Every year, the 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 42 years since satellites began taking measurements. Snow covers only 50% of the area today, covering a late summer of 40 years.
This year’s minimum snow limit is the lowest in a 42-year satellite record other than 2012, reinforcing a long-term decline in Arctic ice cover. Each of the last four decades has had progressively lower summer sea ice. NSIDC
As revealed by the Intergovernmental Panel on Climate Change, carbon dioxide levels in the atmosphere are higher than at any time in human history. The last time atmospheric CO2 concentrations reached today’s levels – about 412 parts per million – was 3 years ago, during the Pliocene era.
As geologists who study the evolution of the Earth’s climate and how it creates conditions for life, we see the evolving conditions in the Arctic as an indicator of how climate change can change the planet. If global greenhouse gas emissions continue to increase, they can return the Earth to Pliocene conditions, with high sea levels, changing weather conditions and altered conditions in both the natural world and human society.
We are part of a team of scientists who analyzed sediment cores from Lake L”ngyen in northeastern Russia in 2013 to understand the climate of the Arctic under high atmospheric carbon dioxide levels. The fossil pollen preserved in these cores suggests that the Pliocene Arctic was very different from its current position.
Today the Arctic is a sparse plain, with only sparse tundra vegetation, such as grasses, sage, and some flowering plants. In contrast, the Russian sediment core contained pollen from trees such as larch, spruce, cedar, and hemlock. This suggests that the Boreal forests, which today end up hundreds of miles to the south and west of Russia and into the Arctic Circle of Alaska, once reached all of Russia and much of North America en route to the Arctic Ocean.
Because the Arctic was warmer in the Pliocene, the Greenland ice sheet was not present. Small glaciers along the hilly east coast of Greenland were among the few places in the Arctic with year-round snow. The Pliocene Earth had ice at only one end – Antarctica – and it was less widespread and susceptible to melting ice.
Because the oceans were warm and the Northern Hemisphere did not have large ice sheets, sea level levels were 30 feet to 50 feet (9 to 15 meters) higher worldwide than today. The shorelines were far from their current locations. The areas now in California’s Central Valley, the Florida Peninsula, and the Gulf Coast were all underwater. So was the land where major coastal cities such as New York, Miami, Los Angeles, Houston and Seattle stood.
Winters have been reduced by the snowfall in the western US, which supplies most of the region’s water these days. Today’s Midwest and Great Plains were so hot and dryer that it would have been impossible to grow corn or wheat there.
Why was there so much CO2 in the Pliocene?
How did CO2 concentrations reach levels similar to today during the Pliocene? Man will not be visible on Earth for at least another million years, and our use of fossil fuels has been even more recent. The answer is that some natural processes on Earth in 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 natural global thermostat, regulated by rocks that chemically react with CO2 and pull it out of the atmosphere.
The greenhouse effect causes an increase in surface temperature and precipitation in some places. Together these silicates accelerate rock weathering. Rapid weathering in turn extracts more CO2 from the atmosphere (yellow arrow). The strength of the greenhouse effect depends on atmospheric CO2 levels. Gretashum / Wikipedia
In soil, some rocks are constantly broken down into new materials in reactions that consume CO2. These reactions intensify when temperatures and rainfall are high – exactly climatic conditions when atmospheric greenhouse gas concentrations increase.
But this thermostat has an inherent control. When CO2 and temperature increase and rock weathering intensifies, it draws more CO2 from the atmosphere. If CO2 starts to fall, globally the temperature cools and rock weathering slows down, removing less CO2.
Rock weathering reactions can also work rapidly where soils have many newly exposed mineral surfaces. Examples include areas with high erosion or duration when the Earth’s tectonic processes push the land upward, forming major mountain ranges with steep slopes.
Rock weathering thermostats are geologically driven. For example, at the end of the dinosaur’s age about 65 million years ago, scientists estimated that atmospheric CO2 levels ranged between 2,000 and 4,000 parts per million. It took 50 million years for the Pliocene to naturally reduce them to about 400 parts per million.
Because natural changes in CO2 levels occurred very slowly, cyclical changes in the Earth’s climate system also occurred very slowly. The ecosystem had millions of years to adapt, adjust and respond slowly to the changing climate.
A Pliocene-like future?
Human activities today are overwhelming the natural processes that take CO2 out of the atmosphere. At the dawn of the industrial era in 1750, atmospheric CO2 was around 280 parts per million. It has only taken humans 200 years to completely reverse the trajectory that began 50 million years ago and the planet has not experienced CO2 for millions of years.
Most of that innings has been since World War II. An annual increase of 2-3 parts per year is now common. And in response, the Earth is warming at a fast pace. The planet has warmed by 1 ° C (2 ° F) since about 1880 – several times faster than any warming episode in the last 65 million years of Earth’s history.
In the Arctic, the loss of reflective ice and snow cover has increased this warming to +5 C (9 F). As a result, summer Arctic sea ice coverage is running lower and lower. Scientists predict that over the next two decades, the Arctic at Summerwithin will be completely ice-free.
This is not the only evidence of harsh Arctic warming. Scientists have recorded extreme heat melt rates in the Greenland Ice Sheet. In early August, Canada’s last remaining ice shelf, in the region of Nunavut, fell into the sea. A group of Norwegian islands in the Arctic Ocean, part of Arctic Siberia and Svalbard, reached record-high temperatures this summer.
The coastal city, agricultural breadbasket area and water supply for many communities will all be fundamentally different if this planet returns to a pliocax CO2 world. This future is not inevitable – but big steps will now be needed to reduce fossil fuel use and to reduce the Earth’s thermostat to avoid it.
Julie Brigham-Grete is Professor of Geosciences at the University of Massachusetts Amherst. Steve Petsch is an Associate Professor of Geosciences at the University of Massachusetts Amherst. It was first published by Conversation – “The Arctic hasn’t warmed for 3 million years – and it foreshadows major changes for the rest of the planet”.