Each year, sea ice cover in the Arctic Ocean shrinks to a low point in mid-September. This year, it measures just 1.44 million square miles (3.74 million square kilometers) – the second lowest value in the 42 years since satellites started taking measurements. The ice today only covers 50% of the area it covered 40 years agoat the end of summer.

This year’s minimum ice extent is the lowest in 42-year satellite records except for 2012, reinforcing a long-term declining trend in Arctic ice cover. Each of the past four decades has produced, on average, less and less summer sea ice. NSIDC

As the Intergovernmental Panel on Climate Change has shown, levels of carbon dioxide in the atmosphere are higher than at any time in human history. The last time atmospheric CO2 concentrations reached today’s level – about 412 parts per million – it was 3 million years ago, in the pliocene era.

As geoscientists who study the evolution of the Earth’s climate and how it creates living conditions, we see changing conditions in the Arctic as an indicator of how climate change could transform the planet. If global greenhouse gas emissions continue to rise, they could return Earth to Pliocene conditions, with higher sea levels, altered weather patterns, and altered conditions both in the world. natural world and human societies.

The Pliocene Arctic

We are part of a team of scientists who analyzed sediment cores from El’gygytgyn Lake in northeastern Russia in 2013 to understand the arctic climate with higher atmospheric carbon dioxide levels. The fossil pollen preserved in these cores shows that the Pliocene Arctic was very different from its current state.

Today the Arctic is a treeless plain with only tundra vegetation, such as grasses, sedges and some flowering plants. In contrast, Russian sediment cores contained pollen from trees such as larch, spruce, fir and hemlock. This shows that boreal forests, which today ends hundreds of miles further south and west in Russia and the Arctic Circle in Alaska, once reached the Arctic Ocean through much of Arctic Russia and America North.

Because the Arctic was much warmer in the Pliocene, the Greenland ice sheet did not exist. Small glaciers along the mountainous east coast of Greenland were among the few places with year-round ice in the Arctic. Pliocene Earth only had ice at one end – in Antarctica – and that ice was less extensive and more likely to melt.

Because the oceans were warmer and there were no large ice caps in the northern hemisphere, the sea level was 9 to 15 meters (30 feet to 50 feet) higher in the world than ‘today. The coasts were far inland from their present locations. The areas that are now the Central Valley of California, the Florida Peninsula, and the Gulf Coast were all underwater. The same goes for the territory where the big coastal cities like New York, Miami, Los Angeles, Houston and Seattle are located.

Warmer winters in what is now the reduced snowpack of the western United States, which these days provides much of the region’s water. Today’s Midwest and Great Plains were so much hotter and drier that it would have been impossible to grow corn or wheat there.

Why was there so much CO2 in the Pliocene?

How did Pliocene CO2 concentrations reach levels similar to today? Humans would not appear on Earth for at least another million years, and our use of fossil fuels is even more recent. The answer is that some natural processes that have occurred on Earth throughout its history release CO2 into the atmosphere, while others consume it. The main system that keeps these dynamics in balance and controls the Earth’s climate is a global natural thermostat, regulated by rocks that react chemically with CO2 and remove it from the atmosphere.

The greenhouse effect results in increased surface temperatures and, in some places, increased precipitation. Together, they accelerate the weathering of silicate rocks. Faster weathering in turn removes more CO2 from the atmosphere (yellow arrow). The strength of the greenhouse effect lies in the levels of atmospheric CO2. Gretashum / Wikipedia

In soils, some rocks are continuously broken down into new materials during CO2-consuming reactions. These reactions tend to accelerate when temperatures and precipitation are higher – exactly the climatic conditions that occur when atmospheric concentrations of greenhouse gases increase.

But this thermostat has a built-in control. As CO2 and temperatures rise and weathering of rocks accelerates, they pull more CO2 from the atmosphere. If CO2 starts to drop, temperatures cool and weathering of rocks slows down overall, removing less CO2.

Rock weathering reactions can also work faster when the soil contains many newly exposed mineral surfaces. Examples include areas of high erosion or times when the Earth’s tectonic processes pushed the land upward, creating large mountain ranges with steep slopes.

The rock weathering thermostat operates at a geologically slow rate. For example, at the end of the dinosaur era about 65 million years ago, scientists estimate that atmospheric CO2 levels were between 2,000 and 4,000 parts per million. It took over 50 million years to naturally reduce them to about 400 parts per million in the Pliocene.

Since the natural changes in CO2 levels occurred very slowly, cyclical changes in the Earth’s climate system were also very slow. Ecosystems have had millions of years to adapt, adjust and respond slowly to climate change.

A near future of the Pliocene?

Today, human activities are overwhelming the natural processes that extract CO2 from the atmosphere. At the dawn of the industrial era in 1750, atmospheric CO2 amounted to about 280 parts per million. It took only 200 years for humans to completely reverse the trajectory started 50 million years ago and bring the planet back to levels of CO2 not seen in millions of years.

Most of this change has occurred since World War II. Annual increases of 2 to 3 parts per million are now common. And in response, the Earth is heating up at a rapid rate. Since about 1880, the planet has warmed by 1 degree Celsius (2 degrees Fahrenheit)– several times faster than any warming event in the last 65 million years of Earth’s history.

In the Arctic, losses of reflective snow and ice cover amplified this warming to +5 C (9 F). As a result, the summer arctic sea ice cover tends to decline more and more. Scientists predict that the Arctic will be completely ice free in summerover the next two decades.

This is not the only evidence of drastic warming in the Arctic. Scientists have recorded extreme summer melt rates across the Greenland ice cap. In early August, Canada’s last ice shelf, in the territory of Nunavut, collapsed in the sea. Parts of Arctic siberia and Svalbard, a group of Norwegian islands in the Arctic Ocean, hit record high temperatures this summer.

Coastal cities, agricultural regions, and water supplies to many communities will all be radically different if this planet reverts to a Pliocene CO2 world. This future is not inevitable – but to avoid it, it will now take big steps to reduce the use of fossil fuels and lower the Earth’s thermostat.

Julie brigham-grette is professor of geosciences at the University of Massachusetts Amherst. Steve petsch is an associate professor of geosciences at the University of Massachusetts Amherst. This was first published by The Conversation – “The Arctic hasn’t been this hot in 3 million years – and that portends big changes for the rest of the planet“.


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