But these two processes are not always in step.
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Each event itself lasted between 50 thousand and 2.76 million years.
The Ordovician eventseems to have been the resultof two climate phenomena.
First, a planetary-scale period of glaciation (a global-scale ice age), then a rapid warming period.
The second mass extinction occurred during the Late Devonian period around 374 million years ago.
To establish a mass extinction, we first need to know what a normal rate of species loss is.
This wiped out more than 95 percent of all species in existence at the time.
These could have blocked the sun and generated intense acid rains.
The Conversation,CC BY-ND
Is todays biodiversity crisis a sixth mass extinction?
The Earth is currently experiencing an extinction crisis largely due to the exploitation of the planet by people.
But this estimated rate is highly uncertain, ranging between 0.1 and 2.0 extinctions per million species-years.
Otherwise, its difficult to compare Earths situation today with the past.
An endangered Indian wild dog, or Dhole.
Before extinction comes a period of dwindling numbers and spread.
In fact, the Earth has lost about60 percent of all vertebrate individuals since 1970.
And more than 300 animals and 1,000 plant species arenow considered threatened with imminent extinction.
All these conditions together indicate that a sixth mass extinction isalready well underway.
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