The last time CO2 levels were as high as today, in a time called the Pliocene, was some 3 million years ago when sea levels were around 30 feet higher. The Pliocene was a significantly warmer world, likely around 3 degrees warmer than pre-industrial temperatures of the late 1800’s. Much of the Arctic had melted. Heat-trapping carbon dioxide levels hovered around 400 parts per million, or ppm. Today these levels are 418 ppm and relentlessly rising.
However, sea levels won’t instantly rise by tens of feet: Kilometer thick ice sheets take many centuries to thousands of years to melt. When CO2 naturally increases in the atmosphere, pockets of ancient air trapped in ice show this CO2 rise happens gradually over thousands of years. But today, carbon dioxide levels are skyrocketing as humans continue to burn fossil fuels.
Some of the human-driven changes happening on Earth today won’t be reversed for hundreds or thousands of years, even if we instantly stop adding carbon to the atmosphere. They will have impacts upon the planet – like gradually rising sea levels and acidifying oceans – for at least centuries.
The Pliocene shows us how sensitive parts of the Earth are to just a few degrees of warming. For example, much of the vast Greenland ice sheet melted during the warmer Pliocene. “That means the ice sheets are really sensitive to a modest amount of warming,” said Rob DeConto, a professor of climatology at the University of Massachusetts.
Long before the Pliocene, CO2 levels were extremely elevated during the age of the dinosaurs, perhaps at some 2,000 to 4,000 ppm. But over millions of years, Earth’s natural processes of rocks absorbing CO2 from the atmosphere, gradually reduced CO2 levels to around 400 ppm during the Pliocene. In 150 years, humans have completely reversed what took the Earth 3 million years.