Global Decarbonization: Current Status and What It Will Take to Achieve Net Zero by 2050

: A review of global CO 2 emissions over the last century shows that emissions from 80 economies contributed to 95% of global emissions. Among them, 55 economies were decarbonizers, where CO 2 emissions had either plateaued or were declining, while 25 economies were polluters, where CO 2 emissions were still increasing. In 2021, the global CO 2 emissions were 37.1 Gtpa, with 56% coming from polluters and 39% from decarbonizers. If current trends continue, global CO 2 emissions will reach 49.6 Gtpa by 2050, with 81% coming from polluters and 14% from decarbonizers. Only 14 economies will reach net zero. The decarbonization target, over and above current efforts, to achieve net zero is calculated for each economy. Decarbonizers need to mitigate 230 Mtpa CO 2 and polluters 1365 Mtpa CO 2 beginning in 2021 to reach the net-zero target by 2050. This target will increase each year decarbonization is delayed. Analyses show that renewable energies’ share in the total ﬁnal energy consumption in most economies increased by an average of only 4 percentage points in the last decade, which is inadequate for achieving net zero by 2050. Other means of decar-bonization, including low-carbon fossil solutions through carbon capture and storage, will be needed. Pathways to accelerate decarbonization are proposed and their policy implications are discussed.


Introduction
Since the signing of the Paris Agreement in 2015, 196 countries have agreed to reduce their anthropogenic greenhouse gas (GHG) emissions in order to maintain the global rise of atmospheric temperature below 1.5 • C above pre-industrial times [1].This is driven by the belief of many scientists that global warming is caused by anthropogenic GHG emissions [2][3][4].Among GHGs, CO 2 is the most important due to its vast quantity and greenhouse gas effect.Some scientists believe the effect of CO 2 on global warming is irreversible [5,6].
In the last 120 years, global CO 2 emissions from the combustion of fossil fuels have grown 19-fold from 1.95 Gtpa in 1900 to 37.1 Gtpa in 2022 [7].To achieve the 1.5 • C target, many climate scientists believe that global GHG emissions should be reduced by 45% by 2030 and reach net zero by 2050 [8].Hitherto, over 70 countries have pledged to achieve net zero by 2050 [9,10].They include the biggest emitters, such as China [11], the USA [12], India [13] and the European Union [14], and cover 76% of global emissions.Today, more and more businesses are committed to achieving net zero by 2050, as this has become a license to operate in many countries [15][16][17][18][19][20].
Global decarbonization has received much attention in the academic literature in the last five years.In a recent review paper on energy transition, Lau et al. (2021) argued that decarbonization should be considered according to energy-consumption sectors, with power (electricity), transport and industry being the three major ones [21].Furthermore, each country will choose an energy transition pathway depending on its particular energy

Objectives and Methodology
The objectives of this study were threefold.First, a look back at global CO 2 emissions over the last 120 years was conducted to understand the historical trend and current status.Second, using recent historical trends, we forecasted CO 2 emissions by 2050 and determined the extra decarbonization efforts, over and above current efforts, that will be needed to achieve net zero by 2050 for the top 80 CO 2 emitters of the world.Third, we discuss pathways to accelerate decarbonization for various economies and their policy implications.
A linear history matching and forecasting method was used to analyze CO 2 emissions data.The methodology is illustrated in Figure 1.From the global database of CO 2 emissions from Our World in Data [7], we selected the top 80 CO 2 emitters and classified them into two groups.The first group was called decarbonizers and consisted of economies where CO 2 emissions had either peaked or were in decline.The second group was called polluters and consisted of economies where CO 2 emissions were still increasing.In this study, economies were either countries or territories that reported accurate data on CO 2 emissions.After determining the historical CO 2 increase rate from the historical data, we extended the historical CO 2 emissions trends linearly into the future to determine the CO 2 emissions by 2050.We also calculated the CO 2 decline rate needed to achieve net zero by 2050 using the CO 2 emissions rate in 2021.The difference between this rate and the historical rate gave the target decarbonization rate in 2021, over and above current efforts, to achieve net-zero emissions by 2050.Assuming decarbonization was delayed beyond 2021, the decarbonization target was recalculated based on the CO 2 emissions forecast.Thus, we determined the decarbonization targets for every polluter and decarbonizer between 2021 and 2050.We then grouped economies into coal-dominated, gas-and-oil-dominated, and non-fossil-energy-dominated economies and discussed their respective decarbonization pathways and policy implications.
Energies 2023, 16, x FOR PEER REVIEW 3 of 24 study, economies were either countries or territories that reported accurate data on CO2 emissions.After determining the historical CO2 increase rate from the historical data, we extended the historical CO2 emissions trends linearly into the future to determine the CO2 emissions by 2050.We also calculated the CO2 decline rate needed to achieve net zero by 2050 using the CO2 emissions rate in 2021.The difference between this rate and the historical rate gave the target decarbonization rate in 2021, over and above current efforts, to achieve net-zero emissions by 2050.Assuming decarbonization was delayed beyond 2021, the decarbonization target was recalculated based on the CO2 emissions forecast.Thus, we determined the decarbonization targets for every polluter and decarbonizer between 2021 and 2050.We then grouped economies into coal-dominated, gas-and-oil-dominated, and non-fossil-energy-dominated economies and discussed their respective decarbonization pathways and policy implications.

Comparison of Study Methodology with IEA Scenarios
It is worthwhile to compare our forecast methodology with the four global energy forecast scenarios proposed by the IEA (Figure 2).The business-as-usual (BAU) scenario assumes that few or no steps are taken to limit greenhouse gas emissions.Therefore, unabated CO2 emissions will continue to increase with the increased use of fossil energies due to the growth in population and prosperity.The second is the stated policies scenario (STEPS), which reflects the current policy settings of each country as of August 2023 [42].The announced pledges scenario (APS) assumes that all climate commitments made by governments and industries as of August 2023 will be met [41].The sustainable development scenario (SDS) assumes that significant actions will be taken to limit the global temperature increase to 1.5 °C and thereby reduce global CO2 emissions to about 10 Gtpa by 2050 [43].The net-zero (NZE) scenario assumes that net zero will be achieved by 2050 [49].Although the IEA scenarios are global scenarios, they may be adapted to individual countries or economies.

Comparison of Study Methodology with IEA Scenarios
It is worthwhile to compare our forecast methodology with the four global energy forecast scenarios proposed by the IEA (Figure 2).The business-as-usual (BAU) scenario assumes that few or no steps are taken to limit greenhouse gas emissions.Therefore, unabated CO 2 emissions will continue to increase with the increased use of fossil energies due to the growth in population and prosperity.The second is the stated policies scenario (STEPS), which reflects the current policy settings of each country as of August 2023 [42].The announced pledges scenario (APS) assumes that all climate commitments made by governments and industries as of August 2023 will be met [41].The sustainable development scenario (SDS) assumes that significant actions will be taken to limit the global temperature increase to 1.5 • C and thereby reduce global CO 2 emissions to about 10 Gtpa by 2050 [43].The net-zero (NZE) scenario assumes that net zero will be achieved by 2050 [49].Although the IEA scenarios are global scenarios, they may be adapted to individual countries or economies.
Figures 3 and 4 show how our forecast method fits in the context of IEA's four scenarios for a decarbonizer and polluter, respectively.Our forecast did not assume any of the four IEA scenarios for any economy.It just extrapolated the current trend into the future.For a decarbonizer, the current trend may lie between STEPS and APS (Figure 3), which still falls short of the NZE scenario.Our forecasting method showed what it will take to accelerate the current trend to meet net zero by 2050 by assuming different years for the start of this acceleration.Each year a decarbonizer delays in starting the extra effort will make it more difficult to achieve the net-zero goal by 2050.For a polluter, the current trend may fall between BAU and STEPS (Figure 4).Our forecasting method calculated the extra effort needed to achieve the NZE scenario by beginning the extra effort at different years.The extra effort to achieve net zero by 2050 will be much greater for a polluter than a decarbonizer.The validity of our forecast was based on the length of time used to establish Figures 3 and 4 show how our forecast method fits in the context of IEA's four scenarios for a decarbonizer and polluter, respectively.Our forecast did not assume any of the four IEA scenarios for any economy.It just extrapolated the current trend into the future.For a decarbonizer, the current trend may lie between STEPS and APS (Figure 3), which still falls short of the NZE scenario.Our forecasting method showed what it will take to accelerate the current trend to meet net zero by 2050 by assuming different years for the start of this acceleration.Each year a decarbonizer delays in starting the extra effort will make it more difficult to achieve the net-zero goal by 2050.For a polluter, the current trend may fall between BAU and STEPS (Figure 4).Our forecasting method calculated the extra effort needed to achieve the NZE scenario by beginning the extra effort at different years.The extra effort to achieve net zero by 2050 will be much greater for a polluter than a decarbonizer.The validity of our forecast was based on the length of time used to establish the current trend, which was at least one decade and may be as long as three decades.See figures in Section 5 for the USA and China, respectively.Figures 3 and 4 show how our forecast method fits in the context of IEA's four scenarios for a decarbonizer and polluter, respectively.Our forecast did not assume any of the four IEA scenarios for any economy.It just extrapolated the current trend into the future.For a decarbonizer, the current trend may lie between STEPS and APS (Figure 3), which still falls short of the NZE scenario.Our forecasting method showed what it will take to accelerate the current trend to meet net zero by 2050 by assuming different years for the start of this acceleration.Each year a decarbonizer delays in starting the extra effort will make it more difficult to achieve the net-zero goal by 2050.For a polluter, the current trend may fall between BAU and STEPS (Figure 4).Our forecasting method calculated the extra effort needed to achieve the NZE scenario by beginning the extra effort at different years.The extra effort to achieve net zero by 2050 will be much greater for a polluter than a decarbonizer.The validity of our forecast was based on the length of time used to establish the current trend, which was at least one decade and may be as long as three decades.See figures in Section 5 for the USA and China, respectively.

Look Back and Classification
Figure 5 shows the annual CO2 emissions history for decarbonizers over the last 120 years [7].There were 55 economies where CO2 emissions had either peaked or were declining.The complete list is given in Table 1.Among the decarbonizers, the biggest CO2 emitters were the USA, Japan, Germany, South Korea, Canada, Brazil, South Africa, Mexico, Australia and the United Kingdom (UK).

Look Back and Classification
Figure 5 shows the annual CO 2 emissions history for decarbonizers over the last 120 years [7].There were 55 economies where CO 2 emissions had either peaked or were declining.The complete list is given in Table 1.Among the decarbonizers, the biggest CO 2 emitters were the USA, Japan, Germany, South Korea, Canada, Brazil, South Africa, Mexico, Australia and the United Kingdom (UK).

Look Back and Classification
Figure 5 shows the annual CO2 emissions history for decarbonizers over the last 120 years [7].There were 55 economies where CO2 emissions had either peaked or were declining.The complete list is given in Table 1.Among the decarbonizers, the biggest CO2 emitters were the USA, Japan, Germany, South Korea, Canada, Brazil, South Africa, Mexico, Australia and the United Kingdom (UK). Figure 6 shows the CO2 emissions history for polluters over the last 120 years [7].There were 25 economies where CO2 emissions were still increasing.The complete list is given in Table 2.Among the polluters, the biggest CO2 emitters were China, India, Russia, Iran, Saudi Arabia, Indonesia, Turkey, Vietnam, Malaysia and Egypt.
Figure 7 shows the aggregate CO2 emissions for polluters, decarbonizers and the rest of the world (others).It can be seen that between 2001 and 2021, CO2 emissions have dropped by 1.520 Gtpa for decarbonizers but have increased by 12.49 Gtpa for polluters.The net change between the two was 10.97 Gtpa.Global CO2 emissions were still increasing at 407 Mtpa in 2021, as calculated from the slopes of the curves for polluters, decarbonizers and "others" in Figure 7   Figure 6 shows the CO 2 emissions history for polluters over the last 120 years [7].There were 25 economies where CO 2 emissions were still increasing.The complete list is given in Table 2.Among the polluters, the biggest CO 2 emitters were China, India, Russia, Iran, Saudi Arabia, Indonesia, Turkey, Vietnam, Malaysia and Egypt.
Figure 7 shows the aggregate CO 2 emissions for polluters, decarbonizers and the rest of the world (others).It can be seen that between 2001 and 2021, CO 2 emissions have dropped by 1.520 Gtpa for decarbonizers but have increased by 12.49 Gtpa for polluters.The net change between the two was 10.97 Gtpa.Global CO 2 emissions were still increasing at 407 Mtpa in 2021, as calculated from the slopes of the curves for polluters, decarbonizers and "others" in Figure 7 Energies 2023, 16, x FOR PEER REVIEW 6 of 24

Global CO 2 Emissions Forecast
For each economy, we used a straight line to best fit the CO 2 emissions data for the last two to three decades.We extended this line into the future to determine the CO 2 emission by 2050 and the net-zero time for decarbonizers.The results are given in Table 1 for decarbonizers and Table 2 for polluters.When the results for all 55 decarbonizers were summed, we obtained the CO 2 emissions forecast for decarbonizers, as shown in Figure 8.The same was done for the 25 polluters, with the results given in Figure 9.It is worthwhile to emphasize that a linear trend appeared to be the best fit for historical CO 2 emissions data for most countries.A nonlinear history match was usually not a good match and gave an unreliable forecast.ground on decarbonization.
Our calculations showed that if current CO2 emission rates continue, only 14 economies will achieve net zero by 2050.They were the UK, Italy, Spain, Ukraine, Greece, Portugal, Finland, Serbia, Denmark, Estonia, Trinidad and Tobago, Venezuela, South Korea and Japan.Together they contributed only 8.4% of the global CO2 emissions in 2021.Therefore, a drastic change in the way economies tackle decarbonization will be needed over and beyond what they are doing, if they are to achieve net zero by 2050.Using this methodology, we estimated that the CO 2 emissions for decarbonizers, as a group of economies, were changing at a rate of −261 Mtpa.Thus, their CO 2 emissions will reach 6.74 Gtpa by 2050 and net zero will be reached by 2076 (Figure 8).Similarly, our calculations show that the CO 2 emissions for polluters will increase at a rate of +644 Mtpa and will reach 39.6 Gtpa by 2050 (Figure 9).Using this method, we estimated that the CO 2 emission for other economies was increasing at a rate of 23.7 Mtpa and will reach 2.6 Gtpa by 2050 (Figure 10).As of 2021, the global CO 2 emissions were 37.1 Gtpa, with 56% coming from polluters, 39% from decarbonizers and 5% from other economies (Figure 11a).If current CO 2 emission rates continue, our study forecasted that the global CO 2 emissions will increase to 49.3 Gtpa in 2050 (Figure 11b), with 80% coming from polluters, 14% from decarbonizers and 5% from other economies.Consequently, the current emission rates of CO 2 will not allow the world to achieve peak CO 2 emissions by 2050, let alone reach net zero, as evidenced by Figure 7.In short, the increasing CO 2 emissions by polluters will more than offset the CO 2 abatement by decarbonizers, and the world as a whole is losing ground on decarbonization.Our calculations showed that if current CO 2 emission rates continue, only 14 economies will achieve net zero by 2050.They were the UK, Italy, Spain, Ukraine, Greece, Portugal, Finland, Serbia, Denmark, Estonia, Trinidad and Tobago, Venezuela, South Korea and Japan.Together they contributed only 8.4% of the global CO 2 emissions in 2021.Therefore, a drastic change in the way economies tackle decarbonization will be needed over and beyond what they are doing, if they are to achieve net zero by 2050.

Decarbonization Target to Achieve Net Zero by 2050
To quantify what efforts, over and above current ones, will be needed to meet the net-zero target by 2050, for each of the 80 economies, we estimated the decarbonization target rate to achieve this.The method to calculate this is illustrated for a decarbonizer (USA) in Figure 12 and a polluter (China) in Figure 13.In these figures, we drew a straight line connecting the emissions in 2021 to the x-axis at 2050.This is the decarbonization rate needed to achieve net zero by 2050 assuming the 2021 CO 2 rate was neither increasing nor decreasing.This rate was −172.7 Mtpa for USA (Figure 12) and −395.6 Mtpa for China (Figure 13).The target decarbonization rate in 2021 to achieve net zero by 2050 was the difference between this rate and the current decarbonizing rate in 2021.This target rate was −101.3Mtpa for the USA (Figure 12) and −771.4Mtpa for China (Figure 13).To obtain the target decarbonization rate for subsequent years, we drew a straight line from the CO 2 emissions forecast for subsequent years to the x-axis at 2050 and repeated the same calculations.
decreasing.This rate was −172.7 Mtpa for USA (Figure 12) and −395.6 Mtpa for China (Figure 13).The target decarbonization rate in 2021 to achieve net zero by 2050 was the difference between this rate and the current decarbonizing rate in 2021.This target rate was −101.3Mtpa for the USA (Figure 12) and −771.4Mtpa for China (Figure 13).To obtain the target decarbonization rate for subsequent years, we drew a straight line from the CO2 emissions forecast for subsequent years to the x-axis at 2050 and repeated the same calculations.(Figure 13).The target decarbonization rate in 2021 to achieve net zero by 2050 was the difference between this rate and the current decarbonizing rate in 2021.This target rate was −101.3Mtpa for the USA (Figure 12) and −771.4Mtpa for China (Figure 13).To obtain the target decarbonization rate for subsequent years, we drew a straight line from the CO2 emissions forecast for subsequent years to the x-axis at 2050 and repeated the same calculations.Figure 14 shows the target decarbonization rates for decarbonizers and polluters as groups of economies.For decarbonizers, the target decarbonization rate started at 230 Mtpa in 2021 and increased to 500 Mtpa by 2030.As time approaches 2050, this target rate will increase rapidly.For polluters, the target decarbonization rate started at 1365 Mtpa in 2021 and will increase to 2000 Mtpa for the year 2030 and 4400 Mtpa for the year 2040.Thereafter, it will increase sharply as 2050 was approached.The target decarbonization rates for economies with the biggest targets are shown in Figure 15.Figures 14 and 15 show clearly that there was a big difference in target decarbonization rates to achieve net zero by 2050 between polluters and decarbonizers.Polluters required a much bigger target decarbonization than decarbonizers.For both, the target decarbonization rate will increase with time.As 2050 approaches, the target decarbonization rate will be too high to be achievable.This means that decarbonization must begin as soon as possible.Each year of delay in beginning decarbonization will make subsequent decarbonization more difficult and expensive.
show clearly that there was a big difference in target decarbonization rates to achieve net zero by 2050 between polluters and decarbonizers.Polluters required a much bigger target decarbonization than decarbonizers.For both, the target decarbonization rate will increase with time.As 2050 approaches, the target decarbonization rate will be too high to be achievable.This means that decarbonization must begin as soon as possible.Each year of delay in beginning decarbonization will make subsequent decarbonization more difficult and expensive.show clearly that there was a big difference in target decarbonization rates to achieve net zero by 2050 between polluters and decarbonizers.Polluters required a much bigger target decarbonization than decarbonizers.For both, the target decarbonization rate will increase with time.As 2050 approaches, the target decarbonization rate will be too high to be achievable.This means that decarbonization must begin as soon as possible.Each year of delay in beginning decarbonization will make subsequent decarbonization more difficult and expensive.

Decarbonization Pathways
The target decarbonization rates in 2021 to achieve net zero by 2050 are shown in the second-to-last column in Tables 1 and 2. Figure 16 compares the target decarbonization rates in 2021 for 20 economies with the highest targets.The economies with the highest decarbonization targets are China, India, the USA, Russia, Saudi Arabia, Iran and Indonesia.Their efforts will be critical for global decarbonization.It is worthwhile to note that there are some big CO 2 emitters, e.g., Japan, South Korea, the UK and Italy, that do not appear on this list because they have low decarbonization targets since their current decarbonization rates are relatively high.
carbonization rates are relatively high.
To determine the pathways for decarbonization, we divided economies into three categories: coal-dominated (Table 3), gas-and-oil-dominated (Table 4), and non-fossildominated (Table 5) and discuss decarbonization pathways for each.Out of the 80 economies included in the aforementioned analysis, only 66 reported data on their composition of total final energy consumption (TFEC) in 2021 [50].Hence, in the following sections, we focus on these 66 economies.To determine the pathways for decarbonization, we divided economies into three categories: coal-dominated (Table 3), gas-and-oil-dominated (Table 4), and non-fossildominated (Table 5) and discuss decarbonization pathways for each.Out of the 80 economies included in the aforementioned analysis, only 66 reported data on their composition of total final energy consumption (TFEC) in 2021 [50].Hence, in the following sections, we focus on these 66 economies.

Decarbonization Technologies
Decarbonization technologies can be divided into demand-side and supply-side technologies.On the energy demand side, increasing energy efficiency [51] and reducing demand [52] are key areas but are outside the scope of this paper.Decarbonization technologies on the energy supply side can be classified into four types (Table 6).The first type is renewable energies (REs), including wind, solar, bioenergy, geothermal and hydroelectricity [53].They are mostly applied to the power (electricity) sector but can be applied to the transport sector through the use of electric vehicles, which have been gaining acceptance in recent years.In fact, renewable energies have been applied in many economies and contributed to 27.9% of global electricity and 13.5% of global total final energy consumption (TFEC) in 2021 [50].Hitherto, most of the efforts on decarbonization have been focused on RE.
The second type of decarbonization technology is low-carbon fossil energies.They include (1) switching from coal to gas for power and heat generation (coal → gas), applying CCS to coal-fired power plants (CP-CCS), gas-fired power plants (GP-CCS) and industrial plants (Ind-CCS) [28,54,55].Coal → gas is a mature technology and has the potential to halve CO 2 emissions in power generation, as the combustion of gas produces approximately half the CO 2 compared with coal [56].It has been implemented in many advanced and growing economies.CCS has been applied commercially in Norway [57,58] and is currently being implemented in tens of projects worldwide [59], especially in countries that have a carbon tax or credit.It has been gaining momentum in the USA, where economic incentives have been increased through the 45Q tax regulations [60].Another type of low-carbon fossil decarbonization technology is carbon capture and utilization, which seeks to turn anthropogenic CO 2 into useful commercial products [61].However, these technologies are still in the R&D stage and not ready for commercialization [61].
The third type of decarbonization technology is hydrogen.Though hydrogen is not an energy source, it is an energy carrier like electricity.It can be used to generate electricity through a hydrogen fuel cell or combusted to produce heat.Consequently, hydrogen can be used for the transport sector through the use of hydrogen fuel cell vehicles or used to replace fossil fuels for industrial heating and power generation.Green hydrogen is produced via the electrolysis of fresh water with renewable electricity.It is called "green" because this process does not produce CO 2 [62].However, this is the most expensive form of hydrogen and currently costs USD 3-7/kg [63].Blue hydrogen is produced from natural gas through steam methane reforming or coal through coal gasification with emitted CO 2 mitigated by CCS.Both processes are technologically mature [21].Currently, blue hydrogen is roughly half (USD 1.5-2.3/kg)as expensive as green hydrogen [63].Besides being used to fuel hydrogen fuel cell vehicles, hydrogen can be used to replace coal or gas in industries requiring high temperatures (over 1000 • C), such as steelmaking, cement manufacturing, natural gas processing and raw material in the petrochemical processes.Hydrogen is therefore useful to decarbonize the "hard-to-decarbonize" industrial sector.At present, neither green nor blue hydrogen is manufactured at a commercial scale.
The fourth type of decarbonization technology is nuclear energy.The use of conventional nuclear energy is a country-specific issue.Some countries, e.g., France and Ukraine, favor the use of nuclear energy, whereas many others, e.g., Southeast Asian countries, do not.Conventional nuclear energy suffers from a long lead time from proposal to government approval, which could take up to a decade.In addition, the disposal of nuclear waste is also an issue.Recently, there has been interest in the use of advanced small modular reactors (SMRs) of capacity up to 300 MW e , which are faster to build and deploy and can be used for industrial applications or remote areas with limited grid capacity.However, SMRs are still in the research and development stage [64].Table 6.Supply-side decarbonization technologies.

Renewable Energies Low-Carbon Fossil Energies Nuclear Energy Hydrogen
Main types Main use

Slow Pace of Addition of Renewable Energy Capacity
In the last column of Tables 3-5, we have listed the change in RE's share of TFEC over the last decade (2011 to 2021) [50].For coal-dominated economies, Australia's gain in RE's share of TFEC was 9.36 percentage points (pp) over a decade and was the largest (Table 3).This was still less than 1 pp per year.The average increase was only 4.31 pp over a decade.For oil-and-gas-dominated economies, European economies had the biggest gain in RE in the last decade (Table 4).The biggest gain was achieved by the UK (13.98 pp), Germany (13.77 pp), Greece (13.77 pp), Austria (10.39 pp) and the Netherlands (9.68 pp).Ecuador was the only economy in South America with a sizeable gain in RE (15.47 pp).Both the USA and Canada achieved small gains of 5.39 pp and 2.78 pp, respectively.The average increase in RE's share of TFEC was 4.58% over a decade (Table 4).All non-fossil-dominated economies exhibited sizable gains in the RE share of TPEC in the last decade, ranging from 7 pp to 15 pp (Table 5).The average increase in RE's share of TFEC was 10.42 pp over a decade (Table 5).These numbers show that the pace of the addition of RE's share of TFEC was relatively slow and was about 0.4 pp per year for most economies.Even if this rate were tripled, the goal of a near 100% in RE's share of TFEC by 2050 cannot be achieved.There are two reasons for this slow growth in RE's share of TFEC.First, renewable energies' (RE) share of TFEC will only increase if the growth in RE is larger than the growth in fossil fuels.However, in many economies, the growth in the fossil fuel capacity surpasses that of the RE capacity.Therefore, unless more RE capacity is installed, the overall growth in the RE share of TPEC is negative.Second, wind and solar have a lower capacity utilization rate (<20%) than fossil fuel (>40%) in power generation [55].Therefore, replacing fossil power generation capacity with wind and solar requires a much higher wind or solar capacity.

Decarbonization for Coal-Dominated Economies
These are economies where coal's share of TFEC was 25% or higher in 2021 [65].There were 15 economies in this group.They included China, India, Australia, Indonesia and Japan, which are some of the biggest CO 2 emitters in the world.The compositions of TFEC by fuel type in these economies are listed in Table 3.In these economies, the majority of CO 2 is emitted by the combustion of coal.Consequently, the decarbonization effort should be directed at reducing CO 2 emissions from the combustion of coal.
In general, there are two demonstrated ways to decarbonize coal-dominated economies.The primary way is to replace the use of coal for power and heat generation with gas.This has the potential to reduce CO 2 emissions by one-half [56].To achieve this, coal-fired power plants have to be repurposed for using gas as fuel.An alternative decarbonization method is to install CCS in coal-fired power plants to mitigate the emitted CO 2 .This will require compressing the captured CO 2 and transporting it to nearby oil or gas reservoirs, or using saline aquifers for permanent storage.A recent study by Lau showed a potentially large contribution of retrofitting CCS in existing coal-fired power plants in several Asian countries [66].Which method is preferred will depend on the availability of gas to replace coal and the availability of suitable subsurface storage sites for CO 2 storage.The secondary decarbonization method is to install CCS in gas-fired power plants to further reduce CO 2 emissions.

Decarbonization of Gas-and-Oil-Dominated Economies
These are economies where the share of oil and gas in TFEC was 50% or higher in 2021.There were 45 economies in this group.The aggregate CO 2 emissions from them were only 58% of that of coal-dominated economies (Tables 3 and 4).The compositions of TFEC in 2021 for these economies are given in Table 4.In these economies, gas was used mostly for power and heat generation, whereas oil was mostly used as fuel for transport.The main decarbonization pathway was to use CCS in natural gas-fired power plants (GP-CCS) and, secondarily, CCS in industry plants (Ind-CCS) and coal-fired power plants (CP-CCS).For road transport, the main decarbonization pathway was to use electric vehicles (EVs) to replace internal combustion engine vehicles.This will shift mobile CO 2 emissions from vehicles to stationary emissions at power plants.The latter may be mitigated by CCS if fossil fuels are used for power generation.
The biggest CO 2 emitters from this group are the USA, Russia, the UK, Iran, Saudi Arabia, Germany and South Korea (Table 4).Except for Belgium and South Korea, nuclear energy's share of TFEC in 2021 in this group of economies was less than 10%.

Decarbonization of Non-Fossil-Energy-Dominated Economics
These economies were dominated mostly by renewable and nuclear energies and their aggregate share of TFEC was larger than 45%.There were six economies in this category and their compositions of TFEC in 2021 are given in Table 5.Among them, France was the one with the most nuclear energy (36.4%).Other economies were mostly dominated by renewable energies.Among them, Brazil and France were the biggest CO 2 emitters in 2021.Decarbonization of these economies will require the further expansion of nuclear power, possibly by using SMR [64] and renewable electricity, to replace gas for power generation.The secondary decarbonization pathway is the further electrification of road transport to replace gasoline with electricity as a transportation fuel.

Discussion
The decarbonization pathways for an economy will vary according to the composition of its TFEC.The suggested pathways are summarized in Table 7.It should be emphasized that these pathways are over and above current efforts to increase energy efficiency and install renewable energy capacities.Many of these pathways can be classified as low-carbon fossil pathways that include switching from coal to gas for power and heat generation (coal → gas) and mitigating CO 2 emission from the combustion of fossil fuels using CCS (CP-CCS, GP-CCS, Ind-CCS).Given the urgency for decarbonization, there is a need to use all available tools in our toolkit to accelerate decarbonization.Low-carbon fossil solutions should be considered, together with RE and nuclear energy.The optimal path for each economy will likely depend on the availability of fossil, renewable, and nuclear energy resources and CCS options.It should also be noted that the decarbonization pathways in Table 7 are only suggestions and could be incomplete.Other decarbonization options can be included depending on their technology and commercial readiness level.

Policy Implications
If current CO 2 emission rates continue, our analysis shows that only 14 economies will achieve net zero by 2050.There is, therefore, an urgent need for most economies to accelerate the pace of decarbonization over and above the current pace of the addition of RE capacity.Consequently, low-carbon fossil solutions, including switching from coal to gas for power and heat generation (coal → gas) and installing CCS in coal-and gas-fired power plants (CP-CCS, GP-CCS) and industrial plants (Ind-CCS) will be needed.According to the Global CCS Institute, as of September 2022, there were 30 CCS projects operating in the world mitigating 43 Mtpa; another 164 projects are in various phases of development and will add 201 Mtpa of CCS capacity [59].Moreover, CCS has been gaining momentum over the last few years.The already sanctioned Longship project in Norway will be in operation by 2024 and will sequester 0.8 to 5.0 Mtpa CO 2 [67,68].It may become the first cross-border CCS project in the world.The Porthos [69,70] and Aramis [71] CCS projects in the Netherlands will sequester 2.5 Mtpa and 5.0 Mtpa CO 2 , respectively.The East Coast Cluster CCS project in the UK will sequester 26 Mtpa CO 2 , almost 50% of CO 2 emissions from the UK's industrial sector [72,73].Talos Oil has started operations in the first offshore CCS project in the Gulf of Mexico near Port Arthur, Texas, in cooperation with several other companies [74][75][76].The Houston CCS Alliance plans to sequester 100 Mtpa CO 2 by 2040 [77].These projects are all located in countries that have imposed a substantial carbon tax or credit.In 2022, the carbon taxes in Norway, the Netherlands and the UK were USD 88, USD 46 and USD 24, respectively [78].In 2022, the US government increased the carbon credit from USD 45 to USD 85 per tonne CO 2 [79].Outside of Europe and North America, most countries do not have a carbon tax or credit.However, Aramco has announced it will establish a CCUS hub in Jubail [80][81][82][83].Malaysia already has several CCS projects under development [84][85][86] and Indonesia has expressed interest in CCS projects and passing related regulations [87,88].China is also aggressively installing CCS projects, both onshore and offshore [11,89,90].Imposing a credible carbon tax or credit will be the most important energy policy to incentivize companies to reduce their CO 2 emissions.Next, the passing of CCS regulations on CO 2 injection and monitoring will be needed.Furthermore, the transfer of long-term liability from the operator to the government will be needed to de-risk CCS projects and to obtain financing [21,72,73].More public engagement on the benefits of CCS will also be needed to raise the level of public acceptance of CCS.Government funding of CCS R&D should include subsurface characterization of saline aquifers for CO 2 storage [28].
In the transport sector, there appears to be a consensus among economies to replace the use of internal combustion engine vehicles with electric vehicles (EVs) [91,92].This will drastically reduce CO 2 emissions if the electricity used for EVs is generated using renewable energy or low-carbon fossil fuels.If the latter, CCS will be needed.To encourage the wider use of EVs, policies need to be enacted to incentivize the building of charging facilities [91,93], upgrading of the electric grid [94] and R&D of battery technologies [95].
In the longer-term future, the use of hydrogen to replace the use of fossil fuels to decarbonize the "hard-to-decarbonize" industrial sector will be needed [63].Therefore, national governments will need to propose a hydrogen strategy or roadmap to underpin the increased use of hydrogen [96][97][98].Since green hydrogen currently costs twice as much as blue hydrogen, it will probably not be available in large quantities for some time [63].The use of blue hydrogen should therefore be considered [99].

Conclusions
The following may be concluded from this study: 1.
The top 80 CO 2 emitters contributed 95% of the global emissions in 2021.Among them, 55 were decarbonizers, where emissions had either peaked or were in decline; 25 were polluters, where emissions were increasing.2. A linear history matching and forecasting method was applied to historical CO 2 emissions data.The results show that global CO 2 emissions will increase from 37.1 Gtpa in They are the UK, Italy, Spain, Ukraine, Greece, Portugal, Finland, Serbia, Denmark, Estonia, Trinidad and Tobago, Venezuela, South Korea and Japan.3. The target decarbonization rates to achieve net zero by 2050, over and above current efforts, were estimated for the 80 economies.These rates will increase rapidly as 2050 is approached.Therefore, decarbonization must start as soon as possible and any delay will only make future efforts more costly and difficult.4. China, India, the USA, Russia, Saudi Arabia, Iran, Indonesia, Turkey, Vietnam and Canada have the largest decarbonization targets.Their efforts will determine the future of global decarbonization.5.The average increase in RE's share in TFEC was 0.4 pp per year for most economies over the last decade.Even if this rate were tripled, it is still inadequate to allow RE's share in TFEC to approach 100% by 2050.Therefore, other decarbonization methods are urgently needed.They will vary with each country's energy mix.Pathways for coal-dominated, gas-and-oil-dominated, and non-fossil-energy-dominated economies are suggested and their policy implications are discussed.

Figure 3 .
Figure 3.Comparison of current trend study methodology with IEA scenarios for a decarbonizer.

Figure 3 .
Figure 3.Comparison of current trend study methodology with IEA scenarios for a decarbonizer.Figure 3. Comparison of current trend study methodology with IEA scenarios for a decarbonizer.

Figure 3 .
Figure 3.Comparison of current trend study methodology with IEA scenarios for a decarbonizer.Figure 3. Comparison of current trend study methodology with IEA scenarios for a decarbonizer.Energies 2023, 16, x FOR PEER REVIEW 5 of 24

Figure 4 .
Figure 4. Comparison of current trend study methodology with IEA scenarios for a polluter.

Figure 4 .
Figure 4. Comparison of current trend study methodology with IEA scenarios for a polluter.

Figure 12 .
Figure 12.Graphical method to determine decarbonization targets between 2021 and 2050 for USA.

Figure 13 .
Figure 13.Graphical method to determine decarbonizer targets between 2021 and 2050 for China.

Figure 12 .
Figure 12.Graphical method to determine decarbonization targets between 2021 and 2050 for USA.

Figure 12 .
Figure 12.Graphical method to determine decarbonization targets between 2021 and 2050 for USA.

Figure 13 .
Figure 13.Graphical method to determine decarbonizer targets between 2021 and 2050 for China.Figure 13.Graphical method to determine decarbonizer targets between 2021 and 2050 for China.

Figure 13 .
Figure 13.Graphical method to determine decarbonizer targets between 2021 and 2050 for China.Figure 13.Graphical method to determine decarbonizer targets between 2021 and 2050 for China.

Figure 15 .
Figure 15.Target decarbonization targets for various countries.Figure 15.Target decarbonization targets for various countries.

Figure 15 .
Figure 15.Target decarbonization targets for various countries.Figure 15.Target decarbonization targets for various countries.

Figure 16 .
Figure 16.Target decarbonization rate in 2021 to achieve net zero by 2050.

Figure 16 .
Figure 16.Target decarbonization rate in 2021 to achieve net zero by 2050.

CO2 Emissions Current CO2 CO2 Increase Rate to Reach Decarbonization Target in 2021 to Achieve Net Confidence Level of Figure
[7]Annual CO 2 from polluters[7].Energies 2023, 16, x FOR PEER REVIEW 6 of 24

Table 1 .
CO2 emissions forecast and decarbonization targets for decarbonizers.Economy CO2 Emissions in 2021 Estimated Net-Zero Est.

Table 2 .
CO 2 emissions forecast and decarbonization targets for polluters.
a -total, b -average.NA = not available
a -total, b -average.NA = not available

Table 7 .
Suggested decarbonization pathways for various classes of economies.

Coal-Dominated Economies Oil-and-Gas-Dominated Economies Non-Fossil-Energy- Dominated Economies
Gtpa in 2050 with polluters contributing to 81% and decarbonizers 14% of global CO 2 emissions.Furthermore, only 14 economies will achieve net zero by 2050.