Ancient Warming: Forests to Grasslands

News release
University of Florida
Monday, December 17, 2007.

Ancient global warming changed earth from ‘icehouse to greenhouse’

GAINESVILLE, Fla. – Earth literally turned over a new leaf 15 million years ago when an earlier version of global warming changed large parts of the planet from lush forests to open grasslands, a new study by scientists at the University of Florida and other institutions shows.

In a portent of today’s global warming, fossilized leaves tell the story of a carbon dioxide induced warm-up at the end of the Miocene age that melted much of the polar icecaps and led to the spread of animals that thrive in the wide open spaces, such as horses, camels and other grazers, said David Dilcher, a UF paleobotanist and one of the study’s authors.

“Our findings clearly demonstrate that past climate changes were tied to carbon dioxide fluctuations in the atmosphere, which influenced the major vegetation patterns occurring on earth and in turn affected the evolution of major animal groups,” Dilcher said.

The work by Dilcher, Wolfram Kurschner, a paleobotanist at Utrecht University in the Netherlands, and Zlatko Kvacek, a paleobotanist at Charles University in the Czech Republic, appears in a paper published this week in the online edition of the Proceedings of the National Academy of Sciences.

“The relevance for today is that the Antarctic ice sheets are reversing again,” said Dilcher, who works at the Florida Museum of Natural History. “As carbon dioxide and other gasses increase in the atmosphere, we’re emerging from a cooler or icehouse-type period into a greenhouse-type period with ice-free poles. The Earth is gradually going to undergo major changes just as we saw major changes in the upper Miocene Epoch.”

The Miocene Epoch is characterized by weather extremes, from the Earth plunging into its present “icehouse” state with glaciers at the north and south poles to periods of tropical temperatures.

While use of fossil fuels has been blamed for today’s global warming, the likely source of this ancient episode was carbon dioxide belched from widespread volcanic eruptions in the Columbia River Flood Basalt region of the United States and in Central Europe, Dilcher said.

The researchers were able to track the fluctuating levels of carbon dioxide in the atmosphere by taking fossilized leaves and measuring the number of stoma or small pores, through which carbon dioxide is taken in and oxygen released during photosynthesis. The more carbon dioxide in the atmosphere, the fewer stomata there are on the undersides of leaves.

Using three different species of leaves from the Charles University collection, with most of the specimens collected from the brown-coal basins in the Czech Republic, the researchers found a correlation between the number of stoma in the leaves and carbon dioxide levels in the air with climate patterns over time. The carbon dioxide fluctuations coincided with temperature changes recorded in the ocean record – as measured by isotope concentrations in the shells of marine organisms – which, in turn, corresponded with drastic changes in plant and animal life, Dilcher said.

“It was at the very end of the Miocene Age that modern vegetation emerges in the world, and we find that atmospheric carbon dioxide was the forcing factor,” he said.

Fluctuating levels of carbon dioxide combined with reduced available moisture, in the rain shadow of the rising Rocky Mountains, pressured the forest vegetation and photosynthesis of some plants to be altered. As a result, the closed forests of palm and bamboo trees that had dominated interior North America gave way first to savannas and open woodlands and later to grasslands, which also sprouted up across the ocean around the eastern Mediterranean, Dilcher said. These changes occurred gradually, over a few thousand to millions of years, he said.

The Great Plains began to form, leading to a diverse mix of large hoofed herbivores such as extinct species of horses, camels, rhinoceroses and elephants that fed on the lush grasses, he said.

“Preliminary data suggest that this pattern of elevated ungulate diversity is a global phenomenon, and therefore a global driving force such as climate change is the most likely explanation,” he said.

While carbon dioxide levels fluctuated between 370 and 600 parts per million during the Miocene Epoch, today’s levels are at about 375 parts per million, Dilcher said.

“We are in a period of accelerated climate change that is quite unlike anything that we have seen in the fossil record,” he said. “When carbon dioxide levels go up to 400 and then on to 500 parts per million, we will be at the same point that we were in the Miocene age when the poles were ice-free.”

© University of Florida, Gainesville, FL 32611; (352) 392-3261.

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