How Plate Tectonics Influences the Earth's Climate: Greenhouse and Icehouse States

Climate change deniers love to argue that climate has always changed without human cause and that we're arrogant to think that we can cause sweeping, global changes to the planet through our energy consumption. Another common argument is that CO2 has been higher in Earth’s history, yet the Earth has been cooler than the present day. Conversely, lower CO2 has coincided with warmer temperatures. Of course, climate scientists and geologists who study past climates (paleoclimatology) understand that CO2 is certainly not alone in moderating the climate. This common argument fails to consider the timescales involved in the other factors relative to the rate at which CO2 is being released into the atmosphere via. human activity.

These arguments are attractive because they are true on the surface but, upon further investigation, they crumble. They fail to take into account that the Earth’s climate 300 million years ago was moderated by a different tectonic setting that skewed the balance between the variables that influence climate. The Earth’s overall climate reflects a combination of the: (1) solar output, (2) the reflectivity of the Earth’s surface, and (3) the composition of the atmosphere. On longer, millions of year, timescales the second and third of these variables are constantly shifting, moderated by plate tectonics - which controls the distribution of heat across the surface e.g. by restricting ocean transport or by clustering the continents around the equator (which increases weathering rates and sucks CO2 out of the atmosphere). Over these long timescales, a pattern emerges, where the planet has two types of climate states - an icehouse and a greenhouse.

Since the Eocene-Oligocene boundary ~35 million years ago (Ma), Earth has been in an icehouse state characterised by the presence of Antarctic ice, which has been a permanent feature of its surface since the mid-Miocene. Additionally, the northern hemisphere became permanently glaciated during the mid-Pliocene (2.7 Ma) following further intense cooling. Before the present icehouse world, at the peak of the last Greenhouse phase ~50 Ma, the average global temperatures were 10°C warmer than the present day (although this was distributed unevenly globally with much of the warming confined to polar regions) (McInerney & Wing, 2011). Over geologic time, many greenhouse-icehouse transitions (and vice-versa) occur in the sedimentary record. A greenhouse Earth is a period where no permanent continental ice sheets are present, while the opposite is true during an icehouse. The most extreme case of the latter (Snowball Earth events), in which equatorial latitudes experience ice cover, has occurred at least three times during the Proterozoic.

The major differences between an icehouse and a greenhouse world are as follows: (1) the presence or absence of ice sheets (and associated sea-level change), (2) atmospheric CO2 concentrations, (3) tectonic forcing and the long-term carbon cycle, (4) differences in duration of icehouses and greenhouses, (5) distribution of continents which influence ocean circulation and deep water formation.

Origins and Characteristics of Greenhouse and Icehouse Worlds:

Palaeoclimate reconstructions indicate that greenhouse/icehouse periods occur synchronously with supercontinent cycles over the past ~1 Ga, where greenhouse conditions last for ~250 My and icehouses ~100 My (Craig et al., 2009). Evidence shows that the early Earth experienced extremely high palaeotemperatures compared with today. However, there is difficulty in reconstructing the conditions of an early Earth due to preservation biases and erosion/tectonic reworking of the rocks that record information about palaeoclimate.

The early Earth's atmosphere was conducive to greenhouse conditions until ~2.3 Ga due to: (1) extraterrestrial bombardment in the Hadean/early Archean, (2) residual heat from planetary accretion, and (3) a reducing atmosphere enriched in CH4. These favoured a greenhouse state until the first glaciation, which occurred during a significant period of change in the Earth system, coinciding with estimates for both early plate tectonic activity (Van Kranendonk, 2011; Tang et al., 2016) and the oxygenation of the atmosphere (Gumsley et al., 2017).

Following the Great Oxidation Event, where the atmosphere began to accumulate O2, the first snowball Earth event (the Huronian glaciation), and potentially the first transition from a greenhouse to an icehouse world, occurred. Evidence for this includes dropstones and diamictites in low palaeoaltitudes (Chen et al., 2019), cap carbonates (Bekker et al., 2005), and matrix-supported clasts of underlying strata in conglomerate beds (Melezhik, 2006). In addition to oxygenation of the atmosphere, flood basalt eruptions ~2.4 Ga have also been suggested to be an equally significant cause of the Huronian and subsequent greenhouse-icehouse trends (Melezhik, 2006). Flood basalts produce more fresh surfaces for weathering, resulting in the drawdown of atmospheric CO2 and subsequent cooling.

While data is limited for Hadean, Archean, and early Proterozoic climates, there is evidence for cyclicity between icehouse and greenhouse worlds during the past 1 Ga. Icehouse climates are relatively short-lived compared to greenhouse climates and the magnitude of icehouses, indicated by latitudinal ice extent, has declined gradually since the Precambrian. Another difference between icehouse and greenhouse worlds is a marked difference in atmospheric CO2. Once CO2 levels dip below a certain threshold (which depends on the tectonics that govern the climate at the time), this gives way to the positive feedback that no longer enables ice sheets to form. It also appears that the changes in CO2 at the icehouse-greenhouse or greenhouse-icehouse boundaries become less sharp in more recent transitions, implying that, since the Triassic, the Earth has become more sensitive to forcing than during the Precambrian and Palaeozoic.

In addition to the absence of ice sheets, greenhouse climates coincide with high levels of atmospheric CO2. CO2 plays an essential role in governing climate in terms of its short-term (biological, atmospheric, and oceanic) and long-term, tectonically influenced cycles. To transition from an icehouse to a greenhouse, or vice-versa, it is necessary to overcome the stabilising influence of weathering on climate, where an increase in temperature enhances the hydrological cycle and increases weathering, resulting in a drawdown of atmospheric CO2 and overall cooling. During periods of supercontinent assemblage, extensive mountain building and uplift cause weathering rate to increase, removing CO2 from the atmosphere, overwhelming the stabilising influence previously mentioned, leading to an icehouse state taking over. During periods of supercontinent breakup, elevated volcanic outgassing results in higher atmospheric CO2 concentrations associated with greenhouse periods.

Furthermore, the distribution of continental land masses plays a vital role in governing icehouse/greenhouse conditions. If their distribution enables the exchange of polar and tropical waters, greenhouse conditions are favoured as ocean circulation can transport nutrients and heat from equatorial regions poleward. However, when continents cluster around the equator and polar currents encircle the globe preferentially, an icehouse state is more likely (Craig et al., 2009). For an icehouse or greenhouse climate to take hold, many of these variables discussed must be in alignment. Tectonics exerts the strongest influence on this as the cyclicity highlights.

Conclusion:

To summarise, the Earth’s climate over the past 1 Gy has been characterised by the cyclic greenhouse and icehouse periods paced with the supercontinent cycle of assemblage and breakup. The origins of this behaviour are not well-resolved due to uncertainties in the timing of the origin of plate tectonics and early photosynthetic life, although the first recorded icehouse (the Huronian) appears to have occurred around 2.3 Ga, coinciding with significant global changes in the biosphere and lithosphere. Unravelling the origins of tectonics in the Archean will undoubtedly reveal how the planet came out of a lasting greenhouse and eventually was able to support complex life. Additionally, this understanding can provide further insights into how the current greenhouse-icehouse cycles originated and evolved.

While climate change deniers have framed this as a scientific problem and implied that we can’t reconcile these periods where CO2 is high and temperatures are low (or vice versa), this is completely untrue. Understanding that CO2 is cycled via. separate short term cycles and long term, tectonically paced cycles is the key to understanding the complex history of the Earth’s climate.

Sources

Bekker, A., Kaufman, A. J., Karhu, J. A. & Eriksson, K. A., 2005. Evidence for Paleoproterozoic cap carbonates in North America. Precambrian Research, 137(3-4), pp. 167-206.

Chen, Y. et al., 2019. Discovery of the Huronian Glaciation Event in China: Evidence from glacigenic diamictites in the Hutuo Group in Wutai Shan. Precambrian Research, Volume 320, pp. 1-12.

Craig, J. et al., 2009. Global Neoproterozoic petroleum systems: the emerging potential in North Africa. Geological Society, London, Special Publications, Volume 326, pp. 1-25.

Gumsley, A. P. et al., 2017. Timing and tempo of the Great Oxidation Event. PNAS, 114(8), pp. 1811-1816.

McInerney, F. A. & Wing, S. L., 2011. The Paleocene-Eocene Thermal Maximum: A Perturbation of Carbon Cycle, Climate, and Biosphere with Implications for the Future. Annual Review of Earth and Planetary Sciences, Volume 39, pp. 489–516.

Melezhik, V. A., 2006. Multiple causes of Earth’s earliest global glaciation. Terra Nova, Volume 18, pp. 130-137.

Tang, M., Chen, K. & Rudnick, R. L., 2016. Archean upper crust transition from mafic to felsic marks the onset of plate tectonics. Science, 351(6271), pp. 372-375.

Van Kranendonk , M. J., 2011. Onset of Plate Tectonics. Science, 333(6041), pp. 413-414.