A recent study at Rice University provides a possible explanation for the Earth’s long-term back-and-forth between greenhouse and icehouse states over the past 500 million years. Cin-Ty Lee, Professor of Earth Science at Rice, led the just-released four-year study which draws on a new cause for the longest climate cycle.
Currently, we are in an icehouse state, and have been for the last 50 million years. Ice is present at the poles, and we experience periodic glacial activity, on a timescale of thousands of years. When the Earth is in the warmer greenhouse state, there is no ice at the poles, and much more carbon dioxide present in the atmosphere.
What causes this oscillation between hot and cold? The 10 to 100 million year timescale far surpasses any influence from solar or orbital patterns. The standard theory is that greenhouse states come about from carbon dioxide releases caused by volcanic activity along mid-ocean ridges. However, the massive amounts of atmospheric CO2 found in the fossil record from past greenhouse states are suspiciously high for this type of tectonic activity alone. Lee found that the planet’s long-term climate cycle comes naturally from plate tectonics. The research suggested that tectonic activity drives a periodic flare-up of continental-arc volcanoes, especially during periods when oceans are forming and continents are breaking apart. The continental-arc volcanoes that arise during these periods are located on the edges of continents, and the magma that rises through the volcanoes releases large quantities of carbon dioxide as it passes through layers of carbonates in the continental crust.
While only a theory, the findings explain how the Earth can remain in a greenhouse or icehouse state for so many millions of years. The number of arc volcanoes doesn’t change, but it is in the continental-arc stage rather than the oceanic stage that CO2 is released from a deep bank of continental carbonates.
