5.2 Pressures: emissions

People alter the atmosphere primarily by generating emissions. Trends in human-caused emissions are driven by changes in population, urbanization, economic activity, technology and climate (‘the drivers’), as well as by behavioural choices, including lifestyle, and conflict. In turn, these drivers are influenced by policies (‘responses’). Natural emission sources, including emissions from vegetation, soils, wildfires, and windblown sand and dust, also contribute to emissions, but can be affected by people (e.g. through land-use change).

Although an increasing amount of emissions information in some GEO regions is publicly available, there is no global reporting programme applicable to all sources and pollutants and no comprehensive emissions data repository. The Aarhus Convention and its Protocol on Pollutant Release and Transfer Registers (PRTR) aspires to establish a global network, building on the work of the United Nations Economic Commission for Europe (UNECE) and the Organisation for Economic Co-operation and Development (OECD) (see http://prtr.net). Currently, compiling a consistent global emissions inventory requires research effort. This assessment uses the latest anthropogenic emissions data developed using the Community Emissions Data System (CEDS), an open source, global emissions inventory data system that was developed to provide consistent long-term emission trends for use in global atmospheric modelling efforts, such as those supporting the preparation of Intergovernmental Panel on Climate Change (IPCC) 6th Assessment Report (Hoesly et al. 2018). Open biomass burning emissions, whether anthropogenic or natural, are drawn from a separate inventory created for global modelling efforts by merging information from satellite-based estimates, sedimentary charcoal records, historical visibility records and multiple fire models (van Marle et al. 2017). Together, these data sets provide an up-to-date and consistent basis to examine trends for most air pollutants and greenhouse gases (GHGs) (see Figure 5.3).

Figure 5.3: Annual emission trends from 1990 to 2019 in kilotons by pollutant, region and sector
Source: Hoesly et al. (2018).

Globally, anthropogenic carbon dioxide (CO2) emissions increased by more than 40 per cent over the period 1990-2014, driven by large increases in Asia and counteracted by small declines in North America and Europe. Sulphur dioxide (SO2) emissions are the only ones to have declined globally during this period, with increases of more than 50 per cent in Asia offset by a more than 75 per cent decrease in North America and Europe. In recent years, emissions of SO2 and nitrogen oxides (NOX) have begun to decline in East Asia. The inclusion of wild and agricultural fires significantly increases the interannual variability of emissions of non-methane volatile organic compounds (NMVOC), carbon monoxide (CO), black carbon (BC) and organic carbon (OC).

The emissions data presented here are best estimates with different degrees of uncertainty depending on pollutant, sector, region and time period. Hoesly et al. (2018) found that CEDS estimates are slightly higher than previous global inventories (e.g. Lamarque et al. 2010; European Commission 2016). In general, estimates of CO2 and SO2 emissions have uncertainties on the order of ±10 per cent for a 5-95 per cent confidence interval, whereas BC and OC emissions have uncertainties on the order of a factor of two. Uncertainties for CO, NOX, NMVOC and ammonia (NH3) emissions lie in between these two endpoints (Hoesly et al. 2018). Uncertainty also varies by sector: emissions from large electricity generation plants are well characterized, whereas emissions generated by military conflicts are not well understood or commonly included in inventories.

There are considerable gaps in available emissions data for POPs, which include pesticides, industrial chemicals and products of incomplete combustion or chemical reactions. Available data in Europe, America and Central Asia indicate that emissions decreased significantly between 1990 and 2012 for the most studied POPs, due to regulations, including the Stockholm Convention (UNEP 2014a; UNEP 2014b; UNEP 2015a; UNEP 2015b). Nevertheless, alongside the growing number of listed POPs and candidate substances, unregulated POPs emissions may be increasing. Many commercial products contain unknown quantities and types of unregulated POPs, often with unknown effects (see also Section 4.3.3).

The UNEP Global Mercury Assessment estimated that anthropogenic Hg emissions to air were 2,220 (2,000-2,820) (metric) tons/year for 2015 (UNEP 2013a). Globally, artisanal and small-scale gold mining (ASGM) was responsible for about 38 per cent of total anthropogenic Hg emissions to air in 2015, followed by coal combustion (about 21 per cent), non-ferrous metal production (about 15 per cent) and cement production (about 11 per cent). Asia is the main source region, contributing about 49 per cent of 2015 global anthropogenic Hg emissions, followed by South America (18 per cent) and sub-Saharan Africa (16 per cent). Current anthropogenic sources contribute about 30 per cent of annual Hg emissions to air, while natural geological sources contribute about 10 per cent. The remaining 60 per cent comes from ‘re-emissions’ of previously released Hg from soils and oceans, mostly from anthropogenic sources (UNEP 2013a).

Globally, both the production and consumption of ODS, and thus ODS emissions, declined by more than 99 per cent between 1990 and 2016 (UNEP 2017b). Chlorofluorocarbons (CFCs) and halons, the most potent ozone depleters, have been replaced by shorter-lived hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), although recent measurements suggest that new emissions of trichlorofluoromethane (CFC-11) may be occurring (Montzka et al. 2018). The less-depleting HCFCs are now being phased out in favour of chemicals that do not contribute to ozone depletion. Concerns about the potential future contribution of HFCs to climate change led to the 2016 Kigali Amendment to the Montreal Protocol, which will limit future HFC emissions.