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June 19, 2009 — Scientists have unearthed
striking evidence for a sudden ancient collapse in plant biodiversity.
A trove of 200 million-year-old fossil leaves collected in East
Greenland tells the story, carrying its message across time to us today.
Results of the research appear in the journal Science.
The researchers were surprised to find that a likely candidate
responsible for the loss of plant life was a small rise in the
greenhouse gas carbon dioxide, which caused Earth's temperature to rise.
Global warming has long been considered as the culprit for
extinctions--the surprise is that much less carbon dioxide gas in the
atmosphere may be needed to drive an ecosystem beyond its tipping point
than previously thought.
"Earth's deep time climate history reveals startling discoveries
that shake the foundations of our knowledge and understanding of
climate change in modern times," says H. Richard Lane, program director
in the National Science Foundation (NSF)'s Division of Earth Sciences,
which partially funded the research.
Jennifer McElwain of University College Dublin, the paper's lead
author, cautions that sulfur dioxide from extensive volcanic emissions
may also have played a role in driving the plant extinctions.
"We have no current way of detecting changes in sulfur dioxide in
the past, so it's difficult to evaluate whether sulfur dioxide, in
addition to a rise in carbon dioxide, influenced this pattern of
extinction," says McElwain.
The time interval under study, at the boundary of the Triassic and
Jurassic periods, has long been known for its plant and animal
Until this research, the pace of the extinctions was thought to have been gradual, taking place over millions of years.
It has been notoriously difficult to tease out details about the
pace of extinction using fossils, scientists say, because fossils can
provide only snap-shots or glimpses of organisms that once lived.
Using a technique developed by scientist Peter Wagner of the
Smithsonian Institution National Museum of Natural History in
Washington, D.C., the researchers were able to detect, for the first
time, very early signs that these ancient ecosystems were already
deteriorating--before plants started going extinct.
The method reveals early warning signs that an ecosystem is in trouble in terms of extinction risk.
"The differences in species abundances for the first 20 meters of
the cliffs [in East Greenland] from which the fossils were collected,"
says Wagner, "are of the sort you expect. "But the final 10 meters show
dramatic loses of diversity that far exceed what we can attribute to
sampling error: the ecosystems were supporting fewer and fewer species."
By the year 2100, it's expected that the level of carbon dioxide in
the modern atmosphere may reach as high as two and a half times today's
"This is of course a 'worst case scenario,'" says McElwain. "But
it's at exactly this level [900 parts per million] at which we detected
the ancient biodiversity crash.
"We must take heed of the early warning signs of deterioration in
modern ecosystems. We've learned from the past that high levels of
species extinctions--as high as 80 percent--can occur very suddenly,
but they are preceded by long interval of ecological change."
The majority of modern ecosystems have not yet reached their tipping
point in response to climate change, the scientists say, but many have
already entered a period of prolonged ecological change.
"The early warning signs of deterioration are blindingly obvious,"
says McElwain. "The biggest threats to maintaining current levels of
biodiversity are land use change such as deforestation. "But even
relatively small changes in carbon dioxide and global temperature can
have unexpectedly severe consequences for the health of ecosystems."
The paper was co-authored by McElwain, Wagner and Stephen Hesselbo of the University of Oxford in the U.K.
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