Editing IPCC:AR6/WGIII/Chapter-2 (section)

=== 2.2.2 Trends in the Global GHG Emissions Trajectories and Short-lived Climate Forcers ===

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==== 2.2.2.1 Anthropogenic Greenhouse Gas Emissions Trends ====

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Global GHG emissions continued to rise since AR5, but the rate of emissions growth slowed ( ''high confidence'' ). GHG emissions reached 59 ± 6.6 GtCO 2 -eq in 2019 (Table 2.1 and Figure 2.5). In 2019, CO 2 emissions from the FFI were 38 (±3.0) Gt, CO 2 from LULUCF 6.6 ± 4.6 Gt, CH 4 11 ± 3.2 GtCO 2 -eq, N 2 O 2.7 ± 1.6 GtCO 2 -eq and F-gases 1.4 ± 0.41 GtCO 2 -eq. There is ''high confidence'' that average annual GHG emissions for the last decade (2010–2019) were the highest on record in terms of aggregate CO 2 -eq emissions, but ''low confidence'' for annual emissions in 2019 as uncertainties are large considering the size and composition of observed increases in the most recent years ( [[#UNEP--2020a|UNEP 2020a]] ; [[#Minx--2021|Minx et al. 2021]] ).

'''Table 2''' '''.1 |''' '''Total anthropogenic GHG emissions (GtCO''' 2 '''-eq yr''' –1 ''') 1990–2019.''' CO 2 from fossil fuel combustion and industrial processes (FFI); CO 2 from Land Use, Land Use Change and Forestry (LULUCF); methane (CH 4 ); nitrous oxide (N 2 O); fluorinated gases (F-gases: HFCs, PFCs, SF 6 , NF 3 ). Aggregate GHG emissions trends by groups of gases reported in GtCO 2 '''-''' eq converted based on global warming potentials with a 100-year time horizon (GWP100) from the IPCC Sixth Assessment Report (AR6). Uncertainties are reported for a 90% confidence interval. Source: [[#Minx--2021|Minx et al. (2021)]] .

{| class="wikitable"
|-
| rowspan="2"|
| colspan="6"| Average annual emissions (GtCO 2 -eq)

|-
| CO 2 FFI

| CO 2 LULUCF

| CH 4

| N 2 O

| Fluorinated gases

| GHG

|-
| 2019

| 38 ± 3.0

| 6.6 ± 4.6

| 11 ± 3.2

| 2.7 ± 1.6

| 1.4 ± 0.41

| 59 ± 6.6

|-
| 2010–2019

| 36 ± 2.9

| 5.7 ± 4.0

| 10 ± 3.0

| 2.6 ± 1.5

| 1.2 ± 0.35

| 56 ± 6.0

|-
| 2000–2009

| 29 ± 2.4

| 5.3 ± 3.7

| 9.0 ± 2.7

| 2.3 ± 1.4

| 0.81 ± 0.24

| 47 ± 5.3

|-
| 1990–1999

| 24 ± 1.9

| 5.0 ± 3.5

| 8.2 ± 2.5

| 2.1 ± 1.2

| 0.49 ± 0.15

| 40 ± 4.9

|-
| 1990

| 23 ± 1.8

| 5.0 ± 3.5

| 8.2 ± 2.5

| 2.0 ± 1.2

| 0.38 ± 0.11

| 38 ± 4.8

|}

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[[File:0d5efdd376c1afc4183a6931dc85857c IPCC_AR6_WGIII_Figure_2_5.png]]

'''Figure 2.5''' '''|''' '''Total anthropogenic GHG emissions (GtCO''' 2 '''-eq yr''' –1 ''') 1990–2019.''' CO 2 from fossil fuel combustion and industrial processes (FFI); net CO 2 from land use, land use change and forestry (LULUCF); methane (CH 4 ); nitrous oxide (N 2 O); fluorinated gases (F-gases: HFCs, PFCs, SF 6 , NF 3 ). '''Panel (a):''' Aggregate GHG emissions trends by groups of gases reported in GtCO 2 -eq converted based on global warming potentials with a 100-year time horizon (GWP100) from the IPCC Sixth Assessment Report. '''Panel (b):''' Waterfall diagrams juxtaposes GHG emissions for the most recent year (2019) in CO 2 equivalent units using GWP100 values from the IPCC’s Second, Fifth, and Sixth Assessment Reports, respectively. Error bars show the associated uncertainties at a 90% confidence interval. '''Panel (c):''' individual trends in CO 2 -FFI, CO 2 -LULUCF, CH 4 , N 2 O and F-gas emissions for the period 1990–2019, normalised to 1 in 1990. Source: data from [[#Minx--2021|Minx et al. (2021)]] .

GHG emissions levels in 2019 were higher compared to 10 and 30 years earlier ( ''high confidence'' ): about 12% (6.5 GtCO 2 -eq) higher than in 2010 (53 ± 5.7 GtCO 2 -eq) (the last year of AR5 reporting) and about 54% (21 GtCO 2 -eq) higher than in 1990 (38 ± 4.8 GtCO 2 -eq) (the baseline year of the Kyoto Protocol and frequent nationally determined contribution (NDC) reference). GHG emissions growth slowed compared to the previous decade ( ''high confidence'' ): From 2010 to 2019, GHG emissions grew on average by about 1.3% per year compared to an average annual growth of 2.1% between 2000 and 2009. Nevertheless the absolute increase in average annual GHG emissions for 2010–2019 compared to 2000–2009 was 9.1 GtCO 2 -eq and, as such, the largest observed in the data since 1970 (Table 2.1) – and most likely in human history ( [[#Friedlingstein--2020|Friedlingstein et al. 2020]] ; [[#Gütschow--2021b|Gütschow et al. 2021b]] ). Decade-by-decade growth in average annual GHG emissions was observed across all (groups of) gas as shown in Table 2.1, but for N 2 O and CO 2 -LULUCF emissions this is much more uncertain.

Reported total annual GHG emission estimates differ between the WGIII contributions in AR5 (Blanco et al. 2014) and AR6 (this chapter) mainly due to differing global warming potentials ( ''high confidence'' ). For the year 2010, total GHG emissions were estimated at 49 ± 4.9 GtCO 2 -eq in AR5 (Blanco et al. 2014), while we report 53 ± 5.7 GtCO 2 -eq here. However, in AR5 total GHG emissions were weighted based on GWP100 values from IPCC’s Second Assessment Report. Applying those GWP values to the 2010 emissions from AR6 yields 50 GtCO 2 -eq (Forster et al. 2021a). Hence, observed differences are mainly due to the use of most recent GWP values, which have higher warming potentials for methane (29% higher for biogenic and 42% higher for fugitive methane) and 12% lower values for nitrous oxide (Cross-Chapter Box 2 in this chapter).

Emissions growth has been persistent but varied in pace across gases. The average annual emission levels of the last decade (2010–2019) were higher than in any previous decade for each group of GHGs: CO 2 , CH 4 , N 2 O, and F-gases ( ''high confidence'' ). Since 1990, CO 2 -FFI have grown by 67% (15 GtCO 2 -eq), CH 4 by 29% (2.4 GtCO 2 -eq), and N 2 O by 33% (0.65 GtCO 2 -eq), respectively (Figure 2.5). Growth in fluorinated gases (F-gas) has been by far the highest with about 254% (1.0 GtCO 2 -eq), but it occurred from low levels. In 2019, total F-gas levels were no longer negligible with a share of 2.3% of global GHG emissions. Note that the F-gases reported here do not include CFCs and HCFCs, which are groups of substances regulated under the Montreal Protocol. The aggregate CO 2 -eq emissions of HFCs, HCFCs and CFCs were each approximately equal in 2016, with a smaller contribution from PFCs, SF 6 , NF 3 and some more minor F-gases. Therefore, the GWP-weighted F-gas emissions reported here (HFCs, PFCs, SF 6 , NF 3 ), which are dominated by the HFCs, represent less than half of the overall CO 2 -eq F-gas emissions in 2016 (Figure 2.3).

The only exception to these patterns of GHG emissions growth is net anthropogenic CO 2 -LULUCF emissions, where there is no statistically significant trend due to high uncertainties in estimates (Figures 2.2 and 2.5; [https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-2 Chapter 2] Supplementary Material). While the average estimate from the bookkeeping models report a slightly increasing trend in emissions, NGHGI and FAOSTAT estimates show a slightly decreasing trend, which diverges in recent years (Figure 2.2). Similarly, trends in CO 2 -LULUCF estimates from individual bookkeeping models differ: while two models (BLUE and OSCAR) show a sustained increase in emissions levels since the mid-1990s, emissions from the third model (Houghton and Nassikas (HN)) declined (Figure 2.2 in this chapter; [[#Friedlingstein--2020|Friedlingstein et al. 2020]] ). Differences in accounting approaches and their impacts CO 2 emissions estimates from land use is covered in [[IPCC:Wg3:Chapter:Chapter-7|Chapter 7]] and in the [https://www.ipcc.ch/report/ar6/wg3/chapter/chapter-2 Chapter 2] Supplementary Material (2.SM.2). Note that anthropogenic net emissions from bioenergy are covered by the CO 2 -LULUCF estimates presented here.

The CO 2 -FFI share in total CO 2 -eq emissions has plateaued at about 65% in recent years and its growth has slowed considerably since AR5 ( ''high confidence'' ). The CO 2 -FFI emissions grew at 1.1% during the 1990s and 2.5% during the 2000s. For the last decade (2010s) – not covered by AR5 – this rate dropped to 1.2%. This included a short period between 2014 and 2016 with little or no growth in CO 2 -FFI emissions, mainly due to reduced emissions from coal combustion ( [[#Jackson--2016|Jackson et al. 2016]] ; [[#Qi--2016|Qi et al. 2016]] ; [[#Peters--2017a|Peters et al. 2017a]] ; Canadell et al. 2021). Subsequently, CO 2 -FFI emissions started to rise again ( [[#Peters--2017b|Peters et al. 2017b]] ; [[#Figueres--2018|Figueres et al. 2018]] ; [[#Peters--2020|Peters et al. 2020]] ).

Starting in the spring of 2020 a major break in global emissions trends was observed due to lockdown policies implemented in response to the COVID-19 pandemic ( ''high confidence'' ) ( [[#Forster--2020|Forster et al. 2020]] ; [[#Le%20Quéré--2020|Le Quéré et al. 2020]] , 2021; Z. [[#Liu--2020b|]] [[#Liu--2020b|Liu et al. 2020b]] ; [[#Bertram--2021|Bertram et al. 2021]] ). Overall, global CO 2 -FFI emissions are estimated to have declined by 5.8% (5.1%–6.3%) in 2020, or about 2.2 (1.9–2.4) GtCO 2 in total ( [[#Friedlingstein--2020|Friedlingstein et al. 2020]] ; Z. [[#Liu--2020b|]] [[#Liu--2020b|Liu et al. 2020b]] ; [[#BP--2021|BP 2021]] ; [[#Crippa--2021|Crippa et al. 2021]] ; [[#IEA--2021a|IEA 2021a]] ). This exceeds any previous global emissions decline since 1970, both in relative and absolute terms (Figure 2.6). Daily emissions, estimated based on activity and power-generation data, declined substantially compared to 2019 during periods of economic lockdown, particularly in April 2020 – as shown in Figure 2.6 – but rebounded by the end of 2020 ( ''medium confidence'' ) ( [[#Le%20Quéré--2020|Le Quéré et al. 2020]] , 2021; Z. [[#Liu--2020b|]] [[#Liu--2020b|Liu et al. 2020b]] ). Impacts were differentiated by sector, with road transport and aviation particularly affected. Inventories estimate the total power sector CO 2 reduction from 2019 to 2020 at 3% ( [[#IEA--2021a|IEA 2021a]] ) and 4.5% ( [[#Crippa--2021|Crippa et al. 2021]] ). Approaches that predict near real-time estimates of the power sector reduction are more uncertain and estimates range more widely, between 1.8% ( [[#Le%20Quéré--2020|Le Quéré et al. 2020]] , 2021), 4.1% (Z. [[#Liu--2020b|]] [[#Liu--2020b|Liu et al. 2020b]] ) and 6.8% ( [[#Bertram--2021|Bertram et al. 2021]] ); the latter taking into account the over-proportional reduction of coal generation due to low gas prices and merit order effects. Due to the very recent nature of this event, it remains unclear what the exact short- and long-term impacts on future global emissions trends will be.

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[[File:c9cdfa6eb4d9c0d64b7eebd78e29045d IPCC_AR6_WGIII_Figure_2_6.png]]

'''Figure 2.6''' '''|''' '''Global CO''' 2 '''emissions from fossil fuel combustion and industry (FFI) in 2020 and the impact of COVID-19. Panel (a)''' depicts CO 2 -FFI emissions over the past five decades (GtCO 2 yr –1 ). The single year declines in emissions following major economic and geopolitical events are shown, as well as the decline recorded in five different datasets for emissions in 2020 (COVID-19) compared to 2019 (no COVID-19). '''Panel (b)''' depicts the change in global daily carbon emissions (MtCO 2 per day) in 2020 compared to 2019, showing the impact of COVID-19 lockdown policies. Source: [[#Friedlingstein--2020|Friedlingstein et al. (2020)]] , [[#Le%20Quéré--2020|Le Quéré et al. (2020)]] , Carbon Monitor ( [[#Liu--2020b|]] [[#Liu--2020b|Liu et al. 2020b]] ), [[#BP--2021|BP (2021)]] , [[#Crippa--2021|Crippa et al. (2021)]] , [[#IEA--2021a|IEA (2021a)]] .

From 1850 until around 1950, anthropogenic CO 2 emissions were mainly (&gt;50%) from land use, land-use change and forestry (Figure 2.7). Over the past half-century CO 2 emissions from LULUCF have remained relatively constant around 5.1 ± 3.6 GtCO 2 but with a large spread across estimates ( [[#Le%20Quéré--2018|Le Quéré et al. 2018]] a; [[#Friedlingstein--2019|Friedlingstein et al. 2019]] , 2020). By contrast, global annual FFI-CO 2 emissions have continuously grown since 1850, and since the 1960s from a decadal average of 11 ± 0.9 GtCO 2 to 36 ± 2.9 GtCO 2 during 2010–2019 (Table 2.1).

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[[File:42dd9ed63404adf8021acc48b41abfb9 IPCC_AR6_WGIII_Figure_2_7.png]]

'''Figure 2.7''' '''|''' '''Historic anthropogenic CO''' 2 '''emission and cumulative CO''' 2 '''emissions (1850–2019) as well as remaining carbon budgets for limiting warming to 1.5°C and 2°C. Panel (a)''' shows historic annual anthropogenic CO 2 emissions (GtCO 2 yr –1 ) by fuel type and process. '''Panel (b)''' shows historic cumulative anthropogenic CO 2 emissions for the periods 1850–1989, 1990–2009, and 2010–2019 as well as remaining future carbon budgets as of 1 January 2020 to limit warming to 1.5°C and 2°C at the 67th percentile of the transient climate response to cumulative CO 2 emissions. The whiskers indicate a budget uncertainty of ±220 GtCO 2 -eq for each budget and the aggregate uncertainty range at one standard deviation for historical cumulative CO 2 emissions, consistent with Working Group 1. Sources: [[#Friedlingstein--2020|Friedlingstein et al. (2020)]] and Canadell et al. (2021).

Cumulative CO 2 emissions since 1850 reached 2400 ± 240 GtCO 2 in 2019 ( ''high confidence'' ). [[#footnote-007|7]] More than half (62%) of total emissions from 1850 to 2019 occurred since 1970 (1500 ± 140 GtCO 2 ), about 42% since 1990 (1000 ± 90 GtCO 2 ) and about 17% since 2010 (410 ± 30 GtCO 2 ) ( [[#Friedlingstein--2019|Friedlingstein et al. 2019]] ; [[#Friedlingstein--2020|Friedlingstein et al. 2020]] ; Canadell et al. 2021) (Figure 2.7). Emissions in the last decade are about the same size as the remaining carbon budget of 400 ± 220 (500, 650) GtCO 2 for limiting global warming to 1.5°C and between one-third and half the 1150 ± 220 (1350, 1700) GtCO 2 for limiting global warming below 2°C with a 67% (50%, 33%) probability, respectively ( ''medium confidence'' ) (Canadell et al. 2021). At current (2019) levels of emissions, it would only take 8 (2–15) and 25 (18–35) years to emit the equivalent amount of CO 2 for a 67th percentile 1.5°C and 2°C remaining carbon budget, respectively. Related discussions of carbon budgets, short-term ambition in the context of Nationally Determined Contributions (NDCs), pathways to limiting warming to well below 2°C and carbon dioxide removals are mainly discussed in Chapters 3, 4, and 12, but also [[#2.7|Section 2.7]] of this chapter.

Even when taking uncertainties into account, historical emissions between 1850 and 2019 constitute a large share of total carbon budgets from 2020 onwards for limiting warming to 1.5°C with a 50% probability as well as for limiting warming to 2°C with a 67% probability. Based on central estimates only, historical cumulative net CO 2 emissions between 1850–2019 amount to about four fifths of the total carbon budget for a 50% probability of limiting global warming to 1.5°C (central estimate about 2900 GtCO 2 ), and to about two thirds of the total carbon budget for a 67% probability to limit global warming to 2°C (central estimate about 3550 GtCO 2 ). The carbon budget is the maximum amount of cumulative net global anthropogenic CO 2 emissions that would result in limiting global warming to a given level with a given likelihood, taking into account the effect of other anthropogenic climate forcers. This is referred to as the total carbon budget when expressed starting from the pre-industrial period, and as the remaining carbon budget when expressed from a recent specified date. The total carbon budgets reported here are the sum of historical emissions from 1850 to 2019 and the remaining carbon budgets from 2020 onwards, which extend until global net zero CO 2 emissions are reached. Uncertainties for total carbon budgets have not been assessed and could affect the specific calculated fractions (IPCC 2021 [Working Group 1 SPM], Canadell et al., 2021 [Working Group 1 Ch5]).

Comparisons between historic GHG emissions and baseline projections provide increased evidence that global emissions are not tracking high-end scenarios ( [[#Hausfather--2020|Hausfather and Peters 2020]] ), and rather followed ‘middle-of-the-road’ scenario narratives in the earlier series, and by combinations of ‘global-sustainability’ and ‘middle-of-the-road’ narratives in the most recent series (IPCC Special Report on Emissions Scenarios (SRES) and Shared Socioeconomic Pathways (SSP)-baselines) ( [[#Pedersen--2020|Pedersen et al. 2020]] ; Strandsbjerg Tristan [[#Pedersen--2021|Pedersen et al. 2021]] ). As countries increasingly implement climate policies and technology costs continue to evolve, it is expected that emissions will continually shift away from scenarios that assume no climate policy but remain insufficient to limit warming to below 2°C ( [[#Vrontisi--2018|Vrontisi et al. 2018]] ; [[#Hausfather--2020|Hausfather and Peters 2020]] ; [[#Roelfsema--2020|Roelfsema et al. 2020]] ; [[#UNEP--2020b|UNEP 2020b]] ).

The literature since AR5 suggests that compared to historical trends baseline scenarios might be biased towards higher levels of fossil fuel use compared to what is observed historically (Cross-Chapter Box 1 in Chapter 1; [[#Ritchie--2017|Ritchie and Dowlatabadi 2017]] , 2018; [[#Ritchie--2019|Ritchie 2019]] ; [[#Creutzig--2021|Creutzig et al. 2021]] ;). [[#Ritchie--2017|Ritchie and Dowlatabadi (2017)]] show that per-capita primary energy consumption in baseline scenarios tends to increase at rates faster than those observed in the long-term historical evidence – particularly in terms of coal use. For example, SSP5 envisions a six-fold increase in per capita coal use by 2100 – against flat long-term historical observations – while the most optimistic baseline scenario SSP1-Sustainability is associated with coal consumption that is broadly in line with historical long-term trends ( [[#Ritchie--2017|Ritchie and Dowlatabadi 2017]] ). In contrast, models have struggled to reproduce historical upscaling of wind and solar and other granular energy technologies ( [[#Wilson--2013|Wilson et al. 2013]] ; [[#van%20Sluisveld--2015|van Sluisveld et al. 2015]] ; [[#Creutzig--2017|Creutzig et al. 2017]] ; [[#Shiraki--2020|Shiraki and Sugiyama 2020]] ; [[#Sweerts--2020|Sweerts et al. 2020]] ; [[#Wilson--2020b|Wilson et al. 2020b]] ).

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==== 2.2.2.2 Other Short-lived Climate Forcers (SLCFs) ====

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There are other emissions with shorter atmospheric lifetimes that contribute to climate changes. Some of them (aerosols, sulphur emissions or organic carbon) reduce forcing, while others – such as black carbon, carbon monoxide or non-methane volatile organic compounds (NMVOC) – contribute to warming (Figure 2.4) as assessed in WGI (Forster et al. 2021c; [[#Szopa--2021a|Szopa et al. 2021a]] ). Many of these other SLCFs are co-emitted during combustion processes in power plants, cars, trucks, airplanes, but also during wildfires and household activities such as traditional cooking with open biomass burning. As these co-emissions have implications for net warming, they are also considered in long-term emission reduction scenarios as covered in the literature ( [[#Harmsen--2020|Harmsen et al. 2020]] ; [[#Rauner--2020b|Rauner et al. 2020b]] ; [[#Smith--2020|Smith et al. 2020]] ; [[#Vandyck--2020|Vandyck et al. 2020]] ) as well as [[IPCC:Wg3:Chapter:Chapter-3|Chapter 3]] of this report. These air pollutants are also detrimental to human health (e.g., [[#Lelieveld--2015|Lelieveld et al. 2015]] , 2018; [[#Vohra--2021|Vohra et al. 2021]] ). For example, [[#Lelieveld--2015|Lelieveld et al. (2015)]] estimate a total of 3.3 (1.6–4.8) million premature deaths in 2010 from outdoor air pollution. Reducing air pollutants in the context of climate policies therefore leads to substantial co-benefits of mitigation efforts (Von Stechow et al. 2015; [[#Rao--2017|Rao et al. 2017]] ; [[#Lelieveld--2019|Lelieveld et al. 2019]] ; [[#Rauner--2020a|Rauner et al. 2020a]] ). Here we briefly outline the major trends in emissions of SLCFs.

Conventional air pollutants that are subject to significant emission controls in many countries include sulphur dioxide (SO 2 ), nitrogen oxides (NO x ), black carbon (BC) and carbon monoxide (CO). From 2015 to 2019, global SO 2 and NOx emissions declined, mainly due to reductions in energy systems (Figure 2.8). Reductions in BC and CO emissions appear to have occurred over the same period, but trends are less certain due to the large contribution of emissions from poorly quantified traditional biofuel use. Emissions of CH 4 , OC and NMVOC have remained relatively stable in the past five years. OC and NMVOC may have plateaued, although there is additional uncertainty due to sources of NMVOCs that may be missing in current inventories ( [[#McDonald--2018|McDonald et al. 2018]] ).

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'''Figure 2.8''' '''|''' '''Air pollution emissionsby major sectors from CEDS (1970–2019) and EDGAR (1970–2015) inventories.''' Source: Crippa et al. (2019a, 2018); [[#O’Rourke--2020|O’Rourke et al. (2020)]] ; [[#McDuffie--2020|McDuffie et al. (2020)]] .

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