Scientists are puzzled today by the discovery that millions of years ago levels of carbon dioxide in the atmosphere were lower - and yet, temperatures were higher than today's.
The revelations come in a new paper just published by hefty boffinry mag Nature, in which geoscientists probed the temperatures experienced by the Earth during the Miocene era - the period from 12 to 5 million years ago. The boffins write:
Comparatively little is known about the climate of the late Miocene (~12–5 Myr ago), an interval with pCO2 values of only 200–350 parts per million by volume but nearly ice-free conditions in the Northern Hemisphere and warmer-than-modern temperatures on the continents.
Today CO2 stands at 390 parts per million, well up on Miocene levels, and yet the planet - and in particular its oceans, engines of the climate - are significantly cooler. This wasn't known until now, as it is only recently that the team led by Jonathan P LaRiviere of UC Santa Cruz have looked into the matter. A statement from San Francisco State uni, where another of the scientists works, explains the methods used:
They used an organic compound called unsaturated alkenone as their "fossil thermometers." The compound is produced by tiny phytoplankton and preserved in cores of ocean sediment drawn from the mid-latitude Pacific Ocean basin. Ratios of the compound preserve a record of the water temperature in which the plankton lived.
The results of the study show that "sea surface temperatures were significantly warmer than today", according to San Francisco State University geosciences prof Petra Dekens. The Nature paper, indeed, states that in the boffins' opinion oceanic temperature and atmospheric carbon levels - generally considered to be firmly connected in today's climate science - were "decoupled".
"It's a surprising finding, given our understanding that climate and carbon dioxide are strongly coupled to each other," LaRiviere says.
"In the late Miocene, there must have been some other way for the world to be warm."
The paper suggests:
A relatively deep global thermocline, reductions in low-latitude gradients in sea surface temperature, and cloud and water vapour feedbacks may help to explain the warmth of the late Miocene.
It can be read (by subscribers) here. ®