Trends in Global Emissions: Carbon, Sulfur, and Nitrogen

Global anthropogenic emissions of carbon, sulfur, and nitrogen are reviewed by major category. Both present and historical emissions are reviewed including respective uncertainty ranges. The article concludes with a brief discussion of how to relate trends in emissions to the evolution of their underlying driving forces using energy-related carbon emissions as an example. Main conclusions for the three gases are summarized below: Anthropogenic emissions of carbon are small compared to the sizes of natural carbon reservoirs and the annual flow rates between them. Yet they constitute important perturbations of the natural carbon cycle, well quantified for industrial carbon emissions (mostly the burning of fossil fuels), but remaining uncertain for carbon emissions arising from biomass burning and land-use changes. Since the onset of the Industrial Revolution, some 530 billion tons of carbon (PgC) gross emissions have been released by human activities to the atmosphere, part of which was balanced by (highly uncertain) terrestrial biospheric carbon sinks. Net carbon emissions over the period 1800-2000 are estimated at 280 PgC. After accounting for oceanic uptake, these emissions have increased the atmospheric carbon loading by 200 PgC, or by one-third natural levels. In the absence of stringent climate policies, between 770 and 2540 PcG could be released to the atmosphere over the next 100 years. A significant amplification of humankind's "discernible influence on the climate system" is thus likely. Anthropogenic emissions of sulfur have surpassed natural flows ever since the first quarter of the 20th century. Since about 1975 global emissions (mostly from burning of sulfur-rich fuels such as coal and oil) have stayed roughly constant. A continued rise in sulfur emissions in developing countries has been compensated by drastic declines in emissions in Organization for Economic Cooperation and Development (OECD) countries as a result of environmental policies. Similar policies will be required also in developing countries to counter local and regional environmental impacts of high sulfur emissions. A slowdown and ultimately a trend reversal of sulfur emission growth trends also in developing countries could thus be likely over the next few decades. Emissions of nitrogen take a variety of forms and originate from a wide array of different sources including: nitrogen oxides, principally formed in high temperature combustion (burning of fossil fuels); ammonia, principally arising from animal manure; and nitrous oxide (N2O), a powerful greenhouse, principally arising from soil microbial process as well as agricultural activities and animal manure. Because of the multitude of gases and sources, emission estimates are poorly understood and uncertain. There is also a paucity of both past and present inventory data and future emission scenarios. More research is therefore needed before considering policy interventions to control these gases beyond well established source categories (e.g. nitrogen oxide emissions from automobiles, or nitrous oxide emissions from adipic acid production)