1 Introduction

Achieving the goals of the Paris Climate Agreement (2015) will require the total decarbonization of the global energy system by 2050, with an emissions peak between 2020 and 2025 [1] and a drastic reduction in non-energy-related greenhouse gases (GHGs), including land-use-related emissions [2]. Based on the Agreement, countries have agreed to regularly report their GHG emissions and submit their ‘Nationally Determined Contributions’ (NDC), describing their planned measures to reduce GHG emissions. In 2021, the ‘Global Stocktake’ process began to collect the latest data on emissions and assess future developments based on NDCs already commenced. According to the United Nations Framework Convention on Climate Change (UNFCCC), the Global Stocktake ‘enables countries and other stakeholders to see where they’re collectively making progress toward meeting the goals of the Paris Agreement—and where they’re not. It’s like taking inventory’ [3].

This paper documents the decarbonization pathways for all G20 member countries,Footnote 1 broken down into key industry sectors and the energy-related national carbon budgets required to achieve the remaining global carbon budget of between 400 GtCO2 and 500 GtCO2 are calculated. The historic CO2 emissions for all G20 countries are also considered in determining the overall carbon budget until 2050, as a basis for the allocation of national carbon budgets and a contribution to the UNFCCC's ‘Global Stocktake’ process. Additionally, a sensitivity analysis examines the impact of the delayed implementation of the 1.5 °C decarbonization scenarios on the global carbon budget.

2 Methodology

The decarbonization pathways for all G20 countries, and the global scenario, were developed with the One Earth Climate Model (OECM), noting that the overall remaining carbon budget cannot exceed 500 GtCO2 between 2020 and 2050 if we are to limit the mean global temperature rise to + 1.5 °C with a likelihood of 50%. The OECM is an integrated energy assessment model that covers the entire energy system of a country or region, broken down into 16 industry-specific sectors. The methodology was developed by the Institute for Sustainable Futures at the University of Technology Sydney, with the support of the Institute of Networked Energy Systems and Energy Systems Analysis of the German Aerospace Center and in close co-operation with the finance sector of the Net-Zero Asset Owners Alliance and the United Nations Principles for Responsible Investment.

The methodology of the OECM provides high technical resolution and has been extensively documented in the scientific literature [4,5,6,7]. The sectorial breakdown is based on the Global Industry Classification Standard (GICS) (MSCI, 2020) and defines the system boundaries for the sector-specific decarbonization scenarios (Teske and Guerrero, 2022). The term ‘technical resolution’ refers to the level of detail with which the model captures specific technical processes, such as steel, aluminium, and cement production, transport modes, and vehicle types, and the energy-generation technologies used for power, (process) heat, and fuel supply [8].

The existing national industry sectors are calculated bottom up as part of the national energy scenario development. It is assumed that global market shares of national industry sectors will remain constant between 2020 and 2050. If a country produces 50% of the global steel in 2020, it is assumed that this global market share will remain constant until 2050. This assumption was applied to all analysed branches of industry in the manufacturing sector and raw material extraction.

The annual energy-related CO2 emissions between 2020 and 2050 of all country-specific OECM 1.5 °C scenarios have been combined to calculate the total G20 carbon emissions. The accumulated carbon emissions for all industries, by country and globally, are also calculated. The results for all G20 countries are compared with the global emissions—both on national and sectorial levels. The historic emissions [9] of all G20 countries are included in the overall emissions balance. The total emissions—historic plus the projected energy-related CO2 emissions until 2050 under the OECM 1.5 °C scenarios—are then divided by the countries’ populations in 2020 to determine a per capita carbon emission index. Finally, we calculate the additional emissions for each sector and country if implementation is delayed by 5 or 7 years, assuming that a delay freezes emission at the 2022 levels for the period of the delay.

3 Remaining carbon budgets for G20 countries

In this study, we compare the calculated energy-related CO2 emissions of all G20 countries with the global scenario under a defined carbon budget limit. The details of all countries’ scenarios—in terms of energy demand and supply pathways and the assumed market and/or demand developments for 16 industry sectors (outlined in Table 1)—are documented in a separate open-access paper, which allows the input and output of data (Teske et al., 2023). Figure 1 shows the historic cumulative energy-related CO2 emissions in 1750–2019, as previously documented, and the calculated CO2 emissions under the OECM 1.5 °C pathways in 2020–2050. The historic and projected energy-related CO2 emissions in 2020–2050 for each G20 country are divided by the current population to determine the country-specific emission index. The Per Capita Carbon Index (PCCI) is introduced as a benchmark with which to compare cumulative historic and projected emissions. However, the PCCI is a simplified calculation because historic and future population developments are not included. Therefore, the index is only an indication. The global OECM 1.5˚C pathway calculation in this analysis resulted in a total cumulative energy-related CO2 emission of 426 GtCO2 (2020–2050). The maximum value of 500 GtCO2 was deliberately undercut in order to maximize the probability of achieving the 1.5 °C target.

Table 1 Cumulative energy-related CO2 emissions by sector (2020–2050) for all G20 OECM 1.5 °C pathways compared with the global remaining budget and the remaining carbon budgets for other countries
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Fig. 1
figure 1

G20 carbon stocktake: cumulative energy-related CO2 emissions 1750–2020 and 2020–2050 under the OECM 1.5 °C pathways and Per Capita Carbon Index (PCCI)

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3.1 Carbon budgets

Historic (1750–2019) and 1.5 °C pathway emissions (2020–2050) will lead to the total global emission of 2091 GtCO2, 75% (1570 GtCO2) of which will be emitted by the G20 countries. In terms of overall emissions, the USA is by far the greatest emitter, with 471 GtCO2—87% are historic emissions and the future emissions allowance under the OECM pathway is 58.9 GtCO2. The second greatest emitter is the 27 countries of the European Union (EU27), with 286 GtCO2 of historic emissions, 30% less than the USA.

Under the OECM 1.5 °C pathway, the EU27 will decarbonize with a total remaining carbon budget of 32.9 GtCO2, 44% less than the USA, even though the EU27 population is 448 million, 26% larger than the USA. The European economy is significantly more energy efficient than the USA economy, and the renewable energy share (final energy) in 2020 was 28%, more than twice that of the USA (11%). Therefore, the faster decarbonization of Europe’s energy system is assumed. China is the third greatest emitter, with 227 GtCO2 of historic emissions and a projected remaining carbon budget of 151 GtCO2.

The USA, EU27, and China represent 28% of the global population (in 2020) and are responsible for 56% of historic emissions (926 GtCO2). The 1.5 °C OECM decarbonization pathways for these three regions lead to a remaining carbon budget of 243 GtCO2, 57% of the total global carbon budget of 426 GtCO2. China is assumed to require the largest carbon budget to reach decarbonization. Under the national OECM 1.5 °C decarbonization pathway, China—with a population of 1.4 billion people, compared with 0.8 billion for the EU27 and USA combined—will cumulatively emit 151 GtCO2 in 2020–2050, more than 1.5 times the remaining carbon budget of the EU27 and USA combined.

In terms of total combined historic and projected emissions, the fourth largest emitter is Russia, with 138.2 GtCO2, followed by Germany (100.3 GtCO2). All other G20 countries—including populous India—have significantly lower cumulative emissions, although some had significantly higher per capita emissions in 2020.

3.2 Per capita carbon index

The total carbon emitted determines the increase in global temperature, so an increase in global CO2 emissions, and consequently the global carbon budget, will inevitably cause us to miss the 1.5 °C target. Therefore, increasing in the carbon budgets of countries with low historic emissions is not possible. To still determine fair CO2 budgets for developing countries with little responsibility for climate change, the Per Capita Carbon Index (PCCI) is introduced. The historic emissions and future emissions under the OECM 1.5 °C pathway for each G20 member are divided by each country’s population in 2020. The ‘historic PCCI’ shows the level of responsibility and the ‘1.5 °C PCCI’ provides a way to compare the remaining carbon budgets. In combination, the total PCCI provides a way to compare emissions, as required in the Global Stocktake. The overall PCCI for each country is compared with the global PCCI under the global OECM 1.5 °C scenario plus global historic emissions.

G20 countries are the world's major economies, representing 85% of global GDP, 75% of international trade, and two-thirds of the global population [10]. However, the national historic emissions of the G20 member countries vary significantly, and their individual PCCIs differ greatly: India has the lowest, with only 60 tCO2/capita, whereas the USA has the highest, with 1430 tCO2/capita—almost 24 times higher. The United Kingdom, Germany, and Canada have PCCIs > 1000 tCO2/per capita. The global average PCCI is calculated to be 269 tCO2/per capita—almost exactly the PCCI of China (268 tCO2/per capita). The results for all other developing countries are below the global average.

3.3 Carbon budget by sector

The 1.5 °C carbon budgets (2020–2050) for all 16 industry sectors analysed are shown in Table 1. Each sectorial carbon budget is presented as the sum for G20 member countries and compared with the results for the global pathway. The difference between the global and G20 emissions represents the carbon budget for all remaining countries. The OECM 1.5 °C pathways for all G20 countries reflect the current distribution of industry sectors. The G20 steel industry, for example, represents almost 90% of the current production capacity of the global steel industry, and is assumed to remain so until 2050.

3.4 Impact of implementation delay of the 1.5 °C OECM pathways

The total remaining global carbon budget required to limit the temperature increase to 1.5 °C (with 67% likelihood) is 400 GtCO2. An increased carbon budget of 500 GtCO2 will decrease the likelihood to 50%, and one of 650 GtCO2 will reduce it to only 33%, according to the Intergovernmental Panel on Climate Change [1]. The impacts of delaying the implementation of the 1.5 °C pathways by 5 or 7 years are shown, by sector, in Table 2. A delay of 5 years—across all sectors and countries—will increase global carbon emissions from 426 GtCO2 under the 1.5 °C OECM scenario to 585 GtCO2, and the likelihood of meeting the + 1.5 °C target will decrease to < 50%. A delay of 7 years will increase the total emissions to 649 GtCO2 in 2020–2050, reducing the likelihood of achieving the target to 33%.

Table 2 Total cumulative energy-related CO2 emissions (2020–2050) for the G20, global, and ‘rest of the world’ if implementation of the 1.5 °C pathway is delayed by 5 or 7 years
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A closer look at the development of sector-specific emissions and the increase in emissions if implementation is delayed shows that those of the transport sector and building sector will be significant. A 7-year delay in the road transport sector alone will increase emissions in in 2020–2050 by 39 GtCO2.

4 Discussion and conclusion

It will be essential to remain within a global carbon budget well under 500 GtCO2 if we are to meet the Paris Climate Agreement target of maintaining ‘the global temperature rise this century well below 2 degrees Celsius above pre-industrial levels’ [11]. ‘The IPCC 6th Assessment Report (AR6) [1] highlighted that a global temperature rise of 1.5 °C would already lead to significant climate impacts. Therefore, the One Earth Climate Model (OECM) pathways aim to stay within a carbon budget to limit global temperature rise to 1.5 °C with a probability of 50% or more.’

The OECM 1.5 °C decarbonization scenarios were developed for countries, regions, and the world to provide possible mitigation pathways with high technical resolution and system boundaries that reflect the sector definitions used by the global finance industry. In this research, individual scenarios for all G20 countries have been created to assess how countries can decarbonize as quickly as possible while maintaining economic growth. Decarbonization pathways differ significantly because the demand and supply structures of each country are very specific. The level of industrialization, the dominant industry sectors, and the level of private consumption define their current emissions. Restructuring the energy sector to decarbonize a country’s economy is possible in all OECM 1.5 °C pathways, but the national requirements for a carbon budget that allows decarbonization differ. Therefore, a fair distribution of the remaining carbon budget must consider the technical and economic situations of each country, and their historic emissions, in total and per capita, are also important factors. Although the remaining global carbon budget required to remain well under + 2 °C is fixed and cannot be negotiated, the Global Stocktake must go beyond defining national carbon budgets. It must include financial and technical support from those countries with the highest historical emissions to countries that have minor historical emissions but still require energy for economic development.

The Per Capita Carbon Index is one possible tool with which to develop such a global support mechanism. For instance, if a line is drawn at the global average PCCI of 269 tCO2/capita, all countries above that value must provide financial and technical support to those countries below that value. The higher the PCCI, the greater the responsibility to actively invest in decarbonization beyond a country’s national borders. The determination of the carbon budget including the PCCI supports low and middle-income countries (LMIC) in particular in the allocation of funds for climate change adaptation as well as climate mitigation measures.