Variation in atmospheric gas content and climate change

Direct measurements of the atmospheric concentration of the greenhouse gas carbon dioxide (C02) began in 1958, and this figure has shown a significant increase from 315 parts per million by volume (ppmv) to 364 ppmv in 1997 (data from Keeling et al, 1995; Keeling and Whorf, 1998; Fig. 2.2). The increase in atmospheric CO2 is caused by burning of fossil fuels and a change in land use. Previous investigations showed that the atmospheric C02 concentration was about 280 ppmv

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Figure 2.2 Atmospheric carbon dioxide concentrations (ppmv) derived from in situ air samples collected at Mauna Loa Observatory, Hawaii. (Data from C.D. Keeling and T.P. Whorf's (1998) web page)

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10 20 30 40 50 Ice or gas age (thousands of year ago)

Figure 2.3 The Byrd ice-core record of C02, CH4 and N20 during the last 50,000 years, with the <S180 climatic record for comparison. (Adapted from Raynaud et al, 1992)

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10 20 30 40 50 Ice or gas age (thousands of year ago)

Figure 2.3 The Byrd ice-core record of C02, CH4 and N20 during the last 50,000 years, with the <S180 climatic record for comparison. (Adapted from Raynaud et al, 1992)

before industrialization, and that only minor changes of approximately 5ppmv occurred throughout the pre-industrial part of the last millennium (Etheridge et al, 1996). The atmospheric COz concentration, however, increased from about 200 to 270 ppmv during the transition from the last glacial maximum (ca. 20,000 yr bp) to the beginning of the Holocene (11,000 yr bp) (Neftel et al., 1988). Between 46,000 and 18,000 yr bp, fluctuations of about 20 ppmv occurred on a millennial time scale.

The causes of these variations are considered to reflect variations in production rates and operations of sources and sinks of different gases (Sundquist, 1993). During glacial periods of sparse vegetation cover, the oceans acted as a C02 sink. In contrast, warming of the oceans in the Late-glacial and early Holocene caused C02 degassing to the atmosphere, where it was subsequently included in terrestrial vegetation and organic soils. Natural emission of methane occurs from peatland, animals, burning of organic material, oceans and lakes, of which variations in wetland extent and composition seem to have been the major source (Chappelaz et al, 1990; Blunier et al, 1995).

Records of gas concentrations from ice cores seem to correspond closely with temperature curves inferred from oxygen isotopes from the ice sheets (Fig. 2.3). Both carbon dioxide and methane are closely in phase with the climatic signal during deglaciation periods. At the beginning of a glacial, in contrast, methane is in phase but carbon dioxide lags behind (Raynaud et al,

VOLCANIC ACTIVITY AND CLIMATE VARIATIONS 13

1992). Spectral analysis of the C02 and methane contents in the Vostok ice core (Antarctica) shows a maximum at ca. 21,000 yr bp and a weaker maximum at 41,000 yr bp (precessional and obliquity periodicities) (Barnola et al, 1987; Lorius et al, 1988; 1990). Variations in the C02 and methane concentrations are estimated to have caused about half of the temperature fluctuations recorded in the Vostok ice core (Lorius et al, 1988, 1990). Fluctuations in the greenhouse gases probably represent a complicated response to orbitally driven climate changes, and are therefore important elements in the cause (s) of long- and short-term climatic variations.

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