State: atmospheric composition and climate

5.3.4 Climate change

In 2016, global averaged concentrations of CO2, CH4 and N2O reached 403.3±0.1 ppm, 1853±2 ppb and 328.9±0.1 ppb, respectively, corresponding to 145 per cent, 257 per cent and 122 per cent above pre-industrial levels (WMO 2017c). The global CO2 growth rate from 2015 to 2016 was the largest of the last 30 years (partly driven by El Niño) and the CO2 concentration was the highest in at least the last 800,000 years. CH4 concentrations plateaued during 1999-2006 but have been increasing since then. Studies point to a variety of different processes driving the change in CH4, mainly changes in anthropogenic sources, permafrost melting or wetland emissions (Dean et al. 2018). N2O concentrations have been increasing steadily since the mid-1980s. Concentrations of CFC replacements, HCFCs and HFCs, which are potent GHGs, have been increasing exponentially since 2005, though these remain low overall and currently contribute to less than 4 per cent combined of the radiative forcing due to all GHGs. According to the National Oceanic and Atmospheric Administration Annual Greenhouse Gas Index (AGGI), radiative forcing by long-lived GHGs increased by 78 per cent between 1979 and 2016, with CO2 accounting for about 72 per cent of this increase.

Since 1901 almost the whole globe has experienced surface warming, and it is extremely likely that anthropogenic activities caused more than half the observed increase in global mean surface temperature since the mid-20th century (Bindoff et al. 2013). The global mean surface temperature increase over the 1901-2012 period (see Figure 4.2) was approximately 0.89°C, but some regions experienced warming of greater than 2°C (Hartmann et al. 2013).

Trends in precipitation are less clear and differ by locations. In general, dry areas are becoming drier, and wet areas are becoming wetter, but multiple exceptions exist (Trenberth 2011; IPCC 2014; Feng and Zhang 2015). For tropical land areas, observations show a decreasing trend from the mid-1970s to mid-1990s and an increasing trend the following decade, resulting in no significant overall trend from 1951 to 2008 (Hartmann et al. 2013). A statistically significant increase in precipitation occurred from 1901 to 2008 for the northern mid-latitudes (30°N to 60°N) land areas; in contrast, there is only limited evidence of a long-term increase in the southern mid-latitudes (Hartmann et al. 2013). Observed changes in the latitudinal distribution of precipitation over land are suggestive of human influence; however, the results are still inconclusive, due to incomplete data and model uncertainties (Bindoff et al. 2013).

Climate change can also impact atmospheric circulations and features at global and regional levels. Observations indicate a widening of the tropical belt, a poleward shift of storm tracks and jet streams, and a contraction of the northern polar vortex since the 1970s are likely (Hartmann et al. 2013). Stratospheric O3 depletion and GHG warming may have contributed to the poleward shift of the southern Hadley cell and positive trend in the Southern Annular Mode, which characterizes the north-south movement of the belt of westerly winds that circles Antarctica, during the austral summer (Bindoff et al. 2013). Attribution of anthropogenic influence on the poleward shift of the Hadley cell in the Northern Hemisphere is less certain (Bindoff et al. 2013). While many studies have indicated changes in the El Niño-Southern Oscillation (ENSO) and monsoon circulations, there are large observational and modelling uncertainties such that there is low confidence that changes, if observed, can be attributed to anthropogenic activities (Bindoff et al. 2013).

There is increasing evidence that climate change has led to changes in the frequency and intensity of extreme events since the mid-20th century (Trenberth 2011; Hartmann et al. 2013; Alexander 2016). It is likely that the frequency of extreme warm days has increased in North America, Central America, Europe, Southern Africa, Asia and Australia, and the frequency of heat waves has increased in Europe, Australia and across large parts of Asia (Hartmann et al. 2013). Observations have shown a general increase in heavy precipitation at the global scale (Trenberth 2011; Hartmann et al. 2013). Regionally, it is likely that the frequency or intensity of heavy precipitation events has increased in North America, Central America and Europe, and it is virtually certain that there has been an increase in the frequency and intensity of the strongest tropical cyclones in the North Atlantic basin since the 1970s (Hartmann et al. 2013). For drought, the frequency and intensity likely have increased in the Mediterranean and West Africa, and likely have decreased in central North America and north-west Australia (Hartmann et al. 2013).

Air pollution, stratospheric O3 depletion, persistent pollutants and climate change are interlinked problems (see Figure 5.1). Climate warming agents such as BC, tropospheric O3, CH4 and HFCs have a relatively short lifetime in the atmosphere compared with long-lived GHGs and are referred to as short-lived climate pollutants (SLCPs) (Haines et al. 2017). Tropospheric O3 contributes to warming directly as a GHG. However, O3 also contributes to warming by impairing vegetation growth and decreasing plant uptake of CO2 (Ainsworth et al. 2012). BC has a warming effect both in the atmosphere and when deposited on snow and ice. Decreasing emissions of SLCPs can decrease warming in the near term, which may be essential for achieving near-term climate targets or avoiding climate tipping points (Shindell et al. 2017). However, decreasing emissions of SLCPs in the near term needs to be combined with mitigation of long-lived GHGs, which dominate climate forcing over the long term (UNEP 2017c).

Other PM constituents (e.g. sulphates and nitrates) also affect climate and may cool the climate by scattering solar radiation. PM also affects climate indirectly by affecting cloud formation, leading to changes in cloud reflectivity, cloud distribution and precipitation patterns. There is still a significant amount of uncertainty on the net radiative effects of aerosols (Fuzzi et al. 2015).

Through its impact on synoptic and local-scale meteorology, climate change impacts air pollution and PBT concentrations in multiple, non-linear ways (UNEP and AMAP 2011; Fiore, Naik and Leibensperger 2015). Higher temperatures can increase the chemical reaction rates involving O3 formation or reduce PM concentrations as components volatilize (Megaritis et al. 2013; Czernecki et al. 2016). Higher temperatures also increase primary emissions of POPs that can volatilize and secondary emissions by revolatilizing previously deposited POPs (Ma et al. 2011). Because particle-bound POPs are more efficiently removed from the atmosphere via deposition, semi-volatile POPs may last longer in the atmosphere at higher temperatures and be transported further from source regions. Higher temperatures may also increase degradation of POPs (Ma et al. 2011). Reduced cloud cover promotes the formation of O3 by increasing photolysis rates (Na, Moon and Kim 2005). Higher temperatures and light intensity can also increase emissions of biogenic NMVOC (Guenther et al. 2012), which are O3 and PM precursors. At the same time, higher temperatures and water stress lower stomatal uptake of O3 and thus reduce O3 deposition (Solberg et al. 2008; Huang et al. 2016). More rain reduces pollution by washing out PM and other pollutants. Extreme events such as heat waves and drought increase risks of high PM pollution associated with wildfires (Bowman et al. 2017) and dust (Achakulwisut, Mickley and Anenberg 2018). Extreme events such as floods and storms can also impact the remobilization and bioavailability of POPs (Ma et al. 2011).

Meteorological parameters that affect air quality often covary with and depend on synoptic-scale or other larger-scale phenomena. For example, surface O3 and PM concentrations are strongly influenced by ventilation and dilution, which are governed by winds and boundary-layer height and are often correlated with temperature and humidity. A decline in the number of summertime mid-latitude cyclones travelling across North America since 1980 has been associated with increases in stagnation and O3 pollution episodes in the eastern United States of America, offsetting some of the air quality improvement in the north-eastern United States of America from reductions in anthropogenic emissions (Leibensperger, Mickley and Jacob 2008). Extreme wintertime stagnation and pollution episodes in eastern China have been associated with melting sea ice in the Arctic during the preceding autumn and increased snowfall across Siberia during early winter (Zou et al. 2017).