3.4. History of the Earth
3.4. The history of the Earth
In the previous sections, we looked at the current state of the Earth system and the potential impact of human activities on the functioning and the future state of the system. However, a true understanding of the system usually requires an understanding of its history: how has the system evolved and how has it changed over time? It should be noted, however, that it is difficult to study things that happened in the distant past, and we cannot be sure that we have described all developments correctly in what follows.
The Earth, and life, has undergone a number of dramatic upheavals in its history, and knowledge of these is important for understanding the present and the future. The figure below shows the dates of the most important events in Earth's history.
Figure. Major events in the history of the Earth. The inner circle shows the first three aeons of the geologic time scale (Hadean, Archaean and Proterozoic) and the three eras of the current (Phanerozoic) aeon (Palaeozoic, Mesozoic and Cenozoic). The arcs in the outer frame show the periods of occurrence of the different life forms and the texts indicate significant individual events. Ga: billion years ago; Ma: million years ago. Credit: Woudloper derivative work by Hardwigg.
Precambrian (4 540 – 541 millions years ago)
The Earth was formed about 4.55 billion years ago. At the beginning of the Hadean period, the Earth is believed to have been hit by a large protoplanet, and the collision resulted in the formation of the Moon from material hurled into space. For the first few hundred million years, the Earth's temperature was very high, and large and small celestial bodies continued to rain down on a frequent basis. The Earth's atmosphere was composed of carbon dioxide, methane and ammonia, among other elements.
At the beginning of the Archean period, 4 billion years ago, life began on Earth in the form of prokaryotic cells (bacteria and archaea). The first organisms derived most of their energy from chemical sources. Life was very small-scale as only a limited amount of energy was available. A little over three billion years ago, the first prokaryotic microbes using sunlight as an energy source emerged. However, these first organisms did not use the same kind of photosynthesis reaction (i.e. the reaction that converts sunlight into chemical energy) as today's plants and algae, nor did they produce oxygen. The oceans and the atmosphere were practically without oxygen.
The Earth's atmosphere began to oxidise just over 3 billion years ago, when cyanobacteria (also called blue-green algae, although they are not algae but bacteria) evolved. The precursors of the photosynthetic reaction of cyanobacteria, like modern plants and algae, are carbon dioxide (CO2) and water (H20), from which chemical energy (organic molecules) is produced using the sun's radiant energy, with oxygen (O2) as a by-product. The oxygen produced in the photosynthesis, however, did not initially end up in the atmosphere, but instead oxidised iron dissolved in the oceans. Oxygen began to accumulate in the atmosphere only some 2.4 billion years ago.
Oxygen was toxic to the organisms that were anaerobic (i.e. requiring anoxic conditions) at the time, many of which probably became extinct. As a result of atmospheric oxidation, the concentration of methane (a powerful greenhouse gas) in the atmosphere also fell, causing the Earth to freeze over about 2.3 billion years ago. On a frozen Earth, cyanobacterial photosynthesis and the weathering of the ground (both of which sequester carbon dioxide) was obviously very limited, allowing carbon dioxide from volcanic activity to accumulate in the atmosphere. Eventually, after about 300 million years, the amount of carbon dioxide in the atmosphere had risen sufficiently for the glaciers to start melting. Once melting began, the warming was rapid because the water and ground exposed from under the ice effectively absorbed solar radiation, boosting the temperature rise and the melting of the glaciers.
Over the next 1.3 billion years or so, global conditions remained relatively stable. The first eukaryotic organisms evolved around 2 billion years ago, probably when an archaeon was ingested by a proteobacterium. Over time, this endosymbiotic proteobacterium evolved into the mitochondria of modern organisms, which produces energy very efficiently by 'burning' organic molecules with oxygen. The next major evolutionary leap occurred about 1.5 billion years ago, when eukaryotic organisms ingested the photosynthetic bacteria that became the chloroplasts of modern plants and algae. However, organisms remained microscopic and very simple for over a billion more years.
The current understanding is that the stagnation of evolution was caused by the conditions prevailing in the oceans (where life was largely confined) at the time. Although the atmosphere started to become oxygenated around 2.4 billion years ago, the oceans - especially the deeper layers of the oceans - remained anoxic for over a billion years. The deeper layers of the oceans were rich in hydrogen sulphide, which removed essential nutrients such as molybdenum from the water. The oxygen-poor seawater also had very low levels of phosphorus. In a nutrient-poor and low oxygen environment, photosynthesis (and hence also oxygen production) was very slow, so all development came to a halt. The period between 1.8 billion and 800 million years ago is known as "the boring billion" in Earth's history.
Around 720 million years ago, another period of extensive glaciation began, known as the Cryogenian, which lasted for nearly 80 million years. During this period, the Earth is thought to have frozen over twice. The probable cause of the glaciations was a drop in atmospheric carbon dioxide levels caused by the weathering of the bedrock and the resulting cooling of the climate. The Cryogenian ended when enough carbon dioxide from volcanoes had again accumulated in the atmosphere to warm the climate and melt the glaciers.
“Snowballing”
One of the most significant reinforcing feedbacks affecting Earth’s climate is the feedback between glacial albedo, or reflectivity, and climate cooling. If ice forms at the poles, it cools the climate, as white ice and snow reflect most of the sun's radiation back into space. Under the right conditions, the chain reaction of cooling and glacier growth can continue until the surface of the entire planet is frozen. This reinforcing feedback loop is the likely explanation for the already mentioned periods when virtually the entire planet was covered in ice. The idiom ‘snowballing’ describes - coincidentally but fittingly - both the self-reinforcing nature of the process and the end result of the process - an ice and snow-covered Earth.
After the Cryogenian glaciations ended around 635 million years ago, the oxygen content of the atmosphere slowly began to rise from its previous level of about 1%, and the oceans also became more oxygenated. The current understanding is that the erosion caused by the Cryogenian glaciations brought abundant phosphorus to the oceans, which allowed algae to photosynthesise efficiently, leading to rising oxygen levels.
Soon afterwards, the first complex multicellular organisms (the so-called Ediacaran fauna) emerged, supporting the idea that the evolution of life had been slowed down by the lack of oxygen and the scarcity of nutrients in the oceans. The energy production process of eukaryotic organisms (aerobic cellular respiration) requires oxygen; once sufficient oxygen was available, the ability of eukaryotic organisms to produce and consume abundant energy allowed them to grow in size and complexity.
The balance of oxygen and fire
Molecular oxygen (O2) on Earth is virtually all derived from the photosynthetic reaction of plants, where oxygen is produced as a by-product. The amount of oxygen in the atmosphere and in water increases as some organisms are buried after death in the layers of the ocean floor, so that the organic material in those organisms remains un-decomposed (i.e. unburned) and thus does not consume the oxygen produced in the process of making that material. Also organic material from dead terrestrial organisms is buried permanently mainly in the layers of the ocean floor.
In principle, therefore, one could imagine that the amount of oxygen would increase continuously, since some of the organic material always ends up in the layers of the ocean floor. However, this has not been the case, and the oxygen content of the atmosphere has remained relatively stable (around 16-30%) since the evolution of terrestrial ecosystems, i.e. for more than 400 million years. The stability of the atmosphere's oxygen content suggests that it is regulated by a balancing feedback mechanism.
This feedback mechanism is fire. Organic dry matter can only burn if there is at least 13% oxygen in the atmosphere, and normal vegetation can only burn if there is more than 16% oxygen. If vegetation does not burn, more dead organic matter will accumulate, so it is very unlikely that oxygen levels would fall below 16%.
On the other hand, if oxygen levels become very high, organic material will ignite easily and burn intensely. When oxygen levels rise above 25%, fires are so frequent and intense that forests are unable to regenerate (in wet forests this limit can be around 30%). As oxygen levels rise, intense forest fires reduce the amount of organic material on the land (and thus the amount of material buried in the ocean floor), thus halting the rise in oxygen levels.
The death of forest ecosystems also reduces the amount of phosphorus entering the oceans, as phosphorus weathering from bedrock is slow without the action of tree root systems. In the oceans, the amount of phosphorus is a limiting factor for growth, so forest fires also limit the burial of marine organic matter on the ocean floor. The interactions between terrestrial plants, fire and marine nutrient levels therefore balance atmospheric oxygen levels.
Phanerozoic Eon (540 million years ago – present)
The term Phanerozoic derives from the Greek words phaneros (visible) and zoi (life), referring to the clear signs of life in the earth strata of this era. The Phanerozoic eon has been assigned to begin with the so-called 'Cambrian explosion' around 540 million years ago, when the lineages of almost all organisms that are still alive today evolved in a relatively short period of time, approximately 10-20 million years.
During the Phanerozoic eon (i.e. the last 540 million years), the average temperature of the Earth has varied between 32°C and 9°C (today's temperature is around 15°C; see figure below). This period includes at least three cooler periods during which continental glaciers have formed on land near the poles. The most recent glacial period, which we are still living through, started about 34 million years ago. During the Eocene thermal peak about 55 million years ago, atmospheric CO2 levels peaked at about 1500 ppm (there are no good estimates for earlier times), and reached a low of about 180 ppm during the last 'ice age' about 22,000 years ago.
Figure. The average temperature of the Earth over the last 540 million years. When the average temperature is below 18°C, glaciers form at the poles. Source Scotese et al. (2021). Reprinted with permission.
While other factors - such as the amount of solar radiation reaching Earth and the concentration of other greenhouse gases - influence the Earth's climate, the amount of carbon dioxide in the atmosphere is the crucial factor: the more carbon dioxide, the higher the temperature. The amount of carbon dioxide in the atmosphere, in turn, is influenced by many different factors. In the very long term (tens of millions of years), the amount of carbon dioxide and the climate are influenced by the formation of new Earth's crust in the rift zones of the tectonic plates (which adds carbon dioxide to the atmosphere) and by the amount of land subject to erosion (which removes carbon dioxide from the atmosphere).
The current understanding is that the amount of carbon dioxide in the atmosphere - and hence the average global temperature - is also controlled by feedbacks from temperature, precipitation, soil erosion and the oceanic carbon storage system, which tend to restore carbon dioxide levels to lower levels, for example after large-scale lava eruptions. However, these restoring mechanisms are very slow and operate on a timescale of millions of years.
As the figure above shows, the Earth's climate has varied greatly over long periods of time. Shifts to hotter temperatures have generally been associated with large lava eruptions, which have released huge amounts of carbon dioxide into the atmosphere.
The most dramatic lava eruption and global warming occurred at the beginning of the Triassic period around 252 million years ago, when about 7 million square kilometres (20 times the area of Finland) in western Siberia was covered by a layer of basaltic lava 1-4 kilometres thick. The eruption lasted about a million years, doubled the amount of carbon dioxide in the atmosphere and raised the average global temperature by well over 10 degrees Celsius. In the equatorial region, the temperature rose from 25°C to almost 40°C. The interior of the supercontinent Pangea was reduced to an uninhabitable desert; land-dwelling organisms survived mainly only on the slopes of mountains on the rainy coasts.
Temperatures at the poles rose to 14°C, which caused ocean currents to cease functioning and the seas eventually became anoxic and toxic. Methane was released from the anoxic oceans, further contributing to global warming. Between 81% and 96% of marine species became extinct; overall, the number of organisms declined by 90-99%. Tropical forest ecosystems took more than 20 million years to recover and the new forests were very different in species composition from the pre-warming forests. Similarly, coral reefs took more than 10 million years to recover and the coral species changed completely. The early Triassic mass extinction is the most devastating of the known mass extinctions, but illustrative, as four out of the five largest mass extinctions have been linked to large lava eruptions, which have caused ocean acidification and anoxia, acid rain and the release of toxic metals to water bodies, among other things.
Along with lava eruptions, the impact of asteroids on Earth is another factor that has caused sudden changes to the climate and life on Earth.
The best-known asteroid-induced catastrophe is the mass extinction at the end of the Cretaceous period, which wiped out the dinosaurs some 66 million years ago. The asteroid that hit the Yucatan Peninsula lifted up to 3000 million tonnes of vaporised rock into the atmosphere, and left behind a crater about 150 kilometres in diameter (Chicxulub Crater). The fine dust that remained in the air blocked sunlight from reaching the Earth's surface, killing plants and phytoplankton. The dimming of sunlight cooled the climate by 10 degrees Celsius, and this 'nuclear winter' lasted up to decades. Without food, herbivores and the predators that depend on them, starved. Around 75% of all species became extinct; in particular, all the larger animals disappeared.
When the dust finally settled, temperatures rose again - even higher than before the asteroid hit. This was because the volcanic region opposite the impact site (now the Indian subcontinent) began to erupt large quantities of lava and carbon dioxide. The shock waves from the asteroid impact travelled around the Earth and met on the opposite side, probably increasing the activity of the already volcanic region.
A celestial body even larger than the asteroid that left the Chicxulub crater probably hit Earth much earlier, around 445 million years ago, and triggered a 4 million year-long ice age (the Hirnantian Ice Age). However, little is known about this impact, and the crater it may have left on the seafloor is likely to have been buried beneath a continental plate. The Hirnantian Ice Age wiped out about half of the world's biota at the time.
For the last few tens of millions of years, the Earth's climate has been on a cooling trend. This cooling has been explained by the rise of the Himalayan mountains and the resulting accelerated erosion, which has moved carbon out of the atmosphere and into the limestone layers of the ocean floor. The opening up of the ocean current route around Antarctica has also played a cooling role, with cold ocean currents insulating Antarctica from warm water masses.
The Pleistocene, a period of more than two million years ending with the Holocene 11 700 years ago, was a period of rapid and continuous alternation between periods of glaciation (polar ice sheet expansion) and interglacials (polar ice sheet contraction), influenced by variations in the shape of the Earth's orbit and the Earth's inclination.
Over the last two million years, sea levels have risen and fallen 50 times. At its lowest point, sea level was 100 metres below present-day levels (when glaciers were at their highest) and at its highest point, 70 metres above present-day levels (when glaciers were at their lowest). Without human influence, the next phase of glacial expansion would have started in about 20 000 years. However, this will not happen, as anthropogenic carbon dioxide emissions will prevent the next glacial expansion phase from starting.
The Earth is moving away from a climatic state that has lasted for more than two million years and has been characterised by regular periods of widespread glaciation. What the future climate will look like will depend on human activities. Nor should we ignore lava eruptions or the rarer asteroid impacts that have drastically shaped the Earth's climate and life in the past.
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