5.3.2 Phanerozoic climate

On timescales of millions of years, the carbon cycle is mainly controlled by the exchanges between rocks and surface reservoirs (ocean, atmosphere, biosphere, see section 2.3.4). As this long-term carbon cycle determines the concentration of atmospheric carbon dioxide ([CO₂]), its change over time can be represented in a very simplified way by:

The first term of the right-hand side of this equation (Volc) describes the outgassing of CO₂ associated with metamorphism during subduction and volcanic eruptions. The second term (Weath) measures the combined influence of silicate weathering and calcium carbonate sedimentation in the ocean, which removes carbon from the atmosphere and the ocean. The last term (Org) is associated with long term burial of organic matter. The imbalance between these three terms has been responsible for changes in the atmospheric CO₂ concentration and the climate for billions of years. Unfortunately, information about the various processes is not precise enough to estimate their magnitude in the Precambrian eon, but the situation is better for the Phanerozoic eon (the last 542 million years).

First, when tectonic activity is intense, high production rates of oceanic crust at the mid-ocean ridges results in more buoyant oceanic plates that push sea water upward. This results in flooding of the low-lying parts of the continents. As such high tectonic activity is related to large subduction rates and more frequent/stronger volcanic eruptions, it has been suggested that reconstructions of sea levels can be used to derive the time evolution of the outgassing of CO₂.

Second, the burial of organic matter can be estimated from the isotopic composition of the carbon in sea water. During photosynthesis, ¹²C is taken preferentially to ¹³C. This implies that organic matter has a lower content of ¹³C than the atmosphere or the ocean. The isotopic composition is commonly measured by delta value $\delta$ ¹³C which is the ratio of ¹³C and ¹²C isotopes in the sample, compared to a reference standard:

The past values of $\delta$ ¹³C in the ocean, which are related to the one in the atmosphere at the same period, are recorded in carbonate sediments and can thus be measured. This provides estimates of the rate the burial of organic matter: a larger organic transfer to sediments is associated with a decrease in the relative amount of ¹²C and thus to an increase in $\delta$ ¹³C. Based on such measurements, it has been possible to determine that burial was particularly high during the transition from the Carboniferous and the Permian period, around 300 million years ago, a period characterised by a relatively large production of fossil fuel-source rocks and relatively low atmospheric CO₂ concentration.

From the information presented above and the estimates of weathering rates based on the exposure of different types of rocks, it is then possible to build models of the long term carbon cycle. Those models can be very complex as they have to estimate the influence of various processes. They can also include, in addition to the carbon cycle, the cycles of other elements such as oxygen or sulphur. However, they still have large uncertainties and some of the hypotheses they use are contentious. For instance, Figure 5.12 illustrates the influence of climate sensitivity on the simulated CO₂ concentration in one of these models. Climate sensitivity affects the stabilizing feedback between the temperature increase due to higher CO₂ concentration and the intensity of weathering that tends to lower the CO₂ concentration (see section 4.3.2). With low climate sensitivity, this feedback is weak, as CO₂ has only a moderate influence on the climate. As a consequence, the variations of CO₂ concentration are large. By contrast, with high climate sensitivities, the feedback is strong enough to restrain the amplitude of the changes in CO₂. For the model presented in Figure 5.12, the best agreement with reconstructions of atmospheric CO₂ concentration based on various proxy records is obtained for values of climate sensitivity around 3^oC. This is in the middle of the range provided by global climate models for present-day conditions (see section 4.1.3), suggesting a relative stability of this number over long time scales.

The relatively good agreement between simulated and reconstructed CO₂ concentration gives us some confidence in the proposed interpretation of the dominant factor influencing the long-term carbon cycle. The production rate of oceanic crust by tectonic activity appears to play a particularly important role since the relatively large divergences that followed the break-up of the super-continents around 200 million years ago (super-continent Pangaea) and 550 million years ago (super-continent Pannotia, see Figure 5.11) are associated with significant increase in CO₂ concentration. Furthermore, the periods of low CO₂ concentration generally correspond well with recorded glaciation, for example during the Carboniferous period 300 million years ago. This gives us some confidence in the validity of the simulated and reconstructed CO₂ history as well as on the long term relationship between CO₂ and climate. However, the link between CO₂ and global temperature can not, on its own, explain all the past climate variations, in particular at regional scale. Other factors, such as the location of the continents must also be taken into account, as briefly discussed in section 5.3.1 above. For instance, when all the continents are grouped together, the interior of the continents tends to be very dry, leading to an extension of desert there.