Policy Update: Observing human CO₂ emissions

The interaction between the Earth’s carbon cycle and climate change remains a top element of uncertainty within climate change projections. In order to better understand this relationship, carbon cycle scientists use a variety of modeling and observational tools to disassemble the many fluxes and reservoirs that constitute the global carbon cycle. In addition to advancing the scientific understanding of carbon cycling, quantitative assessment of the anthropogenic portion is driven by high priority policy needs. For example, monitoring, reporting and verification the phrase used to encapsulate the emissions accounting in international climate change policy, has emerged as a critical need, not just within the international context, but also at national and local scales since these smaller governance levels enact CO2-emissions mitigation legislation and also need to assess progress [1]. Emissions quantification can also provide information for planning as well, identifying where and when emissions occur in order to optimize mitigation strategies. Given the convergence of science and policy needs, authoritative bodies such as the US National Research Council have recommended the construction of a science-driven carbon monitoring system (CMS) [2,3]. Such a system would combine multiple tiers of observations (e.g., ground-based, aircraft and space-based), as well as modeling elements such as ocean and terrestrial biogeochemistry models, and emission inventories, such as national reporting on fossil fuel CO2 emissions or forest accounting [4]. The portion of the global carbon cycle attributable to the dominant anthropogenic carbon-emitting activity, fossil fuel combustion, should have particular focus within the planned monitoring system. This is not to overlook other anthropogenic carbon emission categories (e.g., agricultural activity, deforestation or fire), but to reflect the current and future numerical dominance of the fossil carbon portion. This focus is also driven by the fact that the non-fossil portions of the carbon budget have been traditionally approached as a residual calculation and, hence, are reliant on the accuracy of fossil fuel CO2 emissions estimation [5]. Figure 1 shows a simple schematic of how one might conceptualize the observational opportunities associated with understanding the complete causal chain leading to fossil fuel CO2 in the atmosphere. The ultimate driver of fossil fuel CO2 emissions is the demand for direct energy services (e.g., space heating and lighting) or energy needed to produce or deliver economic goods and services (e.g., computers, transportation and steel). The proximal driver of fossil fuel CO2 emissions, fuel supply and demand, offers an efficient observational opportunity since fuel is a traceable, physical commodity that moves through economic ‘chokepoints’ (e.g., ref ineries and coal trains) but offers far less in terms of process understanding than the more wieldy upstream economic drivers. Furthermore, fuel supply and demand is not necessarily coincident with the physical location of emissions, making its connection to the downstream atmospheric observations complicated. The physical location of combustion/emissions is observationally more dispersed (e.g., an individual home), but is closer to the radiative greenhouse gas entity, CO2 concentration, and avoids additional observational requirements, such as knowing fuel carbon contents “ ...conceptualizing a carbon monitoring system centered primarily on atmospheric concentration measurements, could lead to tremendous ine!ciencies in the allocation of limited scienti"c resources, not