Confronting the abatement paradox: Integrating aerosol cooling within climate change mitigation policy

Anthropogenic climate change is a problem more confounding than commonly appreciated. For while greenhouse gas (GHG) emissions lead to a positive climate forcing and hence rising global mean surface temperature, parallel emission of particulate aerosol species does much the opposite, in parallel to their deleterious effects as local air pollutants. Through direct radiative interaction with incoming sunlight and complex indirect microphysical effects on cloud, anthropogenic aerosols exert a significant negative forcing — a counteracting cooling influence. Global temperature rise to date is thereby substantially less than would have occurred due to past GHG emissions alone. In fact, painstaking investigations find that aerosols mask between about 35 % and as much as 50 % of current GHG warming. What is more, where GHGs remain in the atmosphere for decades to centuries after emission, aerosols are washed out within days to weeks. This large residence time asymmetry means that the ‘protection’ afforded by aerosol masking depends entirely on their continual emission. But that masking comes at the expense of well-documented damage to natural systems, physical infrastructure, and human health. Crucially, many activities that produce GHGs also emit aerosols and their precursor gases, fossil fuel combustion prominent among them. Climate change mitigation policies that abate GHG emissions therefore simultaneously reduce aerosols — not only their emissions, but actual atmospheric loading. Yet due to the residence time asymmetry, the positive forcing of existing GHG concentrations is little affected in the short term, while the aerosol mask weakens in direct consequence of those lost emissions. this weakened mask constitutes a ‘negative abatement feedback' of mitigation efforts. If unmasking is large enough, near the full effect of existing GHG forcing will be exposed, leading to potentially strong and rapid increase in temperature. As thresholds for dangerous climate change are quickly approaching, including a range of climatic tipping points, a lost aerosol mask as an unintended side effect of genuine GHG abatement may hence, paradoxically, exacerbate rather than mitigate against dangerous anthropogenic interference. The current trajectory of ever-rising global GHG emissions and unambitious, ineffective international mitigation efforts therefore suggests that a policy response to lost aerosol cooling will be needed when abatement finally begins in earnest, as it must. However, critical assessment of current policy making and instrument design finds that this systemic risk is largely unrecognised, and that normative ontological bias in the conception of pollutant effects hinders adequate recognition of aerosols’full role. The objective of this thesis is then to investigate policy approaches by which that risk may be contained. To do so, I construct a set of policy design criteria that must be met by any effective mitigation framework that explicitly confronts the aerosol abatement paradox. Chief among these is the need to avoid distortion of underlying real GHG abatement; to account for aerosol emissions via metrics that accurately represent their negative forcing character; to properly recognise temporal constraints on their removal; and to ensure that withdrawal of aerosol masking is ‘managed’. Analysing a range of conceivable implementation models uncovers further critical insights for effective design, in accordance with the physical science. From these foundations, I propose the ‘balancing market’mechanism, a prototype policy instrument compliant with those criteria, integrated with a pre-existing carbon price mechanism under an assumed, and necessary, future international agreement. The central feature of the underlying implementation model is to prioritise abatement pathways in which GHG-emission sources without coupled aerosol emissions are shutdown first. The balancing market does this by paying for the quantity of negative forcing deemed required until mitigation efforts restore a ‘safe’ atmospheric configuration. In a stark illustration of the abatement paradox’s unprecedented challenge, this translates as payment for continued aerosol emissions during a transitionary period while alternative compensative technologies are developed — likely including carbon dioxide removal, and forms of geoengineering. It is my hope that this proposal and the analysis on which it is founded may assist in expanding the policy discourse so as to take full and unvarnished account of all components of anthropogenic interference in the climate system — and the danger they represent. The ethical implications are enormously confronting, but such is the nature of the age of consequences.