Nature, suggests that we may be in the midst of a sixth mass extinction, caused and/ or exacerbated by a variety of human activities, including: hunting, overfishing, deforestation, the introduction of non-native species, the spreading of pathogens, and of course, global warming.
To determine whether a mass extinction is indeed in progress, scientists compared current rate of extinctions to historical, background extinction rates. They determined that the current extinction rates for mammals, amphibians, birds, and reptiles, if calculated conservatlvely over the last 500 years, are well above normal. They estimated that current rates of extinction are faster than, or as fast as, all of the rates that would have produced the past "Big Five" extinctions.
The researchers mention a variety of methods to estimate extinction rates and compare them. One of the most commonly used ways to make this comparison is to estimate the number of extinctions per million species years (E/ MSY). Background rates are estimated from fossil extinctions that took place in million-year time intervals. For current rates of extinction, the proportion of species extinct in a short time interval (a couple of centuries) is extrapolated to predict what the rate would be over a million years.
The authors are quick to point out that measuring mass extinction is not an easy task. There are a number of biases with both fossil data and modern species data that can make comparison difficult and oftentimes, uncertain. However, they point out that even the most conservative analyses still provide evidence that a sixth mass extinction, is indeed, taking place.
These briefs are part of a weekly series of updates to the publication: Climate Change - Past, Present, and Future: A Very Short Guide. The entire series can be found here.