Apr. 22, 2013 — Nearly 34 million years ago, Earth underwent a transformation from a warm, high-carbon dioxide "greenhouse" state to a lower-CO2, variable climate similar to the modern "icehouse" world. Massive ice sheets grew across the Antarctic continent, major animal groups shifted, and ocean temperatures decreased by as much as 5 degrees.
But studies of how this drastic change affected temperatures on land have had mixed results. Some show no appreciable terrestrial Climate-Change'>climate change; others find cooling of up to 8 degrees and large changes in seasonality.
Their results suggest a drop of as much as 10 degrees for fresh water during the warm season and 6 degrees for the atmosphere in the North Atlantic, giving further evidence that the concentration of atmospheric carbon dioxide and Earth's surface temperature are inextricably linked.
"One of the key principles of geology is that the past is the key to the present: records of past climate inform us of how the Earth system functions," says Michael Hren, assistant professor of chemistry and geosciences at the University of Connecticut and the study's lead author. "By understanding past climate transitions, we can better understand the present, and predict impacts for the future."
The transition between the Late Eocene and the Oligocene epochs (between 34 million and 33.5 million years ago) was triggered in part, the authors write in their April 22 paper in Proceedings of the National Academy of Sciences, by changes in the concentration of atmospheric CO2 that enabled ice to build up on the Antarctic continent.
Ice-sheet growth, coupled with favorable changes in Earth's orbit, pushed the planet past a climatic tipping point and led to both the rapid buildup of a permanent Ice-Sheet'>ice sheet in the Antarctic and much larger changes in global climate, says Hren.
But much of what is known about this time period's climate comes from cores drilled deep in the ocean, Hren says. There, organic and inorganic remains of ancient marine creatures retain chemical signatures of ocean temperatures when they were alive.
Now, Hren and his colleagues have used a recently-developed "clumped isotope thermometer" to examine terrestrial fossil shells from this time period. The team collected fossilized snails from the Isle of Wight, Great Britain, and looked for not just the kind and number of carbon and oxygen isotopes present, but how they were bound together.
What makes their results so important, says Hren, is that it's further evidence that CO2 is linked not only to climate by way of the vast oceans and their temperature, but by terrestrial temperatures, too.
Studies have shown that before this drastic cooling event, Earth's atmosphere contained 1,000 parts per million (ppm) of CO2 or more, and by the end of the transition, it was likely lower than 600-700 ppm. Some predictions, notes Hren, suggest that Earth's current CO2 concentrations, currently at close to 400 ppm and climbing, could increase to nearly 1,000 ppm in the next 100 years.
If that turns out to be the case, it's likely that temperature changes on the scale of the Eocene to Oligocene could occur -- but in the other direction, toward a much warmer climate that could again fundamentally alter living things on Earth.
"We are on a path to fundamentally alter our global climate state," says Hren. "These data definitely give you pause."
The other members of the research group are: Nathan Dale Sheldon and Kyger C. Lohmann of the University of Michigan; Stephen T. Grimes and Melanie Bugler of Plymouth University; Margaret E. Collinson of Royal Holloway University; and Jerry J. Hooker of the Natural History Museum.
- Michael T. Hren, Nathan D. Sheldon, Stephen T. Grimes, Margaret E. Collinson, Jerry J. Hooker, Melanie Bugler, and Kyger C. Lohmann. Terrestrial cooling in Northern Europe during the Eocene–Oligocene transition. PNAS, 2013 DOI: 10.1073/pnas.1210930110
Note: If no author is given, the source is cited instead.