Science Briefs

Tracking Carbon Dioxide Emissions from Fossil Fuel Burning

The largest single source of atmospheric carbon dioxide (CO2) is the burning of fossil fuel (coal, oil and gas), which currently accounts for ~80% of the annual emission of CO2 into the atmosphere. Nitrogen is also a byproduct of fossil fuel use. Fossil fuels, therefore, play a pivotal role in the global budget of carbon, acting as both a source of CO2 and of atmospheric nitrogen which can fertilize the biosphere and stimulate it to sequester carbon.

Statistics indicate that fossil fuel use accounts for emissions to the atmosphere of ~5.5 GtC (GtC = billions of metric tons of carbon) annually. Other important processes in the global CO2 budget are tropical deforestation, estimated to generate about 1.6 GtC/yr; absorption by the oceans, removing about 2.0 GtC/yr; and regrowth of northern forests, taking up about 0.5 GtC/yr. However, accurate measurements of CO2 show that the atmosphere is accumulating only about 3.3 GtC/yr. The imbalance of about 1.3 GtC/yr, termed the "missing sink", represents the difference between the estimated sources and the estimated sinks of CO2. This imbalance has increase consistently since around 1920 and has undergone substantial increases since 1950.

Figure
Latitudinal distribution of decadal carbon emission data. Horizontal units are 106 metric tons.

Because CO2 is not chemically active, it either accumulates in the atmosphere or is taken up by the oceans or by the terrestrial biosphere (vegetation and/or soils). Various atmospheric, oceanic, and biospheric modeling studies suggest that Earth's biosphere has been a sink for the "missing" CO2 since the 1950s. This sink may be driven by deliberate human action and/or by feedbacks and interactions between the biosphere and the changing climate and atmosphere. Probable sink mechanisms include climate variability, direct fertilization of the biosphere (i.e., enhanced photosynthetic uptake due to higher CO2 levels in the atmosphere) and indirect fertilization due to enhanced nitrogen deposition from fossil-fuel use and other human activities.

The magnitude, distribution and history of the biospheric carbon sink possible due to these mechanisms has been studied using models which incorporate the various mechanisms. Most of these studies assume that CO2 emissions from fossil-fuel use and deforestation, as well as oceanic uptake and atmospheric accumulation, are fairly well-known, and then evaluate the potential role of other processes in accounting for the increasing difference between known sources and sinks. These studies suggest the existence of a biospheric CO2 sink, but the way it functions and evidence for its operation in ecosystems has not yet been established or measured.

In order to reduce the uncertainty in our knowledge of historical carbon sources and sinks, Bob Andres and Gregg Marland (Oak Ridge National Labs), Elaine Matthews (GISS) and Inez Fung (GISS, Univ. Victoria) have compiled historical statistics on fossil fuel use and combined them with data on locations of population densities and national boundaries to reconstruct the historical and spatial distribution of CO2 emissions between 1950 and 1990 (Andres et al. 1996). The resulting datasets are gridded at 1° latitude/longitude spatial resolution and at decadal increments.

The accompanying figure shows the latitudinal distribution of CO2 emission from fossil fuels for five decades from 1950 to 1990. The data show that the global fossil fuel CO2 source has increased 3.5-fold, going from 1.59 GtC in 1950 to 5.81 GtC in 1990. The data also indicate that while industrialized economies in the north temperate regions continue to account for the major portion of fossil fuel use, developing economies in the subtropics and tropics are contributing an increasing fraction of the total, particularly in the last twenty years.

Reference

Andres, R.J., G. Marland, I. Fung, and E. Matthews 1996. A 1°x1° distribution of carbon dioxide emissions from fossil fuel consumption and cement manufacture, 1950-1990. Global Biogeochem. Cycles 10, 419-429.

Contact

Please address all inquiries about this research to Elaine Matthews.