The Atlantic Gulf Stream was unexpectedly strong during the last ice age – new study

20,000 years ago, the world Great Ice AgeA two-mile-thick ice sheet covered much of North America, Scandinavia, and the British Isles.

Greenhouse gas concentrations were much lower, the world was 6°C colder, and water trapped in ice sheets would have lowered sea levels by at least 120 metres, exposing land that is now submerged. Doggerland Or from Russia to Alaska Beringia.

However, our currently published research Naturehas revealed at least one surprising fact about ice age climate: the Gulf Stream, which carries warm water north through the Atlantic, was stronger and deeper than it is today.

The research began because, as a paleoceanographer (a scientist who studies the oceans of the past), I wanted to understand how the oceans behaved during the last ice age to provide insight into how climate change may change things in the future.

Warm water – from Mexico to Norway

Currently, warm, salty water from the Gulf of Mexico flows north as part of the Gulf Stream, and as it passes through Europe it gives off a lot of heat, helping to maintain the climate of Western Europe. Very mild.

Then, as the surface waters pass north of Iceland, they lose enough heat and become denser, sinking to form the deep ocean. This process sets in motion a deep-sea terrestrial conveyor belt that connects the world’s oceans, slowly moving heat around the planet more than a mile below the surface.

Scientists have so far Atlantic Meridional Gyre The Arctic Ocean, a complex system of deep and surface currents including the Gulf Stream, was weak during periods of extreme cold, such as the last ice age. In theory, more Arctic sea ice should reduce the amount of water sinking from the surface to the deep ocean, slowing the global deep-ocean conveyor belt.

However, our new study reveals that the Gulf Stream was actually much stronger (and deeper) during the last ice age, despite the presence of a cold glacial climate and a large ice sheet around the northern Atlantic Ocean at that time.

In fact, our study suggests that the glacial climate itself was responsible for the strengthening of the Gulf Stream. In particular, the glacial periods were characterized by very strong winds in parts of the North Atlantic, which may have led to the strengthening of the Gulf Stream. So, although less water was sinking from the surface into the deep ocean, the Gulf Stream became stronger, and although it didn’t travel as far north as it does today, it still carried a lot of heat north.

Reconstructing past ocean circulation

Lacking data from weather buoys or satellites, they instead used proxy evidence preserved in marine sediment cores – long tubes of mud on the seafloor – to reconstruct how the oceans circulated during the last ice age.

The cores we used contain mud that has been deposited on the ocean floor for the past 25,000 years and were retrieved by research vessels from multiple locations along the east coast of the United States. Woods Hole Oceanographic Institution Massachusetts, where part of our team is based.

To determine the strength of the Gulf Stream during the ice ages, researchers measured the size of the sediment particles in the mud: larger particles indicate faster currents, and smaller particles indicate faster currents.

From the same mud, the researchers also measured the chemical composition of the shells of tiny, single-celled organisms called foraminifera. By comparing data from multiple sites and different depths in the Northwest Atlantic, they were able to pinpoint the boundary between foraminifera that once lived in warmer, subtropical waters and those that lived in cooler, subpolar waters. This allowed them to determine how deep in the Gulf Stream these organisms lived at the time.

This adds uncertainty to climate projections

Our study shows that the Gulf Stream and the entire Atlantic Ocean current are sensitive to changes in wind strength and meltwater from the Greenland Ice Sheet, which has important implications for future climate change.

Climate models predict that the Gulf Stream will weaken during the 21st century, in part because The wind weakensThis will cause sea levels to rise further along the east coast of the United States. Global Warming in EuropeIf wind patterns change in the future due to climate change, the Gulf Stream will also change, increasing uncertainty about future climate conditions.

“Our findings also highlight why we should not make simple statements about Atlantic currents and future climate change. The Atlantic is made up of a series of interconnected ocean currents, each with its own behaviour and its own response to climate change. Therefore, when describing the impacts of anthropogenic climate change on the climate system, we need to be very clear about which parts we are discussing and what the impacts are for each country.”