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 recognized in the past 539 million years of our planet’s history.
Up to 94% of marine species and 70% 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 that 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 8 million years after.
We hypothesized 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 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 centimeters in height.
Enclaves of larger plants remained near the poles, in coastal and in slightly mountainous regions where the temperature was slightly cooler. After about 5 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 in 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 analyzed our model results, we found that the initial increase in temperature from the Siberian Traps was preserved for 5 to 6 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 in the 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.
Andrew Merdith, DECRA Fellow, School of Earth Sciences, University of Adelaide; Benjamin J. W. Mills, Professor of Earth System Evolution, University of Leeds, and Zhen Xu, Research Fellow, School of Earth and Environment, University of Leeds
This article is republished from The Conversation under a Creative Commons license. Read the original article.
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