When Greenland was green: Ancient soil from under a mile of ice offers warnings for the future

Large parts of Greenland were ice-free about 400,000 years ago. The scrubby tundra basked in the sun’s rays in the island’s northwestern mountains. Evidence suggests that a Katha Vanam Trees buzzing with insects covered the southern part of Greenland. Global sea levels were much higher then, between 20 and 40 m Above today’s levels. All over the world, the land where millions of people live today was under water.

Scientists have known for some time that the Greenland ice sheet disappeared at some point The last million yearsBut not exactly when.

In a new study in the journal scienceWe dated it using frozen soil Extracted during the Cold War From beneath a nearly mile-thick section of the Greenland ice sheet.

Timing — around 416,000 years ago, a period of 14,000 years of ice-free conditions — is important. At that time, the earth and its Early humans It is going through the longest interglacial period since ice sheets first covered high latitudes 2.5 million years ago.

The length, extent, and effects of that natural warming will help us understand the Earth that modern humans are now creating in the future.

A world protected under ice

In July 1966, American scientists and US Army engineers completed a six-year effort Drill through the Greenland Ice Sheet. Drilling took place at Camp CenturyOne of the most unusual bases of the army – it nuclear energy A series of tunnels drilled into the Greenland ice sheet.

The drill site in northwest Greenland was 138 miles off the coast 4,560 feet of snow. When they reached the base of the ice sheet, the team continued to drill 12 feet further into the frozen, rocky soil below.

In 1969, geophysicist Willie Dansgaard’s analysis of the ice sheet at Camp Century revealed the details of what Earth’s climate was like for the first time. changed dramatically For the past 125,000 years. Prolonged cold glacial periods when the ice expanded gave way to warmer interglacial periods when the ice melted and sea levels rose, flooding coastal areas around the world.

For nearly 30 years, scientists ignored the 12 feet of frozen soil from Camp Century. A study The stones were analyzed To understand the rock beneath the ice. Another curiously suggested that it was frozen soil Protected evidence A time warmer than today. But because there was no way to date the material, few people paid attention to these studies. By the 1990s, the frozen soil core had disappeared.

A few years ago, our Danish colleagues discovered lost soil buried deep in a Copenhagen freezer, and we formed it. International team To analyze this unique frozen weather archive.

In the sample above, we found that perfectly preserved Fossil plants – Evidence positive that the land beneath Camp Century was ice-free for some time in the past – but when?

Dating ancient rock, twigs and dirt

Because the material was prepared and analyzed in the dark with samples cut from the center of the sediment core, the material retained a precise memory of its last exposure to sunlight, the ice sheets that cover northwest Greenland — about a mile thick today — as we know it. Disappeared during prolonged natural warm periods known to climate scientists MIS 11Between 424,000 and 374,000 years ago.

To determine more precisely when the ice caps are melting, one of us, Tammy Ritenour, used a technique known as luminescence dating.

Over time, minerals store energy and release radiation as radioactive elements such as uranium, thorium, and potassium. The more the sediment is buried, the more radiation accumulates as trapped electrons.

In the lab, special instruments measure the tiny amounts of energy emitted as light from those minerals. That signal can be used to estimate how long the grains were buried, because the trapped energy would have been released during their last exposure to sunlight.

Laboratory of Paul Biermann At the University of Vermont, the last time near the surface of the sample was determined in another way, using rare radioactive isotopes of aluminum and beryllium.

These isotopes are formed when cosmic rays from our solar system strike Earth’s rocks. Each isotope has a different half-life, meaning it decays at different rates when buried.

By measuring both isotopes in the same sample, glacial geologists Christ was painted It has been determined that melting ice has exposed sediments on the land surface for less than 14,000 years.

Running ice sheet models Benjamin KieslingOur new knowledge now includes that the Camp Century was ice-free 416,000 years ago, and Greenland’s ice sheet must have shrunk significantly by then.

At least, the edge of the ice that surrounded most of the island retreated tens of miles during that period. Water from the melting glaciers raised global sea levels by at least 5 feet and possibly 20 feet above today’s levels.

Warnings for the future

Ancient frozen soil beneath Greenland’s ice sheet warns of trouble ahead.

During the MIS 11 interglacial, Earth was warm and ice sheets were restricted to high latitudes, as they are today. Amount of carbon dioxide 265 to 280 parts per million remained in the atmosphere for about 30,000 years. MIS 11 lasted longer than most interglacials due to the effect that the shape of Earth’s orbit around the Sun had on solar radiation reaching the Arctic. During these 30 millennia, that amount of carbon dioxide generated enough heat to melt most of Greenland’s ice.

Today, our atmosphere contains 1.5 times more carbon dioxide than it did in MIS 11. 420 parts per million, a concentration that is increasing every year. Carbon dioxide traps heat, warming the planet. More of it in the atmosphere raises global warming as the world sees it now.

Explained What is happening to Arctic sea ice?

In the past decade, humans experienced eight of the warmest years on record as greenhouse gas emissions continued to rise. Seen July 2023 Hottest week on record, based on preliminary data. Such heat The ice caps are meltingSnow loss warms the planet even more as the dark rocks absorb sunlight, once reflected by the bright white snow and ice.

Even if everyone stopped burning fossil fuels tomorrow, the amount of carbon dioxide in the atmosphere would increase Stay high Thousands to tens of thousands of years. This is because carbon dioxide takes a long time to move into soil, plants, oceans, and rocks. Like MIS 11, we are creating the perfect conditions for a very long period of warmth.

Unless people drastically reduce atmospheric carbon dioxide concentrations, the evidence we found about Greenland’s past suggests a largely ice-free future for the island.

Everything we can do to reduce carbon emissions Sequester carbon What’s already in the atmosphere makes it more likely that more ice will remain in Greenland.

The alternative is a world that looks like MIS 11 or even more extreme: warming Earth, shrinking ice sheets, rising sea levels, waves rolling in Miami, Mumbai, India and Venice, Italy.