Summary for policymakers

The Working Group I contribution to the IPCC.s Fifth Assessment Report (AR5) considers new evidence of climate change based on many independent scientific analyses from observations of the climate system, paleoclimate archives, theoretical studies of climate processes and simulations using climate models. It builds upon the Working Group I contribution to the IPCC's Fourth Assessment Report (AR4), and incorporates subsequent new findings of research. As a component of the fifth assessment cycle, the IPCC Special Report on Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX) is an important basis for information on changing weather and climate extremes. This Summary for Policymakers (SPM) follows the structure of the Working Group I report. The narrative is supported by a series of overarching highlighted conclusions which, taken together, provide a concise summary. Main sections are introduced with a brief paragraph in italics which outlines the methodological basis of the assessment. The degree of certainty in key findings in this assessment is based on the author teams. evaluations of underlying scientific understanding and is expressed as a qualitative level of confidence (from very low to very high) and, when possible, probabilistically with a quantified likelihood (from exceptionally unlikely to virtually certain). Confidence in the validity of a finding is based on the type, amount, quality, and consistency of evidence (e.g., data, mechanistic understanding, theory, models, expert judgment) and the degree of agreement. Probabilistic estimates of quantified measures of uncertainty in a finding are based on statistical analysis of observations or model results, or both, and expert judgment. Where appropriate, findings are also formulated as statements of fact without using uncertainty qualifiers. (See Chapter 1 and Box TS.1 for more details about the specific language the IPCC uses to communicate uncertainty). The basis for substantive paragraphs in this Summary for Policymakers can be found in the chapter sections of the underlying report and in the Technical Summary. These references are given in curly brackets

Technical summary

The primary purpose of this Technical Summary (TS) is to provide the link between the complete assessment of the multiple lines of independent evidence presented in the 14 chapters of the main report and the highly condensed summary prepared as the WGI Summary for Policymakers (SPM). The Technical Summary thus serves as a starting point for those readers who seek the full information on more specific topics covered by this assessment. This purpose is facilitated by including pointers to the chapters and sections where the full assessment can be found. Policy-relevant topics, which cut across many chapters and involve many interlinked processes in the climate system, are presented here as Thematic Focus Elements (TFEs), allowing rapid access to this information

Belgium’s national emission pathway in the context of the global remaining carbon budget

This Science Brief assesses the implications of the scientific evidence on carbon budgets presented in the latest assessment of the Intergovernmental Panel on Climate Change for national carbon budgets and emissions reductions in Belgium. Based on the best available science, the global remaining carbon budget for limiting global warming to 1.5°C amounts to 400 billion tons of carbon dioxide emissions (GtCO2). The implications of this global remaining carbon budget for Belgium can be explored by using equity and fairness principles to determine a fair national carbon budget share. A variety of principles was applied ranging from approaches that are considered inherently unfair (a grandfathering approach) to approaches that have been proposed by developing country experts. Based on this wide range of distribution keys, the minimum emissions reduction for Belgium that puts their national trajectory in line with limiting global warming to 1.5°C and on track to reach net zero greenhouse gas emissions by mid‐century is ‐69% in 2030 relative to 1990 levels. If Belgium’s net zero greenhouse gas target would be advanced from 2050 to 2042, the corresponding emissions reductions in 2030 would amount to ‐61% relative to 1990 level

Carbon price variations in 2°C scenarios explored

AIM AND SCOPE Clarify the variations in carbon prices found in mitigation scenarios that limit global mean surface temperature increase to below 2°C relative to preindustrial levels. CONTEXT Integrated assessment models1 (IAM) are dominant tools for the development of long-term emissions scenarios in line with climate objectives. There is a large variety of IAMs and, together with variations in socioeconomic and technological assumptions, this variety results in important differences in model behavior. For the achievement of low emissions scenarios, models typically assume or produce an implicit shadow price for carbon or represent policy instruments, including carbon pricing. This briefing aims at exploring and understanding the variation in these carbon price estimates for stringent climate change mitigation scenarios. The focus of this exercise will be on scenarios that limit global mean temperature surface increase (henceforth, warming) to below 2°C relative to preindustrial levels with a greater than 66% probability. This choice is driven by data availability for this particular temperature objective, and does not represent an official or scientific interpretation of the Paris Agreement long-term temperature goal. The assumption is that the qualitative insights would also to

Reply to Comment on 'Unintentional unfairness when applying new greenhouse gas emissions metrics at country level'

This is a companion article to 2021 Environ. Res. Lett. 16 068001 This is a companion article to 2019 Environ. Res. Lett. 14 114039H2020 Societal Challengeshttp://dx.doi.org/10.13039/100010676Peer Reviewe

Co-Benefits and Trade Offs of INDCs (chapter 3)

Climate mitigation can trigger synergies and trade-offs with other policy objectives at the national level, such as poverty reduction, clean air, public health, or energy independence. Synergies (often referred to as co-benefits) are thus important because they influence the national support for climate mitigation policies and more directly impact the life of local populations

Unintentional unfairness when applying new greenhouse gas emissions metrics at country level

The 2015 Paris Agreement sets out that rapid reductions in greenhouse gas (GHG) emissions are needed to keep global warming to safe levels. A new approach (known as GWP*) has been suggested to compare contributions of long- and short-lived GHGs, providing a close link between cumulative CO2-equivalent emissions and total warming. However, comparison factors for non-CO2 GHGs under the GWP* metric depend on past emissions, and hence raise questions of equity and fairness when applied at any but the global level. The use of GWP* would put most developing countries at a disadvantage compared to developed countries, because when using GWP* countries with high historical emissions of short-lived GHGs are exempted from accounting for avoidable future warming that is caused by sustaining these emissions. We show that when various established equity or fairness criteria are applied to GWP* (defined here as eGWP*), perceived national non-CO2 emissions vary by more than an order of magnitude, particularly in countries with high methane emissions like New Zealand. We show that national emission estimates that use GWP* are very sensitive to arbitrary choices made by countries and therewith facilitate the creation of loopholes when CO2-equivalent emissions based on the GWP* concept are traded between countries that use different approaches. In light of such equity-dependent accounting differences, GHG metrics like GWP* should only be used at the global level. A common, transparent and equity-neutral accounting metric is vital for the Paris Agreement's effectiveness and its environmental integrity

Unintentional unfairness when applying new greenhouse gas emissions metrics at country level

The 2015 Paris Agreement sets out that rapid reductions in greenhouse gas (GHG) emissions are needed to keep global warming to safe levels. A new approach (known as GWP*) has been suggested to compare contributions of long- and short-lived GHGs, providing a close link between cumulative CO2-equivalent emissions and total warming. However, comparison factors for non-CO2 GHGs under the GWP* metric depend on past emissions, and hence raise questions of equity and fairness when applied at any but the global level. The use of GWP* would put most developing countries at a disadvantage compared to developed countries, because when using GWP* countries with high historical emissions of short-lived GHGs are exempted from accounting for avoidable future warming that is caused by sustaining these emissions. We show that when various established equity or fairness criteria are applied to GWP* (defined here as eGWP*), perceived national non-CO2 emissions vary by more than an order of magnitude, particularly in countries with high methane emissions like New Zealand. We show that national emission estimates that use GWP* are very sensitive to arbitrary choices made by countries and therewith facilitate the creation of loopholes when CO2-equivalent emissions based on the GWP* concept are traded between countries that use different approaches. In light of such equity-dependent accounting differences, GHG metrics like GWP* should only be used at the global level. A common, transparent and equity-neutral accounting metric is vital for the Paris Agreement's effectiveness and its environmental integrity

Great SCO2T! Rapid tool for carbon sequestration science, engineering, and economics

CO2 capture and storage (CCS) technology is likely to be widely deployed in coming decades in response to major climate and economics drivers: CCS is part of every clean energy pathway that limits global warming to 2C or less and receives significant CO2 tax credits in the United States. These drivers are likely to stimulate capture, transport, and storage of hundreds of millions or billions of tonnes of CO2 annually. A key part of the CCS puzzle will be identifying and characterizing suitable storage sites for vast amounts of CO2. We introduce a new software tool called SCO2T (Sequestration of CO2 Tool, pronounced "Scott") to rapidly characterizing saline storage reservoirs. The tool is designed to rapidly screen hundreds of thousands of reservoirs, perform sensitivity and uncertainty analyses, and link sequestration engineering (injection rates, reservoir capacities, plume dimensions) to sequestration economics (costs constructed from around 70 separate economic inputs). We describe the novel science developments supporting SCO2T including a new approach to estimating CO2 injection rates and CO2 plume dimensions as well as key advances linking sequestration engineering with economics. Next, we perform a sensitivity and uncertainty analysis of geology combinations (including formation depth, thickness, permeability, porosity, and temperature) to understand the impact on carbon sequestration. Through the sensitivity analysis we show that increasing depth and permeability both can lead to increased CO2 injection rates, increased storage potential, and reduced costs, while increasing porosity reduces costs without impacting the injection rate (CO2 is injected at a constant pressure in all cases) by increasing the reservoir capacity.Comment: CO2 capture and storage; carbon sequestration; reduced-order modeling; climate change; economic