6 Topics from Chapter 6: Short-Lived Climate Forcers
IPCC AR6 Chapter 6: Short-Lived Climate Forcers (SLCFs)
Emma Sullivan
Introduction
Chapter 6 of the International Panel on Climate Change Sixth Assessment Report, or IPCC AR6, focuses on emissions and abundances of SLCFs, or short-lived climate forcers (aerosols, methane, ozone, volatile organic compounds, etc.) and the ways in which they amplify or diminish global warming on various time scales. SLCFs are highly variable in both time and space due to varying lifetimes (days to years) and regional emission amounts. Their interactions with the atmosphere are complex and can either have a warming or cooling effect as well as influence precipitation and other climate factors.
It has been determined that over timescales of 10-20 years, SLCFs contribute just as much warming per year as CO2 emissions. However, effects of SLCF’s generally decay rapidly over the course of a few decades compared to CO2 which is responsible for net long term global temperature effects. It is predicted that by 2040, SLCFs could account for 0.06° C to 0.35° C of warming from present temperatures. There is little dependence on scenario because scenarios with more warming attributed to gases such as methane and ozone and have more cooling from aerosols. Futures changes to SLCF emissions are likely to cause additional warming but may become stable after 2040 as long as efforts to mitigate air pollution and methane emissions are implemented.
The following sections assess both tropospheric and stratospheric SLCF abundances and various ways of measuring (satellites, ground based measurements, remote sensing) and modeling (Chemistry Climate Models, Coupled Model Intercomparison Projects, etc) their global distribution and associated climate forcings and feedbacks. Climate forcings are best described as the energy imbalance associated with a change to a certain atmospheric variable. Climate feedbacks are then how changes in an atmospheric variable amplify or diminish an energy imbalance.
6.2 Global and regional temporal evolution of SLCF emissions
SLCFs are produced by both human activities such as agriculture, industry and recreation as well as natural process such as vegetation, fire, lighting, and volcanoes. However, changes in SLCF emissions from natural sources are largely caused by human activities and widespread global changes. Because they are sensitive to climate change, changes to SLCF concentrations can produce climate feedbacks. This means that an increase or decrease in their emissions can amplify or diminish the effects of climate change. This section focusses on historical and future changes to SLCF emissions from human caused, natural, and biomass burning sources.
Anthropogenic Sources
The IPCC AR6 report uses the updated Community Emissions Data System (CEDS) in the Coupled Model Intercomparison Project 6 (CMIP6) to assess global and regional anthropogenic, or human caused, SLCF emission trends. The updated CMIP6 results shows different trends than its predecessor CMIP5 for most SLCF emissions other than sulfur dioxide (SO2), carbon monoxide (CO), and nitrogen oxides (NOx). This is likely due to the exponential industrial growth of East and South Asia. Though the CEDS has improved, it is still estimated the CMIP6 slightly overestimates anthropogenic SO2, NOx, black carbon (BC), and organic carbon (OC) by 10-15%.
Anthropogenic SLCF emissions as reported by the CMIP6 are summarized here. SO2 emissions have decreased overall because of new air pollution regulations in places like North America and Europe, despite industrial growth in many Asian countries. NOx emissions are still increasing despite efforts to decrease emissions in North America and Europe. Non-methane Volatile Organic Compounds (NMVOCs) have increased since the 1990s despite a decrease in emissions from motor vehicles in North America and Europe. This increase is attributed to amplified oil and gas extraction in North America as well as industrialization in Asia. However, sources of NMVOCs have very different spatial and temporal distributions which causes uncertainty when measuring and modeling emissions. BC and OC emissions have doubled since the 1950s with 80% of emissions now originating from Asia and Africa but discrepancies among emission quantiy data in these regions cause large global emission uncertainties. Both methane (CH4) and ammonia (NH3) emissions both continue to grow largely due to expansions in the agricultural and livestock industry in places like Asia and the USA.
Spatial and temporal patterns of human caused SLCFs are widely variable causing uncertainty in models. Until the 1950s NA and Europe were the largest contributors until legislation pushed for reductions to improve air quality. Since the 1990s there has been a large redistribution of SLCF emissions, now most emissions originate from Asia as there is continual and rapid economic growth (now accounts for 50% of emissions of each SLCF).
Emissions by Natural Systems
Natural SLCF sources include volcanoes, lightning, vegetation, fires, etc. However, many of these natural emission sources have been amplified by human activities such as deforestation and agriculture. The spatial and temporal distribution of natural SLCF sources are highly variable and can only be modeled. Additionally, the natural processes of some SLCFs are not well understood and add to the uncertainty of the effects of natural SLCF emissions.
Nitrogen Oxide
10% of NOx emissions globally originate from lightning. Most lightning NOx is released in the upper atmosphere and can have large effects on O3, OH and CH4 lifetimes. Models show that climate change may affect lightning NOx emissions but are unsure about whether it is a positive or negative effect. Soil NOx is created through nitrification and denitrification of biogenic and microbial processes. These soil forming processes are sensitive to temperature, precipitation, and nutrient content and are thus affected by climate change in complex ways.
Land emissions of dust particles
Emissions of dust particles into the atmosphere result from both natural and human-driven processes. On the natural side, emissions are controlled by soil properties, vegetation, and near surface winds. Humans can affect dust emissions through land-use changes, agriculture, roads, and hydrologic changes for irrigation. The human contribution to global dust emissions is uncertain, though models show that dust emissions have increased ~25% between the late 1800s to present due to agriculture, land use, and climate change. There is high confidence that dust emissions are sensitive to climate change, however the estimates of how much dust emissions will change is uncertain.
Open biomass burning emissions
The contributions of biomass burning (forest, grassland, peat fires, and agricultural waste burning) to global emissions of SLCFs are ~30% of CO, ~10% of NOx, ~15% BC, and ~10% of OC. Wildfires are also large contributors in this area and affect atmospheric chemistry and regional air quality. Changes to climate, especially changes in temperature and precipitation will increase the risk of fires, and thus increase biomass burning emissions. However, projections of future emissions vary greatly due to different land-use and land management scenarios.
6.3 Evolution of Atmospheric SLCF abundances
In addition to emissions, atmospheric chemistry, deposition, and transport processes all play a major role in determining the atmospheric distribution, budget and lifetime of SLCFs. This distribution and lifetime are also influenced by the modification of chemical and physical processes in response to climate change. This section assesses the changes of abundances of atmospheric SLCFs based on observations and modeling, data collections, and overall understanding of long-term trends.
Methane (CH4)
The global mean surface mixing ratio of methane has increased by approximately 156% since 1750 which is primarily driven by anthropogenic activities. The surface mixing ratio is the fraction of methane in air at the surface. The atmospheric lifetime of methane varies depending on the type of sink but considering the range of individual lifetimes, methane was assessed to be 9.25 ± 0.6 years with its primary sink being tropospheric OH.
Tropospheric Ozone (O3)
Ozone, or O3 is a molecule that is beneficial in the stratosphere in the form of the ozone layer which aids in blocking harmful ultra-violet (UVC and UVB) rays from the sun, but can be harmful to human health in the troposphere and acts as a potent greenhouse gas. Approximately 10% of atmospheric ozone exists in the troposphere with concentrations ranging from 10 ppb near the surface, 100 ppb in the upper troposphere, and much more than 100 ppb downwind of ozone emission regions. The lifetime of ozone can range from a few hours to a few months depending on location, and its climate forcing is related to its vertical and regional distribution. Models have consistently been able to simulate present-day distribution and seasonality of ozone concentrations, however there is a slight bias in the models that overestimates seasonal concentrations in the Northern Hemisphere and underestimates it in the Southern Hemisphere. The CMIP6 model approximates the global average lifetime of tropospheric ozone to be 25.5 ± 2.2 days with high confidence.
Because distribution of tropospheric ozone is not homogeneous, it is difficult to estimate preindustrial abundances. However, chemistry climate models (CCMs) complemented by modern observations help to predict long term changes in tropospheric ozone burden. Generally, it is agreed that the tropospheric ozone burden has increased by less than 40% due to human activities.
Stratospheric Ozone (O3)
90% of atmospheric ozone resides in the stratosphere and has lifetimes ranging from less than a day in the upper stratosphere to several months in the lower stratosphere. The concentration of stratospheric ozone is described as the total column ozone (TCO) and is reported in Dobson units (DU). Dobson units correspond to how many .01 mm thick layers of ozone there would be in a given area if all of the ozone was layered together. The CMIP6 model ensemble shows that global TCO changed slightly between 1850-1960, but then had a rapid decrease in the 1970s-1990s due to halogenated ozone-depleting substances (ODSs). Slight increase in TCO following the 1990’s was due to the implementation of the Montreal Protocol to decrease halogenated ODS emissions. Overall, CMIP6 climate models accurately characterize the seasonal and climatological variations of global stratospheric ozone, but disagreements in individual models cause uncertainties of up to 60 DU for pre-industrial TCO values. Overall, there has been a net decrease in stratospheric ozone column of 14.3 ± 8.7 DU since the 1850’s, though there is slow recovery.
Nitrogen Oxides (NOx)
Improved satellite data reveals that the distribution of atmospheric NOx is widely variable across the globe but tends to be concentrated over large urban and industrial areas. Additionally, the lifetime of NOx in the atmosphere is short due to the formation of various nitrates as well as atmospheric transport and deposition. The wide varieties of NOx distribution in both space and time make it difficult to constrain the magnitude and potential climate forcings related to historical and future emissions.
There is high confidence that global tropospheric NOx has increased from 1850- present due to evidence from ice cores and satellites. NO2 concentrations have decreased in places such as the US, western Europe since the 1990’s followed by China in 2012 but continues to increase in South Asia since 2005.
Carbon Monoxide (CO)
The primary sink of CO is OH. Advances in techniques for observing and measuring OH have resulted in better characterization of modern-day global CO distributions. Overall understanding of modern global CO distribution has increased and implies that the CO burden has been decreasing since 2000 through reductions in human caused CO emissions.
Non-methane volatile organic compounds (NMVOCs)
The term NMVOC encompasses thousands of compounds that have widely varying lifetimes and concentrations. These compounds are generally produced by biogenic sources, but human activities perpetuate long-term trends in abundances. Though there was a decrease in emissions between 1980 and 2008, there has since been a slight increase due to oil and gas extraction in North America and industrialization in East Asia.
Ammonia (NH3)
Satellite and ground observations have aided in the understanding of the spatial distribution of ammonia as well as seasonal and annual trends. Generally, the largest ammonia concentrations are focused in areas with extensive livestock and agricultural industries as well as biomass burning regions, however many of these ammonia hotspots are missing emissions data or greatly underestimate emissions. Because of this there is a large range in modern ammonia burden estimated by chemistry climate models (CCMs).
Ammonia concentrations have been increasing over the last few decades in regions such as Western Europe, China, and the USA which is likely due to the expansion of livestock and agricultural industries as well as the decrease of SO2 and NOx emissions which chemically react with ammonia in the atmosphere. CCMs predict that the ammonia burden has increased by a factor of 2 to 7 since pre-industrial times.
Sulphur Dioxide (SO2)
Both surface and satellite observations of SO2 show that there are strong regional trends of SO2 emissions. Generally, SO2 emissions increased in Western Europe and North America until the 1980 and in Asia until 2005 but have since been decreasing at various rates globally.
Short-lived Halogenated Species
Halogenated species include synthetic chlorofluorocarbons (CFCs), halons, hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and others. While many are long-lived, some have lifetimes less than 6 months, which are considered short-lived. These gases affect chlorine and bromine abundances in the stratosphere, and thus contribute to ozone depletion. Between 2011-2019 atmospheric chlorine from hydrochlorofluorocarbons has continued to increase and bromine from hydrofluorocarbons has decreased, though the emission rates continue to increase and decrease respectively. These trends are due to the transition away from ozone-depleting substances (ODSs).
Aerosols
Remote sensing instruments provide a more robust view of aerosol trends and concentrations than ground-based monitoring. Remote sensing methods collect information about the aerosol optical depth (AOD) which is related to how aerosol particles interact with incoming solar radiation (or insolation), which helps in understanding the radiative forcing associated with aerosol concentration and distribution.
CMIP6 models display a positive trend in global mean aerosol optical depth since 1850 with a sharp increase in the 1950s. These increases are associated with the increase in human SO2 emissions. However, in order to properly assess the climate effect of various aerosols, the various aspects of their distribution, optical properties, and chemical composition need to be further investigated.
Sulphate (SO42-)
Global tropospheric sulphate burden has increased from 1850 to 2005. Between 1980 to 2015 sulphate emissions have decreased by 47% in North America and 40% in Europe. Though there was a sharp increase in emissions in Asia until 2005, emissions have since decreased in China, and steadily increased in India.
Ammonium (NH4+) and Nitrate (NO3–) Aerosols
Ammonium sulphate and ammonium nitrate aerosols form in the atmosphere through various chemical reactions. NH4+ and NO3– burdens have increased since the Industrial Revolution though there is much uncertainty as to the magnitude of increase. Additionally, it is challenging to model these emissions due to uncertainties of aerosol pH and other factors.
Carbonaceous Aerosols
Black carbon (BC) and organic aerosols (OA) compose carbonaceous aerosols and absorb solar radiation. Knowledge of optical properties, as well as mixing, coating and ageing, are all essential to assess the climate effect of these aerosols. However, lack of global scale observations, the complex atmospheric chemistry and variations of global budget burdens mean that we can not characterize global trends in carbonaceous aerosols.
Implications of SLCF Abundances for Atmospheric Oxidizing Capacity
Atmospheric oxidizing capacity is determined by the availability of tropospheric OH, which is the main sink of many SLCFs. However, OH abundances are sensitive to both changes in SLCFs (with decreases due to methane, CO, and NMVOCs, and increases due to NOx and ozone) and climate (with increases due to temperature and water vapor), and have an extremely short lifetime of about 1 second, so it is extremely difficult to characterize. There is much uncertainty, but it is generally agreed that OH abundances have increased or remained stable in recent decades due to a decrease in CO emissions and an increase in NOx emissions.
6.4 Short-lived Climate Forcer (SLCF) Radiative Forcing and Climate Effects
Emma Heitmann
Recall that short-lived climate forcers (SLCFs) are gases or aerosols (microscopic particles suspended in the atmosphere) that affect climate but whose lifetime in the atmosphere is relatively short (from a few hours to a few months to a few years). SLCFs can either have a cooling or warming effect on the climate, which in turn can affect other aspects of the climate (e.g., rainfall). Short-lived climate forcers (SLCFs) differ from long-lived greenhouse gases (LLGHGs) not only in timescale, but also in the ways that SLCFs can affect the Earth’s energy balance and climate, because forcings are generally more challenging to constrain, and because distribution across the globe can be spatially uneven and variable over time.
Aerosols fall in the category of SLCFs, and have been the most difficult to constrain. However, this IPCC report shows that global climate models have improved somewhat in predicting how aerosols and clouds interact. Remember that aerosols are microscopic particulate matter that are suspended in the atmosphere. Aerosols can include dust, sea salt, sulfur dioxide (from coal burning or volcanic eruptions), soot (from industry), and other materials. Some aerosols have natural sources, but increasing aerosol concentrations in the atmosphere since the Industrial Revolution are driven by human activities (Figure 6.11). Aerosols can cause a cooling or warming effect on the climate. Cooling aerosols (e.g., sulfates) reflect or scatter radiation (e.g., incoming solar radiation) back to space, and thus have a net negative radiative forcing and cause a cooling effect on Earth’s climate. In contrast, warming aerosols (e.g., black carbon (BC)) can absorb radiation, and thus have a net positive radiative forcing and cause a warming effect on the climate. Aerosols may also help cloud formation, by providing a solid surface for water vapor to condensate. This can result in either a positive or negative feedback of aerosols, because depending on the cloud type, clouds can either have a net negative or positive radiative effects. This is part of the reason why constraining the climate effects of aerosols is so challenging for scientists.
6.4.1 Historical Estimates of Regional Short-lived Climate Forcing
Aerosol radiative forcing is variable across both space and time historically (1850-2014; Figure 6.11). Globally, there was a shift from increasing to decreasing net negative radiative forcing of aerosol emissions driven by trends in industry and policy, but the timing of this shift varies by region and has not occurred in some places. The global mean radiative forcing of aerosols reached maximum negative radiative forcing values in the mid-1970s, but then gradually decreased thereafter with major decreases in the 1990s. In other words, the global cooling effect of aerosols peaked in the mid-1970s, and then started to decrease, particularly rapidly in the 1990s. The reduction in aerosol negative radiative forcing is attributed to reductions in sulfur dioxide (SO2) emissions following the Clean Air Acts, combined with an increase in black carbon emissions (from fossil/biofuels) which has a positive radiative forcing.
6.4.2 Emission-based Radiative Forcing and Effect on Global Temperature
Additional reasons why the radiative forcing and thus effect on global temperature of SLCFs are difficult to constrain (Figure 6.12) are that a change in emissions of one SLCF can have non-linear effects in both subsequent chemical reactions in the atmosphere affecting the concentrations of other climate forcers, as well as in aerosol-cloud interactions. In other words, it is difficult to define a radiative forcing value for an SLCF because of variable and difficult to predict feedback effects, which can depend on other parameters in the model (e.g., initial concentrations of other CFs).
Figure 6.12a (subject to final editing) from IPCC AR6 WG1 Report.
For example, the net radiative forcing of N2O (nitrous oxide, a long-lived climate forcer) emissions is estimated to be 0.24 (+0.10/-0.11) W/m2. However, there are multiple indirect effects of N2O which lead to both negative and positive radiative forcings. N2O can impact ozone distribution (thus a positive radiative forcing) but can also decrease the methane lifetime in the atmosphere (resulting in a negative radiative forcing). Another example is black carbon, whose net radiative forcing estimate ranges from negative to positive (-0.28 to 0.42 W/m2). Black carbon creates a positive radiative forcing from aerosol interactions and a negative radiative forcing from its indirect impacts on clouds and atmospheric water vapor.
The magnitude of radiative forcing of various SLCFs generally is proportional to its contribution to mean global near-surface air temperature. However, the full temperature response to a forcing is somewhat delayed due to the inertia of the climate system. This implies that the cooling effects of SO2 emissions are felt more (proportional to their radiative forcing) than other forcers, because SO2 emissions are decreasing. This also implies that the warming effects due to the present amount of CO2 in the atmosphere have yet to be felt.
6.4.3 Climate responses to SLCFs
While there remains a lot of uncertainty about the effects of aerosols on regional and global climate, it is certain that both warming and cooling aerosols (e.g., black carbon and sulfate, respectively) can have effects on shortwave and longwave radiation, lapse rates of the troposphere, and cloud formation. These effects can result in climatic responses in both temperature and precipitation. Consistent with past IPCC reports, AR6 states that it is likely that aerosols have caused a global decrease in global near-surface air temperatures since the Industrial Revolution (late 1800s) (Figure 6.13). This implies that sulfate aerosols are the dominant driver of near-surface air temperature compared to black carbon, and that the net negative radiative forcing and cooling effects of aerosols has partly mitigated warming from anthropogenic emissions of GHGs.
Recall that there are some aerosols that warm the atmosphere by absorbing radiation (e.g., black carbon), and some aerosols that cool the atmosphere by reflecting or scattering radiation (e.g., sulfates). These aerosols not only have opposing effects on temperature, but they can have alternate effects on atmospheric circulation, clouds, and precipitation. This is evidenced by regional variations in SLCF emissions and thus variations in respective regional climatic responses. For example, AR6 reports that aerosol-driven global cooling has led to large-scale water cycle changes since the mid-1900s in the way of reduced global precipitation and by altering large-scale atmospheric circulation patterns which can impact where precipitation occurs.
The asymmetry of aerosol radiative forcing due to higher emissions of aerosols in the Northern Hemisphere has led to a range of regional climate impacts. This asymmetry has led to dampened warming of the northern hemisphere, which has led to a shift of the tropical rain belt southward (also known as the intertropical convergence zone (ITCZ)), and has even been shown to have contributed to the infamous Sahel drought from the 1970s-80s. Another example of regional impacts of aerosols on climate is in the Arctic. The Arctic is the fastest warming region in the world, partially due the contributions of aerosol radiative forcing both locally and at the Northern Hemisphere midlatitudes. Sulfate and black carbon can be transported to the Arctic from lower latitudes which can cause warming by radiative forcing and cloud interactions. Additionally, black carbon can be deposited on snow and cause an increase in snow melt because it darkens the snow color and thus decreases its albedo and leads to radiation absorption and thus heating of the snow.
6.4.4 Indirect Radiative Forcing through Effects of SLCFs on the Carbon Cycle
SLCFs can affect vegetation and the carbon cycle via deposition and effects on radiation. However, it remains uncertain what the magnitude of these effects are on land carbon sinks, ecosystem productivity and indirect CO2 forcing. For example, nitrogen is an important nutrient for plants. So, in places where there is limited nitrogen (e.g., forests, ocean), the deposition of reactive nitrogen from the atmosphere can increase plant growth and thus carbon sequestration, but can also cause eutrophication and a decrease in biodiversity.
6.4.5 Non-CO2 Biogeochemical Feedbacks
Feedbacks are responses to climate perturbations that either amplify (positive feedback) or diminish (negative feedback) the initial perturbation. There is a need to quantify the many biogeochemical feedbacks due to changes in the carbon-cycle to estimate effects of SLCF emissions, but it’s difficult to do so because some SLCFs can either warm or cool the climate. For example, wildfire frequency and intensity could increase under a warmer climate. Wildfires are an important source of SLCFs, and thus are an important feedback mechanism. Overall, AR6 estimates that the net feedback for SLCFs is negative.
6.4.5 ERF by Aerosols in Proposed Solar Radiation Modification
Solar Radiation Modification (SRM) is a form of geoengineering, in which humans intentionally alter global climate to mitigate warming from GHGs. One method is by injecting cooling aerosols into the stratosphere, which has potential to have a high net negative global radiative forcing. Another proposed solution is marine cloud brightening, which is achieved by releasing aerosols (e.g., sea salt) to the atmosphere that promotes cloud formation to reflect radiation.
6.5 Implications of Changing Climate on Air Quality
Future global warming can affect air pollutants by changes in atmospheric circulation, in chemical reaction rates, and in precipitation. For example, precipitation is a primary method of aerosol removal from the atmosphere and is expected to increase in many regions under future warming. Another impact from a warmer climate with more water vapor in the atmosphere is the increased potential to destroy ozone in unpolluted areas, but warmer temperatures can also lead to more ozone in polluted areas. Additionally, climate-change-driven events such as heatwaves and stagnations could help increase extreme air pollution episodes (i.e., when the concentration of an air pollutant that is above a given threshold value) in areas with a lot of pollution.
6.6 Air Quality and Climate Response to SLCF Mitigation
There are multiple motivations for SLCF removal (e.g., climate change, public health) and thus appears within climate policies, air pollution control, and the UN Sustainable Development Goals. While the management of LLGHGs is important for mitigating long term warming, the management of many SLCFs is important for curbing short-term climate change (e.g., black carbon, CH4, tropospheric O3, and HFCs). To mitigate SLCF and LLGHG emissions, policies often target these sectors: industry, energy production, agriculture, waste management, and residential fuel use. Depending on which sector is limited and by how much will affect the policy’s impact on SLCFs. Thus it’s important to understand the implications of SLCF management on emissions and climate when designing policies. This section evaluates the effects of mitigating SLCFs on temperature response time, the effect of economic sectors on temperature and air quality, and the effects of different mitigation strategies.
6.6.1 Implications of Lifetime on Temperature Response Time Horizon
The impacts of mitigating SLCF emissions on global near-surface temperature over time depends on the lifetimes of the SLCFs in the atmosphere, their radiative forcing, how quickly emissions are reduced, how long reductions last, and the inertia of the climate system. For SLCFs with short lifetimes (days to 1-2 years), the global surface temperature response is quick and strong at first as soon as a permanent change in emissions occurs. Once the response is near its max, the response slows down and it can take centuries for the climate system to go back to equilibrium. For SLCFs with longer lifetimes (decade or longer) there is a delayed temperature response in the reduction of SLCFs proportional to its lifetime (Figure 6.15). In addition, the inertia of the climate system often delays the short-term and long-term responses to perturbations.
6.6.2 Attribution of Temperature and Air Pollution Changes to Emission Sectors and Regions
This section evaluates studies on what the temperature and air quality response would be if you were to limit specific sectors that emit SLCFs, which is important for developing mitigation strategies. There are many ways to evaluate the effects of emissions reductions, one of which is to model what the climate would be like if emissions occurred for one year and then abruptly stopped. This strategy helps to isolate specific SLCFs, sectors, and climate variables to learn more about the climate system, polluters, and pollutants (Figure 6.16).
From these studies, the largest contributors to warming on a long (50-100 year) time scale are the energy, industrial, and on-road transportation sectors. The largest contributors to warming over a short (20 year) timescale are agriculture (CH4), waste management (CH4), and residential biofuel (black carbon). These sectors were evaluated by modelling the effects on climate following a one year pulse of emissions. For agriculture, the results show that it is the second largest contributor to warming on short timescales due to CH4 emissions. Residential and commercial cooking and heating emits a mix of warming and cooling SLCFs (e.g., aerosols, CO, SO2, NOx, CH4), but the net climate impact is warming in the short term which decreases but persists long term. Aviation also emits a range of SLCFs with complex feedbacks, but causes a net warming on short timescales (10 years). The largest impacts are condensation trails (contrails) and NOx emissions. Contrails are ice crystal clouds, formed around aircraft aerosols, and can spread to form cirrus clouds and have a positive radiative forcing. Shipping caused a net global cooling on 10-20 year timescales due to the high volume of cooling aerosol emissions (including sulfates). Land-based transport emissions caused net global warming on all timescales and negatively impacted air quality.
To summarize, the sectors that contribute the most to global warming on short timescales are the CH4 dominated sources (e.g., energy production, waste management, agriculture). Other net warming sectors include residential fuel and aviation. The sectors that contribute the most to the net cooling effect on short timescales are industry and shipping because of aerosol emissions, despite industry being one of the biggest contributors to long-term warming effects due to CO2 emissions. The sector that contributes the most to air pollution in the form of particulate matter globally is residential and commercial cooking and heating, except in the Middle-East and in Asia. Agriculture is the dominant source of particulate matter in North America and Europe. Energy and industry are important air pollution contributors too, except in Africa. The energy and land transport sectors are also major sources of the ozone pollutant globally.
6.6.3 Past and Present SLCF Reduction Policies and Future Mitigation Opportunities
The effects of various past and present policies and strategies have been evaluated with climate models, and this section evaluates these results of SLCF mitigation strategies and its effects regionally and globally. Recall that air quality policies emerged in the 1970s, driven by concerns over ecosystem damage and health impacts. Studies show that air pollution policies in Europe established in the 1970s has led to the decrease in aerosols and as a result has led to a small warming effect and increase in precipitation over Europe. There is consensus in the literature that SLCF mitigation is central to mitigation efforts of climate change, air quality, and other SDGs. However, there is less agreement on how to implement mitigation and the associated climate impacts.
6.7 Future Projections of Atmospheric Compositions and Climate Response in SSP Scenarios
Monica Hill
The primary focus of Chapter 6 so far has been centered on the evolution of different short lived climate forcers (SLCFs) at scales ranging from global to regional (6.2-6.3); the historical and current abundances of SLCFs in the atmosphere; and at the overall effects of these types of climate forcers on Earth’s energy balance. Sections 6.7-6.8 examine the specific and overall contribution of SLCFs to climate change based on different potential social and political scenarios, referred to as Shared Socio-economic Pathways (SSPs) in the IPCC Report.
Descriptions of each of the Shared Socio-economic Pathways (SSPs) are covered in detail in Chapter 1 of the IPCC Report, but for the sake of the projection comparisons presented in Section 6.7, it is critical to understand the assumptions between the different scenarios. The SSPs are based on results from models that take societal factors like economic growth, population growth and movement patterns, rate of technological advancement, education, regional policy-making, etc. into account. The SSPs do not directly represent climate trends or projections but serve to provide broad-scale perspectives on how social and political choices, in the absence of any new, overarching climate policy, can affect emissions and climate change.
The IPCC Report described five different SSPs, referred to as SSP1-SSP5. SSP1 is the most progressive scenario, with an emphasis on sustainable growth and global economic/environmental equality. SSP2 represents the “middle of the road” scenario in which global socioeconomic trends continue to follow current patterns. Development and economic growth increase in some regions more than others, and international development of climate and sustainability goals continues, but not quickly. SSP3 (the “regional rivalry” pathway) presents a challenging scenario where competition between nations gives way to nationalism and security concerns, and international development and coordination declines through the 21st century. SSP4 represents a future of inequality, where gaps in regional socioeconomic status continue to grow; development and technological advances rapidly increase in developed nations while low-income regions do not. SSP5 is centered around a future society that prioritizes sustainable development and technological growth yet continues to be primarily driven by fossil-fuels. Each of these pathways is a plausible future society, and each presents unique challenges to climate goals.
Each of these storylines can be thought of as a baseline for the degree to which society in the future could mitigate greenhouse gas emissions or adapt to a changing climate. In the report, each SSP is also paired with a modelled Representative Concentration Pathway (RCP) scenario that describes the radiative forcing of greenhouse gas emissions, or the amount of energy from the sun trapped by the Earth’s atmosphere. For example, SSP3-1.9 would describe the SSP3 pathway with a radiative forcing of 1.9 W/m2 by 2100. SSPs are also important in the creation of Integrated Assessment Models (IAMs), which determine possible future scenarios for specific SLCF emissions from energy, land use, air pollution, etc. based on the framework of each SSP.
Projections of Emissions and Atmospheric Abundances
Short lived climate forcers are pollutants that remain in the atmosphere for a relatively shorter period compared to other greenhouse gases like CO2. The SLCFs of primary concern include methane, ozone, aerosols, black carbon, hydrofluorocarbons (HFCs) and sulphur dioxide. On average, the direct effects of SLCFs are felt by the atmosphere from within days to up to two decades after emission, depending on the specific pollutant. SLCFs have also been found to have longer term effects on climate if they cause shifts in other climate cycles (water cycle, biogeochemical, etc.). Because of the short life-span of the pollutants, societal or policy-driven changes over the rest of the century could result in high variability of regional abundances and future climate effects by 2100. Nearly all the SSPs except for SSP3-7.0 (high emission, ‘rivalry’ scenario) predict that all SLCF emissions will decrease from 2019 levels by 2100 except for HFCs and ammonia.
All SSPs agree that sulfur dioxide (SO2) emissions will be reduced in the short and long term. Currently, SO2 emissions primarily come from fossil fuel combustion for electricity/heat production, industry, and transportation, as well as from volcanic activity. Sulfate aerosols have a negative radiative forcing, or cooling effect on climate. The most significant reductions in SO2 emissions are expected to occur because of more strict policies for power plants in China and India, reductions in global coal use, and reductions to the amount of sulfur in fuels used for shipping.
Nitrous oxide (NOx) emissions projections by the end of the century are more variable under the SSPs due to differences in current regional emissions trends. NOx represents an important group of pollutants including nitric oxide (NO) and nitrogen dioxide (NO2) that are produced primarily from fossil fuel combustion and industrial processes. NOx emissions can have immediate effects on regional air quality when they react with volatile organic compounds (VOCs) in sunlight to create surface ozone. Overall NOx emissions are not predicted to decline globally by 2040; a decline is predicted to continue for OECD countries and East Asia, while trends are expected to stay like present or increase for the rest of the world. In SSP 1, NOx emissions will be reduced by 70% by 2100, which would be comparable to the 1950s level. Emissions from Africa are projected to decrease the least in all SSPs because of the large population growth and burning of biomass for heat, cooking and agriculture.
Methane (CH4) is a potent positive climate forcer that comes from anthropogenic sources (often fossil fuel extraction), microbial processes (ruminant digestion, landfills, standing water/rice paddies), and abiotic sources (wildfires, thawing permafrost). The lifespan of CH4 in the atmosphere is around 12.4 years, but, on a 100 year time scale, has a global warming potential 32 times that of CO2. The IPCC report finds that methane emissions projections are very similar to the RCP scenarios used in AR5, where emissions are expected to increase until about 2040 before eventually decreasing by 2100. However, there are some regional differences in emissions brought forward under the SSPs. In East Asia, predictions for the highest RCP are nearly double the highest SSP by 2100. This discrepancy is due to current and future efforts to mitigate poor air quality, especially in China. Concerningly, in scenarios that do not involve large-scale emissions mitigation, the rapid rise of developing economies like some in Africa, the Middle East and Latin America will result in a rapid rise in CH4 emissions from agriculture, power generation, and waste management. The report remarks that for methane emissions to be significantly reduced worldwide, especially from agriculture/livestock, there will not just need to be changes in policy, but changes within behavioral and institutional structures.
Black carbon is a form of particulate matter composed of pure aerosolized carbon and is produced by incomplete combustion of fossil fuels or biological matter. The largest current sources include burning (of forests or agriculture), open-flame cooking, diesel engines, and industrial/electricity production processes. As of 2019, about half of global anthropogenic black carbon comes from burning solid fuel for heat or cooking, which is most common in parts of Asia and Africa where access to alternatives may not be available or easily accessed. BC emissions have a short atmospheric lifespan of several days to weeks but may have longer effects on climate if it is deposited on snow or glaciers, which would reduce the albedo of that surface for longer periods of time. Projections for black carbon (BC) also decline under all SSPs except SSP3-7.0. In SSP1-1.9 and SSP1-2.6, access to clean energy across most of the world, reductions in automotive emissions, and decarbonization of the energy industry result in global reductions of BC of 70-75% by 2040 and 80% by 2100. Around 30% of the emissions by 2100 are from open waste burning, which may be easily reduced with more targeted waste management policies. In SSP3-7.0, a pessimistic scenario, anthropogenic BC emissions are slow to decline due to regional inequality that makes reducing emissions from diesel engines, waste burning, and open flame cooking more difficult in Asia, Africa, and parts of Latin America.
Hydrofluorocarbons (HFCs) are a group of man-made chemicals that are often used as substitutes for ozone-depleting refrigerants like hydrochlorofluorocarbons (HCFCs) that are being pushed out under the Montreal Protocol. These substances have variable lifespans in the atmosphere from months to decades. They have very high global warming potentials compared to CO2, so although the total emissions are much lower than other greenhouse gases, they result in a substantial amount of warming. The 2016 Kigali Amendment to the Montreal Protocol will begin to phase out HFC emissions, which is accounted for in SSP1, SSP2, and SSP4. In scenarios that do not include mitigation policies, the global HFC emissions are predicted to be 4-7.2 Gt CO2-equivalent per year, while those that do follow under the Kigali Amendment will have total emissions of 0.1-0.35 Gt CO2-equivalent per year by 2100. There is some concern that the reduction of HCFC use in non-OECD countries will result in higher demand for HFCs to supply an increasing demand for refrigerants and industrial materials as populations grow.
The modeled SSP scenarios, as well as the trajectories of short lived climate forcer emissions contain some amount of uncertainty. For many SLCFs, it is difficult to accurately determine the amount of emissions that are from anthropogenic sources vs. natural sources, or how those may change in the future as the climate warms. For example, future increases in natural methane emissions from melting permafrost and ocean chemistry/temperature changes will offset reductions in anthropogenic emissions, but it is difficult to model. The methane evolution from natural sources is not included in the model.
Several SLCFs, like NOx, have direct impacts on air quality and are also precursors for ozone. Surface ozone is produced as the result of several different SLCF emissions and processes, which leads to a high degree of variability in the amount of surface ozone between and within SSPs. Additionally, the SSPs are based on recently enacted policies, and the projections do not take future potential legislation into account. Therefore, the SSPs are not representative of the entire climate mitigation potential. Future policies targeting air quality improvement or other emissions reductions are possible and could cause more rapid improvements in mitigation potentials.
Surface (or tropospheric) ozone is a SLCF that contributes both air pollution and climate warming as a greenhouse gas. It is not emitted directly but is the result of reactions between other precursor gases (like methane, nitrogen oxides, and volatile organic compounds) and sunlight. Surface ozone has an atmospheric lifespan of only several hours to weeks, so air quality effects often are highly localized. Emissions of surface ozone are disproportionately high in the Northern Hemisphere due to increased levels of precursor pollutant emissions. In SSPs 3 and 5, surface ozone increases across most of the world, especially in developing countries, until 2050, after which, moderate decreases are expected. In SSPs where many precursor emissions are reduced but methane emissions remain high, surface ozone is expected to continue to increase. In the SSPs that involve climate mitigation (SSP1, 2, and 4), surface ozone is expected to reduce by 2100. The IPCC report finds that by taking strong measures to reduce ozone precursor emissions other than methane, the average surface ozone content will decrease by 15% worldwide from 2015 to 2055. When methane is also reduced at the same time, the ozone decrease is nearly twice as large, highlighting the importance of reducing methane emissions, especially in developing regions.
Particulate matter (PM) is another major source of air pollution. PM consists of fine particles (solid and liquid) suspended in the air and can include smoke, dust, soot, and ash; PM may also be created from chemical reactions in the atmosphere. PM2.5 specifically refers to fine inhalable particles that have a diameter of 2.5 micrometers or less. Particulate matter is of serious concern for human health. Black carbon and sulfate aerosols are forms of PM2.5. Decreases in PM2.5 at the surface are expected for all SSPs other than SSP3. The decreases are projected to be highest in regions that had the greatest PM2.5 emissions in the beginning of the 21st century (Middle East, South and East Asia) because of increased efforts to combat air pollution. In regions where there are high rates of naturally occurring PM2.5 from dust, the models showed variability. Like surface ozone, a reduction in precursor emissions, not including methane, could reduce the levels of PM2.5 by 25% by 2100.
Effects of SLCFs on ERF and climate response
This section focuses on how the SLCF emissions for different SSPs discussed in section 6.7.1 affect overall climate response. The models are mainly focused on the global surface air temperature (GSAT), effective radiative forcing (ERF), and precipitation changes. To determine the impact on GSAT, the ERF of each SLCF was used in complex models. The models for surface air temperature show the amount of global warming or cooling that is a result of each of the SSPs. Because the lifespan of some SLCFs is in the range of 10s of years, some of the changes in air temperature are due to emissions that happened before 2019.
Regardless of the SSP or total forcing, it is very likely that there will be an increase is GSAT due to SLCFs from 2019-2040. In regions where there is high climate mitigation, the modelled warming trend is most pronounced due to reductions in aerosols. Recall that aerosols are negative climate forcers, meaning that they contribute to cooling effects by reflecting insolation. When aerosols are rapidly reduced, warming occurs. In SSP3, there is overall warming is also predicted, however aerosol emissions are not reduced. In this scenario, the warming is the result of increased methane and ozone. Reducing aerosols can have similar warming effects as increasing other SLCF emissions. SSP5-8.5 has the highest predicted GSAT due to inputs from methane, ozone, HFCs and aerosol reductions. Until 2040, warming is likely regardless of mitigation scenario.
After 2040, the effects of reduced positive and negative forcers begin to level out and become more variable between the scenarios. In SSP1, the warming reaches a maximum around 2040, after which the reduction in ozone and methane dominates and warming decreases. In SSP3-7.0, warming will increase linearly from 2040 to 2100 due to steady increases in emissions, whereas in SSP5-8.5 there is a more rapid early peak in GSAT followed by a gradual reduction.
Regionally, the effects of SLCFs on GSAT vary within the SSPs. Unsurprisingly, North America, East Asia and Europe account for most SLCF emissions. In SSP2-4.5, SLCFs continue to increase in South Asia, but decrease elsewhere. In SSP3-7.0, increased emissions are expected for all regions, especially developing Africa, the highest net warming is in North America due to potentially increased methane and decreased aerosols. Africa is projected to have the highest increase by 2100 due to rapid increases in population and development.
Effect of SLCFs mitigation in SSP scenarios
The amount of potential mitigation of different SLCFs in the SSPs depends largely on the policies that are enacted to limit them. This section assesses how targeted policies that would be plausible in each SSP may impact the total warming over the 21st century. These strategies include efforts to improve air quality in both high continued emissions SSPs as well as high mitigation SSPs. For these models, 2019 is used as the reference year for recent emissions.
The amount of warming in each SSP is a product of the total SLCF emissions as well and the reduction of negative forcers. The greatest warming occurs in SSP5-8.5 where there is no climate change mitigation, but strong regional air quality control. In this scenario, warming is caused by ozone, HFCs, and methane, and reductions in aerosols. In general, the reduction of SO2 and aerosols in low emissions SSPs (SSP1, 2, 4) will result in warming. Although the peak rate of warming is highest in SSP1, where mitigation is highest, the derease in warming will be the greatest as SLCF emissions continue to decrease. In scenarios that implement the Kigali Amendment and reduce short lived HFCs, warming induced by them is negligible (0.02°C) by 2100. In scenarios that do not take steps to reduce HFCs, the warming may be closer to 0.1-0.3°C.
There is strong evidence that air quality will be improved with reductions in non-methane SLCFs, however, short-term warming in the range of 0.1-0.3°C (SSP3) would also result. Reducing methane emissions would have a net cooling effect that has a slower response time than other SLCFs. Methane mitigation could possibly offset the warming due to reduction of other SLFCs, especially in the latter half of the century, however, implementing strict methane reductions is more difficult than non-methane SLCFs due to the deep institutional and structural ties to processes that emit methane. In summary, the warming after around 2040 in scenarios that include emissions mitigation will be stable or reduce, while in scenarios that do not involve climate mitigation, warming will continue to increase over the 21st century.
Surface ozone and PM 2.5 were looked at in order to compare the difference in overall effect on air quality due to targeted air quality policies vs. climate mitigation policy. As ozone and PM2.5 are the primary causes of air pollution, their levels were assessed for each of the SSPs that include different mitigation strategies. The greatest reduction in air pollution by 2100 occurred when both air quality policies and climate mitigation were implemented (SSP1). Air pollutants will experience much more rapid reduction when air quality policy is implemented than climate policy. Air quality policy can be tailored to regional needs and emissions, and often does not require large-scale systemic changes, like stringent climate policy would. Air quality concerns are often concentrated over a handful of regions in Africa, the Middle East, East and South Asia. Reductions in pollutants in the highest producing regions would have the greatest global effect. On the other hand, climate policy would, over the long term, result in greater reductions.