Greenhouse to Icehouse
Published in the EarthSphere Blog
Earth’s normal condition is hot, much warmer than today’s world. The planet formed 4.5 billion years ago, and since then it has spent about 670 million years in glacial conditions. So, the planet has been cold for about 15% of its existence. The presence of polar ice caps is a good criterion for distinguishing between greenhouse and icehouse climatic conditions.
Today we spend much time and energy monitoring and discussing global warming, but we are still on the icehouse side of the climate equation. This does not diminish the threat climate change poses to our societies. But a glance back to the Oligocene lets us see our situation through the lens of geological history.
Humans strode onto the evolutionary stage during the most recent glacial phase, which began about 34 million years ago. Our genetic evolution started in the cold. As a species, a glacial Earth is all we know. It is our reality, so we are rightfully alarmed at the rapid global temperature rise over the past 150 years.
The boundary between the Eocene and Oligocene periods marks the point where Earth reverted from hot tropical conditions to a cooler glaciated planet. Antarctica had settled into its current position over the south pole by the start of the Oligocene, and atmospheric CO2 levels hovered between 300 and 700 ppm. For reference, CO2 levels during the most recent ice age were 180 ppm, and today they are slightly above 400 ppm. So, even though we were moving into a glacial period in the Oligocene, the planet was still warmer than today.
Descent into the Cold
The early Eocene was one of the warmest periods in Earth’s history. Temperatures spiked to 12°C above the present day, and CO2 levels were above 1,000 ppm. The steady fall in global temperatures and CO2 levels during the ensuing twenty million years marked the descent from greenhouse to icehouse conditions. By 34 million years before the present, the beginning of the Oligocene, the world had changed, and the latest geological period of glaciation had begun.
Evolution responds to changes in environmental conditions, and in the Oligocene, a cooler world brought changes to Earth’s fauna. Humans, a category that presumably includes all of my readers, might want to note the appearance of Anthropoids during the late Oligocene.
Primates are traditionally divided into two major groups, prosimians and anthropoids. The prosimians, a group that includes the lemurs of Madagascar, evolved during the Eocene. But the anthropoids are dearer to our history because they include monkeys, apes, and humans. Yes, our very early ancestors popped into existence in the Oligocene. Our entire genetic lineage belongs to an icehouse world. We are genetically bred for the cold, not the heat.
The geological and climatological changes affecting Earth during the Eocene and Oligocene were not catastrophic, and the movement from greenhouse to icehouse conditions was gradual. Many unanswered questions remain about what caused the long descent into a cold Earth, and various theories have emerged from the sparse data available. One line of thinking holds that dropping levels of CO2 and the rearrangement of tectonic plates were the primary drivers of change. In particular, the rearrangement of Earth’s ocean circulation patterns may have been a significant factor.
Plate Tectonics
Our planet’s surface comprises 71% ocean and 29% land. Also, water’s ability to absorb and store heat makes the planet’s oceans the ultimate buffer for surface heat energy. One way to think about climate is in its role as Earth’s engine for heat energy dissipation. Excess heat around the tropical equator gets distributed toward the cooler poles via ocean and atmospheric circulation. Everchanging weather is the end result of this process of heat distribution.
Antarctica took up residence at the south pole in the Eocene, setting the stage for the return of polar ice sheets in the Oligocene. But a final push was needed to create Antarctica as we know it today. This critical event pushing Antarctica to its present-day status occurred when it split off from South America. This split created the Drake Passage that now separates the tip of South America from Antarctica. The timing of this split is a matter of academic debate, but it may have occurred as late as 17 million years ago.
The split from South America paved the way for the formation of Earth’s last great ocean, the Southern Ocean. Today, this ocean is the only one whose currents continuously circumnavigate the globe. Unimpeded by continents and driven by westerly winds, the Antarctic Circumpolar Current endlessly circles the globe flowing from west to east. Its current moves like a mythical ouroboros, circling back upon itself and swallowing its own tail.
The Southern Ocean current extends from the sea surface to depths of 4000 meters and flows at 175 million cubic meters of water per second. For scale, this is about 100 times greater than the combined flow of all the rivers on the planet.
Once established, the Southern Ocean acted much like the atmospheric jet stream in the northern hemisphere, which circles the globe forming a polar vortex and trapping the cold in the northern polar region. The Southern Ocean provides a barrier to heat transfer, sealing in Antarctica’s cold and keeping the warmer northern waters at bay. The ocean is critical in maintaining the Antarctic ice sheet.
The Oligocene represents an important transition between our planet’s past greenhouse conditions and today’s icehouse climate. The current rapid rise in global temperatures is a stark example of how a single species can rapidly alter an entire planet’s ecological and climatic balance. We tell our children not to play with matches lest they get burnt. Perhaps we should remember that this wise advice also applies to us as adults and guardians of the future.
Sources:
https://www.pnas.org/doi/10.1073/pnas.2003914117
news.umich.edu
open.lib.umn.edu
