The explosive end of non-avian dinosaurs is the saddest blow to life on Earth. But there have long been hints that the asteroid may not be the main culprit of this mass extinction, but only the most impressive isolated contribution. Before this dramatic event, 66 million years ago, toxic winds of change were already in the air.

A new analysis by an international team of researchers has added evidence to claims that Earth was nothing short of paradise before the asteroid hit, and that sulfur levels in the atmosphere had reached critical levels. Along with other studies on mercury levels, this study provides evidence of volcanic activity strong enough to cause significant climate change.

In 1991 the timing of this volcanic activity was dismissed as too early to be the cause of the mass extinction; but recent research suggests the timing may be close enough to matter.

“Our data suggest that the removal of volcanic sulfur gas from such activities can cause repeated short-term global temperature declines,” University of Oslo geologist Sara Callegaro and colleagues write in their paper. The team examined rocks from the Deccan Traps, one of the largest volcanic objects in the territory of modern Western India. They applied a new technique they developed to measure sulfur concentrations.

Models show that persistent sulfur emissions from the Deccan Traps are sufficient to significantly alter global climate. This volcanic region alone has released a staggering amount of millions of cubic kilometers of molten rock. The team states that the formation of highly concentrated sulfur-containing lava in this region, from Thakurwadi to Bushe at most, coincides with the cooling of the Cretaceous climate.

Although most basalt in the region tends to be low in sulfur, this may indicate a climate in which the cooling molecule was slowly released into the atmosphere from solidified magma after eruptions.

As a result, global temperatures may have dropped as much as 10°C between periods of rapid recovery in the 100,000 years before the Chicxulub meteor struck the final blow.

“Our study shows that climatic conditions were almost certainly unstable due to repeated volcanic winters that could have lasted decades before the dinosaurs went extinct,” explains McGill University geochemist Don Baker. “This instability would have made life difficult for all plants and animals and paved the way for the extinction of the dinosaurs.”

Fossilized bone fragments and thousands of eggshell remains prove the global decline of non-avian dinosaur species over such a long period.

But these falls have been refuted by other studies, continuing a long-running and sometimes bitter scientific debate between asteroid and volcano theories. Some researchers suggest that the asteroid may have triggered more activity in the Deccan Traps, while others say volcanic activity may even have helped life recover after the asteroid impact.

The pattern of explosive impacts seems to be accumulating, and ultimately volcanoes ended three-quarters of all life on Earth during the previous mass extinction.

“Volcanism in the Deccan Traps has continually disrupted ecological conditions through recurrent short volcanic winters, precipitating a global biotic crisis,” the team said.