Recent Evolution in SLCF Emissions and Abundances
Over the last decade (2010–2019), strong shifts in the geographical distribution of emissions have led to changes in atmospheric abundances of highly variable SLCFs. Evidence from satellite and surface observations show strong regional variations in trends of ozone (O3), aerosols and their precursors. In particular, tropospheric columns of nitrogen dioxide (NO2) and sulphur dioxide (SO2) continued to decline over North America and Europe (high confidence) and to increase over South Asia, but have declined over East Asia. Global carbon monoxide (CO) abundance has continued to decline. The concentrations of hydrofluorocarbons (HFCs) are increasing. Global carbonaceous aerosol budgets and trends remain poorly characterized due to limited observations, but sites representative of background conditions have reported multi-year declines in black carbon (BC) over several regions of the Northern Hemisphere.
There is no significant trend in the global mean tropospheric concentration of hydroxyl (OH) radical–the main sink for many SLCFs, including methane (CH4) – from 1850 up to around 1980 but OH has remained stable or exhibited a positive trend since the 1980s. Global OH cannot be measured directly and is inferred from Earth system and climate chemistry models (ESMs, CCMs) constrained by emissions and from observationally constrained inversion methods. There is conflicting information from these methods for the 1980–2014 period. ESMs and CCMs concur on a positive trend since 1980 (about a 9% increase over 1980–2014) and there is medium confidence that this trend is mainly driven by increases in global anthropogenic nitrogen oxide (NOx) emissions and decreases in anthropogenic CO emissions. The observation-constrained methods suggest either positive trends or the absence of trends based on limited evidence and medium agreement. Future changes in global OH, in response to SLCF emissions and climate change, will depend on the interplay between multiple offsetting drivers of OH.
Effect of SLCFs on Climate and Biogeochemical Cycles
Over the historical period, changes in aerosols and their ERF have primarily contributed to a surface cooling, partly masking the greenhouse gas driven warming. Radiative forcings induced by aerosol changes lead to both local and remote temperature responses. The temperature response preserves the South-North gradient of the aerosol ERF – hemispherical asymmetry- but is more uniform with latitude and is strongly amplified towards the Arctic.
Since the mid-1970s, trends in aerosols and their precursor emissions have led to a shift from an increase to a decrease of the magnitude of the negative globally-averaged net aerosol ERF. However, the timing of this shift varies by continental-scale region and has not occured for some finer regional scales. The spatial and temporal distribution of the net aerosol ERF from 1850 to 2014 is highly heterogeneous, with stronger magnitudes in the Northern Hemisphere.
For forcers with short lifetimes and not considering chemical adjustments, the response in surface temperature occurs strongly as soon as a sustained change in emissions is implemented, and that response continues to grow for a few years, primarily due to thermal inertia in the climate system. Near its maximum, the response slows down but will then take centuries to reach equilibrium. For SLCFs with longer lifetimes (e.g., a decade), a delay equivalent to their lifetimes is appended to the delay due to thermal inertia.
Over the 1750-2019 period, changes in SLCF emissions, especially of CH4, NOx and SO2, have substantial effects on effective radiative forcing (ERF). The net global emissions-based ERF of NOx is negative and that of non-methane volatile organic compounds (NMVOCs) is positive, in agreement with the AR5 assessment. For methane, the emission-based ERF is twice as high as the abundance-based ERF attributed to chemical adjustment mainly via ozone production. SO2 emission changes make the dominant contribution to the ERF from aerosol–cloud interactions. Over the 1750-2019 period, the contributions from the emitted compounds to GSAT changes broadly match their contributions to the ERF. Since a peak in emissions induced SO2 ERF has already occurred recently and since there is a delay in the full GSAT response, changes in SO2 emissions have a slightly larger contribution to GSAT change than for CO2 emissions, relative to their respective contributions to ERF.
Reactive nitrogen, ozone and aerosols affect terrestrial vegetation and the carbon cycle through deposition and effects on large scale radiation. However, the magnitude of these effects on the land carbon sink, ecosystem productivity and hence their indirect CO2 forcing remain uncertain due to the difficulty in disentangling the complex interactions between the individual effects. As such, these effects are assessed to be of second order in comparison to the direct CO2 forcing, but effects of ozone on terrestrial vegetation could add a substantial (positive) forcing compared with the direct ozone forcing.
Climate feedbacks induced from changes in emissions, abundances or lifetimes of SLCFs mediated by natural processes or atmospheric chemistry are assessed to have an overall cooling effect, that is, a total negative feedback parameter, of -0.20 [-0.41 to +0.01] W m−2 °C−1. These non-CO2 biogeochemical feedbacks are estimated from ESMs, which have advanced since AR5 to include a consistent representation of biogeochemical cycles and atmospheric chemistry. However, process-level understanding of many chemical and biogeochemical feedbacks involving SLCFs, particularly natural emissions, is still emerging, resulting in low confidence in the magnitude and sign of most of SLCF climate feedback parameter.
Future Projections for Air Quality Considering Shared Socio-economic Pathways (SSPs
Future air quality (in term of surface ozone and PM concentrations) on global to local scales will be primarily driven by changes in precursor emissions as opposed to climate change and climate change is projected to have mixed effects. A warmer climate is expected to reduce surface O3 in regions remote from pollution sources but is expected to increase it by a few parts per billion over polluted regions, depending on ozone precursor levels. Future climate change is expected to have mixed effects, positive or negative, with an overall low effect, on global surface PM and more generally on the aerosol global burden, but stronger effects are not excluded in regions prone to specific meteorological conditions. Overall, there is low confidence in the response of surface ozone and PM to future climate change due to the uncertainty in the response of the natural processes (e.g., stratosphere–troposphere exchange, natural precursor emissions, particularly including biogenic volatile organic compounds, wildfire-emitted precursors, land and marine aerosols, and lightning NOx) to climate change.
The SSPs span a wider range of SLCF emissions than the Representative Concentration Pathways, representing better the diversity of future options in air pollution management. In the SSPs, the socio-economic assumptions and climate mitigation ambition primarily drive future emissions, but the SLCF emission trajectories are also steered by varying levels of air pollution control originating from the SSP narratives, independently from climate mitigation. Consequently, SSPs consider a large variety of regional ambition and effectiveness in implementing air pollution legislation and result in wider range of future air pollution levels and SLCF-induced climate effects.
Air pollution projections range from strong reductions in global surface ozone and PM (e.g., SSP1-2.6, with strong mitigation of both air pollution and climate change) to no improvement and even degradation (e.g., SSP3-7.0 without climate change mitigation and with only weak air pollution control). Under the SSP3-7.0 scenario, PM levels are projected to increase until 2050 over large parts of Asia, and surface ozone pollution is projected to worsen over all continental areas through 2100. Without climate change mitigation but with stringent air pollution control (SSP5- 8.5), PM levels decline through 2100, but high methane levels hamper the decline in global surface ozone at least until 2080.
Future Projections of the Effect of SLCFs on GSAT in the Core SSPs
In the next two decades, it is very likely that the SLCF emission changes in the WG1 core set of SSPs will cause a warming relative to 2019, whatever the SSPs, in addition to the warming from long-lived greenhouse gases. The net effect of SLCF and HFC changes on GSAT across the SSPs is a likely warming of 0.06°C–0.35°C in 2040 relative to 2019. Warming over the next two decades is quite similar across the SSPs due to competing effects of warming (methane, ozone) and cooling (aerosols) SLCFs. For the scenarios with the most stringent climate and air pollution mitigations (SSP1-1.9 and SSP1- 2.6), the likely near-term warming from the SLCFs is predominantly due to sulphate aerosol reduction, but this effect levels off after 2040. In the absence of climate change policies and with weak air pollution control (SSP3-7.0), the likely near-term warming due to changes in SLCFs is predominantly due to increases in methane, ozone and HFCs, with smaller contributions from changes in aerosols. SSP5-8.5 has the highest SLCF-induced warming rates due to warming from methane and ozone increases and reduced aerosols due to stronger air pollution control compared to the SSP3-7.0 scenario.
At the end of the century, the large diversity of GSAT response to SLCFs among the scenarios robustly covers the possible futures, as the scenarios are internally consistent and span a range from very high to very low emissions. In the scenarios without climate change mitigation (SSP3-7.0, SSP5-8.5,) the likely range of the estimated warming due to SLCFs in 2100 relative to 2019 is 0.4°C–0.9°C. In SSP3-7.0 there is a near-linear warming due to SLCFs of 0.08°C per decade, while for SSP5-8.5 there is a more rapid warming in the first half of the century. For the scenarios considering the most stringent climate and air pollution mitigations (SSP1-1.9 and SSP1-2.6), the reduced warming from reductions in methane, ozone and HFCs partly balances the warming from reduced aerosols, and the overall SLCF effect is a likely increase in GSAT of 0.0°C–0.3°C in 2100, relative to 2019. The SSP2-4.5 scenario (with moderate climate and air pollution mitigations) results in a likely warming in 2100 due to the SLCFs of 0.2°C–0.5°C, with the largest warming from reductions in aerosols.
Potential Effects of SLCF Mitigation
Over time scales of 10 to 20 years, the global temperature response to a year’s worth of current emissions of SLCFs is at least as large as that due to a year’s worth of CO2 emissions. Sectors producing the largest SLCF-induced warming are those dominated by CH4 emissions: fossil fuel production and distribution, agriculture and waste management. On these time scales, SLCFs with cooling effects can significantly mask the CO2 warming in the case of fossil fuel combustion for energy and land transportation, or completely offset the CO2 warming and lead to an overall net cooling in the case of industry and maritime shipping (prior to the implementation of the revised fuel-sulphur limit policy for shipping in 2020). Ten years after a one-year pulse of present day aviation emissions, SLCFs induce strong, but short-lived warming contributions to the GSAT response while CO2 both gives a warming effect in the near term and dominates the long-term warming impact
The effects of the SLCFs decay rapidly over the first few decades after pulse emission. Consequently, on time scales longer than about 30 years, the net long-term global temperature effects of sectors and regions are dominated by CO2 The global mean temperature response following a climate mitigation measure that affects emissions of both short- and long-lived climate forcers depends on their atmospheric decay times, how fast and for how long the emissions are reduced, and the inertia in the climate system. For the SLCFs including methane, the rate of emissions drives the long-term global temperature effect, as opposed to CO2 for which the long-term global temperature effect is controlled by the cumulative emissions. About 30 years or more after a one-year emission pulse occurs, the sectors contributing the most to global warming are industry, fossil fuel combustion for energy and land transportation, essentially through CO2. Current emissions of SLCFs, CO2 and N2O from East Asia and North America are the largest regional contributors to additional net future warming on both short- and long-time scales.
At present, emissions from the residential and commercial sectors (fossil and biofuel use for cooking and heating) and the energy sector (fossil fuel production, distribution and combustion) contribute the most to the world population’s exposure to anthropogenic fine PM, whereas emissions from the energy and land transportation sectors contribute the most to ozone exposure. The contribution of different emission sectors to PM varies across regions, with the residential sector being the most important in South Asia and Africa, agricultural emissions dominating in Europe and North America, and industry and energy production dominating in Central and East Asia, Latin America and Middle East. Energy and industry are important PM2.5 contributors in most regions, except Africa. Source contributions to surface ozone concentrations are similar for all regions.
Assuming implementation and efficient enforcement of both the Kigali Amendment to the Montreal Protocol on Ozone Depleting Substances and current national plans limit emissions (as in SSP1-2.6), the effects of HFCs on GSAT, relative to 2019, would remain below +0. 02°C from 2050 onwards versus about +0.04–0.08°C in 2050 and +0.1–0.3°C in 2100 considering only national HFC regulations decided prior to the Kigali Amendment (as in SSP5-8.5). Further improvements in the efficiency of refrigeration and air-conditioning equipment during the transition to low-global-warming potential refrigerants would bring additional GHG reductions resulting in benefits for climate change mitigation and to a lesser extent for air quality due to reduced air pollutant emissions from power plants.
Future changes in SLCFs are expected to cause an additional warming. This warming is stable after 2040 in scenarios leading to lower global air pollution as long as methane emissions are also mitigated, but the overall warming induced by SLCF changes is higher in scenarios in which air quality continues to deteriorate (induced by growing fossil fuel use and limited air pollution control). If a strong air pollution control resulting in reductions in anthropogenic aerosols and non methane ozone precursors was considered in SSP3-7.0, it would lead to a likely additional near-term global warming of 0.08 [0.00–0.10] °C in 2040. An additional concomitant methane mitigation (consistent with SSP1’s stringent climate mitigation policy implemented in the SSP3 world) would not only alleviate this warming but would turn this into a cooling of 0.07 with a likely range of [-0.02 to 0.14] °C (compared with SSP3-7.0 in 2040). Across the SSPs, the collective reduction of CH4, ozone precursors and HFCs can make a difference of GSAT of 0.2 with a very likely range of [0.1–0.4] °C in 2040 and 0.8 with a very likely range of [0.5–1.3] °C at the end of the 21st century (comparing SSP3-7.0 and SSP1-1.9), which is substantial in the context of the Paris Agreement. Sustained methane mitigation, wherever it occurs, stands out as an option that combines near- and long-term gains on surface temperature and leads to air quality benefits by reducing surface ozone levels globally.
Rapid decarbonization strategies lead to air quality improvements but are not sufficient to achieve, in the near term, air quality guidelines set for fine PM by the World Health Organization (WHO), especially in parts of Asia and in some other highly polluted regions. Additional CH4 and BC mitigation would contribute to offsetting the additional warming associated with SO2 reductions that would accompany decarbonization. Strong air pollution control as well as strong climate change mitigation, implemented separately, lead to large reductions in exposure to air pollution by the end of the century. Implementation of air pollution controls, relying on the deployment of existing technologies, leads more rapidly to air quality benefits than climate change mitigation, which requires systemic changes. However, in both cases, significant parts of the population are projected to remain exposed to air pollution exceeding the WHO guidelines. Additional policies envisaged to attain Sustainable Development Goals (SDG) (e.g., access to clean energy, waste management) bring complementary SLCF reduction. Only strategies integrating climate, air quality, and developments goals are found to effectively achieve multiple benefits.
Implications of COVID-19 Restrictions for Emissions, Air Quality and Climate
Emissions reductions associated with COVID-19 containment led to a discernible temporary improvement of air quality in most regions, but changes to global and regional climate are undetectable above internal variability. Global anthropogenic NOx emissions decreased by a maximum of about by 35% in April 2020. There is high confidence that, with the exception of surface ozone, these emission reductions have contributed to improved air quality in most regions of the world. Global fossil CO2 emissions decreased by 7% (with a range of 5.8% to 13.0%) in 2020 relative to 2019, largely due to reduced emissions from the transportation sector. Overall, the net ERF, relative to ongoing trends, from COVID-19 restrictions was likely small and positive for 2020 (less than 0.2 W m-2 ), thus temporarily adding to the total anthropogenic climate influence, with positive forcing from aerosol changes dominating over negative forcings from CO2, NOx and contrail cirrus changes. Consistent with this small net radiative forcing, and against a large component of internal variability, Earth system model simulations show no detectable effect on global or regional surface temperature or precipitation.