The last time carbon dioxide levels in the atmosphere were as high as they are today, sea levels were 60 feet higher and it was so warm that trees grew in Antarctica.
Current CO2 levels of 410 parts per million (ppm) were last seen on Earth three million years ago, according to the most detailed reconstruction of the Earth’s climate by researchers at the Potsdam Institute for Climate Impact Research (PIK) and published in Science Advances.
Their in-depth analysis of plant fossils and sediments reveal that such CO2 levels were last seen in the late Pliocene Epoch, a time when there were no ice sheets covering either Greenland or West Antarctica, and much of the East Antarctic ice sheet was gone. Temperatures were up to 7 degrees Fahrenheit warmer globally, at least double that at the poles, and sea levels were some 20 meters (65 feet) higher.
“This is an amazing discovery,” Jane Francis, director of the British Antarctic Survey, told The UK Guardian. “They found fossil leaves of southern beech. I call them the last forests of Antarctica.”
While the discovery is remarkable, it’s implications are dire. “Twenty metres of sea level rise would have a major impact on our all our coastal cities,” Francis warned.
The good news is that the Earth does not warm instantly, and mile-thick ice sheets melt even more slowly. So the temperature rise will take several decades, and tens of feet of sea level rise will take hundreds and hundreds of years. That means the choices we make now can affect the rate of rise and determine whether we blow past 65 feet of sea level rise to beyond 200 feet.
But the bad news is that each threshold of higher temperatures and sea levels is all but irreversible, so absent very aggressive action over the next decade, tens of feet of sea level rise will be unstoppable.
Even worse, the climate policy agenda President Donald Trump is pushing — actions that include rolling back U.S. laws that reduce carbon pollution and abandoning the Paris climate agreement — would lock us in to such high CO2 levels, sea levels would rise a foot per decade in just a few decades.
One aspect of the study that hasn’t received much attention is the researchers’ finding that the Earth’s climate is highly sensitive to small changes in CO2 levels.
“The role of CO2 changes in shaping the glacial cycles has not been fully understood,” explained lead author Matteo Willeit of PIK. “It is a breakthrough that we can now show… that changes in CO2 levels were a main driver of the ice ages, together with variations of how the Earth’s orbits around the sun, the so-called Milankovitch cycles.”
Yet again, their remarkable finding is also quite worrying for present-day inhabitants of Earth. “Our results imply a strong sensitivity of the Earth system to relatively small variations in atmospheric CO2,” Willeit said. “As fascinating as this is, it is also worrying.”
The fact that the Earth’s climate demonstrates a strong sensitivity to CO2 levels is particularly worrisome because it means we are much more likely to face the worst-case scenario when it comes to climate change impacts. And that makes it even more urgent that the nations of the world cut carbon pollution immediately and keep the rise in atmospheric CO2 as small as possible.