Fossils reveal why earth was extremely hot for millions of years

Independent

8 hours ago

Some 252 million years ago, almost all life on Earth disappeared.
Known as the Permian–Triassic mass extinction – or the Great Dying – this was the most catastrophic of the five mass extinction events recognised in the past 539 million years of our planet's history.
Up to 94 per cent of marine species and 70 per cent of terrestrial vertebrate families were wiped out. Tropical forests – which served, as they do today, as important carbon sinks that helped regulate the planet's temperature – also experienced massive declines.
Scientists have long agreed this event was triggered by a sudden surge in greenhouse gases which resulted in an intense and rapid warming of Earth. But what has remained a mystery is why these extremely hot conditions persisted for millions of years.
Our new paper, published in Nature Communications, provides an answer.
The decline of tropical forests locked Earth in a hothouse state, confirming scientists' suspicion that when our planet's climate crosses certain 'tipping points', truly catastrophic ecological collapse can follow.
A massive eruption
The trigger for the Permian–Triassic mass extinction event was the eruption of massive amounts of molten rock in modern day Siberia, named the Siberian Traps. This molten rock erupted in a sedimentary basin, rich in organic matter.
The molten rock was hot enough to melt the surrounding rocks and release massive amounts of carbon dioxide into Earth's atmosphere over a period as short as 50,000 years but possibly as long as 500,000 years. This rapid increase in carbon dioxide in Earth's atmosphere and the resulting temperature increase is thought to be the primary kill mechanism for much of life at the time.
On land it is thought surface temperatures increased by as much as 6°C to 10°C – too rapid for many life forms to evolve and adapt. In other similar eruptions, the climate system usually returns to its previous state within 100,000 to a million years.
But these 'super greenhouse' conditions, which resulted in equatorial average surface temperatures upwards of 34°C (roughly 8°C warmer than the current equatorial average temperature) persisted for roughly five million years. In our study we sought to answer why.
The forests die out
We looked at the fossil record of a wide range of land plant biomes, such as arid, tropical, subtropical, temperate and scrub. We analysed how the biomes changed from just before the mass extinction event, until about eight million years after.
We hypothesised that Earth warmed too rapidly, leading to the dying out of low- to mid-latitude vegetation, especially the rainforests. As a result the efficiency of the organic carbon cycle was greatly reduced immediately after the volcanic eruptions.
Plants, because they are unable to simply get up and move, were very strongly affected by the changing conditions.
Before the event, many peat bogs and tropical and subtropical forests existed around the equator and soaked up carbon.
However, when we reconstructed plant fossils from fieldwork, records and databases around the event we saw that these biomes were completely wiped out from the tropical continents. This led to a multimillion year 'coal gap' in the geological record.
These forests were replaced by tiny lycopods, only two to 20 centimetres in height.
Enclaves of larger plants remained towards the poles, in coastal and in slightly mountainous regions where the temperature was slightly cooler. After about five million years they had mostly recolonised Earth. However these types of plants were also less efficient at fixing carbon in the organic carbon cycle.
This is analogous in some ways to considering the impact of replacing all rainforests at present day with the mallee-scrub and spinifex flora that we might expect to see in the Australian outback.
Finally, the forests return
Using evidence from the present day, we estimated the rate at which plants take atmospheric carbon dioxide and store it as organic matter of each different biome (or its 'net primary productivity') that was suggested in the fossil record.
We then used a recently developed carbon cycle model called SCION to test our hypothesis numerically. When we analysed our model results we found that the initial increase in temperature from the Siberian Traps was preserved for five to six million years after the event because of the reduction in net primary productivity.
It was only as plants re-established themselves and the organic carbon cycle restarted that Earth slowly started to ease out of the super greenhouse conditions.
Maintaining a climate equilibrium
It's always difficult to draw analogies between past climate change in the geological record and what we're experiencing today. That's because the extent of past changes is usually measured over tens to hundreds of thousands of years while at present day we are experiencing change over decades to centuries.
A key implication of our work, however, is that life on Earth, while resilient, is unable to respond to massive changes on short time scales without drastic rewirings of the biotic landscape.
In the case of the Permian–Triassic mass extinction, plants were unable to respond on as rapid a time scale as 1,000 to 10,000 years. This resulted in a large extinction event.
Overall, our results underline how important tropical and subtropical plant biomes and environments are to maintaining a climate equilibrium. In turn, they show how the loss of these biomes can contribute to additional climate warming – and serve as a devastating climate tipping point.
Zhen Xu was the lead author of the study, which was part of her PhD work.
Andrew Merdith is a DECRA Fellow at the School of Earth Sciences, University of Adelaide. Benjamin J. W. Mills is a Professor of Earth System Evolution at the University of Leeds. Zhen Xu is a Research Fellow at the School of Earth and Environment, University of Leeds.