Emissions of BC and tropospheric O3 precur- sors are driven by basic societal forces that govern the production and consumption of energy, food and materials:
1. Population growth;
2. Economic development that drives, for ex- ample, the transition away from residential burning of solid fuels, the extent of private vehicle ownership, and dietary habits such as increased meat consumption that affects CH4 emissions;
3. Consumption patterns, driven by popula- tion pressure, that include land-use changes involving deforestation and wetlands re- moval;
4. Technological change that can help in faster emission reductions through the spread of
new, high-efficiency and low-emission tech- nologies;
5. Environmental regulations driven by re- gional health and ecosystem concerns, resulting in, for example, reductions in PM, SO2 and NOX emissions; and
6. Other policy drivers such as global climate change mitigation agreements.
All these drivers interact with different sectors of human activity and have distinct time- frames for implementation and the achieve- ment of objectives.
There are other driving forces that affect emis- sions but are not so directly linked to human activities. These include future climatological changes that may affect, for example, water availability and drought, which would in turn
have an influence on food production and wild-
fires, and consequently on emissions. Other
examples might relate to the frequency and intensity of lightning and associated NOX, soil
NOX and CH4 released from warming tundra, increased tropospheric O3 formation due to higher temperatures, etc. At the same time, previously unavailable areas may open up to enable some economic activities to shift or ex- pand, such as drilling for oil and opening new shipping routes in the Arctic.
Before the start of the industrial revolution, emissions of BC and tropospheric O3 pre- cursors were essentially driven by natural
episodic events – predominantly wildfires, vol- canoes and other geogenic releases – and by the same kinds of continuous natural releases that are seen today, such as CH4 emissions from wetlands, biogenic NMVOCs, and NOX from lightning. Anthropogenic emissions at that time were limited to biomass burning for cooking and heating and represented only a small fraction of total emissions: for O3 pre- cursors and SO2 less than 10 per cent, while for BC about a third of the total (Lamarque et al., 2010). Some episodic releases, such as the large volcanic eruptions of El Chichon (1982) and Mount Pinatubo (1991), injected enor- mous amounts of ash, dust and gases into the atmosphere and had worldwide effects lasting two or three years (McCormick et al., 1995).
The same is true of the large wildfires that
periodically break out in Siberia and are often initiated by lightning strikes. Large
boreal wildfires may also periodically inject
smoke into the lower stratosphere (Fromm et al., 2005). These episodic natural emissions can be of similar magnitude to anthropo- genic emissions. Even some of the continu- ous natural emissions are on the same scale as present-day human-made emissions, ex- amples of which include CH4 from wetlands, bogs, swamps and tundra (200–400 Mt/yr, from Fung et al., 1991); biogenic NMVOCs (500–750 Mt/yr, from Guenther et al., 2006); volcanic SO2 (13–15 Mt/yr, from Andres and Kasgnoc, 1998); lightning NOX (about 5 Mt Nitrogen/year, from Schumann and Huntrieser, 2007) and NMVOCs from biomass burning (about 80 Mt/yr, from Lamarque et al., 2010).
16
Though emissions from natural sources can show large inter-annual variability, they have
not shown significant long-term trends (except
for open biomass burning which has been growing strongly since the 1950s, largely from deforestation and land clearance, and hence is no longer exclusively natural). Anthropogenic emissions, on the other hand, have increased dramatically since pre-industrial times: for ex- ample, SO2 and NOX emissions had increased nearly 50-fold and emissions of BC and CO by factors of 5 and 10 respectively by the year 2000 (Lamarque et al., 2010).
In the early to mid-19th century, widespread industrialization saw the advent of the use of fossil fuels, initially coal, to provide energy to power the production of goods and ser- vices. The development of the Haber-Bosch process for producing nitrogen fertilizer at the beginning of the 20th century led to an unprecedented growth in food production that supported strong population growth but at the same time caused a cascade of environmental changes, including increased air pollution and the perturbation of green- house-gas levels (Erisman et al., 2008). At the same time, biofuels were being consumed in large amounts wherever human settlement occurred in timber-rich parts of the world (Fernandes et al., 2007). At the beginning, the burning of wood and coal in factories and
homes and on railways was an inefficient pro- cess that paid no regard to smoke emissions; large quantities of PM, rich in the carbona- ceous products BC and OC, were therefore generated. Gaseous species including NOX, CO, NMVOCs and SO2 were produced at the same time in varying quantities.
Anthropogenic emissions began to grow rap- idly, and it was not until the 1920s that atten- tion began to be paid to limiting the emis- sions of PM through various add-on removal devices. The economic depression of the 1930s and installation of rudimentary emis- sion controls led to a modest decline in most emissions. In many parts of the world, how- ever, wartime production reversed this trend in the 1940s, and emissions began to increase rapidly in the post-war economic revival, beginning in about 1950. This trend spread
quickly to most parts of the world, especially in the northern hemisphere, which until now has been responsible for the majority of hu- man-caused pollution. As emissions grew and associated environmental and health impacts – acid rain, respiratory diseases, and heavy metals poisoning – became more evident, se- rious attention started to be paid to prevent- ing atmospheric emissions in the early 1970s, at least in the developed world. The last few decades have seen a strong decline in emis- sions of PM and acidifying pollutants from industrial and transport sources in the devel- oped world. However, since the late 1990s, a rapid increase in emissions in Asia has offset these reductions, often leading to continued growth at global level. Emissions of CO2 have kept growing, and only the most recent economic crisis resulted in their temporary stabilization (Friedlingstein et al., 2010). For the future, it is generally expected that growth in emissions of air pollutants will be tempered by continuing pressure to achieve ever more ambitious environmental targets. In some cases emissions will be ameliorated simply by economic development, which will, for example, gradually replace residential solid fuel combustion with cleaner fuels and remove older vehicles from the road. This will go some way towards reducing emissions of PM, including BC and OC, as well as hydrocarbons and CO. For other species, primarily SO2 and NOX, where there is now a fairly long history of pollution control, the experience of developed countries can be expected to spread gradually throughout the world and hold down future emissions; recent emission inventories and assessments
in Asia seem to confirm this assumption (for
example, Zhang et al., 2009a; Klimont et al., 2009; Xu et al., 2009). For some sources of all
species, however, mitigation is either difficult
or expensive and, without new policies, emissions will persist for the foreseeable future. This may lead to growth in total global emissions if the spread of cleaner technology cannot keep pace with economic and industrial growth. Emission projections are sensitive to the socioeconomic attributes of the scenarios, however, and it is not always clear which pathways will generate the
17 highest emissions. For example, while high
economic growth is likely to result in high levels of emissions, it may also promote the transfer of high-performing technologies to less developed countries.