Roasting Triassic heat exterminated tropical life

Some 249 million years ago, parts of Earth were so hot they were literally uninhabitable. The scorching temperatures directly led to an extinction event ? the only time this has happened in Earth's history. Many species were unable to survive in the tropics and could only cling on close to the poles.

"These were the hottest times the Earth's ever had since it cooled from a molten blob," says Paul Wignall of the University of Leeds, UK.

The extreme heatwave happened in the early Triassic period, when the first dinosaurs evolved. Earth was recovering from the massive end-Permian extinction, which struck 252 million years ago and wiped out 80 to 90 per cent of species. Volcanic eruptions in what is now Siberia had lethal consequences, such as depriving the oceans of oxygen, and probably caused the extinction.

The global ecosystem was mysteriously slow to bounce back after the end-Permian extinction, Wignall says. "Normally things start to recover within tens or hundreds of thousands of years." Instead the "dead zone" lasted nearly 5 million years. "We've never known why."

Wignall and his colleagues used tooth-like fossils called conodonts to reconstruct temperatures in the early Triassic. The ratios of oxygen isotopes in structures like teeth and shells depend on temperature, so measuring these isotopes gives a good indication of climate.

Lethally hot

The team found that temperatures rose steadily for the 3 million years immediately after the end-Permian extinction, perhaps driven in part by a pulse of greenhouse gas emissions from the Siberian volcanic eruptions, and a lack of abundant plants to remove carbon dioxide from the atmosphere. Temperatures reached a peak 249 million years ago ? the time of the Smithian-Spathian extinction. This little-known event wiped out far fewer species than the end-Permian extinction, partly because there were so few left to kill.

At this time, sea surface temperatures in the tropics reached 40 ?C, while deeper waters were a few degrees cooler. Land temperatures fluctuate more than ocean temperatures, so they may have hit 50 ?C or even 60 ?C at times. "Tropical summers must have been lethally hot," Wignall says.

The extreme heat explains peculiar patterns in the fossil record, says Wignall. Large and physically mobile species, like fish, disappeared from the tropics but endured at the poles. Only small, immobile species like molluscs, which would have coped better with the heat, remained in the tropics. Much of the land was virtually devoid of plant life.

Temperatures were back to normal by 247 million years ago, at which point big, active animals returned to the tropics and plants recolonised the land.

Many mass extinctions coincided with global warming, but temperature itself was not the killing factor: related events, such as shifting ecosystems, were to blame. The Smithian-Spathian is the only known case where heat killed.

Overlooked mechanism

"Clearly, widespread heat death is an overlooked and understudied mechanism for mass extinction in Earth's history," says Matthew Huber of Purdue University in West Lafayette, Indiana. "Earth's biota may be vulnerable to very strong warming in the tropics." He adds that the results will need to be independently confirmed.

While the extreme temperatures of the Triassic are unlikely to be repeated, parts of Earth could nevertheless become uninhabitable for humans in the next few centuries. Extreme heat combined with humidity is fatal to humans, because sweating cannot cool us down. Huber has shown that lethally hot and humid conditions could spread over much of the tropics if global temperatures rise more than 7 ?C.

Journal reference: Science, DOI: 10.1126/science.1224126

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