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Spatiotemporal evolution of the hydrological cycle throughout the European continent during past abrupt climate changes

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What a Stone Age climate crisis can tell us about our own future

An abrupt shift in the climate 12 000 years ago holds clues for how global warming might affect different regions of Europe.

Climate Change and Environment

Greenhouse gases from human activity are warming the planet, but how climate change will manifest at the fine scale is still an open question. “We know things are not changing uniformly,” says palaeoclimatologist and geochemist Dirk Sachse, project coordinator of STEEPclim. “It’s more important to know that certain regions have droughts or floods, than a wholesale increase in temperature.”

Ice age

To better understand the properties of a rapidly changing climate, Sachse and his team turned to a historical period known as the Younger Dryas, which occurred ca 12 000 years ago. At the end of the last glaciation, after a period of warmer temperatures and melting continental icesheets, temperatures in the northern hemisphere suddenly plummeted, almost sending Europe back into an ice age. “It was a 4 degree temperature change over 100 years; an extremely abrupt climate change for a geologist,” explains Sachse, head of the Organic Surface Geochemistry Lab at GFZ Helmholtz Centre Potsdam in Germany. “These changes happened over the period of a human lifespan.” To understand how this period of sudden climate change affected different regions in Europe, Sachse and his team, supported by the EU, looked at sediment cores taken from lakes across the continent. Trapped in the layers of mud are the remnants of leaf waxes, the shiny protective layer of hydrocarbons that covers all higher plants.

Historical record

These compounds are referred to as molecular fossils, and the proportions of stable isotopes of carbon and hydrogen in these waxes act as a record of the hydroclimate at the time. “It’s not like we reconstruct temperature, but we can see certain conditions such as droughts and wetter periods,” he says. Through collaborators, sediment cores were taken from 20 lakes, from Estonia to southern Spain. Age estimates were made using laminations in the cores that result from seasonal changes in deposition, acting like rings in a tree. These measurements were calibrated with isochrons – ash deposits from volcanic eruptions that appear across all samples at the same moment in time. The work was supported by the European Research Council. “This would never have been possible without such funding,” adds Sachse. “We’re working on a Europe-wide scale. It could have been done in individual legs, but would have taken 20 years.”

Tipping point

Sachse says the evidence shows that western Europe was more immediately affected by the Younger Dryas, with cooler and dryer conditions spreading from Greenland over a period of 170 years. “It shows global change is always about regional change,” he notes. The findings will be used to generate more accurate and precise climate models in the future. The Younger Dryas is believed to have been driven by changes to the Gulf Stream, which is also expected to weaken in the future as a result of human-induced climate change and which is considered an important climate system tipping point. “If this were to happen today, there would be huge consequences for society,” adds Sachse. “The whole Holocene has been amazingly stable, now we’re turning the knobs on this system.”

Keywords

STEEPclim, climate, ice, Holocene, tipping, point, sediment, core, molecular, fossil

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