For about 100 million years all sorts of animals roamed the then subtropical North and South poles. But then suddenly some 34 million years ago during the Eocene everything changed when temperatures fell dramatically in only a 100,000 year timespan, leading to the extinction of many animal species.
Previously it was assumed that a change in ocean currents was the cause for the sudden drop in temperature. But a new study in Science shows that the culprit was in fact a roughly 40 per cent drop in atmospheric CO2.
By looking at ancient algae remnants from deep-ocean sediments the research team found a change in their biochemical molecules that correspond with a sudden drop in carbon dioxide. The two sites they used were specially selected for their stability in the relevant time-period.
Other sites where ocean dwelling algae were subject to upwelling may have led to the skewed results of previous studies that showed precisely the opposite of this one: a rise on CO2 at the time of Antarctic glaciation onset.
Mother of all climate tipping points
The formation of the ice sheets on the poles is the mother of all climate tipping points in Earth’s history. In geological terms the kilometer-thick ice sheet over Antarctica formed overnight. The polar ice sheets exert a strong influence on global climate. From impacting the circulation of cold and warm air masses to wind strength, precipitation patterns and variability in regional and global temperatures.
That such an event was triggered by a change in CO2 shows the significance of the greenhouse gas on Earth’s climate, not just in the past but also in the present and future. The team found that ice sheets start to form at an atmospheric tipping point of about 600 parts carbon dioxide per million.
Will the ice sheets stay or go?
With the 400 million parts per million that we are currently nearing, it seems that the ice sheets will be around for a while. But unfortunately the threshold for the melting and formation of ice sheets are not linear.
While 600 parts per million might be enough to start ice sheet formation, unfortunately their melting may, and according to recent measurements actually does, start at much lower carbon dioxide levels. Whether we will eventually reach the tipping point to completely melt the ice sheets can as of yet not be predicted. But judging from the trend in atmospheric CO2 emissions it seems more than likely.
© Jorn van Dooren | www.bitsofscience.org