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Residual fossil CO2 emissions in 1.5–2 °C pathways

Profile image of Bert SaveynBert Saveyn

Nature Climate Change

https://doi.org/10.1038/S41558-018-0198-6
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11 pages

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Abstract

The Paris Agreement-which is aimed at holding global warming well below 2 °C while pursuing efforts to limit it below 1.5 °C-has initiated a bottom-up process of iteratively updating nationally determined contributions to reach these longterm goals. Achieving these goals implies a tight limit on cumulative net CO 2 emissions, of which residual CO 2 emissions from fossil fuels are the greatest impediment. Here, using an ensemble of seven integrated assessment models (IAMs), we explore the determinants of these residual emissions, focusing on sector-level contributions. Even when strengthened pre-2030 mitigation action is combined with very stringent long-term policies, cumulative residual CO 2 emissions from fossil fuels remain at 850-1,150 GtCO 2 during 2016-2100, despite carbon prices of US$130-420 per tCO 2 by 2030. Thus, 640-950 GtCO 2 removal is required for a likely chance of limiting end-of-century warming to 1.5 °C. In the absence of strengthened pre-2030 pledges, long-term CO 2 commitments are increased by 160-330 GtCO 2 , further jeopardizing achievement of the 1.5 °C goal and increasing dependence on CO 2 removal.

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Assessing Intended Nationally Determined Contributions to the Paris Climate Agreement
Michel den Elzen

By 29 October 2015, 156 Parties (representing approximately 90% of global greenhouse gas emissions in 2012) had submitted their Intended Nationally Determined Contributions (INDCs) to the UNFCCC in preparation for the adoption of the Paris Agreement in December 2015 (UNFCCC, 2015). In this report, we assess the mitigation components of the INDCs of 102 of these Parties (representing approximately 89% of global greenhouse gas emissions in 2012), including the EU28 Member States. The main findings of this assessment are: Findings regarding global emissions: The collective ambition of the INDCs put forward in 2030 falls short of what is needed to put the world directly on a cost-effective pathway to keep the global temperature increase below 2 °C; however, if fully implemented, the INDCs will deliver significant emission reductions from business-as-usual trends. – Full implementation of all unconditional INDCs is projected to reduce yearly global greenhouse gas emissions by approximately 9 (5–10) GtCO2eq by 2030, relative to the PBL business-as-usual scenario of about 65 GtCO2eq in 2030. – Additional implementation of all conditional INDCs (which would require some form of international climate finance or international cooperation mechanisms) would increase this projected reduction to about 11 (7–13) GtCO2eq in 2030 below PBL business-as-usual levels. – Despite these projected reductions relative to business-as-usual levels, global emissions are projected to increase until at least 2030, to about 56 (54–60) GtCO2eq if the unconditional INDCs are implemented, or to about 54 (52–58) GtCO2eq if the conditional INDCs are implemented as well. The median estimates for 2030 are 13% to 18% above 2010 emission levels. – Implementation of the unconditional INDCs would still leave an emission gap of 14 (13–18) GtCO2eq relative to the global emission level needed for keeping the temperature increase below 2 °C with a likely chance (42 GtCO2eq in 2030, as estimated in the UNEP Gap Report of 2014). Implementation of the conditional INDCs would further reduce this gap to 12 (10–16) GtCO2eq. Findings regarding national INDCs (G20 members): the presented analysis of individual INDCs shows how INDC implementation could enable a transition from business-as-usual trends to lower emission levels, lower emissions per capita and lower emission intensities at both national and global levels. – Based on national INDCs, emissions in middle-income countries such as Mexico, Indonesia, Brazil and South Korea are expected to peak before 2025. Emissions in China, India and South Africa are expected to peak by 2030 or later. Emissions in most high-income countries have already peaked in the past. – The largest emission reductions in 2030 relative to the PBL business-as-usual scenario (which assumes that no new climate policies will be implemented) are projected for Australia, Brazil and Canada. – Full implementation of submitted INDCs (unconditional and conditional) would decrease global net land-use (LULUCF) emissions by 2.6 GtCO2eq in 2030 compared to national business-as-usual projections, which is approximately one third of the total global emission reduction that would result from the full implementation of all unconditional INDCs. The largest absolute reductions in land-use emissions are expected for Brazil and Indonesia, followed by the United States, China, Ethiopia, Gabon and Democratic Republic of the Congo. – Based on the submitted INDCs, per capita emission levels are projected to decline between 2010 and 2030 in all G20 countries except Argentina, China, India, the Russian Federation and Turkey. – Similarly, emission intensities (emissions per GDP) are projected to decline between 2010 and 2030 in all G20 countries except Turkey, indicating a relative decoupling of economic growth and greenhouse gas emissions. Decoupling – implying absolute emission reductions in a growing economy – is projected for 13 to 15 of the G20 countries, depending on the conditionality of their INDCs. – While the aggregate effect of submitted INDCs is projected to be insufficient to close the global emission gap, INDCs of some countries are nevertheless consistent with national cost-optimal below-2 °C pathways (EU28, United States) and/or below-2 °C pathways based on converging per capita emissions (United States).

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Enhancing global climate policy ambition towards a 1.5 °C stabilization: a short-term multi-model assessment
Bert Saveyn

Environmental Research Letters

The Paris Agreement is a milestone in international climate policy as it establishes a global mitigation framework towards 2030 and sets the ground for a potential 1.5 • C climate stabilization. To provide useful insights for the 2018 UNFCCC Talanoa facilitative dialogue, we use eight state-of-the-art climate-energy-economy models to assess the effectiveness of the Intended Nationally Determined Contributions (INDCs) in meeting high probability 1.5 and 2 • C stabilization goals. We estimate that the implementation of conditional INDCs in 2030 leaves an emissions gap from least cost 2 • C and 1.5 • C pathways for year 2030 equal to 15.6 (9.0-20.3) and 24.6 (18.5-29.0) GtCO 2 eq respectively. The immediate transition to a more efficient and low-carbon energy system is key to achieving the Paris goals. The decarbonization of the power supply sector delivers half of total CO 2 emission reductions in all scenarios, primarily through high penetration of renewables and energy efficiency improvements. In combination with an increased electrification of final energy demand, low-carbon power supply is the main short-term abatement option. We find that the global macroeconomic cost of mitigation efforts does not reduce the 2020-2030 annual GDP growth rates in any model more than 0.1 percentage points in the INDC or 0.3 and 0.5 in the 2 • C and 1.5 • C scenarios respectively even without accounting for potential co-benefits and avoided climate damages. Accordingly, the median GDP reductions across all models in 2030 are 0.4%, 1.2% and 3.3% of reference GDP for each respective scenario. Costs go up with increasing mitigation efforts but a fragmented action, as implied by the INDCs, results in higher costs per unit of abated emissions. On a regional level, the cost distribution is different across scenarios while fossil fuel exporters see the highest GDP reductions in all INDC, 2 • C and 1.5 • C scenarios.

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Climate and carbon budget implications of linked future changes in CO2 and non-CO2 forcing
Corinne Hartin

Environmental Research Letters

The approximate proportional relationship between cumulative carbon emissions and instantaneous global temperature rise (the carbon budget approximation) has proven to be a useful concept to translate policy-relevant temperature objectives into CO 2 emissions pathways. However, when non-CO 2 forcing is changing along with CO 2 forcing, error in the approximation increases. Using the GCAM model to produce an ensemble of ~3,000 scenarios, we show that linked changes in CO 2 forcing, aerosol forcing, and non-CO 2 greenhouse gas forcing lead to an increase in total non-CO 2 forcing over the 21 st century across mitigation scenarios. This increase causes the relationship between instantaneous temperature and cumulative CO 2 emissions to become more complex than the proportional approximation often assumed, particularly for low temperature objectives such as 1.5 °C. The same linked changes in emissions also contribute to a near-term increase in aerosol forcing that effectively places a limit on how low peak temperature could be constrained through greenhouse gas mitigation alone. In particular, we find that 23% of scenarios that include CCS, but only 1% of scenarios that do not include CCS, achieve a temperature objective of 1.5 °C without temperature overshoot. The stated aim of the Paris Agreement is to keep the global temperature increase in this century "well below" 2 °C (compared to preindustrial levels) and to "pursue efforts" to limit the temperature increase to 1.5 °C. 1 In understanding such objectives, the concept of a carbon budget linking temperature increases to cumulative CO 2 emissions has proven useful, because it enables policy objectives (stated in terms of temperature increases) to be related to potential policy actions (stated in terms of CO 2 emissions reductions) (

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From Zero to Hero?: Why Integrated Assessment Modeling of Negative Emissions Technologies Is Hard and How We Can Do Better
Jay Fuhrman

Frontiers in Climate

Climate change mitigation strategies informed by Integrated Assessment Models (IAMs) increasingly rely on major deployments of negative emissions technologies (NETs) to achieve global climate targets. Although NETs can strongly complement emissions mitigation efforts, this dependence on the presumed future ability to deploy NETs at scale raises questions about the structural elements of IAMs that are influencing our understanding of mitigation efforts. Model inter-comparison results underpinning the IPCC's special report on Global Warming of 1.5 • C were used to explore the role that current assumptions are having on projections and the way in which emerging technologies, economic factors, innovation, and tradeoffs between negative emissions objectives and UN Sustainable Development Goals might have on future deployment of NETs. Current generation IAM scenarios widely assume we are capable of scaling up NETs over the coming 30 years to achieve negative emissions of the same order of magnitude as current global emissions (tens of gigatons of CO 2 /year) predominantly relying on highly land intensive NETs. While the technological potential of some of these approaches (e.g., direct air capture) is much greater than for the land-based technologies, these are seldom included in the scenarios. Alternative NETs (e.g., accelerated weathering) are generally excluded because of connections with industrial sectors or earth system processes that are not yet included in many models. In all cases, modeling results suggest that significant NET activity will be conducted in developing regions, raising concerns about tradeoffs with UN Sustainable Development Goals. These findings provide insight into how to improve treatment of NETs in IAMs to better inform international climate policy discussions. We emphasize the need to better understand relative strength and weaknesses of full suite of NETs that can help inform the decision making for policy makers and stakeholders.

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Mid-century emission pathways in Japan associated with the global 2 °C goal: national and globalmodels’ assessments based on carbon budgets
Pedro Rochedo

Climatic Change

This study assesses Japan’s mid-century low-emission pathways using both national and global integrated assessment models in the common mitigation scenario framework, based on the carbon budgets corresponding to the global 2 °C goal. We examine high and low budgets, equal to global cumulative 1600 and 1000 Gt-CO2 (2011–2100) for global models, and 36 and 31 Gt-CO2 (2011–2050) in Japan for national models, based on the cost-effectiveness allocation performed by the global models. The impacts of near-term policy assumption, including the implementation and enhancement of the 2030 target of the nationally determined contribution (NDC), are also considered. Our estimates show that the low budget scenarios require a 75% reduction of CO2 emissions by 2050 below the 2010 level, which is nearly the same as Japan’s governmental 2050 goal of reducing greenhouse gas emissions by 80%. With regard to near-term actions, Japan’s 2030 target included in the NDC is on track to meet the high budget s...

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Environmental co-benefits and adverse side-effects of alternative power sector decarbonization strategies
Robert Pietzcker

Nature Communications

A rapid and deep decarbonization of power supply worldwide is required to limit global warming to well below 2 °C. Beyond greenhouse gas emissions, the power sector is also responsible for numerous other environmental impacts. Here we combine scenarios from integrated assessment models with a forward-looking life-cycle assessment to explore how alternative technology choices in power sector decarbonization pathways compare in terms of non-climate environmental impacts at the system level. While all decarbonization pathways yield major environmental co-benefits, we find that the scale of co-benefits as well as profiles of adverse side-effects depend strongly on technology choice. Mitigation scenarios focusing on wind and solar power are more effective in reducing human health impacts compared to those with low renewable energy, while inducing a more pronounced shift away from fossil and toward mineral resource depletion. Conversely, non-climate ecosystem damages are highly uncertain ...

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Geological storage capacity for green excess energy readily available in Germany
Michael Kühn

Advances in Geosciences

Energy supply in Germany is subject to a profound change. The present paper addresses the German potential of storing excess energy from renewable power sources in the geological subsurface. Wind and solar electricity can be transformed into hydrogen, and with carbon dioxide subsequently into methane. When needed, electricity is regained in a gas turbine power plant combusting the methane. Here, we are taking into account the actual German storage capacity for natural gas and show that the outlined technology is ready for operation and economically competitive. The current potential for combined storage of methane and carbon dioxide allows to store around 80 TWh renewable excess energy. This is far more than required to date and estimated to provide the entire coverage in 2050.

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A framework for national scenarios with varying emission reductions
Salony Rajbhandari

Nature Climate Change

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