Chapter 3: Negative Emissions: The Role and Response of the Climate System

Our climate is changing and the role of human activity in this is unequivocal. There is now a well-defined relationship between the amount of carbon we emit as CO₂ and the global temperature rise. This allows us to quantify a global carbon budget that is consistent with the goals of the Paris Agreement to limit warming to well below 2 °C above pre-industrial levels. Almost all scenarios of future socioeconomic activity and carbon emissions that meet the Paris targets rely on society developing and deploying techniques to directly remove some of the emitted carbon. These negative emissions technologies (NETs) and greenhouse gas removal (GGR) are receiving much scientific attention in terms of their feasibility, costs, limitations, effectiveness, and unintended consequences. But there are considerable uncertainties in our knowledge surrounding how much CO₂ removal, i.e., negative emissions, would be required to achieve targets, and also how much is feasible. Feedback between climate and the carbon cycle represent the main processes which determine the remaining carbon budgets, and our imperfect knowledge of them represents a key gap in being able to inform mitigation policy more quantitatively. This chapter aims to highlight the role of the physical Earth System in this scientific debate – showing that it is a major control of the amount of negative emissions which are required to achieve climate goals (i.e., the “demand” for negative emissions), and also that many NETs and GGR are affected by the climate which therefore regulates the potential supply of negative emissions.