Joint CO2 and CH4 accountability for global warming

We propose a transparent climate debt index incorporating both methane (CH₄) and carbon dioxide (CO₂) emissions. We develop national historic emissions databases for both greenhouse gases to 2005, justifying 1950 as the starting point for global perspectives. We include CO₂ emissions from fossil sources [CO₂(f)], as well as, in a separate analysis, land use change and forestry. We calculate the CO₂(f) and CH₄ remaining in the atmosphere in 2005 from 205 countries using the Intergovernmental Panel on Climate Change’s Fourth Assessment Report impulse response functions. We use these calculations to estimate the fraction of remaining global emissions due to each country, which is applied to total radiative forcing in 2005 to determine the combined climate debt from both greenhouse gases in units of milliwatts per square meter per country or microwatts per square meter per person, a metric we term international natural debt (IND). Australia becomes the most indebted large country per capita because of high CH₄ emissions, overtaking the United States, which is highest for CO₂(f). The differences between the INDs of developing and developed countries decline but remain large. We use IND to assess the relative reduction in IND from choosing between CO₂(f) and CH₄`control measures and to contrast the imposed versus experienced health impacts from climate change. Based on 2005 emissions, the same hypothetical impact on world 2050 IND could be achieved by decreasing CH₄ emissions by 46% as stopping CO₂ emissions entirely, but with substantial differences among countries, implying differential optimal strategies. Adding CH₄ shifts the basic narrative about differential international accountability for climate change.The United Nations Framework Convention on Climate Change (UNFCCC), which was the basis for the Kyoto Protocol and the post-Kyoto negotiations initiated at the Copenhagen Conference of Parties in December 2009, calls for allocating accountability for action on mitigation and adaptation based on “common but differentiated responsibilities” (1). Part of the reason that this concept has not been fully implemented is lack of an acceptable metric, often termed “climate debt,” that allows differentiated responsibility to be transparently measured. The most frequently applied measure of a country’s responsibility for global warming is its current annual emissions of greenhouse gases (GHGs)^*. The second most common is cumulative emissions, simply the sum total of all past emissions. Neither metric, however, fully reflects the causes of global warming, because the amount of global warming occurring at any time is actually due to the anthropogenic GHGs from past emissions still remaining in the atmosphere at a given time, a quantity that is usually intermediate between current and cumulative emissions.Climate debt discussions have focused on carbon dioxide (CO₂) emissions, the most important GHG. In particular, the emphasis has been on CO₂ emissions consequent to fossil fuel combustion and cement manufacture, which we term CO₂(f), referring to fossil carbon. Increasingly, net CO₂ emissions from land use change and forestry (LUCF) have also garnered attention. It is not just CO₂, however, that is causing climate change, but a suite of human activities resulting in the emissions of GHGs and aerosols, together termed climate-altering pollutants (CAPs). Indeed, CO₂’s current impact on global warming is only about one-half the total, depending on how calculated (8). There is relatively little in the published climate debt literature, however, incorporating the impact of any GHG except CO₂(f) (2, 9).In this paper, we first briefly review the rationale behind the use of climate debt metrics for determining “differentiated responsibility” and then offer an alternative to past approaches by presenting a global database of climate debt at the country level that integrates CO₂(f) and CH₄, the two GHGs with the most radiative forcing (RF) associated with human activity (10), into a single combined climate debt metric.We illustrate how a combined climate debt metric can bring GHGs with different atmospheric lifetimes together into a common measure that is a function of the GHGs’ different depletion functions and RFs, but without use of arbitrary time horizons or discount rates. Difficulties in choosing among time horizons and discount rates, for example, have plagued the calculation of global warming potentials, which are used to compare current emissions of different GHGs in a common metric of carbon dioxide equivalents (11). To investigate the issues posed by excluding or including LUCF, we also calculate a combined climate debt metric that consolidates both CO₂(f) and LUCF as well as CH₄ by region, as this is the finest geographic scale available for the LUCF dataset.We characterize the global landscape of historical accountability illuminated by the combined climate debt metric and demonstrate two of its applications. First, we reveal how, in addition to facilitating international comparisons, a combined climate debt metric can help prioritize among mitigation options that target different GHGs. Second, we reanalyze global patterns of health impacts from climate change to show how a combined climate debt metric alters relationships that have been examined from the standpoint of CO₂(f) alone in previous analyses.Although linked to the concept of “differential responsibility” in UNFCCC, we use the term “accountability” here so as to distance the concept from a moral judgment, which is a subject for another venue.     

Climate forcings in the Industrial era

The forcings that drive long-term climate change are not known with an accuracy sufficient to define future climate change. Anthropogenic greenhouse gases (GHGs), which are well measured, cause a strong positive (warming) forcing. But other, poorly measured, anthropogenic forcings, especially changes of atmospheric aerosols, clouds, and land-use patterns, cause a negative forcing that tends to offset greenhouse warming. One consequence of this partial balance is that the natural forcing due to solar irradiance changes may play a larger role in long-term climate change than inferred from comparison with GHGs alone. Current trends in GHG climate forcings are smaller than in popular “business as usual” or 1% per year CO₂ growth scenarios. The summary implication is a paradigm change for long-term climate projections: uncertainties in climate forcings have supplanted global climate sensitivity as the predominant issue.     

From the Cover: Setting cumulative emissions targets to reduce the risk of dangerous climate change

Avoiding “dangerous anthropogenic interference with the climate system” requires stabilization of atmospheric greenhouse gas concentrations and substantial reductions in anthropogenic emissions. Here, we present an inverse approach to coupled climate-carbon cycle modeling, which allows us to estimate the probability that any given level of carbon dioxide (CO₂) emissions will exceed specified long-term global mean temperature targets for “dangerous anthropogenic interference,” taking into consideration uncertainties in climate sensitivity and the carbon cycle response to climate change. We show that to stabilize global mean temperature increase at 2 °C above preindustrial levels with a probability of at least 0.66, cumulative CO₂ emissions from 2000 to 2500 must not exceed a median estimate of 590 petagrams of carbon (PgC) (range, 200 to 950 PgC). If the 2 °C temperature stabilization target is to be met with a probability of at least 0.9, median total allowable CO₂ emissions are 170 PgC (range, −220 to 700 PgC). Furthermore, these estimates of cumulative CO₂ emissions, compatible with a specified temperature stabilization target, are independent of the path taken to stabilization. Our analysis therefore supports an international policy framework aimed at avoiding dangerous anthropogenic interference formulated on the basis of total allowable greenhouse gas emissions.     

The contributions of individual countries and regions to the global radiative forcing

Knowing the historical relative contribution of greenhouse gases (GHGs) and short-lived climate forcers (SLCFs) to global radiative forcing (RF) at the regional level can help understand how future GHGs emission reductions and associated or independent reductions in SLCFs will affect the ultimate purpose of the Paris Agreement. In this study, we use a compact Earth system model to quantify the global RF and attribute global RF to individual countries and regions. As our evaluation, the United States, the first 15 European Union members, and China are the top three contributors, accounting for 21.9 ± 3.1%, 13.7 ± 1.6%, and 8.6 ± 7.0% of global RF in 2014, respectively. We also find a contrast between developed countries where GHGs dominate the RF and developing countries where SLCFs including aerosols and ozone are more dominant. In developing countries, negative RF caused by aerosols largely masks the positive RF from GHGs. As developing countries take measures to improve the air quality, their negative contributions from aerosols will likely be reduced in the future, which will in turn enhance global warming. This underlines the importance of reducing GHG emissions in parallel to avoid any detrimental consequences from air quality policies.

Anthropogenic activities have been the main drivers of climate change since industrialization, and recent climate change has had substantial impact on both humans and natural systems (1). Increase in anthropogenic greenhouse gas (GHG) emissions is the dominant cause of observed climate warming, and those emissions are driven by the energy demand from economic and population growth since the preindustrial era. In addition to GHGs, anthropogenic activities also change the climate through aerosol emissions, such as black carbon (BC) and organic aerosols (both primary and secondary), as well as aerosol and ozone precursor emissions such as SO₂, NH₃, NO_X, VOC, and CO, all of which are termed short-lived climate forcers (SLCFs). Albedo changes are another climate forcer, mainly induced by land-cover change (LCC). Radiative forcing (RF), a natural or anthropogenic perturbation to the earth’s energy budget, is used to quantify the magnitude by which forcers like GHGs and SLCFs change the climate (2).The Paris Agreement, for the first time, brings all nations into a common cause to undertake ambitious efforts to combat climate change and adapt to its effects. Its goal is to limit global warming to well below 2 °C, preferably to 1.5 °C, compared to preindustrial levels. To achieve this goal, individual signatory countries are requested to submit nationally determined contributions (NDCs) every 5 y. Knowing the historical relative contribution of GHGs and SLCFs to global RF at the regional level can help understand how future GHGs emission reductions and associated or independent reductions in SLCFs will affect the ultimate purpose of the Paris Agreement. Since NDCs are submitted without negotiations and given that stock takes of global emissions and commitments are planned in 2023 and 2028 to assess progress toward the Paris Agreement climate goals, it is even more essential to monitor the contributions of individual countries to climate change.Quantifying the individual country and regional historical contributions to global RF was suggested years ago in the Brazilian Proposal (3) to follow the principle of common but differentiated responsibilities and respective capabilities established by the United Nations Framework Convention on Climate Change (UNFCCC). The Copenhagen Conference of Parties in 2009 marked a shift away from this top-down approach, which meant to attribute mitigation responsibilities relatively to each country’s historical contribution to global climate change, and instead all parties involved in the Paris Agreement in 2016 agreed to take a bottom-up approach yet with a global stock take and a process to improve bottom-up intentions of mitigation.In this study, we quantify individual countries’ and regional contributions to the global RF, following a method close to the one established in our previous estimation for China (3). Compared to this previous work, secondary organic aerosols (SOA), aerosol–cloud interaction, and albedo change induced by BC deposition on snow are included in this study, making it more systematic and comprehensive, albeit at the cost of using a simplified model leading to increased uncertainty. The global RFs are calculated using the reduced-complexity Earth system model OSCAR v3.1, enabled by more complex Earth system models (ESMs) upon which its parameters are calibrated (4–6). The individual countries’ or regional contributions are isolated by using factorial simulations in which a small fraction of each country’s or region’s emissions are removed to quantify their marginal effect on the climate system. This attribution method is used in previous studies (3, 7–9), which is referred to as the “normalized marginal attribution method” (see Methods for details).     

From the Cover: Greenhouse gas mitigation by agricultural intensification

As efforts to mitigate climate change increase, there is a need to identify cost-effective ways to avoid emissions of greenhouse gases (GHGs). Agriculture is rightly recognized as a source of considerable emissions, with concomitant opportunities for mitigation. Although future agricultural productivity is critical, as it will shape emissions from conversion of native landscapes to food and biofuel crops, investment in agricultural research is rarely mentioned as a mitigation strategy. Here we estimate the net effect on GHG emissions of historical agricultural intensification between 1961 and 2005. We find that while emissions from factors such as fertilizer production and application have increased, the net effect of higher yields has avoided emissions of up to 161 gigatons of carbon (GtC) (590 GtCO₂e) since 1961. We estimate that each dollar invested in agricultural yields has resulted in 68 fewer kgC (249 kgCO₂e) emissions relative to 1961 technology ($14.74/tC, or ∼$4/tCO₂e), avoiding 3.6 GtC (13.1 GtCO₂e) per year. Our analysis indicates that investment in yield improvements compares favorably with other commonly proposed mitigation strategies. Further yield improvements should therefore be prominent among efforts to reduce future GHG emissions.     

The Copenhagen Accord for limiting global warming: Criteria,constraints, and available avenues

At last, all the major emitters of greenhouse gases (GHGs) have agreed under the Copenhagen Accord that global average temperature increase should be kept below 2 °C. This study develops the criteria for limiting the warming below 2 °C, identifies the constraints imposed on policy makers, and explores available mitigation avenues. One important criterion is that the radiant energy added by human activities should not exceed 2.5 (range: 1.7–4) watts per square meter (Wm⁻²) of the Earth''s surface. The blanket of man-made GHGs has already added 3 (range: 2.6–3.5) Wm⁻². Even if GHG emissions peak in 2015, the radiant energy barrier will be exceeded by 100%, requiring simultaneous pursuit of three avenues: (i) reduce the rate of thickening of the blanket by stabilizing CO₂ concentration below 441 ppm during this century (a massive decarbonization of the energy sector is necessary to accomplish this Herculean task), (ii) ensure that air pollution laws that reduce the masking effect of cooling aerosols be made radiant energy-neutral by reductions in black carbon and ozone, and (iii) thin the blanket by reducing emissions of short-lived GHGs. Methane and hydrofluorocarbons emerge as the prime targets. These actions, even if we are restricted to available technologies for avenues ii and iii, can reduce the probability of exceeding the 2 °C barrier before 2050 to less than 10%, and before 2100 to less than 50%. With such actions, the four decades we have until 2050 should be exploited to develop and scale-up revolutionary technologies to restrict the warming to less than 1.5 °C.     

Sharing global CO2 emission reductions among one billion high emitters

We present a framework for allocating a global carbon reduction target among nations, in which the concept of “common but differentiated responsibilities” refers to the emissions of individuals instead of nations. We use the income distribution of a country to estimate how its fossil fuel CO₂ emissions are distributed among its citizens, from which we build up a global CO₂ distribution. We then propose a simple rule to derive a universal cap on global individual emissions and find corresponding limits on national aggregate emissions from this cap. All of the world's high CO₂-emitting individuals are treated the same, regardless of where they live. Any future global emission goal (target and time frame) can be converted into national reduction targets, which are determined by “Business as Usual” projections of national carbon emissions and in-country income distributions. For example, reducing projected global emissions in 2030 by 13 GtCO₂ would require the engagement of 1.13 billion high emitters, roughly equally distributed in 4 regions: the U.S., the OECD minus the U.S., China, and the non-OECD minus China. We also modify our methodology to place a floor on emissions of the world's lowest CO₂ emitters and demonstrate that climate mitigation and alleviation of extreme poverty are largely decoupled.     

On avoiding dangerous anthropogenic interference with the climate system: formidable challenges ahead

The observed increase in the concentration of greenhouse gases (GHGs) since the preindustrial era has most likely committed the world to a warming of 2.4°C (1.4°C to 4.3°C) above the preindustrial surface temperatures. The committed warming is inferred from the most recent Intergovernmental Panel on Climate Change (IPCC) estimates of the greenhouse forcing and climate sensitivity. The estimated warming of 2.4°C is the equilibrium warming above preindustrial temperatures that the world will observe even if GHG concentrations are held fixed at their 2005 concentration levels but without any other anthropogenic forcing such as the cooling effect of aerosols. The range of 1.4°C to 4.3°C in the committed warming overlaps and surpasses the currently perceived threshold range of 1°C to 3°C for dangerous anthropogenic interference with many of the climate-tipping elements such as the summer arctic sea ice, Himalayan–Tibetan glaciers, and the Greenland Ice Sheet. IPCC models suggest that ≈25% (0.6°C) of the committed warming has been realized as of now. About 90% or more of the rest of the committed warming of 1.6°C will unfold during the 21st century, determined by the rate of the unmasking of the aerosol cooling effect by air pollution abatement laws and by the rate of release of the GHGs-forcing stored in the oceans. The accompanying sea-level rise can continue for more than several centuries. Lastly, even the most aggressive CO₂ mitigation steps as envisioned now can only limit further additions to the committed warming, but not reduce the already committed GHGs warming of 2.4°C.     

Societal shifts due to COVID-19 reveal large-scale complexities and feedbacks between atmospheric chemistry and climate change

The COVID-19 global pandemic and associated government lockdowns dramatically altered human activity, providing a window into how changes in individual behavior, enacted en masse, impact atmospheric composition. The resulting reductions in anthropogenic activity represent an unprecedented event that yields a glimpse into a future where emissions to the atmosphere are reduced. Furthermore, the abrupt reduction in emissions during the lockdown periods led to clearly observable changes in atmospheric composition, which provide direct insight into feedbacks between the Earth system and human activity. While air pollutants and greenhouse gases share many common anthropogenic sources, there is a sharp difference in the response of their atmospheric concentrations to COVID-19 emissions changes, due in large part to their different lifetimes. Here, we discuss several key takeaways from modeling and observational studies. First, despite dramatic declines in mobility and associated vehicular emissions, the atmospheric growth rates of greenhouse gases were not slowed, in part due to decreased ocean uptake of CO₂ and a likely increase in CH₄ lifetime from reduced NO_x emissions. Second, the response of O₃ to decreased NO_x emissions showed significant spatial and temporal variability, due to differing chemical regimes around the world. Finally, the overall response of atmospheric composition to emissions changes is heavily modulated by factors including carbon-cycle feedbacks to CH₄ and CO₂, background pollutant levels, the timing and location of emissions changes, and climate feedbacks on air quality, such as wildfires and the ozone climate penalty.

The effects of the COVID-19 pandemic and associated lockdown measures have provided a way to observationally test predictions of future atmospheric composition. This is illustrated conceptually in Fig. 1. With many people working from home and limiting travel, the pandemic caused a significant decrease in anthropogenic emissions. These emissions reductions can be thought of as a jump forward in time to a future where additional systemic emissions controls have been adopted. However, because these changes occurred in a matter of months, the changes to the concentrations of key air quality (AQ) and climate-relevant gases in the atmosphere were readily observable. Combining these observations with current state-of-science models allows us an important window into the underlying processes governing the response of the Earth system to reductions in anthropogenic emissions and thus a preview of the relative effectiveness of different emissions-control strategies.Open in a separate windowFig. 1Illustration of the conceptual foundation for this study. The COVID-19–induced reductions in human activity led to reduced anthropogenic emissions. The fact that these reductions occurred over months rather than decades allows us to observe how the atmosphere, land, and ocean are likely to respond in a future scenario with stricter emissions controls. This analysis helps to identify effective pathways to mitigate air pollution and climate-relevant GHG emissions. Image credit: Chuck Carter (Keck Institute for Space Studies, Pasadena, CA).Our goal is to synthesize some of the key results from the past year into a coherent understanding of what we have learned about the effectiveness of different strategies to reduce greenhouse gas (GHG) emissions and improve AQ. We briefly highlight individual components of the changes in composition (which are well-described in the literature) but focus on the interactions and feedbacks between different parts of the Earth system. We will do so in four parts. First, we summarize the observed changes in anthropogenic emissions during 2020. Second, we examine how the reduction in CO₂ emissions impacted the atmospheric CO₂ growth rate. Third, we show that the response of AQ to NO_x emissions reductions differs for cities around the world and depends strongly on the interaction with meteorology. We focus on ozone and nitrate particulate matter (PM) as key AQ metrics that are strongly driven by NO_x emissions. Fourth, we discuss the implications of these results for future AQ improvement strategies; our understanding of processes controlling GHG concentrations in the atmosphere; feedbacks between AQ, GHGs, and climate; and, finally, close by identifying strengths and gaps in our current observing networks. We draw three primary conclusions from this synthesis:

• 1.Despite drastic reductions in mobility and resulting vehicular emissions during 2020, the growth rates of GHGs in the atmosphere were not slowed.
• 2.The lack of clear declines in the atmospheric growth rates of CO₂ and CH₄, despite large reductions in human activity, reflect carbon-cycle feedbacks in air–sea carbon exchange, large interannual variability in the land carbon sink, and the chemical lifetime of CH₄. These feedbacks foreshadow similar challenges to intentional mitigation.
• 3.The response of AQ to emissions changes is heavily modulated by factors including background pollutant levels, the timing and location of emissions changes, and climate-related factors like heat waves and wildfires. Achieving robust improvements to AQ thus requires sustained reductions of both air pollutant (AP) and GHG emissions.

     

Disentangling the effects of CO2 and short-lived climate forcer mitigation

Anthropogenic global warming is driven by emissions of a wide variety of radiative forcers ranging from very short-lived climate forcers (SLCFs), like black carbon, to very long-lived, like CO₂. These species are often released from common sources and are therefore intricately linked. However, for reasons of simplification, this CO₂–SLCF linkage was often disregarded in long-term projections of earlier studies. Here we explicitly account for CO₂–SLCF linkages and show that the short- and long-term climate effects of many SLCF measures consistently become smaller in scenarios that keep warming to below 2 °C relative to preindustrial levels. Although long-term mitigation of methane and hydrofluorocarbons are integral parts of 2 °C scenarios, early action on these species mainly influences near-term temperatures and brings small benefits for limiting maximum warming relative to comparable reductions taking place later. Furthermore, we find that maximum 21st-century warming in 2 °C-consistent scenarios is largely unaffected by additional black-carbon-related measures because key emission sources are already phased-out through CO₂ mitigation. Our study demonstrates the importance of coherently considering CO₂–SLCF coevolutions. Failing to do so leads to strongly and consistently overestimating the effect of SLCF measures in climate stabilization scenarios. Our results reinforce that SLCF measures are to be considered complementary rather than a substitute for early and stringent CO₂ mitigation. Near-term SLCF measures do not allow for more time for CO₂ mitigation. We disentangle and resolve the distinct benefits across different species and therewith facilitate an integrated strategy for mitigating both short and long-term climate change.For about two decades, policy-makers have considered options to avoid dangerous anthropogenic interference with the climate system (1). So far, many countries support limiting warming to below a 2 °C temperature limit, but the required global mitigation action to achieve this has been limited (2–4). To inform policy-makers about options and challenges, the United Nations Environment Program (UNEP) published several reports over the past years on three interlinked aspects: climate stabilization and greenhouse gas (GHG) mitigation (3), short-lived climate forcers (SLCFs) and clean-air benefits (5, 6), and hydrofluorocarbons (7) (HFCs). We build here upon the insights of these reports (henceforth referred to as “Gap Report,” “SLCF Reports,” and “HFC Report,” respectively) to disentangle the joint effects of CO₂ and SLCF mitigation for limiting global warming. We evaluate the potential for limiting global-mean warming until 2100 and the rate of near-term warming, with a focus on 2 °C-consistent scenarios (Fig. 1). Reductions in CO₂ and SLCFs also provide important cobenefits like energy security (8), and local health and agricultural benefits (9–12), which fall outside the scope of this paper.Open in a separate windowFig. 1.Influence of SLCF-CO₂ linkages under varying CO₂ mitigation. (A) Global-mean surface temperature implications and interdependence of CO₂ (black), CH₄ (green), HFC (orange), BC-related (blue), and SO₂ mitigation (red). (B) The general effect of SLCF-CO₂ linkages. CO₂ paths show a world “with CO₂ mitigation” (32) and with “no CO₂ mitigation” (24). Early CH₄ mitigation is represented by the combined light and dark green area. HFC mitigation is shown for the lower end of the range assessed in this study. BC-related (and SO₂) measures show the difference between Case 6 and Case 2 (Case 4 and Case 2). Alternative cases are provided in SI Appendix, Fig. S1. Vertical dashed lines are time points relevant to Figs. 2 and ​and33.The main challenge in this exercise is the interdependence of coemitted climate forcers and the differences between their net forcing effects (13). For example, energy-related black carbon (BC) aerosols have an overall warming effect (14), whereas sulfate aerosols and some biomass-related BC emissions together with their coemitted species are cooling (13, 14). Because CO₂ and BC-related emissions often have common combustion sources (14), CO₂ mitigation will also influence the abundance of SLCFs. This linkage has already been well studied for other air pollutants (15, 16). Due to data limitations, the first studies that analyzed the mitigation potential of SLCFs (5, 6, 9, 17–19) did not account for these linkages in the long term and kept post-2030 SLCF forcing constant across a wide range of CO₂ paths. Alternatively, simple relationships between species were used (20). Such approaches, however, cannot guarantee that the long-term SLCF and CO₂ evolutions remain internally consistent. To provide an integrated view, we here account for this linkage and apply relationships (21) derived from detailed energy–environment–economy scenarios that explore various levels of air pollution control and track technological linkages between SCLF and CO₂ sources (8). Each CO₂ scenario in our analysis is thus associated with a consistent evolution of SLCFs at a specific level of pollution control stringency (see below). In policy discussions, methane (CH₄) and BC are often subsumed under the single term “short-lived climate pollutants” (SLCP) but in light of their different influence on the climate, as well as differing technological and policy instruments for mitigation, they are explicitly distinguished here.     

Irreversible climate change due to carbon dioxide emissions

The severity of damaging human-induced climate change depends not only on the magnitude of the change but also on the potential for irreversibility. This paper shows that the climate change that takes place due to increases in carbon dioxide concentration is largely irreversible for 1,000 years after emissions stop. Following cessation of emissions, removal of atmospheric carbon dioxide decreases radiative forcing, but is largely compensated by slower loss of heat to the ocean, so that atmospheric temperatures do not drop significantly for at least 1,000 years. Among illustrative irreversible impacts that should be expected if atmospheric carbon dioxide concentrations increase from current levels near 385 parts per million by volume (ppmv) to a peak of 450–600 ppmv over the coming century are irreversible dry-season rainfall reductions in several regions comparable to those of the “dust bowl” era and inexorable sea level rise. Thermal expansion of the warming ocean provides a conservative lower limit to irreversible global average sea level rise of at least 0.4–1.0 m if 21st century CO₂ concentrations exceed 600 ppmv and 0.6–1.9 m for peak CO₂ concentrations exceeding ≈1,000 ppmv. Additional contributions from glaciers and ice sheet contributions to future sea level rise are uncertain but may equal or exceed several meters over the next millennium or longer.     

From the Cover: Near-linear cost increase to reduce climate-change risk

One approach in climate-change policy is to set normative long-term targets first and then infer the implied emissions pathways. An important example of a normative target is to limit the global-mean temperature change to a certain maximum. In general, reported cost estimates for limiting global warming often rise rapidly, even exponentially, as the scale of emission reductions from a reference level increases. This rapid rise may suggest that more ambitious policies may be prohibitively expensive. Here, we propose a probabilistic perspective, focused on the relationship between mitigation costs and the likelihood of achieving a climate target. We investigate the qualitative, functional relationship between the likelihood of achieving a normative target and the costs of climate-change mitigation. In contrast to the example of exponentially rising costs for lowering concentration levels, we show that the mitigation costs rise proportionally to the likelihood of meeting a temperature target, across a range of concentration levels. In economic terms investing in climate mitigation to increase the probability of achieving climate targets yields “constant returns to scale,” because of a counterbalancing rapid rise in the probabilities of meeting a temperature target as concentration is lowered.     

Using decision pathway surveys to inform climate engineering policy choices

Over the coming decades citizens living in North America and Europe will be asked about a variety of new technological and behavioral initiatives intended to mitigate the worst impacts of climate change. A common approach to public input has been surveys whereby respondents’ attitudes about climate change are explained by individuals’ demographic background, values, and beliefs. In parallel, recent deliberative research seeks to more fully address the complex value tradeoffs linked to novel technologies and difficult ethical questions that characterize leading climate mitigation alternatives. New methods such as decision pathway surveys may offer important insights for policy makers by capturing much of the depth and reasoning of small-group deliberations while meeting standard survey goals including large-sample stakeholder engagement. Pathway surveys also can help participants to deepen their factual knowledge base and arrive at a more complete understanding of their own values as they apply to proposed policy alternatives. The pathway results indicate more fully the conditional and context-specific nature of support for several “upstream” climate interventions, including solar radiation management techniques and carbon dioxide removal technologies.Governments worldwide are facing a host of public policy controversies that involve tough tradeoffs across economic, environmental, temporal, and social objectives. These choices typically involve multiple stakeholders and uncertainty as to the effectiveness of policy responses. Although the acceptance of policy initiatives is never guaranteed, more broadly supported options will emerge when the views of constituent stakeholders are understood in advance and when policy design anticipates and responds to the reasons behind public support or opposition.Nearly all experts agree that human-caused emissions of carbon dioxide (CO₂) and other greenhouse gases (GHGs) are already responsible for significant changes to the earth’s climate. These changes include higher mean temperatures, shifts in rainfall amounts and location, sea-level rise, and more frequent and severe droughts and storm events (1). However, policies aimed at mitigating the effects of climate change are controversial, in large part due to disagreements about the sources and extent of climate change or the perceived quality of the associated policy options (2). Recent reports of the Intergovernmental Panel on Climate Change (IPCC) thus call for further policy initiatives, wherein citizens will be asked about new technological and behavioral initiatives intended to mitigate the worst impacts of climate change.In such contexts, the responsibility of public officials is twofold: help citizens and other stakeholders become better informed about the nature and distribution of the risks and benefits of proposed actions, then find ways to listen to and act on their ideas. A fundamental challenge is to develop methodologies that accurately capture public input, including learning about how different groups within society think through or evaluate a range of policy options. Eliciting and understanding public opinion is challenging, however, because people use diverse mental models to interpret information and make sense of policy options (3). Peoples’ assessments of options are also filtered through what Kahneman (4) and others have referred to as “fast and slow” thinking. Fast and slow thinking includes a variety of cognitive processes that involve deliberative attention to problems as well as heuristics (or “rules of thumb”), which are efficient but can also be responsible for judgmental errors (e.g., anchoring on selected aspects of a problem).New, large-scale technologies that raise difficult ethical questions and involve uncertain outcomes significantly compound this challenge. A primary example is climate engineering technologies designed to capture and store CO₂ or to reflect sunlight away from the earth. Both have recently come under consideration due to rapid increases in global temperatures and increased concerns about the vulnerability of global ecosystems (5, 6).Carefully designed surveys will continue to play an important role in shaping public policies (7, 8). In the context of climate mitigation and adaptation actions, however, we question a primary dependence on conventional surveys. This concern arises because many climate mitigation options, such as large-scale geoengineering technologies, are unfamiliar and could represent an “unprecedented human intervention into nature’’ (9). In such situations, our worry is that some survey approaches may encourage quick responses that fail to incorporate key factual information and overly reflect the automatic choices and political ideologies characteristic of “fast” thinking, in contrast to slower and more deliberative thinking needed for unfamiliar, multidimensional decisions.In addition, survey research reveals two kinds of motivation that reduce the accuracy of participants responses: solution aversion, wherein people contest policies suggested by environmental scientists (10), and social desirability, wherein respondents edit reported behavior to avoid embarrassing themselves (11). Scholars of public participation are calling for new methods that increase response accuracy and can help to “open up” citizens’ analytic and participatory appraisal of new technologies and policies (12). A related trend in public participation is the adoption of more deliberative designs, endorsed in both the United States (13) and the United Kingdom (14), particularly because they are viewed as providing opportunities for discussion, reflection, and learning (15).This paper describes a “decision pathway” approach to surveys that addresses these challenges by combining the strengths of interactive deliberative designs with the larger and more representative sampling provided by surveys. We begin by reviewing the literature on public attitudes toward climate change, including recent shifts to consider climate change in reference to both in situ policy contexts (e.g., urban planning) and emerging mitigation or adaptation options (e.g., greater dependence on nuclear power). We follow with a discussion of the potential contribution of decision-making theory to design and implement deliberative surveys (14, 16). We then summarize empirical results from a climate-change pathway survey of a representative sample of US citizens (n = 800). To ensure informed responses and to address the technical and social complexity of required decisions, the survey design follows the lead of earlier “mental models” work (3) and incorporates tutorials on leading climate engineering techniques (e.g., carbon capture and storage, solar radiation) to encourage reflection on—and possibly changes to—participants’ values, reasoning strategies, and policy choices. We summarize these findings and their implications for the development of a broader methodological toolkit to aid decisions about novel and controversial technologies.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Beyond the hockey stick: Climate lessons from the Common Era

More than two decades ago, my coauthors, Raymond Bradley and Malcolm Hughes, and I published the now iconic “hockey stick” curve. It was a simple graph, derived from large-scale networks of diverse climate proxy (“multiproxy”) data such as tree rings, ice cores, corals, and lake sediments, that captured the unprecedented nature of the warming taking place today. It became a focal point in the debate over human-caused climate change and what to do about it. Yet, the apparent simplicity of the hockey stick curve betrays the dynamicism and complexity of the climate history of past centuries and how it can inform our understanding of human-caused climate change and its impacts. In this article, I discuss the lessons we can learn from studying paleoclimate records and climate model simulations of the “Common Era,” the period of the past two millennia during which the “signal” of human-caused warming has risen dramatically from the background of natural variability.

Clearly, there is a cautionary tale told by the hockey stick curve in the unprecedented warming that we are causing, but the lessons from the paleoclimate record of the Common Era (CE) go far beyond that. What might we infer, for example, about the role of dynamical mechanisms relevant to climate change impacts today from their past responses to natural drivers? Examples are the El Niño phenomenon, the Asian summer monsoon, and the North Atlantic Ocean “conveyor belt” circulation. Are there potential “tipping point” elements within these climate subsystems? How has sea level changed in past centuries, and what does it tell us about future coastal risk? Are there natural long-term oscillations, evident in the paleoclimate record, that might compete with human-caused climate change today? Can we assess the “sensitivity” of the climate to ongoing human-caused increases in greenhouse gas concentrations from examining how climate has responded to natural factors in the past? Also, can better estimates of past trends inform assessments of how close we are to critical “dangerous” warming thresholds? In this article, I seek to address such questions and offer thoughts about ways forward to more confident answers.     

Dependence of hydropower energy generation on forests in the Amazon Basin at local and regional scales

Tropical rainforest regions have large hydropower generation potential that figures prominently in many nations’ energy growth strategies. Feasibility studies of hydropower plants typically ignore the effect of future deforestation or assume that deforestation will have a positive effect on river discharge and energy generation resulting from declines in evapotranspiration (ET) associated with forest conversion. Forest loss can also reduce river discharge, however, by inhibiting rainfall. We used land use, hydrological, and climate models to examine the local “direct” effects (through changes in ET within the watershed) and the potential regional “indirect” effects (through changes in rainfall) of deforestation on river discharge and energy generation potential for the Belo Monte energy complex, one of the world’s largest hydropower plants that is currently under construction on the Xingu River in the eastern Amazon. In the absence of indirect effects of deforestation, simulated deforestation of 20% and 40% within the Xingu River basin increased discharge by 4–8% and 10–12%, with similar increases in energy generation. When indirect effects were considered, deforestation of the Amazon region inhibited rainfall within the Xingu Basin, counterbalancing declines in ET and decreasing discharge by 6–36%. Under business-as-usual projections of forest loss for 2050 (40%), simulated power generation declined to only 25% of maximum plant output and 60% of the industry’s own projections. Like other energy sources, hydropower plants present large social and environmental costs. Their reliability as energy sources, however, must take into account their dependence on forests.     

No buzz for bees: Media coverage of pollinator decline

Although widespread declines in insect biomass and diversity are increasing concerns within the scientific community, it remains unclear whether attention to pollinator declines has also increased within information sources serving the general public. Examining patterns of journalistic attention to the pollinator population crisis can also inform efforts to raise awareness about the importance of declines of insect species providing ecosystem services beyond pollination. We used the Global News Index developed by the Cline Center for Advanced Social Research at the University of Illinois at Urbana–Champaign to track news attention to pollinator topics in nearly 25 million news items published by two American national newspapers and four international wire services over the past four decades. We found vanishingly low levels of attention to pollinator population topics relative to coverage of climate change, which we use as a comparison topic. In the most recent subset of ∼10 million stories published from 2007 to 2019, 1.39% (137,086 stories) refer to climate change/global warming while only 0.02% (1,780) refer to pollinator populations in all contexts, and just 0.007% (679) refer to pollinator declines. Substantial increases in news attention were detectable only in US national newspapers. We also find that, while climate change stories appear primarily in newspaper “front sections,” pollinator population stories remain largely marginalized in “science” and “back section” reports. At the same time, news reports about pollinator populations increasingly link the issue to climate change, which might ultimately help raise public awareness to effect needed policy changes.     

Life cycle air quality impacts of conventional and alternative light-duty transportation in the United States

Commonly considered strategies for reducing the environmental impact of light-duty transportation include using alternative fuels and improving vehicle fuel economy. We evaluate the air quality-related human health impacts of 10 such options, including the use of liquid biofuels, diesel, and compressed natural gas (CNG) in internal combustion engines; the use of electricity from a range of conventional and renewable sources to power electric vehicles (EVs); and the use of hybrid EV technology. Our approach combines spatially, temporally, and chemically detailed life cycle emission inventories; comprehensive, fine-scale state-of-the-science chemical transport modeling; and exposure, concentration–response, and economic health impact modeling for ozone (O₃) and fine particulate matter (PM_2.5). We find that powering vehicles with corn ethanol or with coal-based or “grid average” electricity increases monetized environmental health impacts by 80% or more relative to using conventional gasoline. Conversely, EVs powered by low-emitting electricity from natural gas, wind, water, or solar power reduce environmental health impacts by 50% or more. Consideration of potential climate change impacts alongside the human health outcomes described here further reinforces the environmental preferability of EVs powered by low-emitting electricity relative to gasoline vehicles.Society is in the midst of a great effort to understand and mitigate anthropogenic greenhouse gas (GHG) emissions and their effects on the global climate (1–5). However, GHG damages are not the only environmental impact of human activities, and are often not even the largest. In transportation, for example, non-GHG air pollution damage externalities generally exceed those from climate change (6–8). Here, we explore the air quality impacts of several proposed transportation fuel interventions: liquid biofuels (9), electric vehicles (EVs) powered by conventional and alternative energy sources (3), biomass feedstocks to power EVs (10, 11), compressed natural gas (CNG) powered vehicles (5), and improved vehicle fuel economy.The air quality impacts of biofuels, transportation electrification, CNG vehicles, and improved fuel economy have been studied (refs. 7, 8, and 12–21; results are summarized in Table S1); our work advances prior research by combining estimates of life cycle emissions [i.e., emissions from production (“upstream”) and consumption (“tailpipe”) of the fuel] with an advanced air quality impact assessment. In addition, we incorporate greater spatial, temporal, and chemical detail than have prior research efforts. We also report non-GHG air quality life cycle impacts of biomass-powered EVs, which to our knowledge have not yet been described.We use a spatially and temporally explicit life cycle inventory model (22) to estimate total fuel supply chain air pollutant emissions for scenarios where 10% of US projected vehicle miles traveled in year 2020 are driven in 1 of 11 types of passenger cars: (i) conventional gasoline powered vehicles (abbreviation: “gasoline”); (ii) grid-independent hybrid EVs (“gasoline hybrid”); (iii) diesel powered light-duty vehicles (“diesel”); (iv) internal-combustion CNG vehicles (“CNG”); (v) vehicles powered by ethanol from corn grain through natural-gas–powered dry milling (“corn ethanol”); (vi) vehicles powered by cellulosic ethanol from corn stover (“stover ethanol”); and battery EVs (“EV”) powered by electricity from the following: (vii) the projected year 2020 US average electric generation mix (“EV grid average”); (viii) coal (“EV coal”); (ix) natural gas (“EV natural gas”); (x) the combustion of corn stover (“EV corn stover”); and (xi) wind turbines, dynamic water power, or solar power (“EV WWS”). Because year 2020 electric generation infrastructure is not predetermined, we explore a range of electricity technologies rather than attempting to predict future electrical generation and dispatch deterministically; our approach can inform transportation and electricity generation policies in tandem. Based on prior research, we assume that the difference among scenarios in emissions from manufacturing and disposal of vehicles and from upstream infrastructure is small relative to differences in vehicle operation emissions (8, 23, 24) with the exception of lithium ion EV battery production. To highlight battery-related impacts, we analyze them separately from fuel-related impacts.We use spatially and temporally explicit simulations, including a state-of-the-science mechanistic meteorology and chemical transport model, to estimate for each scenario the changes in annual-average concentrations of the regulated pollutants fine particulate matter (PM_2.5) and ground-level ozone (O₃). We use spatially explicit population data (25) and results from major epidemiological studies (26, 27) to estimate increases in mortalities attributable to each scenario. We estimate monetized externalities from mortalities using a value of statistical life (VSL) metric. Results are given next; methods are described thereafter.     

Tipping elements in the Earth's climate system

The term “tipping point” commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. Here we introduce the term “tipping element” to describe large-scale components of the Earth system that may pass a tipping point. We critically evaluate potential policy-relevant tipping elements in the climate system under anthropogenic forcing, drawing on the pertinent literature and a recent international workshop to compile a short list, and we assess where their tipping points lie. An expert elicitation is used to help rank their sensitivity to global warming and the uncertainty about the underlying physical mechanisms. Then we explain how, in principle, early warning systems could be established to detect the proximity of some tipping points.     

Carbon choices determine US cities committed to futures below sea level

Anthropogenic carbon emissions lock in long-term sea-level rise that greatly exceeds projections for this century, posing profound challenges for coastal development and cultural legacies. Analysis based on previously published relationships linking emissions to warming and warming to rise indicates that unabated carbon emissions up to the year 2100 would commit an eventual global sea-level rise of 4.3–9.9 m. Based on detailed topographic and population data, local high tide lines, and regional long-term sea-level commitment for different carbon emissions and ice sheet stability scenarios, we compute the current population living on endangered land at municipal, state, and national levels within the United States. For unabated climate change, we find that land that is home to more than 20 million people is implicated and is widely distributed among different states and coasts. The total area includes 1,185–1,825 municipalities where land that is home to more than half of the current population would be affected, among them at least 21 cities exceeding 100,000 residents. Under aggressive carbon cuts, more than half of these municipalities would avoid this commitment if the West Antarctic Ice Sheet remains stable. Similarly, more than half of the US population-weighted area under threat could be spared. We provide lists of implicated cities and state populations for different emissions scenarios and with and without a certain collapse of the West Antarctic Ice Sheet. Although past anthropogenic emissions already have caused sea-level commitment that will force coastal cities to adapt, future emissions will determine which areas we can continue to occupy or may have to abandon.Most studies on the projected impacts of anthropogenic climate change have focused on the 21st century (1). However, substantial research indicates that contemporary carbon emissions, even if stopped abruptly, will sustain or nearly sustain near-term temperature increases for millennia because of the long residence time of carbon dioxide in the atmosphere and inertia in the climate system, e.g., the slow exchange of heat between ocean and atmosphere (2–5). Earth system and carbon-cycle feedbacks such as the release of carbon from thawing permafrost or vegetation changes affecting terrestrial carbon storage or albedo may further extend and possibly amplify warming (6).Paleontological records indicate that global mean sea level is highly sensitive to temperature (7) and that ice sheets, the most important contributors to large-magnitude sea-level change, can respond to warming on century time scales (8), while models suggest ice sheets require millennia to approach equilibrium (9). Accordingly, sustained temperature increases from current emissions are expected to translate to long-term sea-level rise (SLR). Through modeling and with support from paleontological data, Levermann et al. (10) found a roughly linear global mean sea-level increase of 2.3 m per 1 °C warming within a time-envelope of the next 2,000 y.This relationship forecasts a profound challenge in light of warming likely to exceed 2 °C given the current path of emissions (11). Although relatively modest in comparison, projected SLR of up to 1.2 m this century has been estimated to threaten up to 4.6% of the global population and 9.3% of annual global gross domestic product with annual flooding by 2100 in the absence of adaptive measures (12). Higher long-term sea levels endanger a fifth of all United Nations Educational, Scientific and Cultural Organization world heritage sites (13). These global analyses depend on elevation data with multimeter rms vertical errors that consistently overestimate elevation and thus underestimate submergence risk (14). Here we explore the challenges posed under different scenarios by long-term SLR in the United States, where highly accurate elevation and population data permit robust exposure assessments (15, 16).Our analysis combines published relationships between cumulative carbon emissions and warming, together with two possible versions of the relationship between warming and sea level, to estimate global and regional sea-level commitments from different emissions totals. The first version, the “baseline” case, employs a minor modification of the warming–SLR relationship from Levermann et al. (10) The second version, the “triggered” case, makes a major adjustment to explore an important possibility suggested by recent research, by assuming that an inevitable collapse of the West Antarctic Ice Sheet (WAIS) already has been set in motion (17–19).For each case, we then use topographic, tidal, and census data to assess the contemporary populations living on implicated land nationwide, by state and by municipality. Although current populations will not experience full, long-term SLR, we use their exposure as a proxy for the challenge facing the more enduring built environment and the cultural and economic activity it embodies, given the strong spatial correlation between population and development. We focus most on cities, identifying and tabulating municipalities where committed sea levels would set land that is home to more than half (or other fractions) of the current population below the high tide line.By “committed” or “locked in” warming or sea level in a given year, we refer to the long-term effects of cumulative anthropogenic carbon emissions through that year: the sustained temperature increase or SLR that will ensue on a time scale of centuries to millennia in the absence of massive and prolonged future active carbon removal from the atmosphere. We call a city “committed” when sea-level commitments would affect land supporting more than half of its current population (or another percentage of the population, if specified). We assume zero future emissions when assessing commitments for a given year, with the exception of one analysis incorporating future emissions implied by current energy infrastructure. When we associate years with warming, sea level, and city commitments, we are referencing the 21st century years when the commitments are established through cumulative emissions, not the years farther in the future when the commitments are realized through sustained temperature increases and SLR.