June 25, 2009 — For climatologists, part of
the challenge in predicting the future is figuring out exactly what
happened during previous periods of global climate change.
One long-standing climate puzzle relates to a sequence of events
33.5 million years ago in the Late Eocene and Early Oligocene. Profound
changes were underway. Globally, carbon dioxide levels were falling and
the hothouse warmth of the dinosaur age and Eocene Period was waning.
In Antarctica, ice sheets had formed and covered much of the southern
But what exactly was happening on land, in northern latitudes? When
and how did Northern glaciation begin, and what does this knowledge add
to the understanding of the relationship between carbon dioxide levels
and today’s climate?
An international team that included Dr. David Greenwood, an
NSERC-funded researcher at Brandon University, now provides some of the
very first detailed answers, and they come from an unusual source.
"Fossils of land plants are excellent indicators of past climates,"
said Dr. Greenwood. "But the fossil plant localities from the Canadian
Arctic and Greenland don’t appear to record this major climate change,
and pose problems for precisely dating their age, so we needed to look
The "where" was in marine sediments entombed when the North Atlantic
Ocean was beginning to open, and lying now at the bottom of today’s
Norwegian-Greenland Sea. Sediment cores taken from there contained a
record of ancient spores and pollen blown from the continent to the
"These marine sediment cores give us a very precise chronology of
the changes in the dominant land plants," said Dr. Greenwood "and since
many of these species have modern relatives, we can assume that the
temperatures and environments they lived in were very similar."
To arrive at a holistic picture of the climate of the transition,
the researchers merged the plant data with physical information about
the state of the atmosphere and ocean taken from chemical and isotopic
information in the same sediments, and compared this to computer
modelling of climate in the period.
"We can see that summer temperatures on land remained relatively
warm throughout the Eocene/Oligocene transition, but that the period
was marked by increasing seasonality," said Dr. Greenwood.
"Mean temperatures during the coldest month dropped by five degrees Celsius, to just above freezing," he said.
"This was probably not enough to create much in the way of
continental ice on East Greenland," he said, "but it did wipe out palms
and other subtropical trees such as swamp cypress. They were replaced
by temperate climate trees such as spruces and hemlock."
The researcher said that, nonetheless, the middle period of the
transition remained fairly warm. "Hickory and walnut were still
present, but these became rare in the final stages," he said.
Although the march to a cooler world was gradual in northern latitudes, it was inevitable according to Dr. Greenwood.
"Changes in the earth’s position in its orbit were leading a much
greater seasonal range in radiation for polar regions and, overall,
heat was becoming more concentrated in the tropics, largely due to a
global drop in carbon dioxide levels in the atmosphere" he said.
The group’s detailed record of the Eocene/Oligocene transition will appear in the June 18 issue of Nature.