A disconcerting new study in the journal Nature has identified a common feature of almost all mass extinction events throughout history, and warns that the portents of the next such catastrophe are now on the rise due to climate change. Specifically, the authors found that all historic episodes of extreme global warming have triggered toxic freshwater algal blooms that persist for hundreds of millennia, and say that the proliferation of these harmful microbes is now detectable once again.

The largest extinction event in Earth’s history occurred some 252 million years and is known as the end-Permian event (EPE), or the Great Dying. Precipitated by a dramatic increase in volcanic activity, the EPE is associated with a sharp elevation in carbon dioxide levels and global temperatures and is believed to have wiped out around 90 percent of all species on the planet.

To study how this apocalyptic episode affected freshwater ecosystems, the researchers analyzed the fossil, sediment, and chemical records in rocks around the Sydney Basin, Australia. In doing so, they detected a massive increase in microscopic algae and cyanobacteria just after the EPE, with recurrent blooms lasting for around 100,000 years.

Under normal circumstances, microbes like these help aquatic ecosystems to thrive by oxygenating the water. However, when blooms spiral out of control they have the opposite effect, depleting oxygen and releasing toxins that make the environment inhospitable to all other life forms.

According to the study authors, the conversion of freshwater bodies into noxious sludge prevented ecosystems from recovering for around 3 million years after the EPE.

They go on to explain that this devastating bloom was triggered by a perfect storm of accelerated greenhouse gas emissions and higher global temperatures as a result of volcanic activity, combined with an influx of nutrients into freshwater bodies. This third ingredient occurred as wildfires and droughts led to mass deforestation, causing compounds in soil to leach into waterways.

Moreover, the authors found that this phenomenon was far from a one-off, and occurred during every mass extinction apart from the famous end-Cretaceous extinction that wiped out the dinosaurs. Occurring 66 million years ago, this asteroid-driven event did not generate sufficient increases in greenhouse gases to spark a major algal bloom.

However, study author Tracy Frank says that modern anthropogenic climate change has the potential to recreate the conditions needed for such a blooming event, and could therefore trigger a major ecological catastrophe.

“We’re seeing more and more toxic algae blooms in lakes and in shallow marine environments that are related to increases in temperature and changes in plant communities, which are leading to increases in nutrient contributions to freshwater environments,” Frank says. “So, a lot of parallels to today. The volcanism was a source of CO₂ in the past but we know that the rate of CO₂ input that was seen back then was similar to the rate of CO₂ increases we’re seeing today because of anthropogenic effects.”

The researchers calculate the optimal water temperature for the growth of these harmful algae to be between 20 and 32°C (68-89.6°F). Worryingly, current climate change models predict mid-latitude continental summer surface air temperatures to fall into this range by the end of this century. Added to this equation is a notable recent increase in wildfires, which has the potential to fuel these blooms by allowing soil nutrients to leak into rivers and lakes.

Spelling it out in simple terms, the study authors state that “our findings from the deep geological record underscore these warnings, and provide a disturbing prediction for the long-term consequences of continued warming and deforestation.”