A Journey Through Earth's Icy Transformation: 34 Million Years Ago
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The Transition from Greenhouse to Icehouse
For 226 million years, Earth basked in greenhouse conditions, beginning in the late Permian and continuing until the early Oligocene. During this era, polar ice caps were nonexistent, and the planet was adorned with lush tropical vegetation from pole to pole. Imagining a tropical Antarctica is difficult today, but roughly 55 million years ago, global temperatures averaged around 30 degrees Celsius—15 degrees warmer than they are now—creating a warm haven on the continent. However, around 34 million years ago, a significant drop in temperatures initiated a prolonged cooling phase, and this shift has persisted to the present day. While the link between decreasing CO2 levels and climate cooling is well-established, the precise trigger for this dramatic climate shift remains elusive.
The planet has experienced five transitions from greenhouse to icehouse conditions throughout its extensive history. Notably, glacial periods account for only about 15% of Earth's 4.5-billion-year timeline, meaning that a warm, largely ice-free planet is the prevailing state. The current cold phase, known as the Late Cenozoic Ice Age, marks a departure from this norm. Although many perceive the last ice age as the one occurring 20,000 years ago, we are still entrenched in an icehouse world, even as we actively work to reverse this trend and raise global temperatures back to historical averages.
The Cooling Trend Begins
As the Oligocene epoch commenced, temperatures began to decline, leading to the formation of polar ice caps. This cooling was primarily driven by decreasing atmospheric CO2 levels and the shifting of continental plates. However, the fundamental causes of changes in greenhouse gas concentrations remain a topic of speculation.
Tectonic Movements and Their Impact
During the late Eocene and early Oligocene, continents were in a state of flux, drifting and separating as Earth transitioned into the Late Cenozoic Ice Age. Antarctica underwent significant changes, with Australia and South America moving away from it, creating the Southern Ocean. This ocean is unique in that it allows the Antarctic Circumpolar Current (ACC) to flow uninterrupted around the globe, driven by relentless westerly winds. This current is a powerful force, stretching from the ocean's surface to depths of 4,000 meters, with a flow rate of approximately 175 million cubic meters per second—about 100 times greater than all the world's rivers combined.
The Influence of Ocean Currents
The ACC's cold, deep waters act as a thermal barrier, preventing heat from warmer northern waters from reaching Antarctica. This barrier is vital in maintaining the continent's icy conditions. The development of the Southern Ocean and the ACC during the Oligocene significantly restricted heat transfer, leading to the establishment of three major ice caps on Antarctica over millions of years. By 14 million years ago, these ice caps reached sizes comparable to the present-day Antarctic ice sheets.
As the Antarctic ice sheet formed, glaciers emerged in the Northern Hemisphere. By the late Pliocene, approximately 2.9 to 2.6 million years ago, the Greenland ice sheet was also taking shape, marking the onset of the most recent phase of icehouse conditions characterized by glacial cycles.
The Role of CO2 Reduction
The transition to the Late Cenozoic Ice Age coincided with a notable decrease in atmospheric CO2 levels, which plummeted from around 1,000 to 2,000 ppm before the Oligocene cooling to between 500 and 300 ppm during the initial 14 million years of the current icehouse phase. For the following 20 million years, CO2 levels remained around 250 ppm or lower—just 12% of the concentrations seen during the preceding greenhouse period.
The exact mechanisms behind this reduction in CO2 levels remain unclear. However, historical insights suggest that the early atmosphere, rich in nitrogen and CO2, evolved over time. Volcanic eruptions and mantle degassing introduced CO2 into the atmosphere, while biological processes and weathering of rocks sequestered it, creating a delicate balance.
The Impact of Weathering
During the Ordovician period, approximately 435 million years ago, the emergence of terrestrial plant life catalyzed increased chemical weathering. Mosses and other primitive plants thrived on land, enhancing weathering processes through the secretion of organic acids. This accelerated weathering led to a decline in atmospheric CO2, contributing to an Ordovician Ice Age.
The Late Cenozoic Ice Age was preceded by significant mountain-building events during the Paleocene and Eocene, which may have further reduced atmospheric CO2 through enhanced weathering of newly uplifted rocks.
Biological Contributions to Carbon Sequestration
Some scientists propose that the late Eocene saw extensive blooms of marine flora and fauna that captured substantial amounts of CO2. Deposits of organic material, such as black shales, indicate significant carbon sequestration. Typically, CO2 absorbed by plants is returned to the atmosphere after decay. However, if organic material is rapidly buried or preserved in anoxic conditions, it can be permanently sequestered.
Additionally, marine organisms contribute to CO2 removal through shell production, effectively acting as a carbon sink. Phytoplankton, for example, utilize carbon to build shells, which, upon their death, sink to the ocean floor, thus contributing to the reduction of atmospheric CO2.
What We Understand Today
We know that Earth's climate shifted from a greenhouse state to an icehouse environment during the Late Cenozoic, a condition that persists today. The substantial decline in CO2 levels during this period was likely influenced by weathering and biological activity, though the dominant process remains uncertain.
Once CO2 levels dropped to approximately 700 ppm, the Antarctic ice sheet began to take shape, aided by the insulating effects of the newly formed Southern Ocean. The reflective ice sheets further enhanced cooling trends.
As Earth cooled sufficiently, the glacial-interglacial cycles of the Pliocene and Pleistocene began to influence the climate, coinciding with the evolution of Homo sapiens. Ironically, while we have adapted to this icehouse world, we are now attempting to revert to the warmer greenhouse conditions that were once the norm.
While efforts to mitigate climate change are crucial, rising temperatures are inevitable, leaving the question of how high they will climb. Perhaps we should also contemplate how to adapt to a warmer Earth.
Sources:
Invasion of the Ordovician plants (Source: ArcheanWeb)
The Antarctic Circumpolar Current: An Ouroboros (Source: ArcheanWeb)
From Greenhouse to Icehouse (Source: EarthDate)
The End of the Hothouse (Source: SkepticalScience)
Read more on Medium publications: EarthSphere and Dropstone
See my recent Rand Soler book