The tap & the tub: A better way to envision climate change mitigation
· 27 min read
Climate change experts confirm that humanity still has a (narrow) chance to prevent global mean temperatures from rising more than 1.5°C by 2035. (1) But it falls on non-experts to turn the experts’ projections and advice into policy—and perhaps more importantly, into public awareness that results in needed behavioral changes and support for new policy initiatives. Unfortunately, the present discourse about climate change mitigation taking place among members of the general public is confused and potentially counterproductive.
In particular, the most common way the current goals of climate change mitigation are articulated in the popular media is to achieve “net zero” CO2 emissions by the early 2050s followed by “net negative” CO2 emissions thereafter. The concept of “netting” or “offsetting” CO2 emissions has become a source of confusion and controversy in public discussions. In principle, netting is a method of carbon accounting that allows a CO2 emitter to combine the positive emissions from their activity—say, burning fossil fuels to operate freight-carrying trucks—with reductions in CO2 that generally have little or nothing to do with the emitter’s main activity. These reductions in CO2 may result either from reducing the amount of CO2 already present in the atmosphere, or reducing emissions from other activities that would have taken place. Summing the positive and negative emissions yields a lower “net” emission that the emitter then uses as if it were the true amount of CO2 emitted.
Measuring emissions on a “net” basis is confusing to the general public. First, measures of “net” emissions do not specify the sizes of the “direct” and “indirect” reductions used to calculate net emissions. Many people only hear the “zero” in “net zero” and are led to believe that an emitter has realized far larger reductions in their emissions than is true. Next, this practice treats reductions in atmospheric CO2 or avoided emissions from other activities as interchangeable with direct reduction of emissions from the emitter’s main activity. This alarms many who recognize that these activities differ in key respects, including (and perhaps especially) their costs to the firms that engage in them. In particular, the practice of netting has given rise to the purchasing of carbon “offsets,” which are credits for reductions in CO2 performed by third parties. The use of offsets is increasingly being met with accusations of greenwashing, because an emitter may be using the purchase of cheaply priced offsets to avoid having to reduce the direct emissions from their main activity. In addition, providers of offsets have come under increasing criticism for not meeting their promised CO2 reductions, or even committing fraud, implying that the netted emissions numbers are not trustworthy or valid.
Because of these issues with netting, many people have become skeptical of or even opposed to the use of reductions in atmospheric CO2 or avoided indirect emissions as tools for mitigating climate change. Although maintaining a focus on having emitters reduce the direct emissions from their main activities is a laudable goal, the increasingly negative public view of other CO2 reduction methods could result in the abandonment of many tools that could prove highly effective if their role and use in climate change mitigation were properly understood. Given the urgency of the need for climate change mitigation, it would be foolish not to take full advantage of all mitigation approaches, provided they can be used well.
In this paper, we present an alternative way to understand and discuss climate-change mitigation strategies. Rather than using the language of netting and offsets, we advocate the adoption of a “tap and tub” metaphor for describing mitigation strategies and activities. We can think of the earth’s atmosphere as a bathtub and the level of atmospheric CO2 as the water in the tub. The tub can hold a large amount of water, but if the amount of water becomes too great, there will be bad consequences. At the same time, we can think of carbon emissions as a tap that is contributing water to the tub. If the flow of water from the tap is too great, it will overfill the tub and lead to bad consequences. Using this metaphor, humanity faces both an “open tap” problem, which is the continued flow (flux) of new CO2 emissions entering the atmosphere; and a “full tub” problem, which is an excess stock (pool) of CO2 already trapped in the atmosphere from past flows. To address these two causes of anthropogenic climate change, two distinct solutions are needed: turn off the tap and drain the tub. In other words, society must accomplish two tasks: rapidly cut gross emissions and reduce the excess carbon in the atmosphere.
In addition to being easy to understand, the tap-and-tub framework shows why cutting emissions (from all sources) and reducing the CO2 already in the atmosphere are not only complementary, but jointly necessary for reaching society’s goal of preventing excessive increases in global mean temperatures. In the main body of the paper, we describe the tap and tub problems in more detail and then use well known results from the theory of economic policy to show why two complementary approaches to resolve the problems are necessary. We then discuss how to interpret and evaluate climate mitigation approaches in light of the tap and tub framework, and how policies need to be redesigned to encourage full implementation of both tap-based and tub-based solutions. (1)
To the extent that human activity is a leading cause of climate change (2), policy design should be driven by lessons from economics, psychology, sociology, systems science, and other fields that focus on human behavior and its multiple connected consequences. An important lesson from economics regards the design of policy at the aggregate level (3). Every policy aims to solve at least one overarching problem, such as preventing global average temperatures from rising more than 1.5 degrees C. The challenge is to find one or more variables that affect the existence or severity of the problem that can also be affected if not modulated by human effort. It is uncommon, however, in complex economic (as well as ecological) systems that such variables exist. Instead, large problems generally have multiple underlying causes or driving forces that are measurable and highly correlated with the severity of the problem but cannot be directly and precisely manipulated by humans. They can, however, be affected by at least one human activity and often by multiple human activities that can be modulated directly and sometimes with good degrees of precision.
Economists call the underlying causes of the problem the targets of policy (3). As the word target suggests, these phenomena become proxies for achieving the overall objective of solving the overarching problem. In the case of the mean global temperature rise problem, the two main targets of policy should be the aggregate flow rate of CO2 emissions and the concentration of CO2 in the atmosphere. Again, these correspond to the rate at which water flows into the tub, and the amount of water already in the tub, respectively. These causes of climate change are measurable and highly correlated with global average temperature changes but cannot be directly manipulated by human actions.
Nonetheless, they can be affected by specific human activities. For example, the quantity of CO2 emissions can be affected by the choice of energy sources used in production and transportation, and the concentration of CO2 in the atmosphere can be affected by human efforts to restore natural carbon sinks. Economists refer to the specific human activities that affect the targets of policy as instruments of policy (3). Making policy then becomes the task of manipulating various instruments in order to achieve a set of measurable effects on the targets of policy, which should then mitigate or solve the overall problem. One of the key lessons from the theory of economic policymaking is that targets and instruments need to be carefully identified, and the number of instruments employed must equal or exceed the number of targets chosen (3).
Educating society that two distinct targets can and should be used to mitigate climate change is an essential task that needs to occur as soon as possible. This requires clear articulation of the two distinct targets that can be used in order to reach society’s climate change mitigation goals, coupled with explanations of how they work. Target 1 is the quantity of CO2 emitted into the atmosphere per year. Reaching this target amounts to closing the “open tap” to reduce the flow of CO2 into the atmosphere. Target 2 is the stock of CO2 already in the atmosphere, measured by its concentration. Hitting this target amounts to draining the “full tub” to remove the excess stock of CO2 from the atmosphere to an appropriate safe limit (Fig 1).
POLICY TARGET 1: CLOSE THE TAP
Stop the FLOW of CO2 into the atmosphere
POLICY TARGET 2: DRAIN THE TUB
Reduce STOCK of CO2 from the atmosphere
Figure 1. Two Climate Policy Targets
Reaching Target 1—reducing carbon dioxide emissions—is the current focus of climate mitigation policy. Many instruments could be used to close the open tap, but one of the main instruments that society has chosen is to pressure both the public and private sectors to make commitments (including legally binding ones) to reduce their overall CO2 emissions to zero (4). This instrument has been successful in the sense that more than one third of the world’s largest companies have made pledges and this fraction is increasing (5). Yet actual progress toward emissions reduction remains alarmingly slow. Reports indicate that annual global CO2 emissions have been leveling off (largely due to land-use change reductions) but have not yet peaked, with total CO2 emissions projections for the year 2022 ranging between 37.5 – 40.5 GtCO2 (6). Steep reductions in emissions are needed, estimated at around 1.4 GtCO2/yr (7, 8). The remaining carbon budget (as of 2022) is estimated at only 210 GtCO2, which could be reached in 5.25 years at current emissions rates (8, 9). In response, society is modifying the “commitment” instrument by imposing additional conditions on companies’ commitments. In particular, entities that make emissions reduction pledges need to now show a credible “decarbonization pathway” that specifies the concrete actions they will take in order to fulfill their promises. Organizations are supporting this process by providing science-based targets (SBTi) for industries and individual businesses (10). Carbon taxes are another instrument that can be used to hit Target 1. They can provide a strong incentive to both households and businesses to economize on GHG-emissions intensive activities as well as to seek out (or create) low-emissions alternatives. To date, only a handful of jurisdictions have implemented this instrument.
It is clear, however, that relying on one target will be insufficient to mitigate climate change. Living up to emissions reductions commitments is proving to be technically challenging and costly in the short run (11). Many companies may not be able to fulfill their pledges within the time needed to limit the rise in average temperatures to only 1.5 degrees C. And even if all CO2 emissions were to stop tomorrow, the world would continue to face climate change impacts because the concentration of CO2 in the atmosphere would remain too high. Historically, CO2 concentrations have risen from 277 ppm over 250 years ago to 417 ppm reported in 2022 (7). Atmospheric concentrations have been increasing at a rate of 2.4 ppm per year during the decade 2010 – 2019 (7). Scientists estimate that to have a 50% chance of staying below a 1.5°C rise in average temperature, the concentration level cannot exceed 430 ppm (7). At the current rate of emissions flows, the remaining carbon budget could be used up before the end of this decade (12). Therefore, the world needs to reduce the stock of CO2 in the atmosphere in addition to reducing CO2 emissions. In other words, the tub is already nearly full and needs to be drained.
Draining the tub has been largely overlooked as a second target of policy, but it must be elevated to this status. The instruments needed to reach Target 2, draining the tub, involve integrating carbon dioxide removal (CDR) technologies into our systems of production and distribution (13). Activities to improve land management by protecting and restoring nature and integrating nature into production systems through nature-based solutions are estimated to provide 37% of the carbon capture needed between now and 2030 to ensure that the 1.5°C ceiling remains attainable (14, 15). Improved protection and management of marine and coastal ecosystems could generate significant additional carbon dioxide removal gains (16, 17), estimated at up to 3 GtCO2 per year (18). A new study outlines the tradeoffs and benefits between biological methods of CDR and mechanical methods, concluding that biological methods are more effective and more resource efficient in achieving a climate-relevant scale of CO2 removal (19, 20). It is also recognized that much more investment will be needed in machine-based CDR solutions before they are ready to deploy at scale. Nonetheless, both nature-based and machine-based technologies will be needed and are included in all the AR6 IPCC climate scenarios. The AR6 summary report for policymakers estimates we will need 8 GtCO2e/-yr of CDR by midcentury with only 2 GtCO2e/year CDR happening now (21, 22).
As the evidence from the IPCC reports shows, the two policy targets are already implicitly recognized by climate scientists. But making them explicit and distinct from each other will lead to better public understanding and more effective policy coordination. In particular, although reducing the stock of carbon dioxide through instruments like CDR cannot substitute for achieving emissions reductions (Target 1), it is needed as a complementary policy to help manage both the time lag before emission reduction goals are realized, as well as the lag between these reductions and the resulting climate impacts. Even if society successfully stabilized atmospheric CO2 concentrations immediately, temperatures would continue to rise for years if not decades (23). Actively reducing the stock of CO2 in the atmosphere now will both accelerate temperature mitigation impacts and help demonstrate the tangible benefits of emissions reductions and thus maintain public support for these efforts.
Making the two targets explicit will also clarify the growing confusion and reduce the resulting backlash surrounding carbon offsets. Currently, carbon offsets are used to allow organizations who have made pledges to reduce their carbon emissions to substitute carbon captured or emissions avoided by third parties for some of their emissions reductions. Offsetting is broadly achieved through the use of carbon credits derived from one of 4 mechanisms: carbon capture through forestry and conservation (Nature Climate Solutions); avoided emissions through using renewable energy (such as solar, wind, and hydroelectric); avoided emissions through certain community projects (such as cookstove replacement); and avoided emissions through converting waste to energy (such as methane capture) (24). Offsetting is allowed when reducing emissions to zero is not feasible in the short run, such as in the case of the airline industry, and has always been designed as a “short-term” transition mechanism except for some hard-to-abate industries, which may have to rely on offsets for much longer (25).
Carbon credits, particularly those arising from carbon capture through nature-based or machine-based technologies, are most properly associated with Target 2 and its instruments. Ideally, the public would recognize that reducing the stock of carbon dioxide in the atmosphere complements emissions reductions as described above and therefore can be appropriate in the short run. In addition, the public would also understand and support the long-term role that Target 2 must play to reach our overall goal of climate change mitigation. Thus, the use of “offsets” would be significantly reduced and limited to “residual emissions” (Target 1) using SBTi guidelines; but carbon “credits” could increase and legitimately be claimed towards efforts to drain the tub (Target 2), contributing to global climate mitigation and avoiding “greenwashing” of emissions reductions claims.
Unfortunately, however, many members of the public who only recognize Target 1 have come to see the use of offsets as a form of greenwashing, in which companies are seen as arbitraging the carbon offset rules by finding legal loopholes that enable them to delay their decarbonization transitions and continue polluting, perhaps indefinitely, while claiming to be fulfilling their emissions reduction pledges (26). Public opinion regarding the use of offsets has been further damaged by discoveries that some projects intended to be long-term carbon sequestration remedies were not performing as claimed (27). These two issues raise legitimate concerns about how the nascent carbon markets are operating in the absence of clear targets, regulations, safeguards and disclosure requirements. They also increase the operational and reputational risks faced by sellers and buyers in the carbon market, which delay and possibly even threaten the development of Target 2 as a policy target as well as the instruments used to reach it (27). Even without these problems, the view that projects which reduce the stock of atmospheric CO2 are merely a source of offsets inherently limits the role and scope of this policy tool in the mind of the public, which has been trained to think of emissions reduction as the best (if not only) way to mitigate climate change.
The Benefits of Having Two Climate Mitigation Targets
Climate change mitigation policy thus needs to be reframed so that it acknowledges both emissions reductions—closing the tap—and reductions in the stock of atmospheric CO2—draining the tub—as distinct and essential policy targets (Fig. 2). This can begin with formal recognition of Target 2 in public statements about climate change mitigation, followed by a focused campaign to explain this target, its role in climate change mitigation, and its relationship to Target 1. Then Target 2 can be formally integrated into climate change mitigation policy. This step includes setting goals for atmospheric carbon dioxide removal as well as guidelines and possibly limits on the acceptable mixture of emissions reductions and carbon dioxide removals in the climate change mitigation efforts of individual organizations and countries.
Separation of emissions reductions, emissions avoidance, and carbon dioxide removal would create additional flexibility for both public and private climate change mitigation planning, as well as increase clarity and precision of monitoring and reporting (28). Establishing carbon dioxide reduction as a separate target will also move society away from the “offset” view of carbon removal and the connotation that it is an inferior substitute for emissions reduction, and lead to a more productive discussion of the optimal mix of climate mitigation efforts at both the aggregate and individual levels. A further consequence is that it should also reduce claims that carbon dioxide removal is being used for greenwashing purposes.
POLICY TARGET 1: CLOSE THE TAP
Stop the FLOW of CO2 into the atmosphere – current emissions
Measures: Quantity of CO2 emissions per year
Instruments:
POLICY TARGET 2: DRAIN THE TUB
Reduce the STOCK of CO2 from the atmosphere – legacy emissions
Measures: CO2 PPM in atmosphere
Instruments:
Two Distinct Policy Targets and Instruments Needed for Climate Mitigation
Acknowledging the importance of draining the tub will result in more attention and resources being directed to this policy target. One of the main recipients should be the market for carbon credits. Reducing the concentration of CO2 will become valuable for its own sake rather than only through its use as an offset, especially once specific goals for carbon dioxide reductions are adopted. Thus, carbon dioxide removal credits should emerge as a marketable asset, which should spur greater interest within the private sector in investing in carbon reduction science and technologies, as well as developing quality standards and ratings for the production and verification of carbon dioxide removal credits. These will complement recent efforts to define “high quality” offsets, including initiatives such as the guidelines recently issued by the Integrity Council for the Voluntary Carbon Market (ICVCM) (29). The additional interest in carbon credits will create the conditions for this market to grow and become truly global.
Explaining the tap-and-tub approach to climate change mitigation will also lead more people to understand the importance of protecting existing stores of carbon, and to value these stores accordingly. As they do, the market for carbon credits should develop further. Awarding carbon credits for protecting biomes, habitats, and species as well as preventing conversion of natural systems into net carbon emitters becomes logical and desirable when people understand that without such protection, these natural assets will add significant carbon dioxide to the full tub. Similarly, protection credits could also be claimed for stabilizing earth systems’ CDR capacity. Sales of carbon credits related to protection can then generate revenues for keeping existing carbon sinks and the stocks of carbon they have sequestered intact. Carbon credits for protection could also provide necessary finance for achieving the global targets of the Kunming Montreal agreement to protect 30% of land and seas and restore 30% of degraded ecosystems by 2030. This new role for carbon credits, moreover, reduces the perverse incentive to destroy old-growth, biodiverse forest and then replant to achieve “additionality” offsets. It also avoids the main problem with avoided loss “offsets” in the current carbon market, which is the difficulty of proving “business as usual” baselines and accurately measuring the additionality derived from offset financing.
One other important consequence of formally incorporating Target 2 into climate mitigation policy is that it could help resolve a key point of contention between the global North and South that may otherwise impede or halt progress on climate change mitigation. Developing countries from the global South have pressed “loss and damage” compensation claims against the advanced economies (30). Their argument for doing so is that the full tub can largely be attributed to past economic growth in developed countries (31). Yet advanced economies have been slow to acknowledge or fund these claims. This has created resistance among some developing economies to undertake climate mitigation actions that are costly or that could impede economic development. But many low-income countries are rich in natural assets that need to be protected so as to prevent carbon from re-entering the atmosphere. They also host natural assets that could remove significant additional carbon dioxide from the atmosphere if protected and restored (32). Therefore, establishing carbon dioxide removal as a target and creating a market for removal credits could facilitate advanced-economy funding for nature-based solutions which address mitigation and adaptation in developing countries (33). When investors from the global North purchase carbon removal credits from developing economies for their efforts to drain the tub, this would essentially provide some compensation towards the legacy of past carbon emissions from advanced economies, a portion of which could begin to fund “loss and damage” claims (34, 35). Because of the relative abundance of carbon-capturing natural assets in many developing economies, the market for carbon dioxide removal credits would channel investments into countries with higher climate impacts and adaptation costs. Carbon dioxide removal credits would thus “recapitalize” the natural capital of these countries, making available funding that could be used to invest in nature-positive development and climate adaptation.
Having multiple targets gives policymakers additional flexibility and generally enlarges the set of instruments they can use to achieve a certain policy objective. In terms of mitigating climate change risk, these benefits in turn increase the likelihood of reaching the ultimate goal of the policy, opening the way for companies to act quickly in relation to Target 2 which provides additional SDG co-benefits to society. Thus, declaring the removal of excess atmospheric CO2 to be an explicit policy target and giving it equal prominence in policy discussions are critical steps toward climate change mitigation that should be taken as soon as possible. Reframing the public conversation on climate mitigation in terms of two objectives—closing the tap and draining the tub—will not only add clarity but also help resolve questions and objections that are currently preventing the full and complementary use of both targets. Uncertainty about the role, value and use of carbon dioxide removal credits has led to charges of greenwashing that inhibit both the use of this target as well as exploration into the optimal mix of emissions reduction and carbon dioxide removal targets for limiting global average temperature increases to 1.5 C.
The adoption of carbon dioxide removal as a formal target of climate change mitigation also presents the opportunity to reconsider the value of assets that are important to climate change mitigation but have been overlooked. For example, carbon-sequestering natural assets are only eligible to earn marketable carbon offsets used to substitute for emissions reductions if they meet a restrictive set of conditions (36). Meanwhile, immense amounts of CO2 captured by existing natural assets are ignored, so that these services have no value and hence cannot be used to fund protection of the assets. Similarly, the value of preserving the carbon-capturing capacity of existing natural assets is also overlooked. Such pronounced differences in the values of natural assets that are otherwise comparable are not economically sustainable, since assets with low values will be put to other uses that are assigned higher values by the market. As this happens, the progress toward climate change mitigation attained by focusing on emissions reductions could be reduced and possibly reversed.
If the role played by the stock of atmospheric carbon dioxide in climate change is understood and promoted, however, reductions in the stock will take on positive social and economic values, which supports the creation of credits that may be sold in order to fund conservation and restoration of natural assets. Credits for avoided emissions are also possible under this system, and moreover, the values of credits obtained through different means will tend to converge so that price variations will be due only to differences in production, transactions, and information costs, as economic theory predicts.
Of course, a range of new issues would need to be thought through if carbon dioxide removal is formally adopted as a policy target. For example, minimum standards specifying how much each company and country would be expected to contribute to draining the tub may be adopted. Regulations governing the use of proceeds from sales of carbon dioxide removal or avoided emissions credits may also be desirable or needed. The structure of the new markets for carbon removal credits would also need to be decided. While such questions are beyond the scope of this paper, we see this two-target pathway as essential to accelerate action (not just pledges) and create a less-risky market environment for both avoidance and removal credits. The payoff to society in terms of progress toward the ultimate goal of limiting global temperature increases will be worth the efforts associated with integrating carbon dioxide removal as an additional formal target of climate change mitigation policy.
illuminem Voices is a democratic space presenting the thoughts and opinions of leading Sustainability & Energy writers, their opinions do not necessarily represent those of illuminem.
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Acknowledgments:
We would like to acknowledge Carlos Duarte and Andreas Merkl for their comments and suggestions, Rayyan Chami for conceptual suggestions, and Audrey Saliba for research assistance.
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