Ancient climate warming likely triggered increased rainfall

Scientists have examined a period of extreme global warming that happened about 56 million years ago to find out how climate change could influence Earth’s water cycle in the future.

The research, led by Cambridge Earth Scientists, finds evidence for increased rainfall and storminess during the Paleocene-Eocene thermal maximum (PETM), when global temperatures increased by roughly 5–8 °C.

This ancient warming event is thought to be analogous to current and future global warming, so the findings could help refine forecasts of how rainfall patterns might change in the future.

Climate change is predicted to intensify the water cycle because warming will increase evaporation and precipitation. But forecasts of precipitation — and how its frequency, seasonality and intensity will change — are uncertain because climate models are less certain about the effects of global warming on rainfall and storminess.   

Studying warm periods from the past may help to address this shortcoming. In a new study, researchers studied sediment deposited in the North Sea during the PETM. They used a new method to isolate the hydroxyl (OH) group locked in the structure of the clay minerals in the sediment, which reflects the ancient water from which the clay formed.

“We have more evidence that, during a period of global warming in the past, we saw increased rainfall intensity and storminess,” said first author of the paper Gregory Walters, a PhD student in the Department of Earth Sciences.

The researchers brought together two pieces of equipment — one to step heat the clay and separate the hydroxyl group and another to measure its isotopic signature. Oxygen and hydrogen isotopes in the hydroxyl showed that rainfall increased during the PETM, “We see a strong negative hydrogen and oxygen isotope excursion right at that time — that’s exactly what you’d expect to see with increased precipitation,” said co-author of the paper, David Hodell from Cambridge’s Department of Earth Sciences.

Aside from the increased precipitation, the hydrological cycle was probably more enhanced as a whole during the PETM, explains Hodell, “Warm temperatures would have meant more rainfall over land — changing weathering profiles, soil types and increasing freshwater runoff into the North Sea.”

The isotopic evidence supports previous work, said Hodell, “It adds another piece to the puzzle, which collectively points to enhanced precipitation during the PETM.” Previous studies of the PETM sections in the North Sea have observed increased amounts of kaolinite, a type of clay found in the warm and humid tropics, in addition to higher numbers of fossilised freshwater-loving dinoflagellates (unicellular algae), indicating increased rainfall and runoff. In the Pyrenees, other researchers have also found evidence of flash flooding and intense rainfall.

That existing data makes the North Sea an ideal place to test out the new method, explains Hodell, “It’s a promising new paleoclimate proxy, developed here in Cambridge.” Now Hodell and the team are looking to apply the method to other sites to understand spatial variability in rainfall.

Walters, G. L., Kemp, S. J., Hemingway, J. D., Johnston, D. T., & Hodell, D. A. (2022). Clay hydroxyl isotopes show an enhanced hydrologic cycle during the Paleocene-Eocene Thermal Maximum. Nature Communications, 13(1), 1-10.