2C or not 2C?

Political attention has increasingly focused on limiting warming to 2 °C. However, there is no consensus on both questions “Is the 2 °C target achievable?” and “What should be done with this target that becomes increasingly difficult to achieve?”. This paper aims at disentangling the points of deep uncertainty underlying this absence on consensus. It first gives simple visualizations of the challenge posed by the 2 °C target and shows how key assumptions (on the points of deep uncertainty) influence the answer to the target achievability question. It then proposes an “uncertainties and decisions tree”, linking different beliefs on climate change, the achievability of different policies, and current international policy dynamics to various options to move forward on climate change.     

Postponing emission reductions from 2020 to 2030 increases climate risks and long-term costs

Substantially postponing the emission reductions, compared to the ranges indicated in IPCC’s recent assessment for 2020 as required for meeting the longterm 2°C target, increases the risk of exceeding this target. The costs of a delay strategy are lower in the short term, but leads to higher costs in the longer term. The analysis shows if the emission reductions are postponed to 2030 it is not likely that higher emissions from the earlier years can be fully compensated in future decades in a so-called ‘delayed action scenario’. A full compensation would require emission reduction rates in the coming decades that are much higher than those found in the scenario literature. Without compensation, the risk of exceeding the global temperature rise target of 2°C will increase. This confirms that it is not only the reduction commitments for 2050 that determine the risk of exceeding the 2°C target, but also the path between now and 2050. To meet this 2°C target, more ambitious 2020 reduction targets are needed for the developed and developing countries than those that have been pledged so far.     

Quality of geological CO2 storage to avoid jeopardizing climate targets

We explore allowable leakage for carbon capture and geological storage to be consistent with maximum global warming targets of 2.5 and 3 °C by 2100. Given plausible fossil fuel use and carbon capture and storage scenarios, and based on modeling of time-dependent leakage of CO₂, we employ a climate model to calculate the long-term temperature response of CO₂ emissions. We assume that half of the stored CO₂ is permanently trapped by fast mechanisms. If 40?% of global CO₂ emissions are stored in the second half of this century, the temperature effect of escaped CO₂ is too small to compromise a 2.5 °C target. If 80?% of CO₂ is captured, escaped CO₂ must peak 300?years or later for consistency with this climate target. Due to much more CO₂ stored for the 3 than the 2.5 °C target, quality of storage becomes more important. Thus for the 3 °C target escaped CO₂ must peak 400?years or later in the 40?% scenario, and 3000?years or later in the 80?% scenario. Consequently CO₂ escaped from geological storage can compromise the less stringent 3 °C target in the long-run if most of global CO₂ emissions have been stored. If less CO₂ is stored only a very high escape scenario can compromise the more stringent 2.5 °C target. For the two remaining combinations of storage scenarios and climate targets, leakage must be high to compromise these climate targets.     

Global climate policy and deep decarbonization of energy-intensive industries

If we are to limit global warming to 2 °C, all sectors in all countries must reduce their emissions of GHGs to zero not later than 2060–2080. Zero-emission options have been less explored and are less developed in the energy-intensive basic materials industries than in other sectors. Current climate policies have not yet motivated major efforts to decarbonize this sector, and it has been largely protected from climate policy due to the perceived risks of carbon leakage and a focus on short-term reduction targets to 2020. We argue that the future global climate policy regime must develop along three interlinked and strategic lines to facilitate a deep decarbonization of energy-intensive industries. First, the principle of common but differentiated responsibility must be reinterpreted to allow for a dialogue on fairness and the right to development in relation to industry. Second, a greater focus on the development, deployment and transfer of technology in this sector is called for. Third, the potential conflicts between current free trade regimes and motivated industrial policies for deep decarbonization must be resolved. One way forward is to revisit the idea of sectoral approaches with a broader scope, including not only emission reductions, but recognizing the full complexity of low-carbon transitions in energy-intensive industries. A new approach could engage industrial stakeholders, support technology research, development and demonstration and facilitate deployment through reducing the risk for investors. The Paris Agreement allows the idea of sectoral approaches to be revisited in the interests of reaching our common climate goals.

Policy relevance

Deep decarbonization of energy-intensive industries will be necessary to meet the 2 °C target. This requires major innovation efforts over a long period. Energy-intensive industries face unique challenges from both innovation and technical perspectives due to the large scale of facilities, the character of their global markets and the potentially high mitigation costs. This article addresses these challenges and discusses ways in which the global climate policy framework should be developed after the Paris Agreement to better support transformative change in the energy-intensive industries.     

Managing carbon-intensive materials in a decarbonizing world without a global price on carbon

Emissions from the production of iron and steel could constitute a significant share of a 2°C global emissions budget (around 19% under the IEA 2DS scenario). They need to be reduced, and this could be difficult under nationally based climate policy approaches. We compare a new set of nationally based modelling (the Deep Decarbonization Pathways Project) with best practice and technical limit benchmarks for iron and steel and cement emissions. We find that 2050 emissions from iron and steel and cement production represent an average 0.28?tCO₂ per capita in nationally based modelling results, very close to the technical limit benchmark of 0.21?tCO₂ per capita, and over 2.5 times lower than the best practice benchmark of 0.72?tCO₂ per capita. This suggests that national projections may be overly optimistic about achievable emissions reductions in the absence of global carbon pricing and an international research and development effort to develop low emissions technologies for emissions-intensive products. We also find that equal per capita emissions targets, often the basis of proposals for how global emissions budgets should be allocated, would be inadequate without global emissions trading. These results show that a nationally based global climate policy framework, as has been confirmed in the Paris Agreement, could lead to risks of overshooting global emissions targets for some countries and carbon leakage. Tailored approaches such as border taxes, sectoral emissions trading or carbon taxes, and consumption-based carbon pricing can help, but each faces difficulties. Ultimately, global efforts are needed to improve technology and material efficiency in emissions-intensive commodities manufacturing and use. Those efforts could be supported by technology standards and a globally coordinated R&D effort, and strengthened by the adoption of global emissions budgets for emissions-intensive traded goods.

Policy relevance

This article presents new empirical findings on global iron and steel and cement production in a low-carbon world economy, demonstrates the risks associated with a nationally based global climate policy framework as has been confirmed in the Paris Agreement, and analyses policy options to deal with those risks.     

Leviathan carbon taxes in the short run

A cap is imposed on the carbon tax rate if the total tax revenue is not allowed to increase. Using recent data on the carbon-intensity of the economy and the overall tax take, I show that this cap constrains almost any climate policy in at least some countries. A larger number of countries, emitting a substantial share of global carbon dioxide, cannot fully participate if the carbon tax (or equivalent alternative regulation) is high enough to meet the 2?°C target. For that target, the carbon tax revenue in 2020 is greater than 10?% of total tax revenue in every country.     

The global impacts of US climate policy: a model simulation using GCAM-TU and MAGICC

To assess the potential impacts of the US withdrawal from the Paris Agreement, this study applied GCAM-TU (an updated version of the Global Change Assessment Model) to simulate global and regional emission pathways of energy-related CO₂, which show that US emissions in 2100 would reduce to ?2.4?Gt, ?0.7?Gt and ?0.2?Gt under scenarios of RCP2.6, RCP3.7 and RCP4.5, respectively. Two unfavourable policy scenarios were designed, assuming a temporary delay and a complete stop for US mitigation actions after 2015. Simulations by the Model for the Assessment of Greenhouse-gas Induced Climate Change (MAGICC) indicate that the temperature increase by 2100 would rise by 0.081°C–0.161°C compared to the three original RCPs (Representative Concentration Pathways) if US emissions were kept at their 2015 levels until 2100. The probability of staying below 2°C would decrease by 6–9% even if the US resumes mitigation efforts for achieving its Nationally Determined Contribution (NDC) target after 2025. It is estimated by GCAM-TU that, without US participation, increased reduction efforts are required for the rest of the world, including developing countries, in order to achieve the 2°C goal, resulting in 18% higher global cumulative mitigation costs from 2015 to 2100.

Key policy insights

• President Trump’s climate policies, including planned withdrawal from the Paris Agreement, cast a shadow on international climate actions, and would lower the likelihood of achieving the 2°C target.

• To meet the 2°C target without the US means increased reduction efforts and mitigation costs for the rest of the world, and considerable economic burdens for major developing areas.

• Active state-, city- and enterprise-level powers should be supported to keep the emission reduction gap from further widening even with reduced mitigation efforts from the US federal government.

     

Impacts of climate change as a function of global mean temperature: maize productivity and water use in China

Projections of future climate change are plagued with uncertainties from global climate models and emission scenarios, causing difficulties for impact assessments and for planners taking decisions on adaptation measure. Here, we developed an approach to deal with the uncertainties and to project the changes of maize productivity and water use in China using a process-based crop model, against a global mean temperature (GMT) increase scale relative to 1961?C1990 values. From 20 climate scenarios output from the Intergovernmental Panel on Climate Change Data Distribution Centre, we adopted the median values of projected changes in monthly mean climate variables for representative stations and driven the CERES-Maize model to simulate maize production under baseline and future climate scenarios. Adaptation options such as automatic planting, automatic application of irrigation and fertilization were considered, although cultivars were assumed constant over the baseline and future. After assessing representative stations across China, we projected changes in maize yield, growing period, evapotranspiration, and irrigation-water use for GMT changes of 1°C, 2°C, and 3°C, respectively. Results indicated that median values of projected decreases in the yields of irrigated maize without (with) consideration of CO₂-fertilization effects ranged from 1.4% to 10.9% (1.6% to 7.8%), 9.8% to 21.7% (10.2% to 16.4%), and 4.3% to 32.1% (3.9% to 26.6%) for GMT changes of 1°C, 2°C, and 3°C, respectively. Median values of projected changes in irrigation-water use without (with) consideration of CO₂-fertilization effects ranged from ?1.3% to 2.5% (?18.8% to 0.0%), ?43.6% to 2.4% (?56.1% to ?18.9%), and ?19.6% to 2.2% (?50.6% to ?34.3%), which were ascribed to rising CO₂ concentration, increased precipitation, as well as reduced growing period with GMT increasing. For rainfed maize, median values of projected changes in yields without (with) consideration of CO₂-fertilization effects ranged from ?22.2% to ?1.0% (?10.8% to 0.7%), ?27.6% to ?7.9% (?18.1% to ?5.6%), and ?33.7% to ?4.6% (?25.9% to ?1.6%). Approximate comparisons showed that projected maize yield losses were larger than previous estimates, particularly for rainfed maize. Our study presents an approach to project maize productivity and water use with GMT increases using process-based crop models and multiple climate scenarios. The resultant impact function is fundamental for identifying which climate change level is dangerous for food security.     

Are estimated costs of stringent mitigation biased?

We take issue with the claim by Tavoni and Tol (Clim Chang 100:769–778, 2010) that reviews of the macroeconomic costs of achieving the 2 °C climate target have been affected by selection bias and have underestimated the costs. Although many more cost estimates are available in the literature, they have restricted their survey to the data in the EMF22 study, with a limited set of model solutions for the 2 °C target. They have applied the methodology of observational meta-analysis inappropriately to policy meta-analysis, where the number of results is often very small and the basis for imputing a statistical distribution does not usually exist. They have mixed direct costs with net costs in terms of %GDP. Their method of “correcting” for missing data with (high) costs of stringent mitigation could equally be applied to correcting the data for omission of mitigation options such as biomass energy with carbon capture so reducing the cost estimates. And finally they implicitly assume that the same policy combinations and mitigation options are applied for all climate scenarios, when more stringent scenarios may require more stringent policies and options, such as regulation or BECCS. The conclusion from the literature is more appropriately that the costs are highly uncertain, that they can equally be positive or negative (gains) and that models which fail to solve for stringent mitigations are not fit for purpose.     

International and national climate policies for aviation: a review

Aviation constitutes about 2.5% of all energy-related CO₂ emissions and in addition there are non-CO₂ effects. In 2016, the ICAO decided to implement a Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA) and in 2017 the EU decided on faster emission reductions in its Emissions Trading System (EU ETS), which since 2012 includes the aviation sector. The effects of these policies on the expected development of air travel emissions from 2017 to 2030 have been analyzed. For the sample country Sweden, the analysis shows that when emissions reductions in other sectors are attributed to the aviation sector as a result of the EU ETS and CORSIA, carbon emissions are expected to reduce by ?0.8% per year (however if non-CO₂ emissions are included in the analysis, then emissions will increase). This is much less than what is needed to achieve the 2°C target. Our analysis of potential national aviation policy instruments shows that there are legally feasible options that could mitigate emissions in addition to the EU ETS and CORSIA. Distance-based air passenger taxes are common among EU Member States and through increased ticket prices these taxes can reduce demand for air travel and thus reduce emissions. Tax on jet fuel is an option for domestic aviation and for international aviation if bilateral agreements are concluded. A quota obligation for biofuels is a third option.

Key policy insights

• Existing international climate policies for aviation will not deliver any major emission reductions.

• Policymakers who want to significantly push the aviation sector to contribute to meeting the 2°C target need to work towards putting in place tougher international policy instruments in the long term, and simultaneously implement temporary national policy instruments in the near-term.

• Distance-based air passenger taxes, carbon taxes on jet fuel and quota obligations for biofuels are available national policy options; if they are gradually increased, and harmonized with other countries, they can help to significantly reduce emissions.

     

Achievability of the Paris Agreement targets in the EU: demand-side reduction potentials in a carbon budget perspective

Limiting global warming to ‘well below’ 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase even further to 1.5°C is an integral part of the 2015 Paris Agreement. To achieve these aims, cumulative global carbon emissions after 2016 should not exceed 940 – 390?Gt of CO₂ (for the 2°C target) and 167 – ?48?Gt of CO₂ (for the 1.5°C target) by the end of the century. This paper analyses the EU’s cumulative carbon emissions in different models and scenarios (global models, EU-focused models and national carbon mitigation scenarios). Due to the higher reductions in energy use and carbon intensity of the end-use sectors in the national scenarios, we identify an additional mitigation potential of 26–37 Gt cumulative CO₂ emissions up to 2050 compared to what is currently included in global or EU scenarios. These additional reductions could help to both reduce the need for carbon dioxide removals and bring cumulative emissions in global and EU scenarios in line with a fairness-based domestic EU budget for a 2°C target, while still remaining way above the budget for 1.5°C.

Key policy insights

• Models used for policy advice such as global integrated assessment models or EU models fail to consider certain mitigation potential available at the level of sectors.

• Global and EU models assume significant levels of CO₂ emission reductions from carbon capture and storage to reach the 1.5°C target but also to reach the 2°C target.

• Global and EU model scenarios are not compatible with a fair domestic EU share in the global carbon budget either for 2°C or for 1.5°C.

• Integrating additional sectoral mitigation potential from detailed national models can help bring down cumulative emissions in global and EU models to a level comparable to a fairness-based domestic EU share compatible with the 2°C target, but not the 1.5°C aspiration.

     

The emissions gap between the Copenhagen pledges and the 2 °C climate goal: Options for closing and risks that could widen the gap

As part of the Copenhagen Accord, Annex I Parties (industrialised countries) and non-Annex I Parties (developing countries) have submitted reduction proposals (pledges) and mitigation actions to the UNFCCC secretariat. Our calculations show that if the current reduction offers of Annex I and non-Annex I countries are fully implemented, global greenhouse gas emissions could amount to 48.6-49.7 GtCO₂eq by 2020. Recent literature suggests that the emission level should be between 42 and 46 GtCO₂eq by 2020 to maintain a “medium” chance (50-66%) of meeting the 2 °C target. The emission gap is therefore 2.6-7.7 GtCO₂eq. We have identified a combined set of options, which could result in an additional 2.8 GtCO₂eq emission reduction. This would lead to an emission level just within the range needed. The options include reducing deforestation and emissions from bunker fuels, excluding emissions allowance increases from land use and forestry rules, and taking into account the national climate plans of China and India. However, there are also important risks that could widen the emissions gap, like lower reductions from countries with only a conditional pledge and the use of Kyoto and/or trading of new surplus emission allowances.     

Probabilistic spatial risk assessment of heat impacts and adaptations for London

High temperatures and heatwaves can cause large societal impacts by increasing health risks, mortality rates, and personal discomfort. These impacts are exacerbated in cities because of the Urban Heat Island (UHI) effect, and the high and increasing concentrations of people, assets and economic activities. Risks from high temperatures are now widely recognised but motivation and implementation of proportionate policy responses is inhibited by inadequate quantification of the benefits of adaptation options, and associated uncertainties. This study utilises high spatial resolution probabilistic projections of urban temperatures along with projections of demographic change, to provide a probabilistic risk assessment of heat impacts on urban society. The study focuses on Greater London and the surrounding region, assessing mortality risk, thermal discomfort in residential buildings, and adaptation options within an integrated framework. Climate change is projected to increase future heat-related mortality and residential discomfort. However, adjusting the temperature response function by 1–2 °C, to simulate adaptation and acclimatisation, reduced annual heat related mortality by 32–69 % across the scenarios tested, relative to a no adaptation scenario. Similar benefits of adaptation were seen for residential discomfort. The study also highlights additional benefits in terms of reduced mortality and residential discomfort that mitigating the urban heat island, by reducing albedo and anthropogenic heat emissions, could have.     

To what extent can a long-term temperature target guide near-term climate change commitments?

The question of appropriate timing and stringency of future greenhouse gas (GHG) emission reductions remains an issue in the discussion of mitigation responses to the climate change problem. It has been argued that our near-term action should be guided by a long-term vision for the climate, possibly a long-term temperature target. In this paper, we review proposals for long-term climate targets to avoid ‘dangerous’ climate change. Using probability estimates of climate sensitivity from the literature, we then generate probabilistic emissions scenarios that satisfy temperature targets of 2.0, 2.5, and 3.0°C above pre-industrial levels with no overshoot. Our interest is in the implications of these targets on abatement requirements over the next 50 years. If we allow global industrial GHG emissions to peak in 2025 at 14 GtCeq, and wish to achieve a 2.0°C target with at least 50% certainty, we find that the low sensitivity estimate in the literature suggests our industrial emissions must fall to 9 GtCeq by 2050: equal to the level in 2000. However, the average literature sensitivity estimate suggests the level must be less than 2 GtCeq; and in the high sensitivity case, the target is simply unreachable unless we allow for overshoot. Our results suggest that in light of the uncertainty in our knowledge of the climate sensitivity, a long-term temperature target (such as the 2.0°C target proposed by the European Commission) can provide limited guidance to near-term mitigation requirements.     

Changes in African temperature and precipitation associated with degrees of global warming

For almost two decades, politicians have been negotiating temperature limits to which anthropogenic global warming should be restricted, and 2 °C has emerged as benchmark for danger. However, there has been a lack of scientific research into the implications of such a change for African climate. This study aims to provide information for mitigation debates; through an examination of temperature and precipitation changes in Africa associated with 1 °C, 2 °C, 3 °C, and 4 °C of global warming. Data from Global Climate Models show little significant precipitation change at 1 °C, then larger anomalies at 2 °C which are strengthened and extended at 3 °C and 4 °C, including a wet signal in East Africa, and dry signals in Southern Africa, the Guinea Coast, and the west of the Sahel. Some of the models project changes with potential for severe societal implications. Despite the uncertainty attached to these projections, they highlight risks associated with 2 °C and beyond. Using these findings as a framework for impact assessment and evaluation, further research has the potential to uncover the implications of global warming for African regions.     

Adaptive and risk-based approaches to climate change and the management of uncertainty and institutional risk: The case of future flooding in England

This paper focuses on how scientific uncertainties about future peak flood flows and sea level rises are accounted for in long term strategic planning processes to adapt inland and coastal flood risk management in England to climate change. Combining key informant interviews (n = 18) with documentary analysis, it explores the institutional tensions between adaptive management approaches emphasising openness to uncertainty and to alternative policy options on the one hand and risk-based ones that close them down by transforming uncertainties into calculable risks whose management can be rationalized through cost-benefit analysis and nationally consistent, risk-based priority setting on the other hand. These alternative approaches to managing uncertainty about the first-order risks to society from future flooding are shaped by institutional concerns with managing the second-order, ‘institutional’ risks of criticism and blame arising from accountability for discharging those first-order risk management responsibilities. In the case of river flooding the poorly understood impacts of future climate change were represented with a simplistic adjustment to peak flow estimates, which proved robust in overcoming institutional resistance to making precautionary allowances for climate change in risk-based flood management, at least in part because its scientific limitations were acknowledged only partially. By contrast in the case of coastal flood risk management, greater scientific confidence led to successively more elaborate guidance on how to represent the science, which in turn led to inconsistency in implementation and increased the institutional risks involved in taking the uncertain effects of future sea level rise into account in adaptation planning and flood risk management. Comparative analysis of these two cases then informs some wider reflections about the tensions between adaptive and risk-based approaches, the role of institutional risk in climate change adaptation, and the importance of such institutional dynamics in shaping the framing uncertainties and policy responses to scientific knowledge about them.     

Comparative assessment of Japan's long-term carbon budget under different effort-sharing principles

This article assesses Japan's carbon budgets up to 2100 in the global efforts to achieve the 2?°C target under different effort-sharing approaches based on long-term GHG mitigation scenarios published in 13 studies. The article also presents exemplary emission trajectories for Japan to stay within the calculated budget.

The literature data allow for an in-depth analysis of four effort-sharing categories. For a 450?ppm CO₂e stabilization level, the remaining carbon budgets for 2014–2100 were negative for the effort-sharing category that emphasizes historical responsibility and capability. For the other three, including the reference ‘Cost-effectiveness’ category, which showed the highest budget range among all categories, the calculated remaining budgets (20th and 80th percentile ranges) would run out in 21–29 years if the current emission levels were to continue. A 550?ppm CO₂e stabilization level increases the budgets by 6–17 years-equivalent of the current emissions, depending on the effort-sharing category. Exemplary emissions trajectories staying within the calculated budgets were also analysed for ‘Equality’, ‘Staged’ and ‘Cost-effectiveness’ categories. For a 450?ppm CO₂e stabilization level, Japan's GHG emissions would need to phase out sometime between 2045 and 2080, and the emission reductions in 2030 would be at least 16–29% below 1990 levels even for the most lenient ‘Cost-effectiveness’ category, and 29–36% for the ‘Equality’ category. The start year for accelerated emissions reductions and the emissions convergence level in the long term have major impact on the emissions reduction rates that need to be achieved, particularly in the case of smaller budgets.

Policy relevance

In previous climate mitigation target formulation processes for 2020 and 2030 in Japan, neither equity principles nor long-term management of cumulative GHG emissions was at the centre of discussion. This article quantitatively assesses how much more GHGs Japan can emit by 2100 to achieve the 2?°C target in light of different effort-sharing approaches, and how Japan's GHG emissions can be managed up to 2100. The long-term implications of recent energy policy developments following the Fukushima nuclear disaster for the calculated carbon budgets are also discussed.     

Higher precision estimates of regional polar warming by ensemble regression of climate model projections

This study presents projections of twenty-first century wintertime surface temperature changes over the high-latitude regions based on the third Coupled Model Inter-comparison Project (CMIP3) multi-model ensemble. The state-dependence of the climate change response on the present day mean state is captured using a simple yet robust ensemble linear regression model. The ensemble regression approach gives different and more precise estimated mean responses compared to the ensemble mean approach. Over the Arctic in January, ensemble regression gives less warming than the ensemble mean along the boundary between sea ice and open ocean (sea ice edge). Most notably, the results show 3?°C less warming over the Barents Sea (~7?°C compared to ~10?°C). In addition, the ensemble regression method gives projections that are 30?% more precise over the Sea of Okhostk, Bering Sea and Labrador Sea. For the Antarctic in winter (July) the ensemble regression method gives 2?°C more warming over the Southern Ocean close to the Greenwich Meridian (~7?°C compared to ~5?°C). Projection uncertainty was almost half that of the ensemble mean uncertainty over the Southern Ocean between 30° W to 90° E and 30?% less over the northern Antarctic Peninsula. The ensemble regression model avoids the need for explicit ad hoc weighting of models and exploits the whole ensemble to objectively identify overly influential outlier models. Bootstrap resampling shows that maximum precision over the Southern Ocean can be obtained with ensembles having as few as only six climate models.