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.
