How Continental Positioning affects Climate: Part II – Possible Snowball Earth Triggering Mechanisms + Regional Effects of Mountain Ranges

In Part 1 of this article, I outlined some of the variables which can affect Earth’s climate, and gave a brief overview of plate tectonics, and how changes in continental positioning can lead to climate change through albedo feedback and via the alteration of ocean circulation and heat distribution patterns. In doing so, I used the example of the Rodinia Supercontinent and the Snowball Earth hypothesis of the Neoproterozoic era in order to relate the concepts to events in Earth’s prehistory. For the sake of completeness, I want to finish up that example by briefly going over a few proposed triggering mechanisms that could have made a runaway albedo feedback loop possible in the Cryogenian period. After that, I want to go over the ways in which the presence of mountain ranges can affect local and regional climate.

Although the exact causal sequence (and even the SnowBall Earth hypothesis itself) is still an unresolved area of active scientific debate, there are multiple candidates for such a cooling trigger mechanism: decreased solar luminosity, global cooling from a Super Volcano eruption, or perhaps a reduction in atmospheric methane – a much stronger greenhouse gas than CO₂ – due to reactions with atmospheric oxygen could plausibly have contributed. There is also evidence suggesting increased sequestration of CO₂ by rocks due to weathering effects from high precipitation levels. The idea behind the latter hypothesis is the following: Carbonate and Silicate rock weathering reactions are important carbon sinks in Earth’s carbon cycle:

Carbonate rock weathering reaction:

CaCO₃ + CO₂ + H₂O → 2HCO₃^– + Ca²⁺

Silicate rock weathering reaction:

CaSiO₃ + 2CO₂ + H₂O → 2HCO₃^– + Ca²⁺ + SiO₂

Precipitation levels tend to be high around the equator, which therefore increases these weathering reactions, and thus increases CO₂ sequestration, thereby decreasing the greenhouse effect, and leading to global cooling. Once sufficient equatorial land ice could accumulate, the aforementioned ice albedo feedback effect could ensue.

The end of the extreme Neoproterozoic glaciation is an interesting story in its own right, but in addition to the breakup of Rodinia via tectonic plate movement, it involves a greenhouse effect facilitated by a very long period of extreme volcanism, which I’ll be covering in a subsequent post. It also directly preceded the Cambrian explosion.

Rodinia was not the last time Earth had a tropical continental arrangement, but a critical change occurred: the evolution of forests. The Devonian Period (419 – 372 mya) featured the Greening of the Continents, and the evolution of forests, which serve as carbon sinks. So, by the time the Pangea Supercontinent formed (about 250 mya), an equatorial continental arrangement did not necessarily mean excessive rock weathering reactions, let alone runaway ice albedo feedback.

Plate tectonics can also influence climate through the formation of mountains (orogeny). Mountains can have profound effects on climate, particularly in their effect on precipitation patterns on the surrounding lands. Higher points on mountains tend to correspond to lower temperatures, so as rising moist warm air makes its way up the windward side of a mountain, it cools down, thus causing its ability to hold water to decline, which in turn leads to precipitation (rain or snow). Consequently, the leeward side of the mountain will often receive less precipitation, and by the time that air reaches the adjacent flat lands, it may not have any moisture left. This is one reason deserts are sometimes located directly next to mountain ranges. Mountains can also affect air circulation patterns great distances away. You can read more about how mountains affect climate and weather patterns at the Mountain Professor.

Mountains and regional climate:

It’s perfectly natural to wonder what (if any) role changes in Continental positioning have played in the global warming and climate change we’ve been experiencing currently. The answer is very little (if any). The issue is the RATE at which the current change has been occurring. These processes I’ve described take place over tens or even hundreds of millions of years. The continents tend to move at roughly 2 cm a year, and even faster moving plates rarely exceed 10 cm even on a fast year. The continents haven’t moved all that much in the last few hundred thousand years, and less than meter or so within the last 150 years during which the recent changes have been occurring. So continental movement is ruled out as a principle causal determinant of the current warming. It simply doesn’t induce climatic changes fast enough to explain the rapidity with which the recent warming has been occurring. However, its effects on the planet over the long term can be quite profound indeed.

References:

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Davies, N. S., Gibling, M. R., & Rygel, M. C. (2011). Alluvial facies evolution during the Palaeozoic greening of the continents: case studies, conceptual models and modern analogues. Sedimentology, 58(1), 220-258.

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Goodale, C. L., Apps, M. J., Birdsey, R. A., Field, C. B., Heath, L. S., Houghton, R. A., … & Nabuurs, G. J. (2002). Forest carbon sinks in the Northern Hemisphere. Ecological Applications, 12(3), 891-899.

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Murphy, J. B., Nance, R. D., & Cawood, P. A. (2009). Contrasting modes of supercontinent formation and the conundrum of Pangea. Gondwana Research, 15(3), 408-420.

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