Working Group I: The Scientific Basis

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3.3 Palaeo CO2 and Natural Changes in the Carbon Cycle 3.3.1 Geological History of Atmospheric CO2

Atmospheric CO2 concentration has varied on all time-scales during the Earth�s history (Figure 3.2). There is evidence for very high CO2 concentrations (>3,000 ppm) between 600 and 400 Myr BP and between 200 and 150 Myr BP (Figure 3.2f). On long time-scales, atmospheric CO2 content is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and vulcanism (Berner, 1993, 1997). In particular, terrestrial vegetation has enhanced the rate of silicate weathering, which consumes CO2 while releasing base cations that end up in the ocean. Subsequent deep-sea burial of Ca and Mg (as carbonates, for example CaCO3) in the shells of marine organisms removes CO2. The net effect of slight imbalances in the carbon cycle over tens to hundreds of millions of years has been to reduce atmospheric CO2. The rates of these processes are extremely slow, hence they are of limited relevance to the atmospheric CO2 response to emissions over the next hundred years.

Figure 3.3:
Fossil fuel emissions and the rate of increase of CO2 concentration in the atmosphere. The annual atmospheric increase is the measured increase during a calendar year. The monthly atmospheric increases have been filtered to remove the seasonal cycle. Vertical arrows denote El Niño events. A horizontal line defines the extended El Niño of 1991 to 1994. Atmospheric data are from Keeling and Whorf (2000), fossil fuel emissions data are from Marland et al. (2000) and British Petroleum (2000), see explanations in text.

It is pertinent, however, that photosynthesis evolved at a time when O2 concentrations were far less than at present. O2 has accumulated in the atmosphere over geological time because photosynthesis results in the burial of reduced chemical species: pyrite (FeS2) derived from sulphur-reducing bacteria, and organic carbon. This accumulation has consequences for terrestrial and marine ecosystems today. Primary production is carbon limited in terrestrial ecosystems in part because of (geologically speaking) low CO2 concentrations, and in part because Rubisco (the enzyme that fixes CO2 in all plants) also has an affinity for O2 that reduces its efficiency in photosynthesis (see Section Primary production is iron limited in some marine ecosystems mainly because of the extreme insolubility of Fe(III), the predominant form of iron in the present, O2-rich environment. These difficulties faced by contemporary organisms represent a legacy of earlier evolution under very different biogeochemical conditions.

In more recent times, atmospheric CO2 concentration continued to fall after about 60 Myr BP and there is geochemical evidence that concentrations were <300 ppm by about 20 Myr BP (Pagani et al., 1999a; Pearson and Palmer, 1999, 2000; Figure 3.2e). Low CO2 concentrations may have been the stimulus that favoured the evolution of C4 plants, which increased greatly in abundance between 7 and 5 Myr BP (Cerling et al., 1993, 1997; Pagani et al., 1999b). Although contemporary CO2 concentrations were exceeded during earlier geological epochs, they are likely higher now than at any time during the past 20 million years.

3.3.2 Variations in Atmospheric CO2 during Glacial/inter-glacial Cycles

The purity of Antarctic ice allows the CO2 concentration in trapped air bubbles to be accurately measured (Tschumi and Stauffer, 2000). The CO2 record from the Vostok ice core is the best available for the glacial/inter-glacial time-scale and covers the past four glacial/inter-glacial cycles (420 kyr) with a resolution of 1 to 2 kyr (Petit et al., 1999; Fischer et al., 1999). The general pattern is clear (Figure 3.2d): atmospheric CO2 has been low (but 180 ppm) during glacial periods, and higher (but 300 ppm) during interglacials. Natural processes during the glacial-interglacial cycles have maintained CO2 concentrations within these bounds, despite considerable variability on multi-millenial time-scales. The present CO2 concentration is higher than at any time during the 420 kyr period covered by the Vostok record.

The terrestrial biosphere stores 300 to 700 Pg more carbon during interglacial periods than during glacial periods, based on a widely accepted interpretation of the 13C record in deep-sea sediments (Shackleton, 1977; Bird et al., 1994; Crowley, 1995). Terrestrial modelling studies (e.g., Friedlingstein et al., 1995b; Peng et al., 1998) have reached the same conclusion. Thus, the terrestrial biosphere does not cause the difference in atmospheric CO2 between glacial and interglacial periods. The cause must lie in the ocean, and indeed the amount of atmospheric change to be accounted for must be augmented to account for a fraction of the carbon transferred between the land and ocean. The mechanism remains controversial (see Box 3.4). In part this is because a variety of processes that could be effective in altering CO2 levels on a century time-scale can be largely cancelled on multi-millenial time-scales by changes in CaCO3 sedimentation or dissolution, as discussed in Section

Box 3.5: The use of O2 measurements to assess the fate of fossil fuel CO2.

The amount of CO2 that remains in the atmosphere each year has been consistently less than the amount emitted by fossil fuel burning. This is because some CO2 dissolves and mixes in the ocean, and some is taken up by the land. These two modes of uptake have different effects on the concentration of O2 in the atmosphere. Fossil fuel burning consumes O2 and causes a decline in atmospheric O2 concentration (Figure 3.4). Dissolution of CO2 in the ocean has no effect on atmospheric O2. Terrestrial uptake of CO2, by contrast, implies that photosynthesis (which releases O2) is exceeding respiration and other oxidation processes, including fire (which consume O2). Thus, net terrestrial uptake of CO2 implies a net release of O2, in a known stochiometric ratio. This difference can be used to partition the total CO2 uptake into land and ocean components, as shown graphically in Figure 3.4. Strictly speaking, the atmospheric O2 � CO2 budget method can only distinguish between net non-biological ocean uptake and net biospheric uptake, which in principle includes both the terrestrial and the marine biosphere. However, since biological oxygen uptake is not expected to have changed significantly during recent decades because of nutrient limitations in most parts of the ocean (see Section, this inferred biospheric uptake is attributed to the land.

Measurement of changes in O2 presents a technical challenge because changes of a few ppm caused by fossil fuel burning have to be determined against a background concentration of 209,000 ppm (about 21%). For technical reasons, O2 is measured relative to N2, the main constituent of the atmosphere, as a reference gas. For simplicity this chapter refers to O2 concentrations, although strictly it is O2 : N2 ratios that are measured. The impact of nitrification-denitrification changes on atmospheric N2 content are assumed not to be problematic because they are small and the inventory of N2 is very large. Increases in ocean temperatures (Levitus et al., 2000), because of their effect on the temperature dependent solubility, induce small outgassing fluxes of O2 and N2 (Keeling et al., 1993) that have to be taken into account (see Figure 3.4) although their magnitude is only approximately known. Impacts on atmospheric O2 caused by changes in the ventilation of deeper, oxygen depleted waters have been observed on interannual time-scales (Keeling et al., 1993, Bender et al., 1996). They could also occur on longer time-scales, e.g., through increased ocean stratification induced by ocean warming.

Orbital variations (Berger, 1978) are the pacemaker of climate change on multi-millenial time-scales (Hays et al., 1976). Atmospheric CO2 is one of many Earth system variables that show the characteristic �Milankovitch� periodicities, and has been implicated as a key factor in locking natural climate changes to the 100 kyr eccentricity cycle (Shackleton, 2000). Whatever the mechanisms involved, lags of up to 2,000 to 4,000 years in the drawdown of CO2 at the start of glacial periods suggests that the low CO2 concentrations during glacial periods amplify the climate change but do not initiate glaciations (Lorius and Oeschger, 1994; Fischer et al., 1999). Once established, the low CO2 concentration is likely to have enhanced global cooling (Hewitt and Mitchell, 1997). During the last deglaciation, rising CO2 paralleled Southern Hemisphere warming and was ahead of Northern Hemisphere warming (Chapter 2).

During glacial periods, the atmospheric CO2 concentration does not track the �fast� changes in climate (e.g., decade to century scale warming events) associated with Dansgaard-Oeschger events, although there are CO2 fluctuations of up to 20 ppm associated with the longer-lived events (Stauffer et al., 1998; Indermühle et al., 2000) (see Chapter 2 for explanations of these terms). During the last deglaciation, atmospheric CO2 concentration continued to increase, by about 12 ppm, through the Younger Dryas cold reversal (12.7 to 11.6 kyr BP) seen in Northern Hemisphere palaeoclimate records (Fischer et al., 1999; Smith et al., 1999). Palaeo-oceanographic evidence shows that the Younger Dryas event was marked by a prolonged shut-down of the thermohaline circulation, which is likely to have been triggered by the release of melt water into the North Atlantic. Similar behaviour, with a slight rise in CO2 accompanying a major Northern Hemisphere cooling and shutdown of North Atlantic Deep Water production, has been produced in a coupled atmosphere-ocean model (Marchal et al., 1998). The observed CO2 rise during the Younger Dryas period was modest, suggesting that atmospheric CO2 has, under natural conditions, been well buffered against abrupt changes in climate, including thermohaline collapse. This buffering is a direct consequence of the large reservoir of DIC in the ocean.

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