The Truth of Global Warming: Part 3 - History of Earth's Atmosphere

Published: Feb. 3, 2024, 9:29 a.m. (UTC) / Updated: May 22, 2024, 12:24 p.m. (UTC) 🔖 3 Bookmarks

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Basic Terminology Clarification

When we hear the term "ice age," many people might recall the animated movie "Ice Age" by Disney, featuring mammoths and saber-toothed tigers. Some might envision a world entirely covered in ice, similar to what is portrayed in the movie. However, a slight modification is needed for a more accurate understanding.

Firstly, an ice age is a period on Earth when large-scale ice sheets exist. To elaborate, an ice age is characterized by a prolonged cooling of the Earth's climate, leading to a sustained decrease in temperatures of the Earth's surface and atmosphere. During an ice age, glaciers proliferate or expand in polar regions and high-altitude areas. Considering the current presence of extensive ice sheets in locations like Greenland and Antarctica, we are technically still in an ice age, known as the Quaternary glaciation, which began around 2.6 million years ago. Mammoths, for instance, inhabited the Earth between approximately 4 million and 10,000 years ago, placing them in the period spanning from the Third Glacial Period to the Fourth Glacial Period.

Within the extended periods of an ice age, particularly cold climate intervals are referred to as glacials, and the intermittent warmer periods between glacials are called interglacials. Examining the last 2.5 million years, interglacials have occurred approximately every 40,000 to 100,000 years, with the intervening periods being lengthy glacials. We are currently in an interglacial period, with the last glacial period believed to have started around 70,000 years ago and ending about 10,000 years ago. As the glacial period ended and the interglacial period ensued, the Earth warmed, marking the transition from the Paleolithic era to the Neolithic era, characterized by the advent of agriculture and settled communities.

Earth's Early Ages

The extremely long-term history of Earth's atmosphere remains an area of ongoing research with many uncertainties. In particular, when drawing parallels in the context of global warming, it might seem straightforward to say, "During ice ages, temperatures and $CO_2$ concentrations are low, and during interglacial periods, the opposite is true. This is because the increase in $CO_2$ concentration is the cause of the temperature rise." However, the reality is far from such a simple narrative.

According to the current scientific consensus, the evolution of the atmosphere is thought to have occurred in three stages. When the Earth was forming, the solar system had a high concentration of helium and hydrogen, which were bouncing back from the Earth's surface at extremely high temperatures. Eventually, these molecules escaped into space, replaced by the second atmosphere consisting of volcanic emissions. Through volcanic eruptions, water vapor, carbon dioxide, and ammonia (one nitrogen and three hydrogen atoms) were released, creating a gaseous blanket in the upper atmosphere and forming early water bodies below. CO2 gradually dissolved into shallow seas, and around 2.7 billion years ago, cyanobacteria began photosynthesis, generating oxygen. This oxygen accumulated, and approximately 2.4 billion years ago, the composition of the atmosphere changed dramatically, leading to the extinction of most existing microorganisms at that time.

The Appearance of "Another Planet Earth" Hundreds of Millions of Years Ago

Transition of $CO_2$ Concentrations

Before the Industrial Revolution, the $CO_2$ concentration was approximately 280 ppm, and currently, it has reached about 420 ppm. How does this level compare to the long-term history of Earth? Let's take a look at the transition of long-term $CO_2$ levels.

The $CO_2$ concentration has undergone significant changes over Earth's 4.54 billion-year history, influencing the average temperature of the planet. The furthest estimated period for $CO_2$ concentration is around 500 million years ago during the Ordovician period, with atmospheric CO2 levels ranging from a staggering 3,000 to 9,000 ppm! The Earth looked vastly different from its current state. Subsequently, 400 million years ago, it was around 2,000 ppm, fluctuating from there. Changes in $CO_2$ concentration are determined by the imbalance in the carbon cycle, involving carbon fixation (burial in sediments, captured by plants) and emission (decomposition, volcanic activity). Due to this imbalance, $CO_2$ levels decreased, leading to an ice age approximately 300 million years ago.

Later, volcanic activity intensified, doubling $CO_2$ concentration to around 1,000 ppm. Around 200 million years ago, massive fern forests emerged, and subsequently, $CO_2$ levels continued to decline until reaching current levels. This occurred during the Eocene epoch, approximately 33 to 23 million years ago when temperatures were 4 to 6 degrees Celsius higher than today. Since dipping below 400 ppm around 20 million years ago, $CO_2$ levels had not reached 400 ppm until the present time, making the current $CO_2$ concentration an event not witnessed in approximately 20 million years.

Temperature Trends

Now, let's take a look at the history of temperature.

Looking at the above figure, the average temperature around 500 million years ago was approximately $14{}^\circ C$ higher than before the Industrial Revolution. During the ice age around 300 million years ago, it dropped by about $4{}^\circ C$ compared to pre-industrial levels. However, around 50 million years ago, it rose again to approximately $14{}^\circ C$ higher than pre-industrial levels, gradually approaching the current climate until 1 million years ago. In this perspective, the current degree of global warming may not seem significant when viewed in the context of Earth's long history. Life on Earth has adapted and evolved to survive in even more challenging environments.

By the way, I invite you to compare the graphs of changes in $CO_2$ concentration and temperature trends. While there are similarities in some patterns, you'll notice significant differences in other aspects. I will delve into this aspect further in another study.

Other Significant Differences in Earth's Conditions

Firstly, the influence of Milankovitch cycles is essential.

Axial Tilt (periodic changes in the tilt of the rotational axis): Due to the gravitational interactions with the Sun and other planets like Jupiter and Mars, Earth's axial tilt fluctuates between approximately 22 degrees to 24.5 degrees in a cycle of about 40,000 years.
Precession of the Equinoxes: The direction of the rotational axis also changes from the current North Star to Vega in the constellation Lyra in a cycle of approximately 20,000 years.
Orbital Eccentricity: The shape of Earth's orbit around the Sun, affecting the proximity to the Sun during different seasons, changes in cycles of 100,000 years, 405,000 years, and 2 million to several million years, leading to further variations in seasonality.

These cycles of Earth's orbital changes are collectively known as the "Milankovitch cycles". Here, obtaining precise information about the past Earth's orbit, especially a specific point like 500 million years ago, would likely be challenging even with modern computers. Simulating the complex interactions in a multi-body planetary system to recreate the state of Earth (and the entire solar system) at a particular time point, such as 500 million years ago, is extremely difficult. Estimations and assumptions would be involved in determining the Earth's orbit and axial conditions at that time. For instance, variations in axial tilt would impact the sunlight conditions at the North and South Poles. Additionally, changes in the Earth's rotation direction due to axial precession would influence sunlight conditions. Furthermore, alterations in the orbital period affecting the distance between the Sun and Earth would naturally lead to significant changes in seasonal conditions. It's worth noting that some hypotheses propose a different rotation speed for Earth around 2 billion years ago, with a day being approximately 20 hours.

By the predictions based on research related to the Milankovitch cycles, it is suggested that the next ice age would begin at least 50,000 years from now, disregarding anthropogenic global warming. It is crucial to understand that in such an extended period, the Earth's conditions, shaped by Milankovitch cycles, have undergone substantial changes. Over these hundreds of thousands of years, variations in sunlight conditions occurred, influencing the evolution and extinction of plants and organisms. The conditions for carbon fixation have changed, atmospheric composition has been significantly altered by volcanic activities, and plate movements, including the formation of mountain ranges affecting climate, have taken place. Additionally, solar temperature has experienced fluctuations.

Comparing the Earth from hundreds of millions of years ago to the present involves vastly different conditions, many uncertainties, and intricate interconnections of various factors. It is essential to understand that a straightforward comparison between the Earth's ancient past on a scale of hundreds of millions of years and the present is not feasible due to the complex and divergent nature of conditions. When addressing discussions that attempt to casually link discussions of Earth's ancient conditions with contemporary global warming discourse, maintaining a sense of distance, considering it akin to discussing conditions on a planet similar to Earth, might be appropriate, especially for those who are not experts in the field.

Ancient $CO_2$ Levels and Climate in Deep History

To be fair, it is crucial to reiterate that recent findings in paleoclimatology suggest that there isn't a clear correlation between past Earth's $CO_2$ levels and the corresponding temperatures during those periods. According to a study published in the Proceedings of the National Academy of Sciences (PNAS), the relationship between historical $CO_2$ concentrations and temperatures is not distinctly observed. The figure below presents relative $CO_2$ concentrations from 500 million years ago to the present, with shaded regions at the top indicating periods of relatively cooler climates and unshaded areas corresponding to warmer periods. If we assume that higher $CO_2$ concentrations should coincide with higher temperatures, the lower $CO_2$ levels during the shaded periods should not align with the inferred cooler climates. However, this is not the case.

Certainly, this does not imply that the discussion on global warming is fake. As mentioned above, delving into ancient facts requires extensive time, and it may take a while for comprehensive facts to emerge and solidify our understanding. The author believes that it will take some time before a thorough understanding is established.

The Earth After the Last Ice Age

When Can We Safely Compare the Earth to the Present?

For instance, Earth around 150 million years ago, at the end of the Jurassic period during the peak of the dinosaur era, exhibited a completely different aspect with a temperature $5-6{}^\circ C$ higher than the current temperature and a $CO_2$ concentration reaching 1,000 ppm. Similarly, 50 million years ago, at the end of the Cretaceous period marking the conclusion of the dinosaur era, the atmospheric temperature was $14{}^\circ C$ higher than today, with $CO_2$ concentration again reaching 1,000 ppm. Comparing Earth during such periods with the present may be risky due to significant differences in underlying assumptions.

The ongoing ice age, known as the Quaternary glaciation, started approximately 2.6 million years ago. Within this ice age, there are both relatively warm interglacial periods and cold glacial periods. The last glacial period, the final ice age of the current ice age, began about 110,000 years ago and ended approximately 10,000–15,000 years ago. If one wishes to engage in discussions related to current global warming by comparing it to the present Earth, it might be more valid to consider data from around 1 million years ago, a time when the cyclical nature of glacial and interglacial periods within the last ice age became more established. However, when contemplating matters on a similar timescale as these glacial-interglacial cycles, it is essential to also consider other Earth-level feedback loops, as indicated below, presenting additional challenges in comparing the environmental conditions of ancient Earth with the current state.

The Earth as a Relatively Soft Solid

Until now, it has been known that over periods of several million years, there is a causal relationship where "solid Earth processes govern the Earth's climate." However, in 2021, a research team led by Kato revealed that in the scale of tens of thousands of years, such as the glacial-interglacial cycles, there is a reverse causal relationship where the "solid Earth responds sensitively to climate change." The study is detailed in their research.

During ice ages, the development of continental ice sheets leads to a reduction in sea levels. Consequently, the hydrostatic pressure on the seafloor decreases, promoting magma generation at seafloor volcanoes such as mid-ocean ridges and island arcs. The climatic variations of ice ages and interglacial periods, characterized by the advance and retreat of continental ice sheets, have been revealed to influence two solid Earth processes: chemical weathering of continental rocks and hydrothermal activity from magmas beneath the seafloor.

This interaction may result in cyclic patterns, and one such cycle could be as follows:

Development of ice sheets during ice ages
Reduction in sea levels
Increased activity of seafloor volcanoes
Heating of the ocean + release of carbon dioxide from volcanoes
Warming of the Earth
Melting and retreat of ice sheets
Rise in sea levels
Subsidence of seafloor volcanoes due to the weight of seawater
Cooling of the ocean, reducing carbon dioxide emissions from volcanoes
Carbon fixation by plants and other organisms
Cooling of the Earth
Development of ice sheets, primarily around the poles (return to the beginning)

Since this cycle operates through a mechanism distinct from the Milankovitch cycles, it will likely be categorized as a completely separate factor. If one wishes to discuss global warming by comparing the current Earth environment with that of a million years ago, it may be necessary to integrate models that consider both the ice-sheet and volcano feedback loop and the Milankovitch cycles. Such integration could be a future research topic.

Regardless, understanding that Earth is a complex and organically interconnected system beyond our imagination is crucial.

So far, I have tried to present as concisely as possible the elements that seem necessary to understand the discussion on global warming in the context of the atmospheric history of Earth. It is essential to recognize that Earth's environment has a highly dynamic history, the mechanisms of which are intricately complex and interconnected. There is still much we do not know, and there are likely to be many exciting discoveries in the future.