Back to the Miocene
October 13th, 2009 by Jim JustThe last time atmospheric carbon dioxide levels were sustained at levels as high as they are today global temperatures were 5 to 10 degrees Fahrenheit higher, sea levels were 75 to 120 feet higher, and there was no permanent sea ice cap in the Arctic and very little ice on Antarctica and Greenland. That was 15 to 20 million years ago.
That’s what UCLA scientist Aradhna Tripati and colleagues report Oct. 8 in the online edition of the journal Science.
Tripati said the findings were “slightly shocking”:
A slightly shocking finding is that the only time in the last 20 million years that we find evidence for carbon dioxide levels similar to the modern level of 387 parts per million was 15 to 20 million years ago, when the planet was dramatically different.
Levels of carbon dioxide have varied only between 180 and 300 parts per million over the last 800,000 years — until recent decades. The seasonally-adjusted level of atmospheric CO2 was 388.00 ppm in September 2009.
One implication of the findings is that 388 ppm is already catastrophic and that the world has to get back to the 280 ppm of the pre-industrial era if global calamities are to be avoided.
Here’s the abstract:
The CO2 content of the atmosphere has varied cyclically between ~180 and ~280 ppmv over the last 800,000 years, closely coupled with temperature and sea level. For earlier periods in Earth’s history, pCO2 is much less certain and the relationship between pCO2 and climate remains poorly constrained. We use boron/calcium ratios in foraminifera to estimate pCO2 during major climate transitions of the last 20 million years (myr). During the Middle Miocene, when temperatures were ~3 to 6°C warmer and sea level 25 to 40 meters higher than present, pCO2 was similar to modern levels. Decreases in pCO2 were synchronous with major episodes of glacial expansion during the Middle Miocene (~14 to 10 million years ago; Ma) and Late Pliocene (~3.3 to -2.4 Ma).