulfate injections are abruptly terminated after a multi- decadal period of implementation while greenhouse gas emissions have continued unabated. We show that upon cessation, an abrupt, spatially broad, and sustained warming over land occurs that is well outside the bounds of 20th century climate variability. We then use an upwelling-diffusion energy balance climate model to further show the sensitivity of these trends to background greenhouse gas emissions, termination year, and climate sensitivity. We find that the rate of warming from cessation of solar radiation management -- of critical importance for ecological and human systems -- is principally controlled by the background greenhouse gas concentrations. It follows that the only way to avoid the risk of an abrupt and dangerous warming that is inherent to the large-scale implementation of solar radiation management is to also strongly reduce greenhouse gas emissions. The climate system responds to radiative forcing on a diverse spectrum of timescales, which will affect what goals can be achieved for a given stratospheric aerosol implementation. We next investigate how different rates of stratospheric sulfate aerosol deployment affect what climate impacts can be avoided by simulating two rates of increasing stratospheric sulfate concentrations in a fully-coupled global climate model. We find that disparate goals are achieved for different rates of deployment; for a slow ramping of sulfate, land surface temperature trends remain small but sea levels continue to rise for decades, whereas a quick ramp-up of sulfate yields large land surface cooling trends and immediately reduces sea level. However, atmospheric circulation changes also act to create a large-scale subsurface ocean environment around Antarctica that is favorable for increased basal melting of ice sheet outlets, thereby leaving the potential open for increased sea level rise even in the absence of large atmospheric surface warming. We show that instead, when greenhouse gases are abruptly returned to preindustrial levels, circulation anomalies are reversed, and the subsurface ocean environment does not pose the same threat to Antarctic ice sheets. We conclude that again, reduction of greenhouse gases is the preferred strategy for avoiding climate impacts of global warming.
- Atmospheric sciences 
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