The Great Oxygenation Event 2.4 to 2 billion years ago caused the extinction of many organisms which were adapted to an environment without oxygen. The oxygen was produced by photosynthesis.
Massive volcanic eruptions 252 million years ago caused the End-Permian mass extinctions. The basaltic layers from those eruptions are in China and Siberia, and are called the Siberian Traps. Hydrocarbon deposits (like peat and coal) were also set on fire, which added to the carbon dioxide and particulates emitted directly by the volcanos.
An asteroid impact 66 million years ago caused the End-Cretaceous mass extinction. The resulting Chicxulub crater is on the Yucatan Peninsula in Mexico. An impact winter from dust and smoke blocked photosynthesis.
After the great oxygenation event, depletion of oxygen and further accumulation of oxygen has happened several times, with lethal effects for organisms not adapted to the new conditions. Ocean anoxia (low oxygen levels) has been a key contributor to several mass extinctions.
War – With our technical abilities come greater devastation risks.
The climate emergency is only just beginning. Feedback loops could inflame it in ways that have received little attention.
Pandemic – It would be a rare bug that could kill all of us. However, any event that substantially reduces a population poses a contributory risk. Many extinctions are the result of multiple factors; a trip and a kick and a tumble can be enough to do away with every last one. For example a pandemic could leave a species confined to a small area, and then a simple flood or landslide could overrun the remaining population.
Takeover by artificial life – This may become a risk, but we are yet far from it. Stupid human decisions amplified by complex technologies seems a better description of current and emerging hazards.
Gamma ray burst – It is unlikely earth will be exposed to a gamma ray burst close enough to threaten us. But such a burst certainly has the energy to do so.
Supernovae are less energetic than gamma ray bursts. But they are more common. If close enough, they have more than enough energy to make earth uninhabitable.
Obliteration by aliens – We don't know why the universe is silent. It might be because there is no intelligent life out there. And it might be because it's risky to be noticed in a predatory universe.
Creation of mirror life – DNA, proteins, and lipids are constructed with branching organic structures which are not symmetrical. They spiral to the left or the right (depending which structure you're talking about). Scientists are curious, and may want to try out artificial cells with the opposite chirality (left-right orientation). I commend curiosity. But not in this case. Such life forms might be invisible to our immune system. The chance of disaster is quite small, but not agreed; and the potential size of the disaster is extremely large. Prudence has a higher commendation.
Magnetic field decay – The earth has a magnetic field which protects the atmosphere from the solar wind. Several times in the past that field has reversed. When it does, there is a period when we have much reduced protection. If that period is prolonged we are at risk.
In some cases the loss of one species does not affect the stability of other species. But if there are dependant species then there are consequential extinctions.
More generally, in a rich ecosystem, the main functions are often spread among diverse species. The extinction of one may leave the services largely intact, carried by the remaining performers of that function. Where a main function is served by a single or a small key group of species, a key restriction or a key extinction may have severe and widespread effects.
Examples of some key ecosystem functions are:
Earlier in earth's history the climate was less stable. Sometimes almost the whole earth was frozen at the surface. Today, life helps stabilise the climate.
The disasters from a mass extinction are beyond anything anyone today has lived through. Ecosystems have profound complexity. We can try to guess, but it is not possible to fully calculate the results of extinction. After extinction of a species, some of its functions are not ever replaced. In many cases it takes several million years for a new species to evolve to fill the role that a lost one used to fill. Biodiversity loss puts at risk the survival of homo sapiens.
The sun is getting brighter, by about 1% every 110 million years. This process will make the earth uninhabitable for humans in about 1.3 billion years.
And before that happens, plants will probably go extinct in about 800 or 900 million years because of a drop in atmospheric carbon dioxide levels.
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