By Julie Brigham-Grethe, University of Massachusetts Amherst And Steve Petsch, University of Massachusetts Amherst
Every year sea ice cover in the Arctic Ocean decreases in mid-September. This year it measures just 1.44 million square miles (3.74 million sq km), the second lowest value in 42 years since satellites began taking measurements. Snow today is only 50% of the area that was covered at the end of summer 40 years ago.
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. This means that the Arctic has not warmed in 3 million years.
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 rise, they can return the Earth to Pliocene conditions, with high sea levels, weather changes and changed conditions in both the natural world and human society.
We are part of a team of scientists who analyzed sediment cores from Lake L’गgiggyen in north-east 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 there were no large ice sheets in the Northern Hemisphere, sea levels were 30 to 50 feet (9 to 15 meters) higher than the world today. The shorelines were far from their present 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 appear 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.
In soil, some rocks break down into new materials in reactions that consume CO2 continuously. 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, drawing out 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.
The rock weathering thermostat operates geographically at a slow rate. For example, at the end of the dinosaur age about 65 million years ago, scientists estimated that atmospheric CO2 levels ranged between 2,000 and 4,000 parts per million. In the Pliocene it took them 50 million years to naturally reduce to around 400 million 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.
Summer heat waves are causing changes in northern Siberia, melting permafrost and creating massive wildfire conditions.
A Pliocene-like future?
Today human activities 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 2 ° F (1 ° C) 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 ice cover has increased this warming to 9 F (+5 C). As a result, summer Arctic sea ice coverage is running lower and lower. Scientists predict that within the next two decades the Arctic will be completely ice-free in summer.
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 usage and to reduce the Earth’s thermostat to avoid it.
Julie Brigham-Great, Professor of Geosciences, University of Massachusetts Amherst And Steve Petsch, Associate Professor of Geosciences, University of Massachusetts Amherst
This article is republished from Conversation under a Creative Commons license. Read the original article.
Bottom Line: The Arctic has not warmed for over 3 million years. These geologists discuss what is going on in the Arctic and what effect it will have on the rest of the world.