The Earth surface carbon cycle consists of regular flows between the major sinks: atmosphere, ocean, soils and biota. It is in balance, except for flows to and from long-term storage (methane formations, deep ocean, lithosphere), which comprise the deep Earth carbon cycle. At the beginning of life in earth history, about 500 million years ago, the atmosphere was carbon-rich, and Earth temperatures were vastly hotter than today. Over time most of this carbon was withdrawn from the surface carbon cycle through ocean sedimentation, reactions with weathering rock and migration of organic matter to the lithosphere, where they were transformed into fossil fuels. This eventually produced the temperate Earth, which facilitated human evolution, but now, by extracting and burning fossil energy, humans are recreating conditions for the hotter Earth of earlier times. These direct carbon emissions can be amplified by feedback mechanisms, whereby initial warming releases carbon stored in formerly stable methane deposits and alters the carbon cycle so as to increase the size of the atmospheric sink relative to other sinks or changes in the Earth’s albedo. The politics of controlling emissions is further complicated by lags in warming and subsequent climate impacts.

This chapter pivots to social science and reviews the leading literature in respect of the governance of climate intervention. It starts with the influential Oxford Principles that derived from a 2009 report on geoengineering issued by the (British) Royal Society. It proceeds to consider the particular challenges presented by collective action problems like emissions reduction. It notes that the all-voluntary nature of nationally determined contributions under the Paris Agreement render this a rather light touch governance architecture, but that the carbon accounting rules being promulgated are essential to success. Carbon removal is a collective action problem very similar to emissions reduction, such that the Paris Agreement may also prove well suited to governing CDR. Nonetheless, several social scientists see risks in mixing emissions and carbon capture targets and would seek to maintain separate targets for each. As CDR may well continue long after emissions have been reduced to zero, the dynamics of post-zero CDR governance are also considered.

Though relatively young, the climate regime has already exhibited a profound transformation. “Old governance”, embodied by the Kyoto Protocol, was largely state-focused and hierarchical. The 2015 Paris Agreement represents “new governance,” with an emphasis on transnational actors and bottom-up approaches. The bet is that new governance is better suited to both the realities of domestic politics and profound uncertainty surrounding climate change than the old governance model. If we are correct, then we have created the enabling conditions for decarbonization. The question is: will it happen fast enough? The answer to this question is critical both for the future of the planet and the legitimacy of global governance. Climate change will likely exacerbate existing problems in world politics such as armed conflict and mass migration. Worse still, those least capable of adapting to a changing climate will be most affected. A shift to a “new” global governance architecture means little if climate change continues apace. To preserve the legitimacy of the climate regime, as well as much of the fabric of the current liberal international order (such as it is), actors will have to move more swiftly and decisively toward a fossil-free world.

Human activities in the land sector produce the second highest level of greenhouse gas emissions globally and the highest in some countries, but the land is also a major sink for anthropogenic emissions. This land carbon sink is at risk because plant growth, survival and distributions are dependent on climate conditions and particularly extreme events: higher temperatures, reduction in water availability in some areas and flooding in others, droughts and wildfires. Agricultural and forest productivity are affected by the combined pressures of climate change, demand for land resources for food production, deforestation and degradation of natural systems, and population pressure. The land sector provides numerous opportunities for mitigation, many of which are cost-effective and can be implemented quickly. However, the land sink has limits because the uptake of carbon represents replacing the stock that was previously depleted by human activities, and productivity is limited by nutrient and water availability. Potential mitigation activities should be assessed in terms of the magnitude, timeframe and cost of the changes in carbon stocks.