It's related to your own argument against climate change, which you have repeated several times: the average temperature of earth has been higher and lower many times, for billions of years.
Yes, we know. There were interchanging ice ages and warm periods, that scientists have several levels of evidence for. I will not go into all the evidence now, I will just mostly talk about the periods.
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The first hot period of earth was 2 billion years long. That's when the earth was formed and after a collision with another planet, leaving it extremely hot, and creating the moon from the rubble in orbit colliding. The average temperature is estimated to be 160–210 °F (70–100 °C). It had intense volcanism, frequent asteroid impacts, and a dense greenhouse atmosphere (mainly CO₂, methane, and water vapor).
[The earliest signs of life are chemical signatures, microbial fossils, and geological structures, aging between 3.5 and 4.2 billion years ago. The earliest widely accepted, or "strong" evidence for life comes from microbial fossils found in rocks in Western Australia that are approximately 3.48 billion years old. This evidence consists of fossilized structures called 'stromatolites', which are layered rock formations created by communities of microorganisms.]
[Before the Great Oxidation Event, the sun was significantly less luminous. The planet was kept warm by an atmosphere rich in methane, a greenhouse gas many times more powerful than CO2.]
During the Great Oxidation Event (about 2.4 to 2.1 billion years ago), the emergence of photosynthetic cyanobacteria introduced free oxygen into the atmosphere. This triggered a chain of events that drastically reduced atmospheric methane concentrations. This resulted in the first ice age, during the Paleoproterozoic Era, called the Huronian glaciation "Snowball Earth".
During "Snowball Earth", there were less bacteria able to use photosynthesis, but there were already bacteria that could use oxygen. There was even some more complex life, like sponges. The periods of low oxygen, then the influx of oxygen from photosynthesis devastating anaerobic life, then ice covering the oceans and making photosynthesis hard, and the ice cap closing off the oceans lowering oxygen again, all forced life to constantly evolve.
After 300 million years, in local pockets of oxygen-rich water, newly adapted aerobic microbes, early eukaryotic cells and methanogenic microbes (converting hydrogen and carbon dioxide into methane and water), together with volcanic activity, the ice melted. It exposed darker surfaces (ocean and rock) that absorbed more solar heat, further accelerating the warming. This created a runaway greenhouse effect that rapidly ended the deep freeze.
The geological record immediately following the glaciation is marked by distinctive cap carbonates, layers of rock that indicate a sudden and drastic shift from extreme cold to a period of intense greenhouse warming. This "hothouse" phase was short-lived on a geologic timescale, as the increased weathering from the newly exposed land eventually helped draw down CO2 levels, allowing the climate to stabilize once again.
The end of the post-Huronian hothouse marked the beginning of a long period of planetary stability known as the "boring billion," which lasted until about 720 million years ago.
Life was then still mostly confined to the oceans and still consisted mostly of microscopic organisms. Life existed only on the edges of land near water, as bacteria and algae formed extensive mats in shallow marine environments. There was some forms of complex life, such as seaweed.
Then the Earth completely froze over again, the second "Snowball Earth" event, called the Sturtian glaciation. It was likely caused by a combination of factors. Continental breakup (the supercontinent Rodinia began breaking apart around 750 million years ago). This triggered large-scale erosion of newly exposed continental rocks. Weathering processes consume atmospheric carbon dioxide. A period of unusually low volcanic activity, further reducing the amount of CO2 being released into the atmosphere, contributing to the cooling effect.
Life continued to exist in deep ocean hydrothermal vents and possibly in meltwater pools or thin areas of sea ice.
The Sturtian glaciation, which occurred from approximately 717 to 660 million years ago, ended due to an extreme buildup of volcanic carbon dioxide in the atmosphere. Weathering of rock was suppressed, because of the ice.
The CO2 concentration reached extremely high levels, perhaps as much as 350 times present-day levels. This trapped an immense amount of heat, eventually overwhelming the ice's high albedo (reflectivity) and causing the planet to thaw.
Afterwards the CO2 plunged Earth into a period of intense heat. The post-Sturtian hothouse, was caused by a "supergreenhouse" climate. Earth's average global temperatures were extremely high, with some estimates suggesting average global temperatures soared to around 122°F (50°C). This period was eventually brought to an end by accelerated silicate weathering, which removed large amounts of CO2 from the atmosphere. The climate did not settle into a long, stable warm period but eventually slipped into the next major "Snowball Earth" event.
Marinoan Glaciation (around 650–635 million years ago).
Then Ediacaran Period (635–541 million years ago): A warmer, more stable period followed the Marinoan glaciation, allowing for the diversification of the first complex, multicellular organisms.
Paleozoic Icehouse (around 450–420 million years ago): A brief but intense glacial period during the Ordovician and Silurian periods, likely caused by a combination of continental drift and plant evolution pulling CO2 from the atmosphere. This the first time that life on land acted on the climate, instead of only the land (silicate weathering) itself.
Devonian Period, a "greenhouse" period (420-360 million years ago). The continents were first colonized by small plants and arthropods, but by the end of the period, the first forests had appeared, and the first vertebrates began to emerge from the water. The iconic Archaeopteris, a progymnosperm, grew into large trees with conifer-like trunks and fern-like leaves. It formed the first forests, with some trees reaching heights of 30 meters (98 feet). Other significant groups that appeared included lycophytes (clubmosses), horsetails, and ferns. By the end of the period, the first seed-bearing plants had also evolved, enabling them to reproduce more easily away from water. The increasing plant life, with its new root systems and decaying organic matter, created the first true soils, fundamentally changing the landscape. Animal fossils from this period include mites, spiders, scorpions, and myriapods (relatives of centipedes and millipedes). The oldest known insect fossils also date to the Early Devonian.
CO2 levels dropped steeply throughout the Devonian, partly due to the expansion of land plants, which sequestered carbon.
Late Paleozoic Icehouse (around 360–260 million years ago): This long icehouse included the Carboniferous and Permian periods and was characterized by lower atmospheric CO2. Unlike the "Snowball Earth" events of the Cryogenian period, the Late Paleozoic Icehouse was not a period of complete global glaciation. Regions closer to the equator, such as what is now North America and Europe, remained moist and tropical. These areas were dominated by vast rainforests, which later became the coal beds for which the Carboniferous period is named.
Mesozoic Hothouse (around 251–66 million years ago): A warm, ice-free period, primarily caused by high levels of atmospheric carbon dioxide and other greenhouse gases, which trapped heat and drove global temperatures approximately 11 to 16°F (6 to 9°C) warmer than it is today. It was the era of the dinosaurs.
Approximately 66 million years ago, a 10-kilometer-wide asteroid struck Earth in the Yucatán Peninsula, creating the 200-kilometer-wide Chicxulub crater. The impact contributed to the mass extinction with global wildfires, massive earthquakes and tsunamis and extreme acid rain. The immediate effects were devastating, but the longer-term environmental consequences proved fatal for most life on Earth. The impact ejected immense amounts of dust, debris, and ash into the atmosphere, creating a thick, planet-encircling shroud. This blocked sunlight from reaching Earth's surface for months or even years. The blockage of solar energy caused global temperatures to plummet. Some studies indicate that average global temperatures dropped by as much as 47°F (26°C). It's important to distinguish this event from the long ice ages driven by changes in Earth's orbit and atmospheric gases. The impact winter was a brief but catastrophically intense event that happened over years, not millions of years.
After the devastation, life entered a new era of opportunity and recovery, leading to the rapid diversification of surviving groups. The disappearance of the dinosaurs left many ecological niches vacant, which paved the way for mammals and other organisms to flourish.
Cenozoic Icehouse (around 34 million years ago to present): Earth's current climatic state, characterized by polar ice sheets and glacial cycles. It was initiated by tectonic shifts that caused the formation of the Antarctic Circumpolar Current and the uplift of mountain ranges, increasing weathering and pulling CO2 from the atmosphere.
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I now described the whole of Earths history in which the climate fluctuated over billions of years, hundreds of millions of years and tens of millions of years, due to geological effects on greenhouse gasses, combined with very long term effects of primitive life that did not evolve into a stable ecosystem. The climate of Earth was dominated by those early geological effects, that have mostly come to rest now.
Today's biosphere plays a powerful role in regulating the climate through the carbon cycle. Plants and soil act as important carbon sinks, taking in CO2, while other biological processes release it.
In summary, the transition from geological to biological dominance of climate was a long, complex, and chaotic process. Early life repeatedly caused catastrophic climate changes, showing a lack of stabilizing feedback. It was the evolution of more complex, integrated ecosystems over billions of years that led to the kind of biological balance we see today, though this balance is now being rapidly disrupted by human activity.
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Modern Ice Ages (last 2.6 million years to present): A series of glacial and warmer interglacial periods driven by Milankovitch cycles (Earth's orbital variations). We are currently in a warmer interglacial period. Those periods are not measured in billions of years, hundreds of millions of years and tens of millions of years.
These cycles of glacial advance and retreat have occurred approximately every 100,000 years.
Last Glacial Period (c. 115,000 to 11,700 years ago): The most recent major cold period saw massive ice sheets cover much of North America, Europe, and Asia.
Current interglacial: Holocene Epoch (11,700 years ago to present).
This is the warm, stable interglacial period we live in today, following the retreat of the last great ice sheets.
The whole evolution of humans into a civilization has been going on in this stable interglacial period. All of the history, with extreme cold and extreme heat has been occurring at a pace of about 1000 times slower than the 'Modern Ice Ages', which
by itself are 10 times slower than the complete history of human civilization.
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