Abstract
Climate change is a major issue regarding the atmospheric environment. It differs from air pollution in spatial and temporal scales. Climate change is due mostly to greenhouse gases that have long atmospheric lifetimes and, therefore, are distributed relatively uniformly in the atmosphere. To the contrary, air pollution shows great spatio-temporal variability and offers a relatively short response time in terms of the relationship between emissions and atmospheric concentrations. Nevertheless, there are many links between climate change and air pollution. First, some air pollutants contribute to climate change. Second, climate change may affect air pollution. Finally, some sources emit both air pollutants and greenhouse gases, whereas other sources may emit mostly air pollutants or mostly greenhouse gases. Therefore, it is essential to identify all the emissions associated with a given source in order to avoid creating one problem when trying to solve another. This chapter first summarizes in general terms the main aspects of climate change and then describes the interactions between climate change and air pollution.
Climate change is a major issue regarding the atmospheric environment. It differs from air pollution in spatial and temporal scales. Climate change is due mostly to greenhouse gases that have long atmospheric lifetimes and, therefore, are distributed relatively uniformly in the atmosphere. To the contrary, air pollution shows great spatio-temporal variability and offers a relatively short response time in terms of the relationship between emissions and atmospheric concentrations. Nevertheless, there are many links between climate change and air pollution. First, some air pollutants contribute to climate change. Second, climate change may affect air pollution. Finally, some sources emit both air pollutants and greenhouse gases, whereas other sources may emit mostly air pollutants or mostly greenhouse gases. Therefore, it is essential to identify all the emissions associated with a given source in order to avoid creating one problem when trying to solve another. This chapter first summarizes in general terms the main aspects of climate change and then describes the interactions between climate change and air pollution.
14.1 General Considerations on Climate Change
As explained in Chapter 5, the Earth’s climate is due in part to the natural presence of greenhouse gases (GHG), which absorb part of the infrared radiation emitted by the Earth and, consequently, lead to an average temperature of about 15 °C at the Earth’s surface. It would be about −19 °C without those GHG.
Climate change results from an increase of the concentrations of GHG either via an increase in concentrations of natural GHG, such as carbon dioxide (CO2), or via the introduction into the atmosphere of new GHG, such as halocarbons.
Since 1988, the Intergovernmental Panel on Climate Change (IPCC) summarizes in reports, which are regularly updated, the state of scientific knowledge on climate change, its causes, the processes involved, and the possible consequences. The Fifth assessment report (also known as AR5) was published in 2013–2014 (IPCC, 2014). Its main conclusions are summarized below.
14.1.1 Observed Changes
A mean global warming since the middle of the 19th century is certain and is without any precedent since decades or even millennia. The GHG concentrations have increased, the mean temperatures of the atmosphere and of the ocean have increased, the total amounts of snow and ice at the Earth’s surface have decreased, and the sea level has increased. These changes are documented in greater detail below.
The atmospheric concentrations of CO2, methane (CH4), and nitrous oxide (N2O) have increased compared to their concentrations in 1750 (i.e., prior to the industrial era) by 40, 150, and 20 %, respectively. The current concentrations are without any precedent since at least the past 800,000 years.
Each of the past three decades has been warmer for the atmosphere than all previous decades since 1850 (i.e., since about the beginning of the industrial era). The period ranging from 1983 to 2012 was likely the warmest 30-year period over the past 1,400 years. Based on the longest time series of available temperature data, the globally averaged (land and ocean combined) surface temperature has increased by 0.72 to 0.85 °C from the 1850–1900 period to the 2003–2012 period. However, there is a large inter-annual and inter-decadal variability.
These changes in atmospheric temperature have been accompanied by changes in precipitation. These latter changes are more difficult to document and are, therefore, less certain. Nevertheless, it seems that on average precipitations have increased in the mid-latitudes. In addition, changes in terms of extreme events (heat waves, hurricanes, flooding …) have been observed since about 1950. It is very likely that the number of cold days/nights has decreased and that the number of warm days/nights has increased globally. It is likely that the frequency of heat waves in Europe, Asia, and Australia has increased. It is likely that there are more regions where the number of heavy precipitation events has increased than there are regions where that number has decreased. Finally, it seems that the frequency or the intensity of heavy precipitation events has increased in North America and Europe.
The ocean dominates in terms of the amount of thermal energy being stored in the climate system, since it accounts for more than 90 % of this increased energy. It is virtually certain that the upper part of the ocean (i.e., from the surface to a depth of about 700 m) has warmed up from 1971 to 2010 and it is likely that it has warmed up from 1870 to 1971. For example, the first 75 meters would have warmed up at the rate of 0.09 to 0.13 °C per decade during the period ranging from 1971 to 2010. Because of limited data and analyses, no trend has been observed in the meridional oceanic circulation of the Atlantic Ocean (AMOC for “Atlantic Meridional Overturning Circulation”), which includes the Gulf Stream in the North Atlantic. The increase in the CO2 concentration (in the atmosphere and, therefore, also in the ocean since it is water soluble as carbonic acid, H2CO3) leads to acidification of the ocean: the pH of surface waters has decreased on average by 0.1 since 1850, i.e., an increase in H+ ions by 25 %.
Concerning the cryosphere (the land surfaces covered with solid water, i.e., ice or snow), the ice sheets of Greenland and Antarctica have lost mass and the glaciers throughout the world, the Arctic sea ice, and the spring snow cover of the northern hemisphere have continued to lose surface area. It is likely, or even very likely, that the loss rates of mass or surface area of the cryosphere have accelerated over the past several years. For example, it is very likely that the annual mass loss rate of the Greenland ice sheet has increased from 1992 to 2011 and it is likely that the annual mass loss rate of the Antarctic ice sheet has increased over the same period.
Sea level has increased by about 19 cm (± 2 cm) from 1901 to 2010. In addition, it is very likely that the rate of increase of sea level keeps increasing. It has been on average in the range of 1.5 to 1.9 mm per year during the period ranging from 1901 to 2010, but it was in the range of 2.8 to 3.6 mm per year for the period ranging from 1993 to 2010. The melting of continental glaciers, the melting of polar caps, and the thermal expansion of the ocean contribute in about equal amounts to this increase in sea level (about one-third each).
14.1.2 Causes of Climate Change
Total radiative forcing is defined as the difference between the radiative energy received and the radiative energy emitted by a given system. Since the Earth system tends to be at equilibrium from an energy viewpoint, radiative forcing in the climate change context concerns the different factors contributing to perturbations of the radiative energy budget. The definition provided by the IPCC is as follows.
Radiative forcing measures the impact of perturbations (for example, an increase in the atmospheric concentration of a greenhouse gas) influencing climate on the energy balance of the Earth system. The term radiative is used because these perturbations modify the balance between incoming solar radiation and infrared radiation outgoing from the atmosphere. Since this radiative balance controls the temperature at the Earth’s surface, the term forcing is used to indicate that the radiative balance of the Earth is being perturbed. Radiative forcing is generally quantified as the rate of energy transfer per unit surface area of the globe, measured in the upper layers of the atmosphere. It is expressed in watts per square meter (W m−2). The standard definition of radiative forcing is the change due to a perturbation in the downward radiative flux at the tropopause after allowing for stratospheric temperatures to adjust to radiative equilibrium, while holding surface and tropospheric temperatures and state variables fixed at their values before the perturbation. The term effective radiative forcing is also used. It is calculated at the top of the atmosphere (instead of being calculated at the tropopause) and it involves allowing some atmospheric variables (temperature, clouds, etc.) to adjust to the new radiative equilibrium, while holding surface conditions (temperature, ice, etc.) fixed. For well-mixed GHG, both definitions lead to the same results, but the uncertainty range is greater for the effective radiative forcing than for the standard radiative forcing (see Myhre et al., 2013, for details). A radiative forcing caused by one or more perturbations is said to be positive when it leads to an increase in energy for the Earth/atmosphere system and, therefore, warming of the system. In the opposite case, a radiative forcing is said to be negative when the energy decreases, which leads to cooling of the system.
The increase in thermal energy in the climate system is directly linked to an increase in total radiative forcing. The greatest contribution to this radiative forcing is due to the increase in the atmospheric concentration of CO2.
Figure 14.1 shows the radiative forcing due to emissions from several GHG, GHG precursors, particulate matter (PM), and PM precursors. For example, the radiative forcing due to emissions of CO2, CH4, halocarbons, and N2O is estimated to be 1.68, 0.97, 0.22, and 0.17 W m−2, respectively. Some GHG such as CO2, halocarbons, and N2O are chemically inert (or quasi-inert); therefore, their radiative forcing contribution is estimated simply from their atmospheric concentration (balance between their emission and atmospheric removal rates). On the other hand, CH4 is slowly oxidized to CO2 and leads to ozone (O3) and water vapor (H2O) formation, two other GHG. Thus, the contribution of CH4 emissions to radiative forcing must also take into account the contribution of CH4 to the whole set of these related chemical species and not be limited to the CH4 concentration.
Figure 14.1. Radiative forcing for the 1750–2011 period calculated for pollutants emitted in the atmosphere. Positive forcing is in black and negative forcing is in gray. Net forcing is shown for species that have both positive and negative forcings. VOC do not include CH4, which is listed separately, nor their effect on secondary organic aerosols (SOA), which is not taken into account here. Halocarbons include here both chlorofluorocarbons and fluorocarbons, which are listed separately in the IPCC report. BC is the black-carbon fraction of atmospheric PM, and OM is the primary fraction of organic compounds present in PM. The effect of these particles on cloud formation is listed as “aerosols/clouds.” The effects of NH3 and SO2 occur through their formation of particulate ammonium and sulfate, respectively. Dust corresponds to soil dust and the contribution of aviation corresponds to contrails.
Similarly, some pollutants are not GHG, but their chemical reactivity influences some GHG concentrations. For example, carbon monoxide (CO) and volatile organic compounds (VOC) are ultimately oxidized to CO2 and form O3 (therefore, a positive radiative forcing). On the other hand, by forming oxidants (particularly the hydroxyl radical, OH), they lead to faster oxidation of CH4 and, therefore, a slight negative radiative forcing. Nitrogen oxides (NOx) lead to O3 formation (positive forcing), but to an increase in the oxidation rate of CH4 and to the formation of particulate nitrate (i.e., negative forcing). Ozone does not appear in Figure 14.1, because it is not emitted in the atmosphere. It is produced by chemical reactions from VOC, NOx, CO, and CH4. Therefore, it appears indirectly via the contributions of these chemical species.
Atmospheric particles (aerosols) lead to absorption of radiation in the case of black carbon (positive forcing), but to scattering of solar radiation in the case of the other components (organic and inorganic) of particulate matter (PM) (negative forcing). These effects are called the direct effects of atmospheric particles on atmospheric radiation. It seems that negative forcing dominates for particles, but there are large associated uncertainties. Particles are also involved in cloud formation and influence precipitation via the size distribution of cloud droplets. This effect is called the indirect effect of atmospheric particles. Clouds have compensating effects. On one hand, they reflect some of the solar radiation back to space and induce some negative forcing. On the other hand, they absorb some of the IR radiation emitted by the Earth, which leads to positive forcing. Overall, clouds are estimated to lead to negative forcing. Therefore, the indirect effect of particles is negative forcing. Furthermore, there is some additional effect due to radiation-absorbing particles. It results from slight associated changes in the atmospheric temperature, which may then decrease the occurrence of low-altitude clouds. If those clouds have a negative forcing contribution, then their decrease would lead to positive forcing. This effect of atmospheric particles on clouds via the perturbation of atmospheric temperatures is called a semi-direct effect.
14.1.3 Future Climate Change
Climate change over the next several years will depend on future emissions of GHG, atmospheric particles, and their precursors. Different scenarios, called representative concentration pathways (RCP), have been used to represent various scenarios of population, energy production and consumption, and technological progress. The conclusions of the IPCC based on scenario calculations are summarized here.
It is likely that in 2100 the mean temperature at the Earth’s surface will have increased by 1.5 °C compared to its value during the second half of the 19th century (1850–1900), except for the most optimistic scenario. This temperature increase could exceed 2 °C for the most pessimistic scenarios. This warming would continue during the following century, except for the most optimistic scenario.
Changes in the water cycle, which are due to this global warming, will not be uniform and the wettest regions and seasons will typically become wetter, whereas the driest ones will become drier. Therefore, larger differences between wet and dry regions and between wet and dry seasons are likely to increase.
Changes currently observed for the ocean temperature, ice, sea ice, and snow cover will continue to increase. The increase in sea level will probably accelerate because of the increase in the ocean temperature (dilatation) and the melting of ice and snow. Estimated increases in sea level from the beginning to the end of the century (i.e., from 1986–2005 to 2081–2100) are in the range of 26 to 55 cm for the most optimistic scenario and in the range of 45 to 82 cm for the most pessimistic scenario.
14.2 Effect of Air Pollution on Climate Change
As described in Section 14.1, some air pollutants have a direct or indirect effect on climate change.
Ozone (O3) is a GHG. It is also an important air pollutant (see Chapter 8). However, its lifetime is short, because it is reactive (destruction by photolysis and reactions with other gases, such as nitrogen oxides and alkenes) and it deposits readily on surfaces. Therefore, any action taken to reduce ozone concentrations has an immediate effect on its radiative forcing. This is a major difference with a GHG such as CO2, which has a lifetime on the order of one century and for which any action taken now will show some notable effect only decades later.
Other air pollutants are involved in climate change in various ways:
– By leading to the formation of a GHG
– By leading to PM formation
– By leading to oxidant formation, which may affect the lifetime of some GHG
CO, NOx, and VOC are precursors of O3. Therefore, their emissions contribute via O3 formation to radiative forcing.
These O3 precursors also lead to hydroxyl (OH) radical formation. OH radicals are the oxidant of CH4. Therefore, an increase in OH radical concentrations due to emissions of CO, NOx, and/or VOC leads to a decrease in the concentration of CH4, i.e., less radiative forcing from this GHG. Therefore, the corresponding contribution is negative radiative forcing.
NOx, VOC, sulfur dioxide (SO2), and ammonia (NH3) are precursors of PM (nitrate, organics, sulfate, and ammonium, respectively).
Particles have mostly a negative radiative forcing, with the exception of black carbon (BC), which absorbs radiation (see Chapter 5) and, therefore, contributes to global warming. The effect of BC on atmospheric radiation depends on its particulate state, i.e., whether it is present as individual particles (external aerosol mixture) or whether it is mixed with other chemical species in particles (internal mixture) (Jacobson, 2001). Therefore, the radiative forcing due to BC is uncertain, but it is estimated to be commensurate with that of CH4 (e.g., Bond et al., 2013). The term “brown carbon” is also used to describe the organic PM fraction that absorbs some radiation (but to a lesser extent than BC). In addition, the absorption of radiation has a semi-direct effect by modifying the thermal budget, which may lead to changes in cloud occurrence (see the semi-direct effect of particles described in Section 14.1.2).
Particles scatter atmospheric radiation and, therefore, they tend to reflect solar radiation back to space, which leads to negative radiative forcing. In addition, given their hygroscopic properties, fine particles act as nuclei for cloud droplets. If there are more particles in the atmosphere, cloud formation will take place on a greater number of condensation nuclei and, for a given amount of liquid water (which depends on the supersaturation of the atmosphere), there will be a greater number of cloud droplets and these droplets will be smaller. Therefore, these clouds will tend to precipitate less and will remain in the atmosphere longer. Since clouds have on average a negative radiative forcing, the result of an increase in the number of particles is negative forcing (indirect cooling effect). The effects of particles on climate are, therefore, complex and require that their physico-chemical and optical properties be well characterized (Boucher, 2015).
Therefore, PM air pollution tends to partially hide global warming. Without any air pollution, global warming would have been greater. However, one should note that the main GHG (CO2) is the major product of combustion and that combustion processes are also the source of many air pollutants and precursors such as NOx, CO, VOC, SO2, and particles. Therefore, one cannot really dissociate the increase of CO2 emissions from the increase in air pollution. However, one may consider that emission control technologies may be used to reduce air pollution (see Chapter 2) without reducing CO2 emissions. In that case, a reduction in PM emissions (other than black carbon) will lead to an increase in global warming.
14.3 Effect of Climate Change on Air Pollution
Climate change has various effects on air pollution through the change in atmospheric temperature and precipitation for example, but also more specifically through the change in the occurrence and intensity of weather types (anticyclones, fronts, etc.). It is important to distinguish two kinds of analyses when addressing the effect of climate change on air pollution:
– The effect of a future scenario (such as an RCP scenario) on air pollution;
– The effect of climate change only (i.e., keeping air pollutant emissions constant) on air pollution.
The first analysis consists of a scenario study that treats jointly climate change and air pollution. However, it does not provide any information on the specific relationships that may exist between the change in meteorological conditions and the change in air pollutant concentrations, because the change in air pollution is typically dominated by the change in the air pollutant emissions, as defined by the scenario being studied.
The second analysis does not correspond to a specific change in the future state of the atmosphere, because one is interested in the effect of meteorological conditions corresponding to a future climate (i.e., resulting from different emissions) on current air pollution (i.e., due to current emissions). Nevertheless, such an analysis isolates the effect of climate change on air pollution and evaluates whether this effect is significant or not. We are interested here in this second analysis. The effect of climate change on air pollution has been studied mainly for ozone and fine particles.
The effects of climate change on ozone have been studied for North America and Europe. Overall, an increase in the frequency of occurrence of anticyclonic regimes combined with a slight increase in temperature (which favors chemical kinetics and some emissions such as biogenic emissions and anthropogenic emissions by evaporation) leads to a slight increase in O3 concentrations. Estimates of the potential increase in O3 concentrations are on the order of a few ppb (1 to 10 ppb) for North America (Jacob and Winner, 2009). For Europe, a range of −1.7 to +1.6 ppb depending on the regions has been estimated for a climate change corresponding to scenario RCP4.5 (Lacressonnière et al., 2016).
The effects of climate change on fine particles are particularly interesting because of some antagonistic effects. An increase in temperature leads on one hand to an increase in biogenic VOC emissions and anthropogenic VOC emissions by evaporation, as well as faster kinetics for the formation of semi-volatile and non-volatile compounds, which are PM precursors. On the other hand, it may favor the volatilization of semi-volatile particulate-phase compounds, such as SVOC and ammonium nitrate. In addition, the change in the frequency of occurrence of weather types will have an effect since anticyclonic conditions favor air pollution and low-pressure systems favor particle scavenging (i.e., removal from the atmosphere). Studies conducted in North America and Europe show that these effects compensate each other to some extent with the net result being that the effects of climate change on PM air pollution are limited (Lecœur et al., 2014; Lacressonnière et al., 2016; Shen et al., 2017). For a climate change corresponding to RCP4.5, the effect of climate change on annual concentrations of fine particles is estimated to range between about −1 μg m−3 and 1 μg m−3 depending on the regions. Larger effects for seasonal concentrations have been estimated for North America, with changes reaching up to about 1 μg m−3 in Europe and ±3 μg m−3 in North America. Nevertheless, it appears that in Europe the annual variations will be less than or similar to the inter-annual variability of PM2.5 concentrations except for a very pessimistic climate change scenario (e.g., RCP8.5) (Lecœur et al., 2014).
Problems
Problem 14.1 Climate and meteorology
Explain briefly the difference between climate and meteorology.
Problem 14.2 Greenhouse gases and air pollutants
Give examples of technological changes that are beneficial for air quality, but are detrimental for climate change, or, conversely, that are beneficial for climate change, but are detrimental for air quality.
Problem 14.3 Particles and climate change
What are the effects of atmospheric particles (aerosols) on climate?