The air and climate system

Context

The layer of air surrounding our planet, better known as the atmosphere, contains water vapour, aerosols, clouds, and many different gases. The dry atmosphere (excluding water vapour) consists mainly of nitrogen (78.1%), oxygen (20.9%), argon (0.93%), and a large number of trace gases such as carbon dioxide and ozone (0.035%). 

The air and climate system is the product of interactions between our atmosphere and our climate. Changes in this system are to an important extent driven by the emission of air pollutants and greenhouse gases (GHGs). These air pollutants and greenhouse gases have a wide range of impacts on health, ecosystems and climate. However, the precise nature of these impacts depends on the location of emissions; chemical reactions of the emitted gases; the atmospheric dispersion of these gases; and their deposition on the earth's surface.

Our understanding of the interaction between air and climate has improved over the last two decades, helped by better knowledge of the role played in this interaction by GHGs and air pollutants. In particular, there has been a greater realisation that some air pollutants act as so-called 'short-lived climate-forcing pollutants' (SLCPs).^[1] Taken together, this improved understanding has led to a growing scientific consensus that it is best to approach air pollution and climate change mitigation as an integrated whole.

Addressing the air and climate system is a challenge for policymakers for a number of reasons. Firstly, complex atmospheric modelling systems are needed to understand the interactions of air pollutants and GHGs. Secondly, international agreements (such as the Montreal Protocol, Kyoto Protocol^[2] and the LRTAP Gothenburg Protocol^[3]) treat air pollutant emissions separately to GHG emissions, even though GHGs and air pollutants are often emitted from the same sources. Thirdly, policy responses in this area can have complex effects. For example, some policies will lead to 'co-benefits' (where policies to reduce air pollution also reduce GHG emissions and vice versa), whereas other policies will lead to trade-offs. These complexities are described in greater detail below.

Key trends

Emissions of air pollutants and greenhouse gases

In Europe, over the last 20 years, emissions of both air pollutants and GHGs have decreased. It is important to emphasise that many air pollutants and GHGs share the same emission sources (Figure 1), and that thematic mitigation policies can simultaneously impact emissions of both GHGs and air pollutants. 

Figure 1: Contribution of anthropogenic sources to total emissions of selected air pollutants and greenhouse gases in the EU-28, 2012^[4][5]

Emissions as drivers of atmospheric composition change

Across the world, emissions of air pollutants and GHGs are changing the composition of the earth's atmosphere. At the global scale, the concentration of the six greenhouse gases included in the Kyoto Protocol has reached 446 ppm CO₂-equivalent, an increase of around 60% compared to pre-industrial levels (CSI013). Changes in land-use and the combustion of fossil fuels from human activities are largely responsible for this increase. In addition, global background ozone pollution levels are increasing as a consequence of the increase in ozone precursor emissions in various parts of the world. Ozone is both a GHG and an air pollutant. Finally, the impacts on European air quality of atmospheric transport of air pollution can also be significant.^[6]

In Europe, air pollutant concentrations are much lower today than 20 years ago and many countries have achieved the emission reduction targets as set under the NEC Directive of 2001 and the Gothenburg Protocol of 1999. But in spite of this improvement, a large percentage of the EU urban population is still exposed to dangerous air pollution from particulate matter and ozone.

Emissions as drivers of climate change

According to the IPCC (2013),^[7] the increase in CO₂ emissions and the resulting increase in the atmospheric concentration of CO₂ is the most important driver of the increase in radiative forcing (RF) between present-day and pre-industrial conditions (Figure 2). Also the air pollutants (CO, NMVOC, NO_X, SO₂ and aerosols) and the other greenhouse gases (CH₄, N₂O and F-gases) have an impact on RF. 

Radiative forcing (RF) is the change in the balance between incoming and outgoing radiation flux (expressed in W/m²) at the top of the atmosphere due to changes in concentrations of greenhouse gases such as CO₂,  incoming solar radiation and changes in the extent the atmosphere and earth are reflecting solar radiation through clouds and reflective land surfaces (albedo). An increase in RF leads to additional warming of the atmosphere, whereas a negative RF results in a cooling of the atmosphere.

Figure 2:  Radiative forcing estimates in 2011 relative to 1750 and aggregated uncertainties for the main drivers of climate change

Source: IPCC (2013)^[7]

It can be difficult to assess the RF effects of GHG and air pollutant emissions. This is because RF is not always the direct effect of the atmospheric concentration of these air pollutants and GHGs, but also the result of their indirect interactions in the atmosphere. These indirect effects include both chemical reactions (ozone formation) and physical reactions (cloud formation and cloud composition) by the air pollutants and GHGs.

In spite of these difficulties, quantitative estimates of these indirect effects can be made, and a recent example of such an estimate is shown in Figure 2. It shows that there is a difference in the scientific understanding of different RF effects and that emissions can result either in an increase or a decrease in RF at the global scale. Over the past 260 years, emissions of CO₂, CH₄, N₂O, F-gases, black carbon, CO, and NMVOC all resulted in an increase in RF. Emissions of SO₂, organic carbon and mineral dust all contributed to a decrease in RF. Emissions of halocarbons had both a positive and negative impact on RF. Also, the emissions of NO_X and NH₃ have had both a positive and negative RF effect, but with a negative net impact on RF. Figure 2 further highlights that interactions between aerosols and clouds resulted in a negative RF, but that the contribution of individual emitted compounds within mixes of aerosols is unknown.

Impact of climate change on air quality 

Climate change could impact future air quality in various ways. One way is through higher temperatures leading to increased ozone formation. Another potential way that climate change can impact air quality is through a change in weather patterns creating 'stagnation events'. In such events, an absence of wind leads to high ozone and PM concentrations. A third possible way that climate change can affect air quality is through a potential change in patterns of hemispheric transport of air.^[8]

Prospects

Emission levels can be affected both by policies that seek to mitigate climate change and by policies that seek to mitigate air pollution. These changes in emission levels can lead to both a decrease or increase of RF over time, and they can have both short-term and long-term effects on climate change. This variety of effects of air policies has been recognised by policy initiatives such as the Climate and Clean Air Coalition (CCAC),^[9] which focuses mitigation efforts on those air pollutants that have a clear warming effect such as black carbon and methane (as shown in Figure 2). At the EU level, the first attempts are now being made to connect the air-pollution and climate-change policy areas through the inclusion of the SLCPs methane and black carbon in the EU Clean Air Policy Package.^[10]

However, the current policy focus on black carbon and methane is only one part of the story for future policy. As shown in Figure 2, some emission reductions will contribute to an increase in RF (SO₂, CO) or will have a mixed RF effect (Halocarbons, NO_X, NH₃), and for a large component of the air and climate system (aerosol-cloud interaction) it is not known what the effect will be. This poses significant scientific and policy challenges to identify those measures that will address air quality but not at the cost of climate change mitigation and vice versa.

Several studies have highlighted the positive effects of climate-change mitigation policies on atmospheric composition change. Not only will these climate-change mitigation policies result in reduced fossil fuel combustion and associated greenhouse gas emissions, they will also reduce air pollutant emissions, their impact on human health and ecosystems, and some of the costs associated with air pollution abatement technologies.^[11][12][13] On the other hand, some climate mitigation measures at present can have a negative effect on air pollution and will continue to do so in their present form. For example, the dieselisation of the European vehicle fleet was the result of policies designed to encourage greater use of diesel vehicles in part because they had lower CO₂ emissions per kilometre compared to gasoline vehicles. However, diesel vehicles generally emit more PM and NO_X per kilometre than gasoline cars, and in some cases this has led to high concentrations of NO₂ measured close to traffic in cities.