There have been many mass extinction events on Earth. The one currently underway is somewhat novel.
There are 2 extents of extinction: background and mass. Background extinction is the demise of a relatively few species. This occurs where adaptation fails, often in a rapidly changing biome. Mass extinction indiscriminately wipes out many species. The difference between the two is a matter of degree. Species, even microbes, are always going extinct.
Background extinction and mass extinction are typically provoked by environmental changes such as climate. The rapidity and severity of change dictate the degree and duration of an extinction event.
The chronicle of mass extinction events is incomplete. New discoveries keep being made of extinction pulses which profoundly affected life.
Conventional accounting tallies only 5 major extinction events prior to the present one. This is an under-count. There have been at least 10 such massive extinction events.
The Devonian period (416–359 million years ago) ended with a severe marine mass extinction owing to invasive species. In the current event, humanity is the invasive species.
Several major mass extinction events resulted from glaciation (Earth going into icehouse). Being toward the cooler end of the “Goldilocks” zone of habitability in its distance from the Sun, Earth tends toward icehouse rather than hothouse in climatic extremes. That said, numerous mass extinctions have occurred as Earth’s climate abruptly went hot.
The rate of climate change is what often determines how severe a mass extinction is. Organisms can adapt in time but be overwhelmed by sudden shifts. Neanderthals when extinct 40,000 years ago as the gyrating climate proved beyond their survival skills.
Most mass extinction events have been on the order of tens of thousands to millions of years. A very few have been geologically “sudden”: occurring over the course of just a few thousand years.
The current event began with industrialization and has picked up steam since. The present mass extinction is extreme in mostly happening within a thousand years or so. The current die-off will continue for centuries, however front-loaded it now seems.
The present global warming is likely to last many centuries at the least; long after humanity has wiped itself from the planet. One reason for this is volcanism.
Earth has ~25 times the water in the oceans within its mantle. As water warms it expands, pressing upon lava plumes which find release on the surface. Volcanos tend to accent the climate change underway, either warming (into hothouse) or cooling (into icehouse).
Volcanism has figured in many mass extinction events. In some, including the current one, volcanos prolonged rather than initiated the die-off.
Marine life typically suffers in a mass extinction event. Marine extinction got a head start this time through overfishing and human pollution, most obscenely sea-faring plastic.
On land, large animals typically get the worst of it in mass extinction events. Insect populations have been badly hit in this one, thanks to pesticides and other human pollutants. This has had hideous knock-on effects for other animals, beginning with insectivores. Many bird populations have plummeted for lack of food.
Rarely have plants suffered severely in mass extinction events (at least in terms of species). Deforestation has been the primary culprit of floral demise in this event. But flora now are down, not out. Plants are likely to recover within centuries, and so moderate the hothouse with their cool lifestyle.
This current mass extinction may be caustic but is likely to be comparatively short-lived. Thankfully for other life, people will be gone, allowing the return of the dinosaurs as the dominant animal life. In the next epoch, birds rule.
Ishi Nobu, The Elements of Evolution, BookBaby (2019).