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.

This is the first of two chapters that focus on mitigation or emissions reduction. Chapter 6 addresses the primary cause of climate change, which is the emission of CO2 that derives from the combustion of fossil fuels. We review the oppositional forces of exploration and consumption in respect of fossil fuel reserves and the concepts of peak oil, gas, and coal. We turn next to the carbon cycle and how it is altered by widespread fossil fuel combustion. The low hanging fruit in mitigation is to electrify everything within reason and to simultaneously decarbonize our electrical supply. However, grid limitations, scant electrical storage capacity, and the intermittent nature of wind and solar combine to make deep decarbonization a more difficult challenge than is often appreciated. These factors along with the distinct consideration of national energy security help explain why fossil fuels are likely to persist late into this century as a source of dispatchable electric power. Finally, there are economic sectors such as aviation and steel making that lend themselves poorly to electrification and will likely remain recalcitrant sectors.

If the incentives to undertake drastic mitigation remain inordinately weak, those in favor of transformational adaptation to climate change will prove very strong, so humanity will spend vast sums of effort and money in this arena. These will range from the personal (buy another air conditioner) to the national (strengthen emergency response capabilities). Agricultural and food systems will need to transform to remain productive in a warmer world. More human intervention including “assisted evolution” will be required to protect natural systems that may increasingly struggle to protect themselves. Cities will need to become more resilient to resist and rapidly recover from the stresses that a changed climate will bring. Infrastructure to hold back rising seas such as the mighty Dutch Delta Works will become common all over the globe. However, over the longer run, humanity will need to surrender lesser value portions of coastline to the waves and execute a broad and painful retreat.

In this pivotal chapter, I examine the 1.5°C emissions pathways urged upon us in the 2018 IPCC SR15 report. I compare those steeply plunging emissions pathways to emissions predictions over the next 30 years made by McKinsey and Bloomberg to demonstrate that the IPCC pathways are unlikely to obtain. To devise what seems a more likely scenario, I utilize the McKinsey/Bloomberg scenarios through 2050 and append to them subsequently the sort of emissions plummet that the IPCC would recommend we commence in 2021. Even in 2050, an emissions nose-dive seems an optimistic scenario but it illustrates a pathway to Net Zero Emissions by 2084. I then translate that emissions pathway into a temperature outcome that shows an increase of roughly 2.7°C above pre-industrial temperatures in the Net Zero year. More importantly, I illustrate that after Net Zero, temperatures and therefore resulting climate damages plateau rather than decline. Worse yet, sea levels would continue to rise for centuries. Having undertaken the enormous sacrifices necessary to achieve Net Zero, I assert that future generations are unlikely to find that state of affairs acceptable. They will demand further action to reduce temperatures and climate damages. They will demand climate intervention.

Sweeping global climate interventions would be nearly impossible to implement without extensive public support. Surveys show an increasing and widespread global consensus that warming is real and constitutes a grave threat. The scientific community is virtually unanimous on these points, though this is poorly understood by the American public, which remains an outlier among developed nations in its degree of continuing climate confusion. Much of American climate exceptionalism derives from a well-funded domestic climate denial industry that has recycled tactics from the tobacco wars of the 1960s to preserve the appearance of scientific controversy where none exists. Despite that, an understanding of the gravity of the situation continues to coalesce. Conversely, knowledge about prospective climate interventions is exceptionally low. The primary exception is the somewhat misguided notion that planting trees can serve as a fully countervailing offset for carbon emissions. Another American oddity is the degree of stock placed in “chemtrailing” conspiracies. SAI is little understood among the general public but is met with strongly negative reactions when described, so much so that there is evidence of a “reverse moral hazard” effect whereby survey participants are more likely to favor strong mitigation if the alternative seems to be SAI.

In this chapter we leap over the divider in our geoengineering toolbox to examine the implements on the other side of our portfolio by considering several prospective solar radiation management techniques. These begin with surface modifications that might in some way increase the albedo of the earth and cause it to absorb less solar energy, including lightening roofs and other built structures, bioengineering crops to be slightly lighter, and manipulating ice to reflect more sunlight. Glaciers could be reinforced to slow their slides into the sea. Turning our gaze upward, marine cloud brightening is a technique that could increase the reflectivity of clouds, while cirrus cloud thinning could reduce the heat-trapping effect of these high wispy clouds. Yet higher up, it may someday be possible to deploy assets in space the might deflect some of the incoming sunlight. However, the surface modification techniques don’t appear to be properly scaled to the problem, and the cloud and space-based ideas remain too speculative to rely upon.