Difference between now and past mass extinctions

Look back to the "Big Five", they were all physically caused events so that they might seem to have little or even nothing to tell us about the Sixth mass extinction event -- which is related to human activities [1]. Obviously, human activities do a strong impact to the ecosystem which may directly lead to species destruction in the modern world. For instance, transformation of the landscape, pollution, overexploitation of species, etc. 
In order to compare the sixth mass extinction(refer to human impact) with the previous events, scientists have identified a "background rate" of species extinctions from the fossil record[2]. A widely-used metric is E/MSY (extinctions per million species years)

Previous extinctions
It is estimated from the fossil extinctions that the background rates took place in million-year-or-more timeframes. Therefore, it is difficult to compare the current rate of extinctions-- just occurring over periods of decades to centuries. According the research，the average E/MSY over the fossil record is about 1.8 which means there were fewer than 2 species became extinct every million species-years on average. To break the time into less than 1,000 years the most common E/MSY is zero.

Current Extinction Rate
Over the past 1,000 years, it has been found the average extinction rate is 24 E/MSY that is about 13 times larger than the background rate. If the data is broken into 1-year bins, the maximum extinction rate over that period is bout 693 E/MSY which is 385 times larger than the background rates.

To sum up, human activities are the most different thing between the 6th mass extinction and the "big five". Their influences are causing faster rate extinctions that may lead to an acceleration for the coming 6th mass extinction. However, there is another hypothesis that the extinction event has been already arrived.  

Reference:
1. http://www.actionbioscience.org/newfrontiers/eldredge2.html.
2. http://www.skepticalscience.com/sixth-mass-extinction.html

A time trip: back to the ancient time

After the blogs written before, we know that the "big five" events have destroyed more than 50% to 95% of all organisms that have ever lived on Earth. Here is a video that gives a conclusion of all "big five" events and leads us back to the ancient time.

This trip made me to think about our current situation carefully. As most of the speculations about what causes a mass extinction are related to the climate change, it is a dangerous signal for us that the concentration of CO2 is rising rapidly and the global temperature is getting higher and higher in recent years. 

Are we causing the sixth mass extinction?

Cretaceous-Paleogene extinction event

Cretaceous-Paleogene extinction event, also called the K-T extinction where K stands for Cretaceous and T for Tertiary, is the most well known extinction event in the world and is the last one of the "Big five". It happened about 65 million years ago. During the this boundary of time, about 40-65% of marine species died out from the Earth ecosystem[1]. Almost all the large vertebrates on Earth, on land, in sea, and in the air suddenly became extinct including coccoliths, planktonic foraminifera, ammonites, gastropods, sponges, marine reptiles, dinosaurs and mammals. 
For this mass extinction event, there are still arguments existed about just how short it was[2]. It was certainly sudden in geological terms and may have been catastrophic by anyone's standards.

Why it happened? There have been many bad theories to explain this extinction event and the causes are still being debated by paleontologists. Based on researches, it is believed that a major factor was an asteroid about 10 kilometers in diameter that struck what is now the Yucatán peninsula in Mexico[3]. The impact led to many serious effects, for instance, global forest fires, possibly a period of cold weather because of sunlight-blocking dust and smoke, etc. Another hypothesis of the cause is a massive bout of volcanism.  Here is a video that gives an explanation to the K-T mass extinction in details.

Reference

1. MacLeod, N. in press. End-Cretaceous extinctions. In R. C. Selley, L. R. M. Cocks and I. R. Plimer (eds.), Encyclopedia of Geology. Academic Press, London.

2. http://www.ucmp.berkeley.edu/education/events/cowen1b.html
3. http://paleobiology.si.edu/geotime/main/htmlversion/cretaceous4.html

Causes for Triassic-Jurassic mass extinction

The Triassic-Jurassic mass extinction is the event owns most of the debates for the causes to it. Several theories for explaining this mass extinction have been suggested, but none of them have answered why it happened.

Generally, evidences have shown there were gradual climate changes, a pulse of oceanic acidification or sea-level fluctuations at that time. They can be causes of mass extinction but according to the TJME situation, those causes cannot explain why event happened so suddenly [1].

Another possible cause-asteroid impact also seems not enough for causing such a sudden big mass extinction as the carbon record presents that there were no big asteroid impacts at that period of time [2].

Then, massive volcanic eruptions, which would emit carbon dioxide or sulfur dioxide and aerosols, could cause global warming or cooling also can be the reason for the event [3]. The exist evidence proofs that there was an increase in  carbon dioxide in the atmosphere at that boundary.

Except the debating of the causes, there are also some one argued for that TJME was not the product of a single major event [4]. They preferred to not treat the event as a mass extinction event but a simply prolonged turnover of species across a considerable amount of time.

Reference:

1. T.M.Quan, et al. (2008) Nitrogen isotope and trace metal analyses from the Mingolsheim core: Evidence for redox variations across the Triassic-Jurassic boundary", Global Biogeochemical Cycles

2. S.Roff. (2011) Dark days of the Triassic: Lost world

3. J.F.Hubert et al. (2001) Stability of atmospheric CO₂ levels across the Triassic/Jurassic boundary

4. http://www.britannica.com/EBchecked/topic/1523109/end-Triassic-extinction