What New Evidence Is There About Processes That Result In Changes In Greenhouse Gases And Aerosol Concentrations In The Atmosphere?


Variations In The Annual Rate Of Increase Of Atmospheric CO2

Highly unusual variations in the global carbon balance have been occurring over the past five years. In the 1960s and early 1970s, the CO2 concentration increased at a rate of about 1.0 part per million by volume (ppmv) per year, varying from even lowest rates to increases of more than 1.5 ppmv/yr. For much of the 1980s the concentration of CO2 in the atmosphere increased at about twice the earlier annual rate of rise, averaging about 1.5 ppmv/yr. [Note that annual fossil fuel emission of 5.5 GtC/yr and biomass emissions of about 1.5 GtC/yr are equivalent to increments of about 2.5 and 0.7 ppmv/yr, or about 3.2 ppmv/yr total. 1 GtC = 1 billion tonnes of carbon] From 1989 to 1993, the growth rate dropped significantly, averaging only about 1.0 ppmv/yr, reaching as low as 0.5 ppmv/year. Such a sustained decrease had not previously been observed in the 35-year record. The downturn does not appear to have been caused by any significant reduction of human-induced CO2 emissions, because global CO2 emissions from fossil fuel have actually increased slightly over this period. It is hypothesized that either an enhanced natural sink in the terrestrial biosphere (or possibly in the oceans) or a reduced natural source is likely responsible for the slowdown in the rate of CO2 increase. Analyses of the carbon-climate record and indications that the CO2 growth rate has returned to higher levels during late 1994 and early 1995 suggest that this unusual downturn in carbon dioxide growth rates may be primarily a climate-related phenomenon resulting from recent variations in the global climate, especially in air temperature and possibly precipitation. Research is underway to understand the effects of climate and other factors on the dynamics of the carbon cycle.
Reference: Interannual Extremes in the Rate of Rise of Atmospheric Carbon Dioxide Since 1980, Keeling, C. D., Nature, Vol. 375, pp. 666-670, 1995.

Worldwide Decrease In Carbon Monoxide Concentration Suggests Important Changes In Tropospheric Oxidation Processes

Carbon monoxide (CO) plays an important role in the oxidizing capacity of the Earth's atmosphere and may therefore indirectly affect the concentration of many man-made and natural trace gases. By changing their concentrations, the CO can, in turn, affect climate, atmospheric chemistry, and the ozone layer. CO is produced in the atmosphere by the oxidation of methane and other hydrocarbons and is released into the atmosphere from automobiles, agricultural waste, and the burning of grasslands and other areas. Recent estimates show that human activities such as those are presently responsible for more than half of the annual global emissions of CO. During the 1980s CO emissions were increasing at roughly 1.2% per year. However, over the period 1988 to 1992, the global CO concentration started to decline, with the rate of decline increasing sharply to about -2.6% per year. Possible explanations for the CO decrease are increases in the atmospheric hydroxyl (OH) concentration, decreased CO emissions from biomass burning, or decreased CO production from a decrease in the oxidation of non-methane hydrocarbons. Whatever the cause, the total amount of CO in the atmosphere is now less it was a decade ago. A continuing trend of decreasing CO may signify important changes in atmospheric oxidation processes, which could affect the rates at which methane and CFC replacements are removed from the atmosphere.
References: (1) Global Decrease in Atmospheric Carbon Monoxide Concentration, Khalil, M. A., and R. A. Rasmussen, Nature, Vol. 370, pp. 639-641, 1994; (2) Recent Changes in Atmospheric Carbon Monoxide, Novelli, P. et al., Science, Vol. 263, pp. 1587-1590, 1994.

New Capabilities To Measure The Oxygen To Nitrogen Ratio Will Help Determine Sources Of CO2

A recent development in research on the global carbon cycle is the ability to measure the atmospheric oxygen/nitrogen ratio with high precision. With this technique, new information on the movement of carbon can be derived by examining variations over the seasonal cycle, the interhemispheric gradient, and the long-term trend of the oxygen/nitrogen ratio of the atmosphere. The long-term trend in the oxygen/nitrogen ratio, for example, can be used to subdivide the overall CO2 sink into its oceanic and terrestrial components. Also, the long-term trend in O2 loss, and the interhemispheric gradient can be used to place limits on CO2 emission estimates resulting from changes in land cover, land use, and land management. This technique will contribute significantly to improved understanding (and, therefore, reduced uncertainty) in determining the sources of atmospheric CO2 and the role of the oceans and oceanic processes in limiting the increase in the atmospheric CO2 concentration.
Reference: What Atmospheric Oxygen Measurements Can Tell Us About the Global Carbon Cycle, Keeling, R. F. et al., Global Biogeochemical Cycles, Vol. 7, pp. 37-67, 1993.

Arctic Ecosystems May Be A Source Of CO2

New evidence indicates that Arctic ecosystems may be acting as a net source of CO2 to the atmosphere and potentially accelerating global climate change. Measurements made at an Arctic tundra ecosystem in Barrow, Alaska in the early 1970s found that it was a net sink for CO2. However, this ecosystem site was re-measured and found to be a source of CO2 to the atmosphere in the early 1990s. Indications that the Arctic tundra is releasing CO2 to the atmosphere are increasing. Despite absorption of carbon in the wettest habitats, many of the geographic regions that have been investigated, including in the Alaskan, Icelandic, and Russian Arctic, have been found to be net sources of CO2 to the atmosphere. Other evidence indicates that losses of CO2 during snow covered periods may even exceed losses during the snow free-period, that the loss of carbon is increased for up to 30 years following fire in tussock tundra, and that Arctic ecosystems adjust relatively quickly adjust to elevated atmospheric CO2, so that there is likely not a CO2 "fertilizer" effect from the increasing atmospheric concentrations. In that the tundra region contains as much as 180 GtC as soil organic matter (equivalent to about 30 years worth of fossil fuel emissions at current emission rates), its release to the atmosphere even in part could have a major positive feedback on the CO2 concentration and on climate change.
References: (1) Recent Changes of Arctic Tundra Ecosystems from a Net Carbon Dioxide Sink to a Source, Oechel, W. C. et al., Nature, Vol. 361, pp. 520-523, 1994; (2) Transient Nature of CO2 Fertilization in the Arctic Tundra, 1995, Oechel, W. C. et al., Nature, Vol. 371, pp. 500-503.

Availability Of Iron Can Limit Atmospheric CO2 Removal By The Tropical Pacific Ocean

An open ocean experiment demonstrated that phytoplankton photosynthesis in certain regions of the equatorial Pacific Ocean is impaired by the lack of iron in sea water, resulting in a significant reduction in the efficiency with which light is converted to stored chemical energy. Consequently, the growth of phytoplankton in the equatorial Pacific is physiologically limited by iron rather than processes such as grazing. These results add credence to theories which suggest that large inputs of dust from the continents to high nitrate, low chlorophyll regions of the open sea could have contributed to greater oceanic uptake of atmospheric CO2 during glacial (global cooling) epochs. These results also suggest that future increases in desertification could bring about increased iron fluxes to certain regions of the open ocean, which could, in turn, stimulate marine phytoplankton growth and increase the rate at which the ocean will absorb CO2.
Reference: (1) Iron Limitation of Phytoplankton Photosynthesis in the Equatorial Pacific Ocean, Kolber, Z., R. T. Barber, K. H. Coale, S. E. Fitzwater, R. M. Greene, K. S. Johnson, S. Lindley, and P. G. Falkowski, Nature, Vol. 371, pp. 145-149, 1994; (2) Dynamical Limitations on the Antarctic Iron Fertilization Strategy, Peng, T.H., and W.S. Broecker, Nature, 349: 227-229, 1991.

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Last updated 04/10/96