This is our climate change status check, and the first three things to understand are that climate change is real, anthropogenic (i.e., caused by humans), and dangerous. To drive home those points, this chapter relies heavily on the science of the landmark IPCC climate assessments and particularly the most recent AR5 report. We drill down on a key figure from the AR5 to clarify the observed data since 1850 in respect of global average surface temperatures, sea level rise, atmospheric concentrations of several greenhouse gases, and annual CO2 emissions. With a second figure we examine the sources of climate forcing since 1950 and clarify the degree to which they are caused by humanity or by nature. We then divert to key facets that we don’t yet fully understand about climate change, including tipping points, “climate sensitivity” and the likely emissions pathway that humanity will choose over the remainder of the century. We conclude with the observation that irrespective of how these mysteries play out, substantial climate change is in our future. It’s coming.

Here I examine flue gas capture of CO2 along with geologic sequestration. While CCS could reasonably be categorized as a form of emissions reduction rather than a climate intervention, I present it as the latter because the capture step is so similar to what would be required for direct air capture, and the sequestration step is identical. I first clarify that the real value in BECCS is the carbon capture, not the inefficient bio energy production. After delving into the mechanics and economics of the CCS process, I review the status of large CCS facilities around the world, noting that we are scaling this technology at a small fraction of the pace required. While there is a small market for carbon use in products, the vast majority of what we might extract needs to be pumped underground in secure storage sites. As there is no market value in such carbon sequestration, this technology will not begin to scale substantially until government policy mandates or incents it via regulation or carbon charges.

On the SRM side of the toolbox, we are at an even earlier stage, so the go-forward imperative here is not development but simply research. We need a much more sound understanding as to whether and how these interventions might work and how we might domesticate them via thoughtful and inclusive governance. The SRM sibling to the National Academies report described above was issued in spring 2021. It recommends an expenditure of $100–200 million over five years, a small fraction of the proposed budget for CDR. Nonetheless, this would be a vast expansion of resources for the field. My personal path forward entails working with colleagues to design aircraft that could be useful in such research.

This is our quick jog through climate science, intended to ground future climate discussions in a solid factual context. We review: the earth’s energy budget via the Trenberth diagram; the role of greenhouse gases, the Keeling Curve, and global warming potential; atmospheric structure and the environmental lapse rate; global atmospheric and oceanic circulations; paleoclimatology over both the last million and the last 1,500 years; and the role of oceans and ice.

Despite the many challenges in achieving complete decarbonization of our energy supply, succeeding in that Herculean task would solve merely two-thirds of our greenhouse gas emissions problem. This chapter addresses how we might tackle the other third. Topics covered include: fugitive methane emissions from fossil fuel production; CO2 emissions from both traditional and industrial biomass combustion; emissions of CO2, methane, and nitrous oxide from the agricultural sector; CO2 emissions from land use changes and the forestry sector; the full range of greenhouse gas emissions from the industrial sector unrelated to energy; and the ongoing release of methane from our waste streams. Tackling these diverse and myriad emission sources will greatly tax the ingenuity and budgets of humanity.

The final carbon capture chapter focuses on DACCS and notes that due to difficulties in remediating ongoing emissions from recalcitrant economic sectors such as aviation, long-distance transport, agriculture, and waste streams, DACCS will almost certainly be necessary simply to get to net zero emissions. However, if by then temperatures are unacceptably high, DACCS will be required on an utterly massive scale to reduce atmospheric carbon concentrations. The mechanics of DACCS are quite similar to those required for flue gas capture, but the much reduced concentration of carbon in ambient air elevates the cost of DACCS substantially. While there remains material uncertainty as to the mature cost of DACCS, the size of a future DACCS industry needed to substantially reduce future atmospheric CO2 concentrations would rival the size of the fossil fuel industry today. The three leading companies in the DACCS field are reviewed to illustrate the promising but nascent status of this industry today.

Our review of climate economics begins with the Kaya identity, which portrays our emissions pathway not as a smooth glideslope to net zero but as a tug of war between opposing forces. We then review the stunning impact that continued compounded economic growth could have on the ability of future generations to adapt to climate change, particularly in the Global South. The concepts of externalities and market failures are considered, along with the widely held view that a significant element in any climate solution will need to be some form of carbon tax. The recognition that mitigation comes at a cost leads to the question of climate cost/benefit analysis and the notion that the economically optimal quantum of further climate change that rational actors might prefer would not necessarily be zero. We close by pivoting to fat tails and the extent to which opting for some economically optimal amount of climate change may expose the future to small probabilities of utterly unacceptable outcomes and therefore unjustifiable risks.

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.

Here we focus on climate impacts, noting first the need to distinguish valid dire climate warnings from breathless scaremongering. Seeking to separate wheat from chaff, we work systematically through the myriad ways in which climate change is expected to impact natural systems including: sea level rise; extreme weather; ocean acidification; ecosystem stress/collapse; water scarcity; and altered global circulations. The resulting human impacts would include: food scarcity; climate migration; health burdens; and heat stress. Predictions in respect of all of these are inherently uncertain and it is therefore difficult to discern how much risk should be acceptable, but it is clear that less risk would be the safer choice.

In contrast to CDR, the institutions seeking to govern emissions reductions seem poorly suited to govern SRM and specifically SAI. SAI does not fall squarely under the jurisdiction of any existing international treaties, and even domestic laws provide few constraints. Voluntary norms such as the Oxford Principles are not yet codified into agreed obligations. Harvard’s intended SCoPEx experiment demonstrates the great trepidation with which SAI is perceived. Attempts to curtail pre-deployment field research generally rest on faulty assumptions, including a presumed slippery slope, technological lock-in, fears of covert or non-state deployment, weaponization, and a blurry line between research and deployment. Without robust research, there is a danger that future climate emergencies may be met with rash and risky deployment. Therefore, global institutions are needed to encourage, fund, and vet research and ensure its transparency. Deployment itself would require new, empowered international institutions to coordinate the program and monitor its effects. While sovereign national deployment efforts by major powers are a salient paradigm, a better outcome for both nature and humanity would be a single global monopolist deployer acting on behalf of the world. Such a unitary global effort would have enormous implications for the international order.

The ethics of climate change are deeply problematical – a “perfect moral storm.” This ethical terrain is characterized by a dispersal of cause and effect, a fragmentation of agency, and an institutional inadequacy. However, the greatest moral pitfall derives from the severely lagged nature of climate change, leading to dire issues of intergenerational equity. Common notions of justice in the climate arena include distributive, reparative, and procedural elements. The precautionary principle is also perceived to be a salient consideration. Ethical challenges particular to climate interventions start with the "moral hazard" or mitigation deterrence reservation as well as the risk of hubris in seeking to engineer the earth system. Some argue that geoengineering shirks responsibility for our emissions and saddles the future with a burdensome climate debt. An alternative though not universally accepted view is that since climate change is likely to have asymmetrical adverse impact on the poor, geoengineering would convey to them asymmetrical benefits. Though ethics would seem to demand not merely societal climate sacrifices but personal ones as well, few among my Yale students or Harvard colleagues have yet to undertake them. I can’t claim to be any holier than they, which is itself a dilemma.

On the CDR side of our toolbox, the go-forward imperative is to stomp the accelerator. We have costly and early-stage but viable technologies by which to remove carbon both from flues and from direct air, with industrial interventions holding more promise than biological ones. The challenge is to mature and scale these technologies as rapidly as possible. A 2019 report from the National Academy of Sciences, Engineering and Medicine laid out a thoughtful agenda for the path forward and suggested an expenditure of $9 billion over the subsequent 10 years to mature the technologies for DACCS, BECCS, several biological removal processes, and geologic storage. The Trump administration did not act on this recommendation, but the imperative remains.