Because climate depends partly on the retention of solar heat by carbon dioxide in the atmosphere, a record of past CO2 fluctuations is important in linking evidence for shifting climate and environments to models. Conversely, models that seek to mimic climates of the past depend heavily on the assumption that the “greenhouse” effect and the carbon cycle underpin global temperature and precipitation. Current theorists consider that shifts in CO2 content of the atmosphere reflect a balance between its release through volcanism (itself a reflection of the rate of plate tectonics) and its removal by weathering of silicate minerals and burial of dead biomass.
The GEOCARB III model predicts rising atmospheric CO2 following the ice-house condition of the late-Precambrian, when rapid sea-floor spreading broke up and began to reassemble supercontinents during the Lower Palaeozoic. In the early Cambrian CO2 levels come out at 25 times the modern amount. Colonization of the land by plants through the Upper Palaeozoic, and the burial of a proportion of the increased amount of carbon fixed by them, allows the model to predict a massive fall in CO2. That tallies very well with the long period of glaciation in southern Pangaea during the Carboniferous and Permian. GEOCARB III suggests a recovery in levels through the Mesozoic, punctuated by extraordinary releases from plume activity, such as that implicated in the formation of ocean plateaux beneath the Pacific about 120 Ma ago.
From GEOCARB modelling stem predictions of the overall forcing of global temperatures. However, only the last 100 Ma can be assessed as regards temperatures, by using accurate proxies provided by oxygen isotopes and the Ca:Mg ratio of marine carbonates. Two of the leading climatic theorists, Thomas Crowley and Robert Berner of Texas A&M and Yale universities usefully summarise the range of other proxies that help validate their kind of modelling (Crowley, T.J. and Berner, R.A. 2001. CO2 and climate change. Science, v. 292, p. 870-872). These include estimates from fossil soils, carbon isotopes in sediments, the pores in plant leaves (see Plant respiration and climate below) and how much boron is taken up in the shells of fossil animals. There are considerable discrepancies with modelling, albeit encompassed by the high uncertainties in the calculations. Crowley and Berner acknowledge the complexity of other factors that affect the global redistribution of heat, such as continental configurations in terms of area, geographic position, their effects on ocean circulation and even on the pace of the carbon cycle. They see the need to expand climate models, taking other factors on board, in an attempt to quantify the discrepancies.
Methane and escape from Snowball Earth
Palaeomagnetic pole positions determined from areas characterized by thick glacigenic deposits around 750 Ma old leave little doubt that large volumes of ice covered the Earth to tropical latitudes. Such evidence suggests an ice-bound world from which escape would have been very difficult because much of the Sun’s energy would have been reflected back to space. Extreme and prolonged frigidity, from which Earth’s climate did escape is seen by a growing number of palaeobiologists as the most profound influence over later evolution and diversification of life. The first fossil metazoans appear in the record shortly after a “Snowball Earth” event at 650 Ma, and the Cambrian explosion of animals with hard parts followed close on the heels of the last. Carbon isotope studies from marine carbonates suggest that each global glaciation witnessed massive extinctions of single-celled organisms, and surviving life was presented with a virtual tabula rasa of niches to fill. Such survivors, possessing characters that had ensured their survival – at which we can only guess – exploited them to the full. It is reasonable to speculate that without such climatic upheavals life would not be as it is now, and that our eventual appearance depended on them.
That Earth’s climate broke out of runaway ice-house conditions is obvious, the question being how was that possible. Volcanic emissions of carbon dioxide, which neither the Neoproterozoic biosphere nor silicate weathering were able to draw down into ocean water and sediments, would have accumulated in the atmosphere, to create “greenhouse” conditions. That simple scenario, envisaging a spectacular shift from frigid to hot conditions, has its problems. In order for climate to stabilize, without rushing into runaway heating along the path followed by Venus, demands implausibly high rates of silicate weathering to draw down CO2 in the period following the end of each “Snowball” event, and strontium isotopes that record the rate of continental weathering shwo no sign of anything so dramatic. It also poses the question of how global ice cover could remain while CO2 slowly built up. The key seems to lie in carbonates that everywhere cap the glacigenic deposits of this age. The cap carbonates record rapid falls in the 13C proportion of the carbon in carbonate. 13C shows a rise in the glacial epochs that signifies massive burial of dead organic matter (enriched in lighter 12C), probably through mass extinction. In a review of the geochemical basis for changes in oceanic carbon isotopes, and high-resolution data from cap carbonates, scientists from the University of California and the Lamont-Doherty Earth Observatory, suggest that the isotopic excursions could reflect massive release of methane from gas-hydrate layers in sediments that were frigid during the Snowball event (Kennedy, M.J. et al. 2001. Are Proterozoic cap carbonates and isotopic excursions a record of gas hydrate destabilization following Earth’s coldest intervals? Geology, v. 29, p. 443-446). Backing up this hypothesis are examples of structures in cap carbonates that are identical to those formed in modern sediments affected by break down of gas hydrates and release of methane from the sea floor.
Plant respiration and climate
Leaf surfaces are pockmarked by pores (stomata), through which cell metabolism draws in the carbon dioxide involved in photosynthesis and transpires its products, including oxygen. When CO2 levels are low, more pores are needed, and vice versa. Surprisingly, museum specimens of leaves collected since the start of the Industrial Revolution do show a decrease in the density of such pores that matches the documented rise in atmospheric CO2 levels. Were it possible to find fossils of the same plant species, pore density would be an excellent proxy for the “greenhouse” effect. That is not possible, because of evolution. However, plants related to the Ginkgo have a pedigree that goes back about 300 Ma. Morphologically, the four genera of Ginkgo-like leaves are very similar, so using them potentially gives an independent record of the “greenhouse” effect.
Gregory Retallack of the University of Oregon has measured the stomatal index of sufficient Ginkgo and related leaves to assess CO2 levels in a broad-brush sense for the period since the early Permian (Retallack, G.J. 2001. A 300-million-year record of atmospheric carbon dioxide from fossil plant cuticles. Nature, v. 411, p, 287-290). His results tally broadly with oxygen-isotope and other proxies for palaeotemperature variations, and to some extent with CO2 modelling (see Phanerozoic CO2 levels above). However, the stomatal record shows changes up to 10 Ma in advance of shifts in temperature. That might be due to coarse resolution in Retallack’s data, but could signify other forces at work other than the “greenhouse” effect. The most significant advance provided by leaf studies is that they help account for mismatches between evidence for cooling and predictions of highCO2 by modelling, for the Jurassic and Cretaceous, that have been a thorn in the side of the modellers. Given fossil leaves more closely spaced in time, and using other plant groups, Retallack’s method potentially could revolutionize climate analyses and extend them back as far as 400 Ma ago.
See also: Kürschner, W.M. 2001. Leaf sensor for CO2 in deep time. Nature, v. 411, p. 247-248.