14.0: Introduction

This chapter provides scientific context for key issues regarding the long-term mitigation of climate change. As such, this chapter first addresses the science underlying the timing of when and how CO2 and other greenhouse gas (GHG) mitigation activities that occur in the present affect the climate of the future. When do we see the benefits of a GHG emission reduction activity? Chapter 4: Projections provides further context for this topic. Relatedly, the present chapter discusses the significance of the relationship between net cumulative CO2 emissions and eventual global warming levels. The chapter reviews recent analyses of global emissions pathways associated with preventing 3.6°F (2°C) or 2.7°F (1.5°C) of warming relative to preindustrial times. And finally, this chapter briefly reviews the status of climate intervention proposals and how these types of mitigation actions could possibly play a role in avoiding future climate change.

14.1: The Timing of Benefits from Mitigation Actions

14.1.1 Lifetime of Greenhouse Gases and Inherent Delays in the Climate System

Carbon dioxide (CO2) concentrations in the atmosphere are directly affected by human activities in the form of CO2 emissions. Atmospheric CO2 concentrations adjust to human emissions of CO2 over long time scales, spanning from decades to millennia.1 ,2 The IPCC estimated that 15% to 40% of CO2 emitted until 2100 will remain in the atmosphere longer than 1,000 years.1 The persistence of warming is longer than the atmospheric lifetime of CO2 and other GHGs, owing in large part to the thermal inertia of the ocean.3 Climate change resulting from anthropogenic CO2 emissions, and any associated risks to the environment, human health and society, are thus essentially irreversible on human timescales.4 The world is committed to some degree of irreversible warming and associated climate change resulting from emissions to date.

The long lifetime in the atmosphere of CO22 and some other key GHGs, coupled with the time lag in the response of the climate system to atmospheric forcing,5 has timing implications for the benefits (i.e., avoided warming or risk) of mitigation actions. Large reductions in emissions of the long-lived GHGs are estimated to have modest temperature effects in the near term (e.g., over one to two decades) because total atmospheric concentration levels require long periods to adjust,6 but are necessary in the long term to achieve any objective of preventing warming of any desired magnitude. Near-term projections of global mean surface temperature are therefore not strongly influenced by changes in near-term emissions but rather dominated by natural variability, the Earth system response to past and current GHG emissions, and by model spread (i.e., the different climate outcomes associated with different models using the same emissions pathway).7 Long-term projections of global surface temperature (after mid-century), on the other hand, show that the choice of global emissions pathway, and thus the long-term mitigation pathway the world chooses, is the dominant source of future uncertainty in climate outcomes.3 ,8

Some studies have nevertheless shown the potential for some near-term benefits of mitigation. For example, one study found that, even at the regional scale, heat waves would already be significantly more severe by the 2030s in a non-mitigation scenario compared to a moderate mitigation scenario.9 The mitigation of non-CO2 GHGs with short atmospheric lifetimes (such as methane, some hydrofluorocarbons [HFCs], and ozone) and black carbon (an aerosol that absorbs solar radiation; see Ch. 2: Physical Drivers of Climate Change), collectively referred to as short-lived climate pollutants (SLCPs), has been highlighted as a particular way to achieve more rapid climate benefits (e.g., Zaelke and Borgford-Parnell 201510 ). SLCPs are substances that not only have an atmospheric lifetime shorter (for example, weeks to a decade) than CO2 but also exert a stronger radiative forcing (and hence temperature effect) compared to CO2 on a per ton basis.11 For these reasons, mitigation of SLCP emissions produces more rapid radiative responses. In the case of black carbon, with an atmospheric lifetime of a few days to weeks,12 emissions (and therefore reductions of those emissions) produce strong regional effects. Mitigation of black carbon and methane also generate direct health co-benefits.13 ,14 Reductions and/or avoidances of SLCP emissions could be a significant contribution to staying at or below a 3.6°F (2°C) increase or any other chosen global mean temperature increase.15 ,16 ,17 ,18 The recent Kigali Amendment to the Montreal Protocol seeks to phase down global HFC production and consumption in order to avoid substantial GHG emissions in coming decades. Stringent and continuous SLCP mitigation could potentially increase allowable CO2 budgets for avoiding warming beyond any desired future level, by up to 25% under certain scenarios.18 However, given that economic and technological factors tend to couple CO2 and many SLCP emissions to varying degrees, significant SLCP emissions reductions would be a co-benefit of CO2 mitigation.

14.1.2 Stock and Stabilization: Cumulative CO2 and the Role of Other Greenhouse Gases

Net cumulative CO2 emissions in the industrial era will largely determine long-term, global mean temperature change. A robust feature of model climate change simulations is a nearly linear relationship between cumulative CO2 emissions and global mean temperature increases, irrespective of the details and exact timing of the emissions pathway (see Figure 14.1; see also Ch. 4: Projections). Limiting and stabilizing warming to any level implies that there is a physical upper limit to the cumulative amount of CO2 that can be added to the atmosphere.3 Eventually stabilizing the global temperature requires CO2 emissions to approach zero.19 Thus, for a 3.6°F (2°C) or any desired global mean warming goal, an estimated range of cumulative CO2 emissions from the current period onward can be calculated. The key sources of uncertainty for any compatible, forward looking CO2 budget associated with a given future warming objective include the climate sensitivity, the response of the carbon cycle including feedbacks (for example, the release of GHGs from permafrost thaw), the amount of past CO2 emissions, and the influence of past and future non-CO2 species.3 ,19 Increasing the probability that any given temperature goal will be reached therefore implies tighter constraints on cumulative CO2 emissions. Relatedly, for any given cumulative CO2 budget, higher emissions in the near term imply the need for steeper reductions in the long term.


Figure 14.1


Global mean temperature change for a number of scenarios as a function of cumulative CO2 emissions from preindustrial conditions, with time progressing along each individual line for each scenario. (Figure source: IPCC 2013;42 ©IPCC. Used with permission).

Between 1870 and 2015, human activities, primarily the burning of fossil fuels and deforestation, emitted about 560 GtC in the form of CO2 into the atmosphere.20 According to best estimates in the literature, 1,000 GtC is the total cumulative amount of CO2 that could be emitted yet still provide a two-thirds likelihood of preventing 3.6°F (2°C) of global mean warming since preindustrial times.3 ,21 That estimate, however, ignores the additional radiative forcing effects of non-CO2 species (that is, the net positive forcing resulting from the forcing of other well-mixed GHGs, including halocarbons, plus the other ozone precursor gases and aerosols). Considering both historical and projected non-CO2 effects reduces the estimated cumulative CO2 budget compatible with any future warming goal,18 and in the case of 3.6°F (2°C) it reduces the aforementioned estimate to 790 GtC.3 Given this more comprehensive estimate, limiting the global average temperature increase to below 3.6°F (2°C) means approximately 230 GtC more CO2 could be emitted globally. To illustrate, if one assumes future global emissions follow a pathway consistent with the lower scenario (RCP4.5), this cumulative carbon threshold is exceeded by around 2037, while under the higher scenario (RCP8.5) this occurs by around 2033. To limit the global average temperature increase to 2.7°F (1.5°C), the estimated cumulative CO2 budget is about 590 GtC (assuming linear scaling with the compatible 3.6°F (2°C) budget that also considers non-CO2 effects), meaning only about 30 GtC more of CO2 could be emitted. Further emissions of 30 GtC (in the form of CO2) are projected to occur in the next few years (Table 14.1).

Table 14.1: Dates illustrating when cumulative CO2 emissions thresholds associated with eventual warming of 3.6°F or 2.7°F above preindustrial levels might be reached. RCP4.5 and RCP8.5 refer, respectively, to emissions consistent with the lower and higher scenarios used throughout this report. The estimated cumulative CO2 emissions (measured in Gigatons (Gt) of carbon) associated with different probabilities (e.g., 66%) of preventing 3.6°F (2°C) of warming are from the IPCC.3 The cumulative emissions compatible with 2.7°F (1.5°C) are linearly derived from the estimates associated with 3.6°F (2°C). The cumulative CO2 estimates take into account the additional net warming effects associated with past and future non-CO2 emissions consistent with the RCP scenarios. Historical CO2 emissions from 1870–2015 (including fossil fuel combustion, land use change, and cement manufacturing) are from Le Quéré et al.20 See Traceable Accounts for further details.
Dates by when cumulative carbon emissions (GtC) since 1870 reach amount commensurate with 3.6°F (2°C), when accounting for non-CO2 forcings:
66% = 790 GtC 50% = 820 GtC 33% = 900 GtC
RCP4.5 2037 2040 2047
RCP8.5 2033 2035 2040
Dates by when cumulative carbon emissions (GtC) since 1870 reach amount commensurate with 2.7°F (1.5°C), when accounting for non-CO2 forcings:
66% = 593 GtC 50% = 615 GtC 33% = 675 GtC
RCP4.5 2019 2021 2027
RCP8.5 2019 2021 2025

14.2: Pathways Centered Around 3.6°F (2°C)

The idea of a 3.6°F (2°C) goal can be found in the scientific literature as early as 1975. Nordhaus22 justified it by simply stating, “If there were global temperatures more than 2 or 3°C above the current average temperature, this would take the climate outside of the range of observations which have been made over the last several hundred thousand years.” Since that time, the concept of a 3.6°F (2°C) goal gained attention in both scientific and policy discourse. For example, the Stockholm Environment Institute23 published a report stating that 3.6°F (2°C) “can be viewed as an upper limit beyond which the risks of grave damage to ecosystems, and of non-linear responses, are expected to increase rapidly.” And in 2007, the IPCC Fourth Assessment Report stated, among other things: “Confidence has increased that a 1 to 2°C increase in global mean temperature above 1990 levels (about 1.5 to 2.5°C above pre-industrial) poses significant risks to many unique and threatened systems including many biodiversity hotspots.” Most recently, the Paris Agreement of 2015 took on the long-term goal of “holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels.” Many countries announced GHG emissions reduction targets and related actions (formally called Intended Nationally Determined Contributions [INDCs]) in the lead up to the Paris meeting; these announcements addressed emissions through 2025 or 2030 and take a wide range of forms. A number of studies have generated projections of future GHG emissions based on these announcements and evaluated whether, if implemented, the resulting emissions reductions would limit the increase in global average temperatures to 3.6°F (2°C) above preindustrial levels. In June 2017, the United States announced its intent to withdraw from the Paris Agreement. The scenarios assessed below were published prior to this announcement and therefore do not reflect the implications of this announcement.

Estimates of global emissions and temperature implications from emissions pathways consistent with targets and actions announced by governments in the lead up to the 2015 Paris climate conference24 ,25 ,26 ,27 ,28 generally find that 1) these targets and actions would reduce GHG emissions growth by 2030 relative to a situation where these goals did not exist, though emissions are still not expected to be lower in 2030 than in 2015; and 2) the targets and actions would be a step towards limiting global mean temperature increase to 3.6°F (2°C), but by themselves, would be insufficient for this goal. According to one study, emissions pathways consistent with governments’ announcements imply a median warming of 4.7°–5.6°F (2.6°–3.1°C) by 2100, though year 2100 temperature estimates depend on assumed emissions between 2030 and 2100.24 For example, Climate Action Tracker,26 using alternative post-2030 assumptions, put the range at 5.9°–7.0°F (3.3°–3.9°C).

Emissions pathways consistent with the targets and actions announced by governments in the lead up to the 2015 Paris conference have been evaluated in the context of the likelihood of global mean surface temperature change (Figure 14.2). It was found that the likelihood of limiting the global mean temperature increase to 3.6°F (2°C) or less was enhanced by these announced actions, but depended strongly on assumptions about subsequent policies and measures. Under a scenario in which countries maintain the same pace of decarbonization past 2030 as they announced in their first actions (leading up to 2025 or 2030) there is some likelihood (less than 10%) of preventing a global mean surface temperature change of 3.6°F (2°C) relative to preindustrial levels; this scenario thus holds open the possibility of achieving this goal, whereas there would be virtually no chance if emissions climbed to levels above those implied by country announcements (Figure 14.2).27 Greater emissions reductions beyond 2030 (based on higher decarbonization rates past 2030) increase the likelihood of limiting warming to 3.6°F (2°C) or lower to about 30%, and almost eliminate the likelihood of a global mean temperature increase greater than 7°F (4°C). Scenarios that assume even greater emissions reductions past 2030 would be necessary to have at least a 50% probability of limiting warming to 3.6°F (2°C)27 as discussed and illustrated further below.


Figure 14.2


Global CO2 emissions and probabilistic temperature outcomes of government announcements associated with the lead up to the Paris climate conference. (a) Global CO2 emissions from energy and industry (includes CO2 emissions from all fossil fuel production and use and industrial processes such as cement manufacture that also produce CO2 as a byproduct) for emissions pathways following no policy, current policy, meeting the governments’ announcements with constant country decarbonization rates past 2030, and meeting the governments’ announcements with higher rates of decarbonization past 2030. INDCs refer to Intended Nationally Determined Contributions which is the term used for the governments’ announced actions in the lead up to Paris. (b) Likelihoods of different levels of increase in global mean surface temperature during the 21st century relative to preindustrial levels for the four scenarios. Although (a) shows only CO2 emissions from energy and industry, temperature outcomes are based on the full suite of GHG, aerosol, and short-lived species emissions across the full set of human activities and physical Earth systems. (Figure source: Fawcett et al. 201527 ).

There is a limited range of pathways which enable the world to remain below 3.6°F (2°C) of warming (see Figure 14.3), and almost all but the most rapid near-term mitigation pathways are heavily reliant on the implementation of CO2 removal from the atmosphere later in the century or other climate intervention, discussed below. If global emissions are in line with the first round of announced government actions by 2030, then the world likely needs to reduce effective GHG emissions to zero by 2080 and be significantly net negative by the end of the century (relying on as yet unproven technologies to remove GHGs from the atmosphere) in order to stay below 3.6°F (2°C) of warming. Avoiding 2.7°F (1.5°C) of warming requires more aggressive action still, with net zero emissions achieved by 2050 and net negative emissions thereafter. In either case, faster near-term emissions reductions significantly decrease the requirements for net negative emissions in the future.


Figure 14.3

Global emissions pathways for GHGs, expressed as CO2-equivalent emissions, which would be consistent with different temperature goals (relative to preindustrial temperatures). INDCs refer to Intended Nationally Determined Contributions which is the term used for the governments’ announced actions in the lead up to Paris. (a) shows a set of pathways where global mean temperatures would likely (66%) not exceed 2.7°F (1.5°C). A number of pathways are consistent with the goal, ranging from the red curve (slowest near-term mitigation with large negative emissions requirements in the future) to the black curve with rapid near-term mitigation and less future negative emissions. (b) shows similar pathways with a 66% chance of exceeding 2.7°F (1.5°C) for only 50 years, where (c) and (d) show similar emission pathways for 3.6°F (2°C). (Figure source: Sanderson et al. 201625 ).

14.3: The Role of Climate Intervention in Meeting Ambitious Climate Targets

Limiting the global mean temperature increase through emissions reductions or adapting to the impacts of a greater-than-3.6°F (2°C) warmer world have been acknowledged as severely challenging tasks by the international science and policy communities. Consequently, there is increased interest by some scientists and policy makers in exploring additional measures designed to reduce net radiative forcing through other, as yet untested actions, which are often referred to as geoengineering or climate intervention (CI) actions. CI approaches are generally divided into two categories: carbon dioxide removal (CDR)29 and solar radiation management (SRM).30 CDR and SRM methods may have future roles in helping meet global temperature goals. Both methods would reduce global average temperature by reducing net global radiative forcing: CDR through reducing atmospheric CO2 concentrations and SRM through increasing Earth’s albedo.

The evaluation of the suitability and advisability of potential CI actions requires a decision framework that includes important dimensions beyond scientific and technical considerations. Among these dimensions to be considered are the potential development of global and national governance and oversight procedures, geopolitical relations, legal considerations, environmental, economic and societal impacts, ethical considerations, and the relationships to global climate policy and current GHG mitigation and adaptation actions. It is clear that these social science and other non-physical science dimensions are likely to be a major part of the decision framework and ultimately control the adoption and effectiveness of CI actions. This report only acknowledges these mostly non-physical scientific dimensions and must forego a detailed discussion.

By removing CO2 from the atmosphere, CDR directly addresses the principal cause of climate change. Potential CDR approaches include direct air capture, currently well-understood biological methods on land (for example, afforestation), less well-understood and potentially risky methods in the ocean (for example, ocean fertilization), and accelerated weathering (for example, forming calcium carbonate on land or in the oceans).29 While CDR is technically possible, the primary challenge is achieving the required scale of removal in a cost-effective manner, which in part presumes a comparison to the costs of other, more traditional GHG mitigation options.31 ,32 In principle, at large scale, CDR could measurably reduce CO2 concentrations (that is, cause negative emissions). Point-source capture (as opposed to CO2 capture from ambient air) and removal of CO2 is a particularly effective CDR method. The climate value of avoided CO2 emissions is essentially equivalent to that of the atmospheric removal of the same amount. To realize sustained climate benefits from CDR, however, the removal of CO2 from the atmosphere must be essentially permanent—at least several centuries to millennia. In addition to high costs, CDR has the additional limitation of long implementation times.

By contrast, SRM approaches offer the only known CI methods of cooling Earth within a few years after inception. An important limitation of SRM is that it would not address damage to ocean ecosystems from increasing ocean acidification due to continued CO2 uptake. SRM could theoretically have a significant global impact even if implemented by a small number of nations, and by nations that are not also the major emitters of GHGs; this could be viewed either as a benefit or risk of SRM.30

Proposed SRM concepts increase Earth’s albedo through injection of sulfur gases or aerosols into the stratosphere (thereby simulating the effects of explosive volcanic eruptions) or marine cloud brightening through aerosol injection near the ocean surface. Injection of solid particles is an alternative to sulfur and yet other SRM methods could be deployed in space. Studies have evaluated the expected effort and effectiveness of various SRM methods.30 ,33 For example, model runs were performed in the GeoMIP project using the full CMIP5 model suite to illustrate the effect of reducing top-of-the-atmosphere insolation to offset climate warming from CO2.34 The idealized runs, which assumed an abrupt, globally-uniform insolation reduction in a 4 × CO2 atmosphere, show that temperature increases are largely offset, most sea ice loss is avoided, average precipitation changes are small, and net primary productivity increases. However, important regional changes in climate variables are likely in SRM scenarios as discussed below.

As global ambitions increase to avoid or remove CO2 emissions, probabilities of large increases in global temperatures by 2100 are proportionately reduced.27 Scenarios in which large-scale CDR is used to meet a 3.6°F (2°C) limit while allowing business-as-usual consumption of fossil fuels are likely not feasible with present technologies. Model SRM scenarios have been developed that show reductions in radiative forcing up to 1 W/m2 with annual stratospheric injections of 1 Mt of sulfur from aircraft or other platforms.35 ,36 Preliminary studies suggest that this could be accomplished at an implementation cost as low as a few billion dollars per year using current technology, enabling an individual country or subnational entity to conduct activities having significant global climate impacts.

SRM scenarios could in principle be designed to follow a particular radiative forcing trajectory, with adjustments made in response to monitoring of the climate effects.37 SRM could be used as an interim measure to avoid peaks in global average temperature and other climate parameters. The assumption is often made that SRM measures, once implemented, must continue indefinitely in order to avoid the rapid climate change that would occur if the measures were abruptly stopped. SRM could be used, however, as an interim measure to buy time for the implementation of emissions reductions and/or CDR, and SRM could be phased out as emissions reductions and CDR are phased in, to avoid abrupt changes in radiative forcing.37

SRM via marine cloud brightening derives from changes in cloud albedo from injection of aerosols into low-level clouds, primarily over the oceans. Clouds with smaller and more numerous droplets reflect more sunlight than clouds with fewer and larger droplets. Current models provide more confidence in the effects of stratospheric injection than in marine cloud brightening and in achieving scales large enough to reduce global forcing.30

CDR and SRM have substantial uncertainties regarding their effectiveness and unintended consequences. For example, CDR on a large scale may disturb natural systems and have important implications for land-use changes. For SRM actions, even if the reduction in global average radiative forcing from SRM was exactly equal to the radiative forcing from GHGs, the regional and temporal patterns of these forcings would have important differences. While SRM could rapidly lower global mean temperatures, the effects on precipitation patterns, light availability, crop yields, acid rain, pollution levels, temperature gradients, and atmospheric circulation in response to such actions are less well understood. Also, the reduction in sunlight from SRM may have effects on agriculture and ecosystems. In general, restoring regional preindustrial temperature and precipitation conditions through SRM actions is not expected to be possible based on ensemble modeling studies.38 As a consequence, optimizing the climate and geopolitical value of SRM actions would likely involve tradeoffs between regional temperature and precipitation changes.39 Alternatively, intervention options have been proposed to address particular regional impacts.40

GHG forcing has the potential to push the climate farther into unprecedented states for human civilization and increase the likelihood of “surprises” (see Ch. 15: Potential Surprises). CI could prevent climate change from reaching a state with more unpredictable consequences. The potential for rapid changes upon initiation (or ceasing) of a CI action would require adaptation on timescales significantly more rapid than what would otherwise be necessary. The NAS29 ,30 and the Royal Society41 recognized that research on the feasibilities and consequences of CI actions is incomplete and call for continued research to improve knowledge of the feasibility, risks, and benefits of CI techniques.


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