Every year, sea ice cover in the Arctic Ocean shrinks to a low point in mid-September.
The Pliocene Arctic
We are part of a team of scientists who analysed sediment cores from Lake El’gygytgyn in northeast Russia in 2013 to understand the Arctic’s climate under higher atmospheric carbon dioxide levels. Fossil pollen preserved in these cores shows that the Pliocene Arctic was very different from its current state.
Because the oceans were warmer and there were no large ice sheets in the Northern Hemisphere, sea levels were nine to 15 metres higher around the globe than they are today. Coastlines were far inland from their current locations. The areas that are now California’s Central Valley, the Florida Peninsula and the Gulf Coast all were underwater. So was the land where major coastal cities like New York, Miami, Los Angeles, Houston and Seattle stand.
Warmer winters across what is now the western US reduced snowpack, which these days supplies much of the region’s water. Today’s Midwest and Great Plains were so much warmer and dryer that it would have been impossible to grow corn or wheat there.
Why was there so much CO2 in the Pliocene?
In soils, certain rocks continually break down into new materials in reactions that consume CO2. These reactions tend to speed up when temperatures and rainfall are higher – exactly the climate conditions that occur when atmospheric greenhouse gas concentrations rise.
But this thermostat has a built-in control. When CO2 and temperatures increase and rock weathering accelerates, it pulls more CO2 from the atmosphere. If CO2 begins to fall, temperatures cool and rock weathering slows globally, pulling out less CO2.
Rock weathering reactions also can work faster where soil contains lots of newly exposed mineral surfaces. Examples include areas with high erosion or periods when Earth’s tectonic processes pushed land upward, creating major mountain chains with steep slopes.
The rock weathering thermostat operates at a geologically slow pace. For example, at the end of the Age of Dinosaurs 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 reduce them naturally to around 400 parts per million in the Pliocene.
Because natural changes in CO2 levels happened very slowly, cyclic shifts in Earth’s climate system were also very slow. Ecosystems had millions of years to adapt, adjust and slowly respond to changing climates.
A Pliocene-like future?
Coastal cities, agricultural breadbasket regions and water supplies for many communities all will be radically different if this planet returns to a Pliocene CO2 world. This future is not inevitable – but avoiding it will require big steps now to decrease fossil fuel use and turn down Earth’s thermostat.