Approximately 2.4 billion years ago, about 200 million years after the first cyanobacteria, atmospheric oxygen increased to about 10% of the present value in the Great Oxygenation Event.
About 540 million years ago, there was still no life on land but single and multi-celled life in the seas increased the amount of oxygen to between 50% and 150% of the present percentage.
The gradual (55 million year) warming period, that followed, began with one of the most significant events in the history of life on Earth, the Cambrian Explosion. This event lasted between 13 and 25 million years and triggered the rapid evolution of symbiotic, multicellular organisms. These consumed oxygen and had complex sensory systems that allowed them to fill new ecological niches.
Before the early Cambrian explosion, most organisms were relatively simple, composed of individual cells, or small multicellular organisms, occasionally organized into colonies. As the rate of evolution accelerated, life forms became much more complex, and almost all present-day animal phyla began to resemble animals that exist today.
But, for most of the past 290 million years, Earth was much warmer than it is now. Between 200 million and 45 million years ago polar ice caps were small or absent. The Eocene Epoch (between 56 and 34 million years ago) was the warmest part of the past 65 million years. The Arctic Ocean was not permanently frozen. Palm trees grew as far north as Canada and alligator predecessors lived on Ellesmere Island near the North Pole.
Fossils from near the poles suggest that the yearly average temperature was as high as 25°C (45°F) warmer than it is today, but the tropics were not much hotter because Earth's warmth was more uniformly distributed.
(Crocodilians first appeared 95 million years ago in the Late Cretaceous period).
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Non-FictionThe eruption of Tonga's underwater volcano, in January 2022, vaporized 150 million tons of seawater and shot a geyser of super heated steam through the stratosphere. The water vapour, a strong greenhouse gas, may increase global temperatures for at...
