Arguments
Earth's five mass extinction events
Posted on 15 April 2010 by John Cook
As climate changes, a major question is whether nature can adapt to the changing conditions? The answer lies in the past. Throughout Earth's history, there have been periods where climate changed dramatically. The response was mass extinction events, when many species went extinct followed by a very slow recovery. The history of coral reefs gives us an insight into the nature of these events as reefs are so enduring and the fossil record of corals is relatively well known (Veron 2008). What we find is reefs were particularly impacted in mass extinctions, taking many millions of years to recover. These intervals are known as "reef gaps".
Figure 1: Timeline of mass extinction events. The five named vertical bars indicate mass extinction events. Black rectangles (drawn to scale) represent global reef gaps and brick-pattern shapes show times of prolific reef growth (Veron 2008).
There have been five mass extinction events throughout Earth's history:
- The first great mass extinction event took place at the end of the Ordovician, when according to the fossil record, 60% of all genera of both terrestrial and marine life worldwide were exterminated.
- 360 million years ago in the Late Devonian period, the environment that had clearly nurtured reefs for at least 13 million years turned hostile and the world plunged into the second mass extinction event.
- The fossil record of the end Permian mass extinction reveals a staggering loss of life: perhaps 80–95% of all marine species went extinct. Reefs didn't reappear for about 10 million years, the greatest hiatus in reef building in all of Earth history.
- The end Triassic mass extinction is estimated to have claimed about half of all marine invertebrates. Around 80% of all land quadrupeds also went extinct.
- The end Cretaceous mass extinction 65 million years ago is famously associated with the demise of the dinosaurs. Virtually no large land animals survived. Plants were also greatly affected while tropical marine life was decimated. Global temperature was 6 to 14°C warmer than present with sea levels over 300 metres higher than current levels. At this time, the oceans flooded up to 40% of the continents.
What caused these mass extinctions? To find the major driver of coral extinction, Veron 2008 looks at the possible options and eliminates many as the primary cause. A meteorite strike is capable of creating huge dust clouds that lead to devastating darkness and cold. However, if this were the cause of coral reef extinction, 99% of the world's coral species would be wiped out in weeks or months. The fossil record shows coral extinction occurred over much longer periods.
Warmer temperatures cause mass bleaching of corals. However, even in a warmer world, deep ocean temperatures would still remain well below surface temperatures and there would be safe havens where cooler water upwells from the deep ocean. That's not to say meteorites or global warming played no part in coral extinction - both have been contributing factors at various times. But they cannot fully explain the nature of coral extinctions as observed in the fossil record.
What Veron 2008 found was each mass extinction event corresponded to periods of quickly changing atmospheric CO2. When CO2 changes slowly, the gradual increase allows mixing and buffering of surface layers by deep ocean sinks. Marine organisms also have time to adapt to the new environmental conditions. However, when CO2 increases abruptly, the acidification effects are intensified in shallow waters owing to a lack of mixing. It also gives marine life little time to adapt.
So rate of change is a key variable in nature's ability to adapt. The current rate of change in CO2 levels has no known precedent. Oceans don't respond instantly to a CO2 build-up, so the full effects of acidification take decades to centuries to develop. This means we will have irretrievably committed the Earth to the acidification process long before its effects become anywhere near as obvious as those of mass bleaching today. If we continue business-as-usual CO2 emissions, ocean pH will eventually drop to a point at which a host of other chemical changes such as anoxia (an absence of oxygen) are expected. If this happens, the state of the oceans at the end Cretaceous 65 million years ago will become a reality and the Earth will enter the sixth mass extinction.
Vonnegut, why did you post the comment from Goreau if not to suggest something? Why did you think SkS readers would find it interesting? Why did you pick that particular quote from all of Goreau's comments in the thread? There comes a time when one no longer receives the benefit of the doubt, even from the most generous.
"Eruptive tectonic activity (volcanoes; igneous province eruptions etc.) were active during much of the Cretaceous at a level that resulted in raised greenhouse gas warming and an increase in the carbon cycle at a rate around double that of present."
Alright, I'm confused. If the Cretaceous had carbon cycle increases at a rate double of the present, that means that volcanoes were putting out twice what's being put out today at any given time, right? So if that's true, how can we cause a faster acidification of the oceans when we are putting out less carbon per year than those volcanoes?
[RH] Please note that we would prefer people not use all caps for emphasis, as it tends to read like shouting. Please use the bold or italics tools at the top of the comment box to add emphasis. Also, make sure you read the commenting guidelines.
All caps changed to bold.
Sorry, I'm used to forums where I don't have bold/italics options. I will use those in the future here.
Varika... The comment you're responding to here is discussing the difference between tectonic and volcanic emissions in the Cretaceous to tectonic and volcanic emissions now, excluding human emissions.
Man-made emissions of CO2 are about 100X that of natural sources.
http://hvo.wr.usgs.gov/volcanowatch/archive/2007/07_02_15.html
The science is settled: It's Not Volcanoes.
Varika
I second what Rob has said. The reference is to changes in tectonic CO2 emissions and a doubling during the Cretaceous of that tectonic rate. Human emissions are much larger than either rate - the US Geological Survey's estimate is that human CO2 emissions are 130 times total volcanic sources. Where elevated tectonic emissions can matter is when they go on for millenia and longer. Even the eruption of a super volcano such as Toba or Yellowstone is thought to only put the equivalent of 1/2 of human emissions into the air, and only for the duration of the eruption.
This is the record of CO2 in the atmosphere from the Mauna Loa observatory:
We might just barely see a wiggle on the annual mean line (black) around the time of the 1992 eruption ofMt Pinatubo if we squint hard and wish a lot. No sign of wiggles around the times of the eruptions of Mt El Chichon in 1982 and Mt Agung in 1964. Those were the big 3 eruptions during the period of the record. Far bigger than volcanic emissions but essentially an annual cycle is the absorption then release again of CO2 by deciduous plants (the wiggles in the red line). We can actully see the forests of the Northern Hemisphere going through their seasonal cycle.
Varika - note too that the sustained high levels of atmospheric CO2, due to this volanic outgassing over geological time scales, would have greatly increased the chemical weathering of rocks at Earth's surface mainly because of the intensified water cycle - more rain washing dissolved inorganic carbon (carbonate & bicarbonate ions in particular) back to the ocean.
Over time this overcompensated for the loss of carbonate ions, and the oceans were actually more conducive to shell formation than they are today, despite the lower than present pH (acidification). This why the Cretaceous (meaning chalk) Period was so favourable to marine calcification.
This doesn't occur with geologically-abrupt increases in atmospheric CO2, pH and carbonate mineral saturation state decline in tandem making seawater corrosive because the rate of weathering is far too slow to provide bicarbonate and carbonate ions back to the ocean. The weathering feedback which counteracts corrosive oceans can take over a hundred thousand years to respond sufficiently.
Hope this helps clear up this apparent paradox. SkS will have a rebuttal on this topic soon.