1 Introduction
Most human activities result in emissions of pollutants that not only affect our health through degrading air quality but also accelerate global warming. Such activities include for example various means of energy production, transportation, agriculture, industrial processes, waste management, cooking and residential heating and cooling. Emissions originating from human activities are called anthropogenic emissions. In contrast, natural emission sources include, for example, forest fires and volcano eruptions. The origin of the term short-lived climate pollutants (slcps) arises from the life cycle of the emitted pollutants: the removal mechanisms of those climate warming pollutants are so effective that they don’t stay in the atmosphere for very long (Table 1.1). The short, or relatively short, lifetimes make emission mitigation of slcps attractive: if their emissions can be reduced, their amounts in the atmosphere will then also rapidly decline. This is very different compared to carbon dioxide (CO2), the lifetime of which is tens to hundreds of years, which means that the effects of emission reductions can be seen only after a very long time in actual atmospheric amounts.
Main sources and lifetimes of the most important slcps
slcp |
Sources |
Lifetime |
|---|---|---|
Black carbon |
Burning of fossil fuel and biomass in energy production, transportation, cooking, agricultural open burning |
Days to weeks |
Hydrofluorocarbons |
Poor maintenance and equipment failure related to refrigeration and air conditioning, aerosolized propellants |
Days to years |
Methane |
Agriculture, landfills, fugitive emissions from natural gas and petroleum systems |
12 years |
Ozone |
Chemical reactions of methane, carbon monoxide, nitrogen oxides, volatile organics (so not an emitted pollutant) |
Hours to weeks |
However, the same processes that emit slcps typically also emit other, co-emitted pollutants, such as sulfate aerosols, which may actually have a cooling effect on climate. If such a pollutant also has a short lifetime in the atmosphere, it is called a short-lived climate forcer (slcf), but not an slcp. slcfs include all short-lived climate altering substances, both warming and cooling, and are thus a more general class of substances than slcps. It is important to consider both slcfs as a whole and slcps in particular because they originate from the same sources, interact within the atmosphere and, therefore, affect each other’s atmospheric abundances. This means that, in general, mitigating of one pollutant entails changes in other pollutants’ emissions and atmospheric abundances. The term ‘slcf’ is more commonly seen in the field
How does the emission of a pollutant lead to a change in the Earth’s climate? To answer this question, an in-depth understanding of many complex and inter-connected processes is needed. These include consideration of the emission of the pollutants, their atmospheric transport and transformation, interaction with solar radiation, and their removal mechanisms. To estimate the climate effects, complex computer programs called Earth-System Models are required to integrate knowledge from several scientific fields, including physics, meteorology, chemistry, and biology. Here, we try to give a general overview of these processes and concepts, especially targeted to readers who are not part of the natural science community.
This chapter first introduces some basic concepts used in assessing climate impacts of pollutants and other changes in the Earth system (Section 2), then introduces slcps and their past, present, and future impacts on the global climate (Sections 3 and 5). In Section 4, we give a short overview on how the climate impacts of slcps are assessed using climate models, the problems which occur in these modelling exercises, and how they are solved. Finally, in Section 6, we explain the meaning and origin of scientific uncertainty about slcps, and how uncertainty estimates can be interpreted.
2 Radiative Forcing and Surface Air Temperature
In this chapter, we describe, how the impact of a change in the Earth system, for instance the increase of atmospheric abundance of a pollutant, leads to a change in the Earth’s climate and more particularly the surface air temperature. We introduce the two central terms of radiative forcing and climate feedback, how they are assessed, and where the related and often cited uncertainties in their estimates come from.
2.1 The Radiative Balance of the Earth System
While much of the public discussion concerning climate change is centred around surface temperatures, the natural sciences deal first and foremost with energy and how it is gained, lost, and re-distributed in a system. Much of the uncertainty in the assessed surface temperature response to an atmospheric pollutant originates from climate feedbacks due to radiative forcings, two terms that will be explained below. By restricting the scientific discussion to
The Earth system constantly gains energy (is warmed) by absorbing a large portion of the incoming sunlight. At the same time, the Earth also loses energy (is cooled) as it emits thermal radiation, i.e., releases heat, back to space. Because these two processes both vary in time and occur unevenly across the Earth, the energy contained in the Earth system is constantly re-distributed. This leads to daily and seasonal variations in, e.g., temperature and precipitation, which we call weather. Even though these weather variables change constantly, we can still find repeating patterns if we look for long enough. On a global scale, such timeframe is of the order of 30 years.3 In an unchanging climate, the Earth’s energy gains and losses, averaged over such time scales, are equal. Hence, the amount of energy in the Earth system remains almost unchanged. In this case, we say that the Earth system is in equilibrium.4 Keep in mind, though, that over shorter time scales, be it days, months or even several years, the Earth’s energy gains and losses are not equal. Therefore, when climate scientists talk about the Earth’s energy balance, it is always with respect to climate-relevant time scales.
If the radiative balance of the Earth system is disturbed (on a climate-relevant time scale), the Earth system will start to either gain or lose energy, which will ultimately lead to a warming or cooling of the planet, respectively. Such a disturbance in the radiative balance is called a radiative forcing: The Earth system is forced out of its equilibrium and starts to change. Over time the Earth system will adjust by emitting respectively more or less thermal radiation to space and thereby find a new equilibrium. This adjustment results in the observed and projected changes in, e.g., long-term average surface temperatures and precipitation patterns. A radiative forcing can be caused by any kind of persistent disturbance in the Earth system including changes in atmospheric composition, vegetation, snow and ice cover, solar irradiation, and cloud cover. In the following part, we will limit the discussion to the change in abundance of an atmospheric pollutant.
Defining and determining a radiative forcing is not easy, and both definitions and methods used have changed over time. It is nonetheless important to do this in a rigorous manner in order to produce comparable and repeatable results. Radiative forcing is usually determined with the help of climate
2.2 Climate Feedbacks and Climate Response
To estimate how much the climate changes in response to the increased amount/abundance of a pollutant, two principal components are to be considered: the radiative forcing that the pollutant induces and the climate feedbacks that occur after the Earth’s surface starts to warm. A climate feedback is defined as a process that occurs in response to the warming of the Earth’s surface. Climate feedbacks can be both negative and positive: a negative climate feedback counteracts global warming, thereby stabilising the climate, while a positive climate feedback enhances global warming. A good example of a negative climate feedback is the fact that a warmer planet emits more thermal radiation, which has a cooling effect on the climate. An example of a positive climate feedback is the Arctic sea ice melt in a warming climate, which increases the amount of sunlight absorbed by the surface, which in turn results in further warming.
In principle, the total response of the Earth system to a given radiative forcing, i.e., the combination of all climate feedbacks, can be estimated with help of a numerical value, called the net feedback parameter. However, the
In practice, it is more feasible to estimate climate response using Earth System Models (see Section 4) since they also model the oceans and hence make it easier to consider the warming of the ocean surface, which in turn allows to model climate feedbacks directly. However, as the use of Earth System Models is computationally very expensive (see Section 4), the scientific discussion is often constrained to the estimation of radiative forcings, thereby omitting the net feedback parameter. This can be done using different models, such as atmospheric global circulation models which, compared to Earth System Models, are computationally less demanding (i.e., fewer processors, less time, and/or data storage capacity are needed to run a global circulation model compared to an Earth System Model). This is because Earth System Models are more complex and include more physical processes to be modelled, and additionally have to model much longer time periods compared to global circulation models. The output from global circulation models provides information mostly about radiative forcings but does not provide rigorous values for surface temperature changes, which may not be as feasible for policy-making. However, by comparing the radiative forcings, the discussion about the climate response to different pollutants can still be held in a scientifically rigorous manner. Furthermore, because climate feedbacks are not (or only partly) included in global circulation model simulations, the overall uncertainty of the results is smaller. Also, physical descriptions of slcfs in Earth System Models are usually not very detailed, because Earth System Models are already computationally demanding. The more light-weight global circulation
Like in the real atmosphere, modelled meteorological variables, such as temperature and precipitation, vary in time. This is both a necessary and wanted feature of a climate model. Scientifically we express this variability as an uncertainty. Such uncertainty can also be estimated when computing radiative forcings. As stated in Sub-section 2.1, radiative forcing estimates as well as the associated uncertainties depend on the type of the radiative forcing. For instance, the definition of sarf allows the stratosphere to adjust to a change in pollutant abundance. This does result in some uncertainty, which is nonetheless rather small. erf, on the other hand, additionally allows for tropospheric adjustment, including changes to atmospheric water vapour, clouds, and transport of pollutants. Especially in the case of modelling the effects of pollutants with relatively short atmospheric lifetimes, atmospheric abundances can vary significantly both in terms of geographical location and time. This can lead to large uncertainties in the resulting erf estimates. For the same reason, the estimated values for sarf and erf can differ quite a lot. During the past years, quantification of climate effects has moved from a quantity closely related to sarf to the nowadays standard erf.8 This is one of the main reasons why the estimated effects of the mitigation of some slcps have changed so much over the last few years.9
3 Short-Lived Climate Pollutants
Here we list the most common slcps—ozone, methane, hydrofluorocarbons, and black carbon—and how they affect the Earth’s radiation budget. Although not considered as an slcp, we also include a short discussion about carbon dioxide (CO2) to provide comparative references.
3.1 Carbon Dioxide
CO2 is the most abundant of all atmospheric greenhouse gases (ghg) which (at least partly) originate from human activities. Without anthropogenic emissions, the natural greenhouse effect (due to clouds, water vapor, CO2 and other
Since pre-industrial times, the abundance of CO2 in the atmosphere has increased by about 50 %. By pre-industrial times we here refer to the year 1750.10 The golden standard of all climate response estimates is the change in average global surface temperatures due to a theoretical doubling of the pre-industrial abundance of CO2. For this change in CO2 abundance, the ipcc best estimate for the resulting erf is 3.93±0.47 W/m2, which is estimated to lead to a global warming between 1.3°C and 3.1°C after 70 years or between 1.5°C and 7.7°C once a new equilibrium in the radiative balance is reached.11 From these values alone it becomes already apparent why discussing radiative forcings instead of temperature changes is useful: radiative forcing values contain uncertainties, but these uncertainties are small compared to the uncertainties in the climate response.
One important consideration to remember is that in these theoretical studies it is assumed that the atmospheric CO2 abundance is doubled instantly. In reality, CO2 abundance has grown gradually, which also means that the radiative balance of the Earth system has been disturbed gradually. Therefore, the observed past climate responses and the projected future climate responses, based on realistic scenarios, differ from the ideal values provided above. The ipcc estimate for the present-day erf value of CO2 is 2.156±0.259 W/m2. Realistic values for the climate response since pre-industrial times (1750) are 0.75–1.41°C for 2019.12
3.2 Ozone (O3)
O3 is a short-lived gas which is not emitted by any natural or anthropogenic sources. Instead, O3 is formed in the atmosphere through photochemical reactions with a range of chemical compounds, including nitrogen oxidants, carbon monoxide (CO) and organic compounds such as methane (CH4). O3 is naturally present in the atmosphere,13 but human activity can significantly
The amount of O3 in the atmosphere depends on various mechanisms, like chemical production and losses. For instance, increased water vapor concentrations in the stratosphere can increase the amount of ice clouds which, in combination with direct sunlight, leads to depletion of ozone. Furthermore, ozone can react directly with various chemical compounds such as chlorides. In addition, there is exchange of ozone between stratosphere and troposphere, and ozone depletion due to deposition at the Earth’s surface.15 O3 acts as a greenhouse gas by trapping some of the thermal radiation escaping to space. The net climatic effect of ozone strongly depends on both its vertical and horizontal location. There is no method to measure the radiative forcing due to ozone directly. Instead, the estimates for O3 radiative forcing are entirely based on models. Besides direct effects on the Earth’s radiative balance, ozone can add a substantial positive radiative forcing via the effects on terrestrial vegetation.16
Near the Earth’s surface, ozone has adverse effects on human health.17 Besides health effects, O3 pollution has negative impacts on agriculture and natural vegetation.18 Ozone can damage plant tissue, which can result in reduced crop yields,19 suppressed forest growth20 and negative impacts on terrestrial ecosystems.21
In the stratosphere, on the other hand, ozone has a vital role for life on the Earth: stratospheric O3 absorbs uv-c and uv-b light, and thereby prevents some portion of the uv radiation from reaching the lower part of Earth’s atmosphere. uv-b is known to have severe effects on human health, plants,
3.3 Methane
Methane is the second most important anthropogenic greenhouse gas. While the abundance of methane in the atmosphere is much lower than that of CO2, the per-molecule warming potential of methane is drastically larger than for CO2. This makes the radiative forcings due to CO2 and methane comparable (see Table 1.2). Because the lifetime of methane is relatively short, its mitigation is a promising option to slow global warming in the next decades. Methane originates from both natural sources, such as peatlands and thawing of permafrost, and from anthropogenic activities, such as agriculture and natural gas production and distribution. In a warming climate, the permanent thawing of permafrost regions may drastically increase natural methane emissions, which makes methane a major player in one of the climate feedback mechanisms which exacerbate global warming (see section 1.2.2).
Methane is involved in a chain of atmospheric chemical reactions which facilitate the production of tropospheric ozone. In the stratosphere methane is a major source of water vapour through oxidisation by hydroxyl (OH).
3.4 Hydrofluorocarbons (hfcs)
Hydrofluorocarbons (hfcs) are a group of compounds that do not have any natural sources but instead are completely synthetic. hfcs are used in both household and industrial air conditioners, refrigerators, and heat pumps. At room temperatures, most of hfcs are in a gaseous form. hfcs are used as a replacement for ozone-depleting substances as they do not directly deplete stratospheric ozone. However, there are indications that hfcs can weaken the stratospheric ozone layer indirectly, and thus, could be considered weak ozone-depleting substances.22
The atmospheric lifetimes of different hfc compounds vary from a year to hundreds of years. Furthermore, hfc molecules have a vast warming potential compared to CO2 and methane. However, the total amount of hfcs in the atmosphere is still very small compared to CO2 and methane, and that is why
3.5 Aerosols and Black Carbon
Aerosols are either solid or liquid particles suspended in a gaseous media, e.g., in the air. There are both primary and secondary sources of atmospheric aerosols: they are either directly emitted from, e.g., industrial processes, road transport and other anthropogenic activities, or they can be formed in the atmosphere via chemical reactions or as a result of the condensation of some atmospheric trace gases. In addition, natural aerosols such as sea salt or mineral dust make up a large portion of the total aerosol burden. The atmospheric properties of aerosols depend on their size and chemical composition, as well as the prevalent atmospheric conditions. The diameter of an aerosol particle can be in the approximate range of 3 nanometres to 10 micrometres.
The lifetime of aerosols varies from days to months, and they can be transported thousands of kilometres from one region to another. During their time in the atmosphere, the composition and size of aerosol particles are subject to changes due to physical processes and chemical reactions. For instance, two small aerosols particles can form one larger particle by colliding with each other. Typically, aerosols are removed from the atmosphere either via wet removal (precipitation) or by depositing on surfaces.
As an opposite to well-mixed greenhouse gases, the climatic effects of aerosols can be more local and more dependent on the emission source horizontal and vertical location. For instance, aerosol emissions from flat terrain in the Arctic region can have very different effects than emissions that originate from elevated mountain areas near the equator. This is due to their relatively short lifetime, which causes the concentration and composition of aerosols to strongly vary with geographic location, which is in strong contrast with, for instance, the concentrations of methane and CO2.
Black carbon is a carbonaceous aerosol compound that is typically formed in incomplete combustion processes. Typical sources of black carbon are forest fires, residential biomass burning, and transportation. In addition, one distinct source of black carbon in the Arctic region is the flaring of natural gas.
In addition, aerosols also have indirect effects on the climate, also called aerosol-cloud interactions. All cloud droplets are formed by condensation of water vapor onto atmospheric aerosol particles. An increased concentration of aerosol particles in general results in a larger amount of cloud droplets. However, because approximately the same amount of water vapour is divided among more cloud droplets, these cloud droplets are also smaller. Clouds with more but smaller droplets both reflect more sunlight back to space and have a longer lifetime than clouds containing a smaller number of larger droplets. Both of these effects have a cooling effect on the climate. Black carbon containing aerosols can also act as such cloud seed aerosols, which can increase the reflectivity of clouds and partly cool the climate. Thus, mitigating black carbon emissions can reduce this indirect cooling effect, and make a warming contribution on the climate.
The direct and indirect effects of black carbon containing aerosols thus affect the climate in opposite directions, which means that the sign of the total effect can only be determined via careful studies. Furthermore, decreasing black carbon emissions also affects the emission strength of co-emitted aerosol species such as organic carbon and sulfate, which are known to have a net cooling effect on global climate. Recent studies have corroborated this.25 It was found that there is large uncertainty in the total climate effect of mitigating black carbon emissions because of these competing processes and especially the uncertainty due to modelling the aerosol-cloud interactions.
3.5.1 Black Carbon and Albedo Effects
The reflectivity of a surface for radiation depends strongly on surface colour: darker surfaces absorb much of the incoming sunlight (more warming) while bright surfaces reflect more sunlight back to space (more cooling). The climate warming effect of black carbon is strongly dependent on this
Besides the impact on surface air temperatures, aerosols can alter both large-scale and regional weather patterns and magnify or cause pollution episodes. For instance, due to various interaction mechanisms in the atmosphere, anthropogenic aerosols have generated anomalies in large-scale precipitation patterns since pre-industrial times.26 Wei et al studied the effects of covid-19 restrictions and how the changes in black carbon emissions affected local weather patterns in Northern India.27 Their results suggest that reductions in black carbon emissions might have led to a delay in the outbreak of Indian summer monsoon. In addition, some studies have shown that black carbon can either enhance or suppress the length and intensity of urban pollution episodes by altering the atmospheric dynamics near the surface.28
3.6 Present-Day and Historical Effects of slcps
In the latest ipcc ar6, there is no estimation for combined effect for slcp contribution on human-induced climate warming.29 However, based on chemistry-climate model simulations (see Section 1.4), the individual erf and surface temperature estimates are provided for each sclf species. The total global surface air temperature change due to anthropogenic activities was estimated to be 1.3 °C between 1750 and 2019.
The erf and corresponding global surface air temperature change estimates between 1750 and 2019
Substance |
Radiative forcing [W/m2] |
Estimated contribution to temperature change |
|---|---|---|
CO2 |
2.156±0.259a |
|
O3 |
0.47 [0.24 to 0.71]a |
~0.2 °C a |
CH4 |
0.54 [0.44 to 0.65]a or 1.19 [0.81 to 1.58]b |
~0.6 °C b |
hfcs |
0.04a |
|
Black Carbon |
0.11 [-0.20 to 0.42]b |
Less than 0.1 °C b |
The values are retrieved from the ipcc ar6 (Chapters 6 and 7)c. The superscript ‘a’ indicates that the numerical value is from the abundance-based estimate (ar6 Chapter 7), and ‘b’ indicates that the value is from the emission-based estimate (ar6 Chapter 6).
Out of all slcps, methane has had the strongest warming effect on the global climate since pre-industrial times. The global mean erf for methane is
The changes in O3 abundance are estimated to have resulted in an erf of 0.47 [0.24 to 0.71] W/m2. This translates into the human-induced changes in O3 to have had a warming effect of 0.23 [0.11 to 0.39] °C on global surface air temperature since pre-industrial times. This makes O3 the second most important anthropogenic compound after well-mixed greenhouse gases. The estimate of the O3 warming effect has increased since ar5, due to improved estimates of its precursor emissions.30
In ar6, the best estimate for global erf due to black carbon is 0.1 [-0.2 to 0.4] W/m2 compared to pre-industrial times, which is lower than in ar5. The negative value of the lower bound highlights the uncertainty related to black carbon-induced warming. Moreover, as mentioned in section 1.3.4, the simultaneous changes in cooling aerosol species, such as oc, can lead to even more negative values when considering combined aerosol effects. The
3.6.1 Present-Day Health Effects Due to slcfs
Climate change and air pollution are strongly interlinked, as many of the pollutants can affect both. Many slcfs are either considered as air pollutants or can contribute to air pollution indirectly. Air pollutants, such as fine particulate matter (pm2.5) and O3, can have severe impacts on human health. pm2.5 originates from both anthropogenic activities and natural sources, and includes various chemical compounds, such as organic carbon, sulfate, black carbon, mineral dust, and sea salt. When inhaled, fine particulate matter enters human lungs and from there can enter the human blood circulation. Both short-and long-term exposure can cause adverse health effects, for instance diseases related to respiratory and lung functioning, or heart-related diseases.32 Similarly, elevated O3 concentrations affect the lungs and cause breathing problems, triggering asthma and other respiratory diseases.
Air pollution is known to provoke millions of excess deaths globally each year.33 For instance, Fuller et al. estimate that in 2019, there were over 6 million (6.67 [5.90–7.49] million) premature deaths due to household and outdoor fine particulate and ozone pollution.34 Out of the 6 million, ozone was associated with 0.4 [0.2–0.6] million, ambient fine particulate matter with 4.1
4 Climate Modelling of slcfs
The most robust method for estimating the climate effects of slcfs is climate modelling. For past and present climate, observational estimates can also be made. However, for obtaining future projections of the climate, a model of some level of complexity must be used. Such models can range from a model which describes the whole Earth as one “box” to Earth system models which aim at including all relevant processes in the Earth system in three-dimensional frameworks. A comprehensive assessment of the climate effects of slcfs can only be made using global three-dimensional models. The climate effects of slcfs depend on a multitude of characteristics: their physical and chemical properties, distribution in all three dimensions in the atmosphere, and how they enter and exit the atmosphere. For example, sizes of atmospheric aerosol particles range over several orders of magnitude and since climate effects of aerosols depend on aerosol size, the whole size range should be simulated in climate models. Another aspect which adds complexity in simulating slcfs is that the number of chemical compounds in the atmosphere is vast. Especially, the number of organic compounds which contribute to aerosol cloud interactions, can be counted in hundreds or even thousands.
Schematic of a global atmospheric model grid (top right) and a schematic of processes that are solved in atmospheric global models
4.1 Structure of Climate Models
The core of Earth System Models consists of atmospheric, land, and ocean models which in turn are coupled with sub-models which simulate, e.g., atmospheric chemistry, carbon cycle, and vegetation. sclps are simulated in the atmospheric aerosol and chemistry sub-models coupled to the atmospheric
Aerosol and chemistry sub-models of Earth System Models and global circulation models describe the full life cycle of slcfs providing their properties to influence incoming sunlight and outgoing thermal radiation emitted by the Earth’s surface. In addition, these sub-models simulate how slcfs alter the physical properties of clouds which in turn also affect incoming and outgoing radiation, i.e., the Earth’s radiation balance. Once these properties are simulated for pre-industrial and present-day conditions, the radiative forcing of different slcfs can be determined.
4.2 Simulating the slcfs in Climate Models
In the models, gases and aerosols are introduced to the atmosphere through emission routines which are either prescribed based on emission data bases or calculated in the model. Emission databases provide emission strengths for individual compounds. These include, e.g., anthropogenic emissions and forest fire emissions. Sea salt aerosol emissions are an example, where aerosol emissions are calculated in the model based on the simulated wind speeds. There are also very complicated emission modules, for example for considering the emission of substances from vegetation, which depends on ambient conditions like temperature and humidity. Regarding future projections of the climate, uncertainties in emission strengths can cause a much larger uncertainty than the lack of knowledge of Earth system processes or model inaccuracies. This is because it is difficult to estimate how humans will change their emissions in the future and what kind of new innovations will emerge from reducing emissions of climate forcers.
Once compounds are emitted into the atmosphere, model routines calculate the evolution of these compounds in the atmosphere, their transport, and modification by chemical and physical processes (see also Section 1.3). For example, the climate effects of black carbon are heavily dependent on such processes. Studies show that different global models have very large variability and uncertainty in simulating the properties of black carbon which dictate the ability of black carbon to affect the Earth’s energy balance through absorbing incoming sunlight. Chemical and physical processing of black carbon aerosol influences both its optical properties and transport from source regions to remote regions (e.g., Arctic regions).38
Parameterizations of aerosol-cloud interactions provides the change in optical properties of clouds due to changes in aerosol concentrations, which allows for estimating the erf of aerosols (see also Section 1.2). In addition to the effects on the Earth’s radiative balance, models simulate how aerosol affects global and regional precipitation which in turn affects the removal of slcfs from the atmosphere. As mentioned above, removal processes affect the transport of slcfs and determine the deposition on ice and snow, e.g., in the case of absorbing aerosol. This in turn affects the albedo of Earth’s surface. Deposition of absorbing aerosol also has implications on melting of snow and ice.
4.3 Estimating Radiative Forcing from Climate Model Simulations
ipcc bases their forcing estimates on model simulated radiative forcing from climate model simulations. Since one model simulation cannot provide robust values, the estimate is based on several Earth System Model and global circulation model simulations. Because climate variability is large, each model runs an ensemble of simulations, i.e., several simulations of the same time span with slightly altered starting conditions. The average over such an ensemble is more robust than a single simulation. The coupled model intercomparison project (commonly referred to as cmip) provides a protocol for multi-model experiments, which provides the model estimates for radiative forcing of slcfs. The best estimates for forcings are calculated from the distribution of values of all model simulations and the uncertainty is derived from the variability of this
The uncertainty in the aerosol radiative forcing in turn impedes our projections of future climate change since it translates into uncertainty of the climate sensitivity. Despite extensive efforts on narrowing down the uncertainty in the temperature change due to a doubling of pre-industrial CO2 concentrations, it has remained persistent.39 Between the ipcc ar5 and ar6, the estimated range has increased from 2.0–4.7 K to 1.8–5.5 K.40 However, there are strong indications that the latter range has an unrealistically high upper bound. Strong aerosol radiative forcing has its part to play in this range of climate sensitivities.41 For example, the coupled model intercomparison project models need to reproduce past temperature records, and models with too high climate sensitivity can still reproduce them well if the model has strong aerosol forcing as it counteracts the greenhouse gas induced warming. One could consider that improving the parameterizations of aerosol processes in climate models would improve the skill of models to simulate atmospheric aerosol and these advances would reduce the uncertainty in aerosol forcing. However, in multi-model experiments even a more detailed description of aerosol size distribution does not translate into improved predictions of the properties of atmospheric aerosol.
5
Because the future development of human behaviour is unknown, analysing effects of anthropogenic activities in climate modelling requires certain assumptions and restrictions. For this purpose, projections of the future have been developed that can be used when estimating plausible emission scenarios for slcps. In the ipcc framework, the future emission scenarios are based on five Shared Socio-economic Pathways (ssps)42 which frame different future assumptions for various socio-economic factors, such as economic growth, population, technological development, and urbanization. The scenarios vary from stringent climate change mitigation (ssp1) to projections with very high level of fossil fuel usage (ssp5), and also consider regional (in)equality in regional development.
For the next two decades (2020–2040), the outcome from ipcc ar6 indicates that the slcf emission changes will result in an additional warming relative to 2019. A net warming effect is present in the case of all scenarios (from ssp1 to ssp5), though with varying magnitude. The warming is caused by increased contributions of methane and ozone and decreased contributions of cooling aerosol species like sulfate. The reductions in sulfur emissions are expected due to more stringent legislation and policies for energy production and the industrial sector, and due reductions of the sulfur content of fuel used in shipping.43 Furthermore, the projections for slcf climatic effects at the end of the century had varying results for global surface air temperature changes. For the “middle way”, moderate scenario ssp2, the temperature changes due to slcf relative to 2019 was estimated to be 0.2 to 0.5 °C, which consists mostly of warming due to aerosol reductions.
Furthermore, in the near future, the ipcc ar6 reports that decarbonization strategies are not projected to be sufficient to achieve the new who air quality guidelines for fine particulate matter. At the end of the century, implementation of air quality controls could lead to significant improvements in air quality compared to climate change-orientated mitigation strategies. Nevertheless, even in the case of air quality-focused mitigation, the air pollution levels are projected to remain at levels that exceed the who guidelines for major parts of the global population.
6
Policy-making benefits from accurate estimates on how much each forcing agent contributes to the total radiative forcing because decision making for, e.g., emission reductions can be targeted more precisely. However, due to the complexity of the Earth system, large uncertainties remain in the estimates of the radiative forcing of individual sclfs. In this section we summarize and expand upon the aspects of uncertainties related to the radiative forcing and the climate effects of slcfs.
In natural sciences, any measured value can never be entirely precise: repeated measurements yield a range of values. This range is expressed using a best estimate (e.g., the mean value) and an uncertainty, which is a measure of the spread of all measured values. Because the computation of the outcomes of physical processes is based on measured values, the uncertainties related to these measured values must be incorporated in the calculations, which means that also the results of computed values are expressed as a best estimate and an uncertainty. For instance, in ipcc ar644 the erf for black carbon is estimated to be 0.1 [-0.2 to 0.4] W/m2, where the mean value is 0.1 and [-0.2 to 0.4] is the associated uncertainty interval.
In climate science there are additional sources of uncertainty. For instance, the climate system is highly chaotic, meaning that small changes in an initial condition can lead to very different final results. Furthermore, many atmospheric processes are not fully understood or suffer from problems like model resolution or lack of computational power. Therefore, different models may treat different atmospheric processes differently, which leads to differences between results from different models and this is ultimately expressed as an uncertainty.
The uncertainty ranges reported in the ipcc assessment reports are a combination of several sources of uncertainty: scientific uncertainties, uncertainties caused by simplifications of Earth’s processes, model structural limitations (e.g., coarse spatial resolution of models), and randomness in the variability of Earth system processes as well as internal variability in climate models. As explained in Section 3, when ipcc reports the model confidence in radiative forcings of individual slcps, the provided uncertainty value is based on the variability between an ensemble of models which also have run an ensemble of simulations.
7 Conclusion
Short-lived climate pollutants, which are a subgroup of slcfs, are known to impact both global climate and human health. Because slcps and slcfs are often emitted from the same sources, their climate and air quality effects are usually considered jointly in natural sciences. Some of the slcps have a stronger contribution to global warming (e.g., methane), whereas other compounds critically affect air quality and are therefore more harmful to human health (e.g., ozone). Therefore, reductions in slcp emissions can bring either improved air quality or reduced warming of the climate, or in some cases, both.
Estimating the global and regional effects of slcf emissions requires a sophisticated modeling framework and sets of numerous climate model simulations, which are then combined to produce more robust climate projections. The outcomes of these simulations, however, include a certain level of uncertainty which is due to various aspects, such as lack of knowledge of emission sources, uncertainty related to the model parametrization, and natural
In the near future, the combined effects of the changes in all anthropogenic slcfs (which includes both reductions and increases in cooling as well as warming agents) are projected to have a warming effect on the global climate. Therefore, reductions in anthropogenic slcps, and especially methane, provide an opportunity to slow down global warming in the near future. However, in the long term, well mixed greenhouse gases like CO2 will play the dominant role in global warming and their mitigation cannot be neglected.
Ibid.
See, for example, John H. Seinfeld and Spyros N. Pandis, Atmospheric chemistry and physics: from air pollution to climate change (John Wiley & Sons, 2016).
ipcc (n1) Annex vii: Glossary, pp. 2215–2256.
Note here that the term equilibrium according to its thermodynamic definition is used quite loosely. A better term would be dynamic steady state, but for simplicity we stick with equilibrium.
ipcc ar6 (n1).
ipcc, Climate Change 2013: The Physical Science Basis. Contribution of Working Group i to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (hereafter ipcc ar5; cup 2013).
ipcc ar6 (n1).
ipcc ar5 (n7); ipcc ar6 (n1).
Arctic Monitoring and Assessment Programme (amap), amap Assessment 2015: Black carbon and ozone as Arctic climate forcers (amap 2015); ipcc ar6 (n1).
Ibid.
Ibid.
ipcc ar6 (n1).
Stephen O. Andersen, Marcel L. Halberstadt & Nathan Borgford-Parnell, ‘Stratospheric ozone, global warming, and the principle of unintended consequences—An ongoing science and policy success story’ (2013) 63 Journal of the Air & Waste Management Association 6, 607–647.
ipcc ar6 (n1).
Ibid.
Ibid.
Ibid.
Gina Mills and others, ‘Evidence of widespread effects of ozone on crops and (semi-)natural vegetation in Europe (1990–2006) in relation to aot40-and flux-based risk maps’ (2011) 17 Global Change Biology, 592–613.
Ibid.
Maxime Cailleret and others, ‘Ozone effects on European forest growth—Towards an integrative approach’ (2018) 106 J Ecol. 4, 1377– 1389.
Evgenios Agathokleous and others, ‘Ozone affects plant, insect, and soil microbial communities: A threat to terrestrial ecosystems and biodiversity’ 6 Science Advances 33, eabc1176.
Margaret M. Hurwitz and others, ‘Ozone depletion by hydrofluorocarbons’ (2015) 42 Geophys. Res. Lett. 20, 8686– 8692.
ipcc ar6 (n1).
Guus J.M. Velders, David W. Fahey, John S. Daniel, Stephen O. Andersen, Mack McFarland, ‘Future atmospheric abundances and climate forcings from scenarios of global and regional hydrofluorocarbon (hfc) emissions’ (2015) Atmospheric Environment, Volume 123, Part A, 2015, Pages 200–209, issn 1352–2310,
ipcc ar6 (n1); Thomas Kühn and others, ‘Effects of black carbon mitigation on Arctic climate’ (2020) 20 Atmos. Chem. Phys., 5527–5546; Tuuli Miinalainen, Harri Kokkola, Kari E.J. Lehtinen, and Thomas Kühn, ‘Comparing the radiative forcings of the anthropogenic aerosol emissions from Chile and Mexico’ (2021) 126 Journal of Geophysical Research: Atmospheres 10, e2020JD033364.
ipcc ar6 (n1).
Linyi Wei and others, Black carbon-climate interactions regulate dust burdens over India revealed during covid-19’ (2022) 13 Nat Commun, Article 1839.
A. J. Ding and others, ‘Enhanced haze pollution by black carbon in megacities in China’ (2016) 43 Geophysical Research Letters 6, 2873–2879; Jessica Slater and others, ‘The effect of bc on aerosol–boundary layer feedback: potential implications for urban pollution episodes’ (2022) 22 Atmos. Chem. Phys. 4, 2937–2953.
ipcc ar6 (n1).
ipcc ar6 (n1).
Knut von Salzen and others, ‘Clean air policies are key for successfully mitigating Arctic warming’ (2022) 3 Commun Earth Environ, Article 222.
gbd 2015 Risk Factors Collaborators, ‘Global, regional, and national comparative risk assessment of 79 behavioural, environmental and occupational, and metabolic risks or clusters of risks, 1990–2015: A systematic analysis for the global burden of disease study 2015’ (2016) 388 The Lancet 10053, 1659–1724.
Ibid.
Richard Fuller and others, ‘Pollution and health: a progress update’ (2022) 6 The Lancet Planetary Health, 6, e535–e547.
Richard Burnett and others, ‘Global estimates of mortality associated with long-term exposure to outdoor fine particulate matter’ 115, Proceedings of the National Academy of Sciences 38, 9592–9597.
World Health Organisation, ‘who global air quality guidelines: Particulate matter (pm2.5 and pm10), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide.’ (who 2021).
G.W. Mann and others, ‘Intercomparison of modal and sectional aerosol microphysics representations within the same 3-D global chemical transport model’ (2012) 12 Atmospheric Chemistry and Physics 10, 4449–4476.
Junfeng Liu and others, ‘Evaluation of factors controlling long‐range transport of black carbon to the Arctic’ 116 Journal of Geophysical Research: Atmospheres D4.
Mark D. Zelinka and others, ‘Causes of Higher Climate Sensitivity in cmip6 Models’ (2020) 47 Geophysical Research Letters 1, p. e2019GL085782.
Clare Marie Flynn and Thorsten Mauritsen, ‘On the climate sensitivity and historical warming evolution in recent coupled model ensembles’ (2020) 20 Atmospheric Chemistry and Physics, 13, 7829–7842.
Meinrat O. Andreae, Chris D. Jones, and Peter M. Cox, ‘Strong present-day aerosol cooling implies a hot future’ (2005) 435 Nature 7046, 1187–1190.
Keywan Riahi and others, ‘The Shared Socioeconomic Pathways and their energy, land use, and greenhouse gas emissions implications: An overview’ (2017) 42 Global Environmental Change, 153–168.
ipcc ar6 (n1).
Ibid.
Reto Knutti and others, ‘Constraints on radiative forcing and future climate change from observations and climate model ensembles’ (2002) 416 Nature, 719–723,
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