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Why did the earth start cooling 34 million years ago?

Earth sweltered in greenhouse heat for 226 million years, starting in the late Permian and ending in the early Oligocene. Polar icecaps and ice sheets were unknown, and tropical plants often grew from pole to pole. Looking at images of Antarctica today, it’s hard to imagine a continent with tropical shorelines and warm ocean breezes. The perfect setting for an afternoon of sunbathing. About 55 million years ago, the average global temperature was 30 degrees Celsius, or 15 degrees warmer than today, and the Antarctic was a paradise. But 34 million years before the present, temperatures dropped, plunging the planet into a big chill, and we have been there ever since. The connection between a cooling climate and dropping carbon dioxide (CO2) levels is clear, but the ultimate question of what flipped this climate switch is uncertain.

Movement from greenhouse to icehouse conditions has happened five times in the earth’s long history. However, our earth has only spent about 670 million years out of its 4.5-billion-year history in glacial periods. So, glacial periods account for 15% of the earth’s existence. This fact means the normal state of the earth is a hot, mostly ice-free planet. The world we live in today is not the norm. This current phase of cold conditions is called the Late Cenozoic Ice Age. Even though we commonly think of the last ice age as the one from 20,000 years ago, the big picture dictates we are still in an icehouse world. Albeit we seem to be doing our best to raise global temperatures back to their historical norm.

Temperatures dropped at the beginning of the Oligocene, and polar ice caps started forming. Dropping levels of atmospheric CO2 and shifting continents were the direct causes of these temperature and environmental shifts. But the root cause of changes in atmospheric greenhouse gases remains speculative.

Tectonic plates on the move

Continents were shifting, drifting, and splitting apart in the late Eocene and early Oligocene as the planet entered into the Late Cenozoic Ice Age. Antarctica was the focus of intense change. Australia and South America both drifted away from Antarctica, isolating the continent and giving rise to today’s Southern Ocean. The only place in the world where an ocean current can circumnavigate the globe and return upon itself like a mythical ouroboros is in the Southern Ocean. There, unimpeded by continents and driven by westerly winds, the Antarctic Circumpolar Current (ACC) endlessly circles the globe flowing from west to east.

The ACC extends from the sea surface to depths of 4,000 meters, and the water flow measures approximately 175 million cubic meters per second. For scale, this rate is about 100 times greater than the combined flow of all the rivers on the planet. The circumpolar-current travels at speeds over three kilometers per hour, and water temperatures hover around freezing.

Keeping the chill in

The world’s oceans exert a powerful influence on climate, and the ACC is no exception. It encircles Antarctica in the same way atmospheric currents, like the jet stream, circle the globe around a polar vortex. This deep, cold mass of circulating water provides a barrier to heat transfer, keeping the warmer northern waters at bay. This barrier helps keep Antarctica frozen.

The Oligocene development of the Southern Ocean and its Arctic Circumpolar Current played a role in restricting heat transfer to the continent. After four million years, three large ice caps formed in the higher elevations of Antarctica. Slowly the ice caps grew and merged, and by 14 million years ago, the Antarctic ice sheets were equivalent in extent and volume to today.

By the time the Antarctic ice sheet was fully formed, glaciers had appeared in the mountains of the Northern Hemisphere. Near the end of the Pliocene, some 2.9 to 2.6 million years before the present, the Greenland ice sheet was rapidly forming, and the latest phase of the Cenozoic icehouse earth was commencing. This final phase includes what we normally think of when we speak of ice ages and Pleistocene glacial cycles.

But the story doesn’t begin and end with plate tectonics and changes in ocean circulation patterns. The change from greenhouse to icehouse conditions required removing carbon dioxide (CO2) from the atmosphere.

CO2 Reversal

The beginning of the Late Cenozoic Ice Age coincided with a dramatic fall in atmospheric CO2 levels. Concentrations of CO2 fluctuated between 1000 and 2000 ppm before Oligocene cooling. During the first 14 million years of the current icehouse world, CO2 levels fell to between 500 and 300 ppm, and then for the next 20 million years, average levels remained at 250 ppm or lower. Simply put, CO2 levels of icehouse earth were about 12% of the levels encountered during the previous greenhouse conditions.

There is no firm answer as to where the CO2 went as earth plunged into the cold. But we can revert to some pre-Anthropocene principles on the basics of atmospheric CO2 for insights. The earth’s earliest atmosphere lacked oxygen, but it was rich in nitrogen, CO2, and water vapor. As the water vapor condensed, it formed oceans, and some of the atmospheric CO dissolved into the seawater. From there, throughout geological history, CO2 concentrations in the atmosphere have fluctuated in response to natural conditions.

Volcanic eruptions and mantle degassing pump CO2 into the air. Biological activity uses the CO2, thus removing it from the atmosphere and sequestering it as biomass or stable chemical compounds. Chemical weathering of rocks at the earth’s surface also removes CO2 from the atmosphere. So, it’s a bit of a tug-a-war between supply and demand. Falling CO2 levels in the Late Eocene and Early Oligocene responded to this battle of supply and demand.

Weathering rock

During a prior icehouse period in the Ordovician (435 million years ago), the emergence of plant life on land triggered an increase in chemical weathering. Mosses, hornworts, and liverworts proliferated across the planet’s land surfaces. These Ordovician plants thrived on both solid rock and rocky soil. Even though their roots were quite shallow, they still secreted a variety of organic acids. These acidic secretions enhanced the natural weathering processes by accelerating the chemical breakdown of rock surfaces.

Faster weathering of the earth’s surface rocks paved the way for another change. During the weathering process, rock chemically alters as it reacts with the atmosphere, removing CO2. The moss and liverworts accelerated the weathering process and gradually lowered the CO2 content of the atmosphere. These CO2 reductions eventually led to an Ordovician Ice Age.

The Late Cenozoic Ice Age was preceded by an intense mountain-building episode, which accelerated in the Paleocene and Eocene. A set of mountain belts measuring 15,000 kilometers in length developed on the southern edge of Europe and Asia, forming the Alps and Himalayas, along with various other mountain ranges. One theory is the weathering of freshly uplifted rock from this tectonic activity pulled CO2 from the atmosphere, reducing atmospheric heat retention and sending the earth into icehouse conditions.

Biomass and long-term carbon sequestration

Other scientists speculate that massive blooms of marine flora and fauna in the late Eocene captured and sequestered significant volumes of CO2. Eocene black shales in the Arctic and other regions point towards large scale preservation of organic material, and these deposits represent a net sequestration of CO2. Usually, the CO2 removed from the atmosphere by plants is returned once they die and decompose. If, however, the dead plant material ends up in anoxic water or is rapidly buried, then the carbon is permanently removed from the planet’s surface.

Another biological method of CO2 removal is via shell production in marine organisms. Phytoplankton in the open ocean, for example, act as a carbon sink. These creatures use carbon to build their tiny shells. The shell-building process effectively extracts CO2 from the ocean waters and sequesters it in their shells. As they die, the shells sink to the ocean bottom. When CO2 levels in the oceans drop, the waters can absorb additional CO2 from the atmosphere, thus reducing atmospheric CO2 and reducing the atmospheric greenhouse effect.

The biological removal theory relies on a combination of massive biological carbon removal and subsequent burial of the organic carbon before it is recycled into the atmosphere. But there is no evidence thus far that biological activity could remove the amounts of CO2 needed to plunge the earth into Cenozoic icehouse conditions.

What do we know?

We know the earth’s climate switched from its normal greenhouse conditions to a cooler icehouse environment in the Late Cenozoic, and we are still in this cold-earth scenario today. Massive amounts of CO2 left the atmosphere at this time, and temperatures dropped. The removal of CO2 was probably a direct result of weathering and biological activity, although it is speculative as to which process dominated.

Once CO2 levels reached about 700 ppm, the Antarctic ice sheet started forming. This process was facilitated by the newly created Southern Ocean, which insulates the continent and retains the cold. The white ice sheets also increased Antarctica’s albedo, further reinforcing the cooling trend.

Once the earth sufficiently cooled, the glacial-interglacial cycles of the Pliocene and Pleistocene controlled the planet’s climate as evolution ran its course and Homo sapiens arrived on the scene. We are a clever species and have managed to break the cycle and start pushing earth back towards its normal sweltering greenhouse conditions. Ironically, we evolved in a rare icehouse world, which we now consider the norm, and we are struggling to prevent a rapid return to the true, long-term geological norm.

Climate change mitigation efforts are reasonable and necessary, but temperatures will continue to rise, and the only real unknown is how high they will go. Perhaps we should also be asking ourselves, “What shall we do to make a hot-earth livable?”

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