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Scientifically rigorous guidance to policy makers on mitigation options for meeting the Paris Agreement long-term temperature goal requires an evaluation of long-term global-warming implications of greenhouse gas emissions pathways. Here, we present a uniform and transparent methodology to evaluate the climate outcome, and hence the Paris Agreement consistency of influential institutional emission scenarios from the grey literature, including those from the International Energy Agency1,2, BP3, and Shell4. We first identify challenges to performing such an assessment and then proceed to outline a sequence of steps to address these challenges by harmonizing5 all emissions to a consistent base-year (2010), extending all pathways to 2100, and filling in missing emission species6. We employ two simple climate models, MAGICC7 and FaIR8,9 to assess peak and end-of-century temperatures, and find that few published scenarios that claim to be compatible with the Paris Agreement are so. References 1. International Energy Agency. World Energy Outlook 2020. (2020).2. International Energy Agency. Net Zero by 2050 - A Roadmap for the Global Energy Sector. (2021).3. BP. Global Energy Outlook 2020. (2020).4. Shell. The Energy Transformation Scenarios. (2021) 5. Gidden, M. J. et al. A methodology and implementation of automated emissions harmonization for use in Integrated Assessment Models. Environ. Model. Softw. 105, 187–200 (2018)6. Lamboll, R. D., Nicholls, Z. R. J., Kikstra, J. S., Meinshausen, M. & Rogelj, J. Silicone v1.0.0 : an open-source Python package for inferring missing emissions data for climate change research. Geosci. Model Dev 13, 5259–5275 (2020)7. Meinshausen, M., Raper, S. C. B. & Wigley, T. M. L. Emulating coupled atmosphere-ocean and carbon cycle models with a simpler model, MAGICC6 - Part 1: Model description and calibration. Atmos. Chem. Phys. 11, 1417–1456 (2011).8. Smith, C. J. et al. FAIR v1.3: A simple emissions-based impulse response and carbon cycle model. Geosci. Model Dev. 11, 2273–2297 (2018)9. Millar, J. R., Nicholls, Z. R., Friedlingstein, P. & Allen, M. R. A modified impulse-response representation of the global near-surface air temperature and atmospheric concentration response to carbon dioxide emissions. Atmos. Chem. Phys. 17, 7213–7228 (2017).

Equity is one of the key principles underpinning the global climate regime and is all the more essential given the heterogenous, self-differentiated nature of the Paris Agreement. The scientific community has proposed several effort-sharing schemes to operationalise equity – reflecting not only the multi-faceted nature of the problem but also a long history of disagreement that continues to date. We outline a synthesis framework that draws on existing estimates in the literature to develop a “fair share range” for each country within which we identify a common position, which, when applied to the ranges of all countries, results in the collective achievement of the desired temperature goal. A series of methodological choices regarding the treatment of the underlying literature and translation into a temperature equivalence are tested. We demonstrate the consistency of this framework (across different methodological choices) with the principle of “Common but Differentiated Responsibilities” – members of the OECD have the most stringent allocations, while only a few African and Asian countries (including India) have emission allocations that are above their 2010 emission levels. A few OECD members, including the European Union and Great Britain, have emission allocations in 2030 that are either close to, or less than zero. Consistency with their fair share of mitigation will require them to provide appropriate levels of international finance and support to facilitate emission reductions in developing countries. These conclusions are robust to various methodological choices.

In 2020, climate target announcements were dominated by net-zero commitments, including by a number of major emitters. Despite the urgency of more ambitious NDCs in the short term, long-term net-zero targets are important for the transition to global zero emissions. Tracking progress towards and assessing the adequacy of these targets requires an assessment of what they mean for transition pathways and associated emissions trajectories at both national and global levels. We present an assessment of net-zero targets of the major emitting countries and their implications for long-term emissions trajectories and warming levels. Based on the work of the Climate Action Tracker, country-specific analyses are aggregated to a global emissions pathway to derive a best estimate for a resulting global warming in 2100. Undertaking this analysis requires assumptions to be made regarding projected emissions and removals from the land-use sector, non-CO2 emissions, and the trajectory of total net emissions after net-zero, which we explain and explore. For example, by computing the cumulative emissions of our aggregated net-zero target emissions pathway, we can compare this pathway with modelled global emissions pathways from the IPCC's SR1.5 Special Report, to draw broad conclusions over what current net-zero commitments might mean for carbon dioxide removal and non-CO2 emissions, and the uncertainties therein.

Nationally Determined Contributions (NDCs) submitted so far under the Paris Agreement are not in line with its long-term temperature goal. To bridge this gap, countries are required to provide regular updates and enhancements of their long-term targets and strategies, based on scientific assessments. The goal of this paper is to demonstrate a policy-support approach for evaluating NDCs and guiding enhanced ambition. The approach rests on deriving national targets in line with the Paris Agreement by downscaling regional results of Integrated Assessment Models (IAMs) to the country level. The method of downscaling relies on a reduced complexity IAM: SIAMESE (Simplified Integrated Assessment Model with Energy System Emulator). We apply the approach to an EU28 member state – Finland – with the aim of providing useful insights for policy makers to consider cost-effective mitigation options. Results over the historical period confirm that our approach is valid when national policies are similar to those across the larger IAM region, but must include country-specific circumstances. Strengths and limitations of the approach are discussed. We assess the remaining carbon budget and analyse the different implications of 2 °C and 1.5 °C global warming limits for the emissions pathway and energy mix in Finland over the 21st century.

Island states are especially at risk of climate impacts and are already feeling the effects of rising sea levels, acidification, climate extremes and other impacts. Small islands face several unique challenges: They usually have limited resources to react, but are exceptionally exposed due to their physical setting and limited livelihood options. In addition, they are remote and not easily reached in time of crisis, making adaptation an imperative. This contribution presents the concept for an integrated database on climate impacts and adaptation, focussing specifically on the requirements of small island states. The database contains information on climate impact projections, linked to examples of existing adaptation projects. The database provides a structured overview of success-factors and limitations, piecing together fragmented knowledge and fostering knowledge exchange across regions in order to support science-based adaptation. While adaptation experience is increasing, including an evolving understanding of prerequisites and limitations to specific forms of adaptation, knowledge is still fragmented, due to the mostly local nature of adaptation. Island states across the world can benefit from a structured exchange, focussing on the transferability of success-criteria for adaptation. An improved knowledge base is also important for other regions, which will face similar challenges in the coming years.

This paper examines different concepts of a ‘warming commitment’ which is often used in various ways to describe or imply that a certain level of warming is irrevocably committed to over time frames such as the next 50 to 100 years, or longer. We review and quantify four different concepts, namely (1) a ‘constant emission warming commitment’, (2) a ‘present forcing warming commitment’, (3) a‘zero emission (geophysical) warming commitment’ and (4) a ‘feasible scenario warming commitment’. While a ‘feasible scenario warming commitment’ is probably the most relevant one for policy making, it depends centrally on key assumptions as to the technical, economic and political feasibility of future greenhouse gas emission reductions. This issue is of direct policy relevance when one considers that the 2002 global mean temperatures were 0.8± 0.2 ∘C above the pre-industrial (1861–1890) mean and the European Union has a stated goal of limiting warming to 2 ∘C above the pre-industrial mean: What is the risk that we are committed to overshoot 2 ∘C? Using a simple climate model (MAGICC) for probabilistic computations based on the conventional IPCC uncertainty range for climate sensitivity (1.5 to 4.5 ∘C), we found that (1) a constant emission scenario is virtually certain to overshoot 2 ∘C with a central estimate of 2.0 ∘C by 2100 (4.2 ∘C by 2400). (2) For the present radiative forcing levels it seems unlikely that 2 ∘C are overshoot. (central warming estimate 1.1 ∘C by 2100 and 1.2 ∘C by 2400 with ∼10% probability of overshooting 2 ∘C). However, the risk of overshooting is increasing rapidly if radiative forcing is stabilized much above 400 ppm CO2 equivalence (1.95 W/m2) in the long-term. (3) From a geophysical point of view, if all human-induced emissions were ceased tomorrow, it seems ‘exceptionally unlikely’ that 2 ∘C will be overshoot (central estimate: 0.7 ∘C by 2100; 0.4 ∘C by 2400). (4) Assuming future emissions according to the lower end of published mitigation scenarios (350 ppm CO2eq to 450 ppm CO2eq) provides the central temperature projections are 1.5 to 2.1 ∘C by 2100 (1.5 to 2.0 ∘C by 2400) with a risk of overshooting 2 ∘C between 10 and 50% by 2100 and 1–32% in equilibrium. Furthermore, we quantify the ‘avoidable warming’ to be 0.16–0.26 ∘C for every 100 GtC of avoided CO2 emissions – based on a range of published mitigation scenarios.

Climatic Change, 75 (1-2), ISSN:0165-0009, ISSN:1573-1480

The rejection of the Kyoto Protocol by President George W. Bush in early 2001 and his subsequent challenging of the basic science underpinning the global response to the threat of climate change, h...

National targets give virtually no chance of constraining warming to 2 °C and no chance of protecting coral reefs.

The goal of the Climate Convention and its Kyoto Protocol is to stabilize greenhouse gas concentrations in the atmosphere at a safe level. This requires both strict limits on emissions from fossil fuels and effective management of biotic carbon stocks. If fossil fuel emissions from 1990 to 2100 are limited to 600 PgC, biotic carbon stocks must increase by 120 PgC to stabilize CO 2 concentrations at 450 ppmv. Establishing an appropriate policy regime to accomplish this goal is complicated by a factor of six discrepancy between estimates of the current biotic sink based on national emissions inventories compared with global carbon cycle model calculations. Appropriate policies must also be designed to create incentives for technological innovation in the energy sector and minimize the risk of granting emission credits for biotic carbon sequestration that proves to be temporary.

Broadening the framework of environmental impact assessment (EIA) involves an examination of the principles and policies which should underline ecologically sustainable development. To work effectively, EIA procedures need to be located within a broad framework of policies and environmental objectives, standards and monitoring. This paper discusses the political and environmental context in Australia within which the debate on sustainable development is occuring. Some important principles which underlie an ecologically sustainable development policy framework are discussed. These include inter-generational equity, conservation of biological diversity and ecological integrity, maintenance of natural capital and a recognition of the biophysical limits on natural resource use. Economic policy needs to emphasise the qualitative aspects of development as opposed to the quantitative growth in resource use. Several key aspects of a broad Australian policy framework are discussed, including reform of Federal/State environmental arrangements and the establishment of a Federal Environment Protection Agency; development of a systematic means of measuring progress towards an ecologically sustainable development; materials resources, energy, waste, transport and urban development policies. The overall objective of these policy areas would be to bring the scale of human activity in Australia within the carrying capacity of the environment.

This note is a final response to the debate raised by Mr. Castles and Mr. Henderson (for brevity, we refer here to the two authors simply as C&H) in this Journal ( vol 14, no 2&3, and no 4) on the issue of economic growth in developing countries in some of the emissions scenarios published in the IPCC Special Report on Emissions Scenarios (SRES) (Nakicenovic et al., 2000). We first outline areas of agreement and then the remaining areas of disagreement. Two important areas of agreement have emerged from the debate according to our view. First, both parties agree that scenarios assuming a conditional convergence in income levels, i.e., a higher growth in per capita income in poorer countries when compared to countries with higher levels of affluence, are both “plausible and well attested in economic history” (C&H, p. 424). Thus, the fundamental, structural characteristic of some of the SRES scenarios contested by C&H are not challenged per se, but rather how fast such trends could unfold in the future. Second, there is agreement on the value of considering purchasing power parities (PPP) in the international comparison of income levels and the need for further research to improve on the paucity of reliable PPP estimates for developing countries within the International Comparisons Project (ICP) (C&H, p. 432). We appreciate that C&H have now acknowledged that PPPs were considered in developing the SRES scenarios and that they are reported in the data appendix of the report (C&H, p. 422–423). Thus, it was not ignorance as suggested by C&H but rather sound empirical and methodological reasons that led the SRES team to use market exchange rates (MER) as the main metric in developing long-term emissions scenarios. This is in agreement with the underlying scenario literature. However, we do agree with C&H on the value of considering PPP as a complementary metric, and have indeed reported corresponding PPP scenarios in SRES. We disagree with C&H that PPP ought to be used as the sole measure in developing long-term emissions scenarios. This leads us to the remaining areas of disagreement. (1) An important area of disagreement is that emissions do not depend on the metric used to measure economic activities. Evidently, historical emissions do not change as a function of whether historical development is measured in PPP or MER and both measures can be used interchangeably given appropriate model calibrations are deployed to assess the resulting emissions. More importantly, future emissions depend first of all on the physical characteristics of the energy system, land use and other human activities that need to be represented in models to calculate future emissions of greenhouse gases. These physical model representations are unaffected by the choice of PPP or MER for measuring economic growth. This fact explains why many of the emissions scenarios in the literature do not include economic development paths but rather determine emissions from human activities, such as energy and food services. We have addressed this argument extensively in the earlier issue of this Journal ( vol 14, no 2&3). (2) There also remains an important disagreement on the issue of using market exchange rates (MER) GDP in developing emission scenarios. C&H hold the extreme view that MER – a directly observable economic variable, as opposed to PPP, which is an elaborate statistical construct – should not be used at all in economic comparisons and in developing scenarios of GDP growth. We reiterate that there are good theoretical, methodological, and empirical reasons for using MER. Contrary to their claim of “unsound” practices, the SRES scenarios are consistent with the underlying literature, available methodologies, and existing practices of economic growth projections of leading international (e.g., the World Bank) and national institutions (e.g., the US DOE Energy Information Administration). (3) A final area of disagreement is whether the C&H criticism is significant or a “red herring”. C&H (p. 428–429) claim that by lowering the economic growth rates for developing countries in the lowest SRES emission-scenarios, one should obtain even lower future emissions. Thus, they claim that the SRES scenarios have failed to represent the lower bound of uncertainty of future emission levels. Here C&H display either a misunderstanding or misrepresentation of economic activity as the sole, independent driver of future emissions. Higher economic growth generally results in higher R&D, more rapid capital turnover, more energy efficiency and higher preferences for pollution controls, all of which tend to reduce GHG emissions. Depending on how these are modeled, lower GDP growth may actually result in higher GHG emissions, and may not, as C&H contend, significantly lower the SRES emissions in the absence of climate policies. We disagree that lower economic development would necessarily result in lower emissions. We conclude our response with some suggestions for improved clarity in the debate and the need to quantify differences in opinion through alternative scenarios published in the peer-reviewed literature.

This report focuses on the risks of climate change to development in Sub-Saharan Africa, South East Asia and South Asia. Building on the 2012 report, Turn Down the Heat: Why a 4°C Warmer World Must be Avoided, this new scientific analysis examines the likely impacts of present day, 2°C and 4°C warming on agricultural production, water resources, and coastal vulnerability for affected populations. It finds many significant climate and development impacts are already being felt in some regions, and in some cases multiple threats of increasing extreme heat waves, sea level rise, more severe storms, droughts and floods are expected to have further severe negative implications for the poorest. Climate related extreme events could push households below the poverty trap threshold. High temperature extremes appear likely to affect yields of rice, wheat, maize and other important crops, adversely affecting food security. Promoting economic growth and the eradication of poverty and inequality will thus be an increasingly challenging task under future climate change. Immediate steps are needed to help countries adapt to the risks already locked in at current levels of 0.8°C warming, but with ambitious global action to drastically reduce greenhouse gas emissions, many of the worst projected climate impacts could still be avoided by holding warming below 2°C.