Info

Age (million years)

Figure 4.7 Climatic megacycles during the Phanerozoic. The hothouse-icehouse cycle has a period of about 300 million years; the warm-mode-cool-mode cycle has a period of about 150 million years. Notice that the two cycle 'conflict' at times. For instance, cool modes during the hothouse phase prevailing during the Late Cambrian, Ordovician, Silurian, and Devonian, and during the hothouse phase prevailing during the Jurassic, Cretaceous, and Late Eocene. Sources: Hothouses and icehouses adapted from Fischer (1981, 1984); warm and cool modes adapted from Frakes et al. (1992); generalized temperature curve after R. E. Martin (1995).

with interglacial terminations, warm mode terminations are gradual and geographically variable; and as with the onset of interglacial conditions, the onset of warm modes is sudden and somewhat puzzling - it is difficult to see why some cool modes ended (Frakes et al. 1992, 195).

Galactic processes may drive the climatic megacycles. The passage of the Solar System about the centre of the Galaxy during the course of a Galactic year may explain the pulse of glaciations. As long ago as 1909, Friedrich Nolke postulated that galactic dust might cause ice ages. Later, Harlow Shapley (1921) expressed the view that cosmic processes might influence the Earth's climate, and William Trowbridge Merrifield Forbes (1931) drew attention to a possible connection between the revolution of the Solar System about the Galactic centre, then estimated to take 230 million years, and an apparent 210-million-year pulse of major glaciations. This link was explored further by several scientists (e.g. Umbgrove 1939, 1940, 1942; Shapley 1949; Steiner 1967, 1973, 1978, 1979; Williams 1972, 1975a, 1981; Steiner and Grillmair 1973). The more recent work had the advantage of a more accurate chronology of events and gave the mean period of mean ages of glaciations as about 155 million years. With a Galactic year of 303 million years, that meant that two glaciations occur during every revolution of the Solar System about the centre of the Galaxy (Williams 1975a, 1981).

Nir J. Shaviv and Jan Veizer (2003) argue that changes in cosmic ray flux through the Phanerozoic have strongly influenced some of the climatic signal, claiming that fluctuations in cosmic ray flux reaching the Earth can explain two-thirds of the Phanerozoic temperature variance and that climate is less sensitive to a carbon dioxide levels than was previously thought (Figure 4.8). They base their argument partly on present modulation of the cosmic ray flux by the solar wind. Research suggests that an increase in solar activity results in an enhanced thermal energy flux, and in a more intense solar wind that weakens the cosmic ray flux reaching the Earth. A weaker cosmic ray flux appears to produce less low-altitude cloud cover over days to decades (sunspot cycle). The hypothesized cause-and-effect sequence runs thus: a brighter sun produces an enhanced thermal flux and enhanced solar wind, which reduces the cosmic ray flux, so generating fewer low-level clouds, a lower albedo, and a warmer climate. A decrease in solar activity has the opposite effect, ultimately resulting in a cooler climate. To extend this line of reasoning to geological time-scales, they reconstruct cosmic ray fluxes for the past billion years using exposure age data on 50 iron meteorites (about 20 of which date from the Phanerozoic) and a simple model estimating cosmic ray flux induced by the Earth's passage through Galactic spiral arms (Shaviv 2002, 2003). Shaviv proposed that the cosmic ray flux reaching the Earth varies owing to attenuation by solar wind and to variations in the interstellar environment. For instance, a nearby supernova may bathe the Solar System with a high level of cosmic rays for many millennia, the increased cloudiness and higher planetary albedo perhaps causing a 'cosmic ray winter' (Fields and Ellis 1999). A particularly large variation in cosmic ray flux should arise from passages of the Solar System through the Milky Way's spiral arms, which harbour most of the star formation activity (Shaviv 2002, 2003). Such passages occur every 143 ±10 million years or thereabouts, intervals similar to the 135 ±9 million-year recurrence of some palaeoclimate data (Veizer et al. 2000).

Interesting though Shaviv and Veizer's (2003) model be, it is not without its critics. Stefan Rahmstorf and his colleagues (2004) argued that the correlation between cosmic ray flux and climate during the Phanerozoic collapses under close inspection. Even when accepting the questionable assumption that meteorite clusters give information on cosmic ray flux variations, they found that the evidence for a link between cosmic ray flux and cli mate 'amounts to little more than a similarity in the average periods of the cosmic ray flux variations and a heavily smoothed temperature reconstruction', and that phase agreement is poor. Dana L. Royer and his associates (2004) considered Shaviv and Veizer's work and came to four conclusions. First, proxy estimates of Phanerozoic carbon dioxide levels agree, within modelling errors, with GEOCARB model results. Second, there is a good correlation between low levels of atmospheric carbon dioxide and the presence of well-documented, long-lasting, and spatially extensive continental glaciations. Third, the uncorrected Veizer temperature curve predicts long periods of intense global cooling that do not match independent observations of palaeoclimate, especially during the Mesozoic, but if corrected for pH effects, the temperature curve shows much closer agreement with the glacial record. Fourth, global temperatures inferred from the cosmic ray flux model of Shaviv and Veizer (2003) do not correlate in amplitude with the temperatures recorded by Veizer et al. (2000) when corrected for past changes in oceanic pH. Royer et al. (2004) own that changes in cosmic ray flux may affect climate but they do not dominant it over a multimil-lion-year time-scale. Nonetheless, if Shaviv and Veizer's model does no more than stimulate healthy debate, it will have served a constructive propose.

The interplay of carbon dioxide addition and withdrawal rates from the atmosphere-ocean system, which perhaps the supercontinent cycle drives (Veevers 1990), may produce similar megashifts in geological climates (Fischer 1981, 1984). Volcanism adds carbon dioxide to the system, and weathering removes it as gaseous carbon dioxide converts to carbonates. Very different factors govern these two processes, but their action will always strive towards a steady-state level of atmospheric carbon dioxide. Volcanism increases during bouts of accelerated mantle convection when plate fragmentation and

Figure 4.8 Phanerozoic climatic indicators, reconstructed pCO2 levels, cosmic ray flux, and tropical temperature anomaly. (a) Detrended running means of S18O values of calcitic shells over the Phanerozoic (Veizer et al. 2000). 3/6 and 10/50 indicate running means at two temporal resolutions (e.g. 3/6 means step 3 million years, window 6 million year averaging). The palaeolatitude of ice-rafted debris (PIRD) is on the right-hand vertical axis. The available, Palaeozoic, frequency histograms of other glacial deposits (OGD), such as tillites and glacial marine strata, are dimensionless. The top bars show cool climate modes (icehouses) and the warm modes (greenhouses), as established from sedimentological criteria (Frakes and Francis 1988; Frakes et al. 1992). The lighter shading for the Jurassic-Cretaceous icehouse reflects the fact that true polar ice caps have not been documented for this time interval. (b) Reconstructed histories of the past pCO2 variations (GEOCARB III) by Berner and Kothavala (2001) and Rothman (2002). The pC02(0) is the present-day atmospheric CO2 concentration. All data are smoothed using a running average of 50 million years with 10-million-year bins. The hatched regions depict the uncertainties quoted in the Rothman and the GEOCARB reconstructions. (c) Tropical temperature anomaly (AT variations over the Phanerozoic. The small-dashed line depicts the 10/50 million year, smoothed temperature anomaly (AT) from Veizer et al. (2000). The solid black line is the predicted AT model for short-dashed line on the cosmic ray flux (d), taking into account also a secular long-term linear contribution. The long-dashed line is the residual. The largest residual is at 250 million years ago, where only a few measurements of S18O exist due to the dearth of fossils subsequent to the largest extinction event in Earth history. (d) The cosmic ray flux, F, reconstructed using iron meteorite exposure age data (Shaviv 2003). The solid black line depicts the nominal cosmic ray flux, with the grey shading delineating the allowed error range. The two long-dashed curves are additional cosmic ray flux reconstructions that fit within the acceptable range (together with the solid black line, these three curves denote the three cosmic ray flux reconstructions used in the model simulations). The short-dashed curve describes the nominal cosmic ray flux reconstruction after its period was fine-tuned to best fit the low-latitude temperature anomaly (that is, it is the 'solid black' reconstruction, after the exact cosmic ray flux periodicity was fine tuned, within the cosmic ray flux reconstruction error). Source: Adapted from Shaviv and Veizer (2003).

Warm Cool Warm Cool Warm Cool Warm Cool

Warm Cool Warm Cool Warm Cool Warm Cool

Phanerozoic Pco2
Age (million years)

plate movement occur. At these times, the supply of carbon dioxide into the atmosphere-ocean system increases. Associated with plate activation is an increase in the volume of mid-ocean ridges leading to marine transgressions. Less land area then being available, the atmosphere cannot lose carbon dioxide by weathering so fast as previously. The net effect of increased mantle convection is thus to pump up carbon dioxide levels in the atmosphere-ocean system, until a new balance is reached wherein the greater intensity of weathering offsets the smaller area being weathered to counterbalance the volcanic additions. The carbon dioxide level of the atmosphere may rise to three or four times its present level by this process, so creating a super-greenhouse effect and a much warmer climate. The mid-Cretaceous superplume may have pumped up atmospheric carbon dioxide levels to 3.7-14.7 times their modern pre-industrial value of 285 ppm (Caldeira and Rampino 1991). During times of sluggish mantle convection, the number of plates becomes smaller, the volume of mid-ocean ridges diminishes, and the continents become aggregated. Volcanism becomes subdued and consequently carbon dioxide emissions decline. Sea level drops, so exposing more land to the atmosphere and increasing the withdrawal of carbon dioxide from the air. When the carbon dioxide content of the atmosphere has fallen low enough, the hothouse state is broken, and the climate system assumes an icehouse state with ice sheets and glaciers.

Exceedingly long cycles of terrestrial processes, admittedly of a rather speculative nature, may follow intergalactic beats. George E. Williams (1975a) thought the properties of the interstellar medium, or the energy output of the Sun (or both), may be sufficiently influenced by the tidal action of the Large and Small Magellanic Clouds, companion galaxies to the Milky Way, to have far-reaching climatic consequences for the Earth and other terrestrial planets. Although the gravitational torque involve is minuscule, it is just possible that the pole of the ecliptic and the plane of the Solar System may track the Large Magellanic Cloud in its orbit around the Galaxy thereby causing secular changes in the obliquity of the ecliptic and consequent very long-term changes in the Earth's palaeoclimates and tecton-ism. It is intriguing that a terrestrial geotectonic megarhythm of roughly 600 to 800 million years, a putative 1,300-million-year pulse of very long-term climatic change, and a 2,500-million-year period of postulated secular change in the Earth's obliquity, are, respectively, about one-quarter, one-half, and equal to the estimated orbital period of the Large Magellanic Cloud (Williams 1981, 13).

Was this article helpful?

0 0

Post a comment