In Earth’s history there have been numerous catastrophic episodes that have altered the course of life on the planet, wiping out many species and paving the way for the rise of others says Sheena Harvey.
- What was the first animal on earth?
- How old is life on Earth? Well, it depends on what you mean by 'life'
However, none were as great as the events of around 250 million years ago, when 81 per cent of marine species and 70 per cent of vertebrate terrestrial animals went extinct, as well as the greatest known number of insect species and a huge number of plants.
What was the Great Dying and what caused it?
It’s hardly surprising that this momentous event has become known as The Great Dying.
The cause of the mass extinction is thought by scientists to have been a massive increase in volcanic activity in present day Siberia. The eruptions created a vast lava plain known as the Siberian Traps, set fire to oil and coal deposits and created dangerous emissions of methane gas.
The resulting combination of atmospheric CO2, sulphur dioxide and methane destroyed much of the ozone layer, let in solar radiation and acidified the oceans to such as extent that the entire globe was affected. The Earth heated up for five million years and life died on an unprecedented scale.
The enormous level of lives lost has been established through the study of layers of sediment where the fossils of creatures have been identified and counted. For example, in a study of two sedimentary zones in south China it was found that 286 of the 329 marine animals present in the first zone had disappeared by the time the second zone had been laid down.
The volcanic trigger for this mass extinction has been established by many different researchers through further studies of the fossil record but a question has always remained - why was the warming of the planet so prolonged on that occasion? Previous and later instances of global warming in the history of Earth did not last nearly as long as five million years.
Could the Great Dying happen again?
Now, a new paper - Early Triassic Super-greenhouse Climate Driven by Vegetation Collapse - published in the journal Nature Communications, puts forward a compelling theory linked to the demise of the tropical forests that was caused by the volcanic activity.
- The Sixth Extinction: what it is, what is causing it - and how many species we are losing
- "99 per cent of the species that have ever lived have gone extinct": Why life needs extinction and how it can be a force for good
It is well known that plants remove and trap much of the atmosphere’s carbon dioxide, keeping the levels in the air appropriate to a gaseous mixture that will sustain life. The loss of plant cover in The Great Dying, argues the paper, made the bad atmospheric situation much worse. It was insufficient carbon capture that sounded the final death knell for a large proportion of organisms.
Lead author, Dr Zhen Xu, from the School of Earth and Environment at the University of Leeds, says: “Critically, this is the only high temperature event in Earth’s history in which the tropical forest biosphere collapses, which drove our initial hypothesis. Now, after years of fieldwork, analysis and simulations, we finally have the data which supports it.”
If we were only considering the events of 250 million years ago this study would possibly just be an interesting addition to our knowledge of the evolution of the planet. But the paper’s authors believe there is a valuable lesson to be drawn from the further evidence they have uncovered of the causes of mass extinction. Our present-day loss of tropical rainforests will potentially contribute even more than previously thought to a long-term damaging rise in Earth’s temperatures.
Scientists believe that we are approaching a tipping point, borne out by Earth’s previous experiences, beyond which the over-heating of the planet and the death of countless species will become inevitable. In other words, without a serious focus on preventing further large losses of the world’s all-important forests, we could be facing a second Great Dying.
In the paper’s conclusion the warning is given: “We believe this case study indicates that beyond a certain global temperature, vegetation die-back will occur, and can result in further warming through removal of vegetation carbon sinks. Our study demonstrates that thresholds exist in the Earth system that can accelerate climate change and have the potential to maintain adverse climate states for millions of years, with dramatic implications for global ecosystem behavior.”