[CO2] Levels similar to those now commonly regarded as adequate to tackle climate change were associated with sea levels 25-40m (80-130 ft) higher than today.
That’s meters. That’s enough to drown a 10-story building. That’s enough to make several billion people move to higher ground or die. Or both. It won’t be pleasant for the people they move in on either.
And that is not conjecture or a probability statement or an extrapolation.
The new research was able to look back to the Miocene period, which began a little over 20 million years ago.
At the start of the period, carbon dioxide concentrations in the atmosphere stood at about 400 parts per million (ppm) before beginning to decline about 14 million years ago – a trend that eventually led to formation of the Antarctic icecap and perennial sea ice cover in the Arctic.
The high concentrations were probably sustained by prolonged volcanic activity in what is now the Columbia River basin of North America, where rock formations called flood basalts relate a history of molten rock flowing routinely onto the planet’s surface.
In the intervening millennia, CO2 concentrations have been much lower; in the last few million years they cycled between 180ppm and 280ppm in rhythm with the sequence of ice ages and warmer interglacial periods.
Now, humanity’s emissions of greenhouse gases are pushing towards the 400ppm range [c. 380ppm now], which will very likely be reached within a decade.
“What we have shown is that in the last period when CO2 levels were sustained at levels close to where they are today, there was no icecap on Antarctica and sea levels were 25-40m higher,” said research leader Aradhna Tripati from the University of California at Los Angeles (UCLA).
“At CO2 levels that are sustained at or near modern day values, you don’t need to have a major change in CO2 levels to get major changes in ice sheets,” she told BBC News.
The elevated CO2 and sea levels were associated with temperatures about 3-6C (5-11F) higher than today.
So, there you have it. The last time greenhouse gases were this high, there wasn’t a 2% chance of melting ice sheets. There was a 100% chance.
Does that mean it will happen again? We’ll probably see. Because the answer to, “Do you want to risk the whole planet to find out?” appears to be “Yes.”
global warming, Aradhna Tripati