The Permian–Triassic mass extinction (PTME), which occurred approximately 252 million years ago, represents the most severe crisis in the history of the Phanerozoic. It is generally accepted that this extinction was caused by intense global warming, caused by volcanic carbon emissions from Siberian traps. Approximately 81–94 % marine invertebrate genera and 89 % terrestrial tetrapod genera became extinct during the PTME. Although one would normally expect atmospheric CO2 and global surface temperature to fall to pre-volcanic levels by about 100,000 years after the volcanic pulses, they surprisingly persisted in the Early Triassic extreme greenhouse conditions for about five million years.

The causes of this unusual persistence of super-greenhouse conditions have been the subject of considerable debate. Previous hypotheses have included changes in the silicate weathering feedback that would prevent efficient removal of CO2 from the Earth system, such as reduced availability of weatherable material or rapid continental weathering accompanied by high rates of back-weathering in a silica-rich ocean. However, it has remained unclear why these processes would take so long.

New study investigates a mechanism that is closely linked in time to the duration of extreme heat: hypothesis that the key cause of the transition to a super-greenhouse Early Triassic was the dramatic and long-lasting reduction of terrestrial biomass at low latitudes, caused by the PTME, and its delayed recovery. Plant communities, especially tropical peatlands, are responsible for significant CO2 sequestration, but these large-scale biomes were lost at the end of the Permian. This is also evidenced by multi-million-year-old "coal gap" in the Early to Middle Triassic, during which peat was not formed from plant material.

To test this hypothesis, the researchers reconstructed spatio-temporal maps of changes in plant productivity during the PTME and Early to Middle Triassic using the fossil record and lithological climate indicators. They found that:

• The most significant extinction occurred tropical and subtropical vegetation in low to mid latitudes (from −45°S to 45°N), where 86 % macrofossil species became extinct, while at high latitudes it was 66 %.
• Before the PTME, plant species richness was greatest in low and mid-latitudes, similar to today. After the crisis, this trend reversed dramatically and higher richness was recorded at high latitudeswhich served as a refuge.
• Extensive arborescent forests up to 50 m high have been replaced lower herbaceous vegetation with a height of 0.05 to 2 m in most low and mid-latitudes, indicating a significant decline in biomass and productivity.
• Estimated net primary productivity on land (NPPL) fell from approximately 54.4–62.5 Pt C/year in the Late Permian to 13.0–19.7 Pt C/year in the Early Triassic (which represents a loss of approximately 70 %).

To quantify the biogeochemical and climatic consequences of these changes in vegetation, a model SCION Earth Evolution ModelIn a control run that did not include vegetation loss, modeled temperatures dropped rapidly after the cessation of volcanic emissions from the Siberian Traps. However, after accounting for the decline in plant productivity in the Early Triassic, the model showed persistent high levels of atmospheric CO2 (around 7000 ppm) and high temperatures (up to 33–34 °C) for approximately five million years, which is consistent with the proxy data. The reduction in the intensity of continental silicate weathering due to a decline in plant productivity had a greater impact on increasing atmospheric CO2 than a direct decrease in organic carbon sequestration.

The study suggests that The progressive recovery of terrestrial ecosystems, beginning in the Olenekian stage and accelerating in the Anisian stage, led to the end of the long-lasting greenhouse environment and subsequent cooling.This dynamic process is consistent with evidence of a more favorable environment during the re-establishment of diverse ecosystems in the Middle Triassic.

This case suggests that beyond a certain global temperature, vegetation dieback may occur, which may lead to further warming through the removal of vegetation as a carbon sink. The study demonstrates the existence of thresholds in the Earth's climate-carbon system that may accelerate climate change and maintain adverse climate conditions for millions of years, with dramatic consequences for global ecosystems. Spring

Results published in the journal Nature Communications,

Glossary of key terms

• Anisian stage: The Middle Triassic stage (~246.7–241.5 million years ago) that saw the gradual revival of tropical biomes and the resumption of coal deposition.
• Biogenic enhancement of weathering (fbiota): A factor in climate-biogeochemical models that represents the extent to which plants increase the rate of continental rock weathering, a key process for removing CO2 from the atmosphere.
• Biogeographic productivity: A metric of plant productivity that does not take into account the effects of CO2 fertilization; it is based on the geographical distribution of biomes.
• Changhsing stage: The last stage of the Permian (~252–251.9 million years ago), which preceded the PTME and was characterized by high plant diversity and productivity in low and mid-latitudes.
• Coal gap: A multi-million-year period in the Early to Middle Triassic where terrestrial plant material was not deposited as peat, indicating a significant decline in terrestrial biomass.
• CO2 fertilization: The process by which higher atmospheric CO2 concentrations can increase the rate of photosynthesis and plant growth, thereby increasing plant productivity (NPPLf).
• δ13C (delta-13 carbon): The ratio of stable carbon isotopes (13C/12C) in carbonates, used to reconstruct changes in the global carbon cycle. Large negative excursions indicate massive release of light carbon.
• Early tremor: The post-PTME period (~251.9–247 million years ago), characterized by persistent super-greenhouse conditions and limited vegetation.
• Indusian stage: The earliest stage of the Early Triassic (~251.9–249.9 million years ago), characterized by particularly inhospitable conditions for plants and a significant decline in productivity.
• Latitudinal diversity gradient: The general pattern in which biodiversity decreases with increasing distance from the equator is observed to reverse after the PTME.
• Net Primary Productivity on Land (NPPL): The total amount of organic matter (carbon) created by land plants through photosynthesis over a given time.
• Olenekian stage: The Early Triassic stage (~249.9–246.7 million years ago) that saw the gradual migration of plants from refuges and the beginning of the revival of ecosystems.
• Palynology: The study of spores and pollen, which provides information about terrestrial floras, often including plants from higher elevations.
• Paleogeography: The study of the historical geography of the Earth, including the distribution of continents and oceans.
• Permian-Triassic mass extinction (PTME): The most extensive mass extinction in Earth's history (~252 million years), which led to the extinction of approximately 81-94 % marine invertebrate genera and 89 % terrestrial tetrapod genera.
• SCION Earth Evolution Model: A global climate-biogeochemical model used to simulate long-term interactions between climate, surface processes, and biogeochemical cycles of the Earth.
• Siberian traps: A vast magmatic province in Russia whose massive volcanic eruptions are widely believed to be the primary cause of the PTME.
• Silicate weathering: A chemical process in which silicate minerals break down and consume CO2 from the atmosphere, a key negative feedback regulating long-term global temperatures.
• Super-greenhouse conditions: A prolonged period of extremely high global temperatures and atmospheric CO2.
• Taphonomy: The study of processes that affect the preservation of organisms in the fossil record, including decomposition, burial, and fossilization.
• Upper temperature steady state: The concept that the Earth system may transition to a state where the climate stabilizes at a much higher global temperature for millions of years due to changes in the carbon cycle.