Global warming 55 million years ago shifted ocean
Jan 04 1:50 PM US/Eastern
An extraordinary burst of global warming that occurred
around 55 million years ago dramatically reversed
Earth's pattern of ocean currents, a finding that
strengthens modern-day concern about climate change, a
The big event, the Palaeocene/Eocene Thermal Maximum
(PETM), saw the planet's surface temperature rise by
between five and eight degrees C (nine and 16.2 F) in
a very short time, unleashing climate shifts that
endured tens of thousands of years.
Scientists Flavia Nunes and Richard Norris of the
Scripps Institution of Oceanography in California
explored how these warmer temperatures might have
affected ocean currents.
They measured carbon-13 isotopes from 14 cores that
had been drilled into the deep floor in four different
ocean basins, taking samples from sediment layers
deposited before, during and after the PETM.
These isotopes are considered to be an indicator of
the nutrients deposited by the water at the time. The
higher the isotope value, the likelier that the source
came from the deep ocean, the prime source for
With a painstaking reconstruction, Nunes and Norris
found that the world's ocean current system did a
U-turn during the PETM -- and then, ultimately,
Before the PETM, deep water upwelled in the southern
hemisphere; over about 40,000 years, the source of
this upwelling shifted to the northern hemisphere; it
took another 100,000 years before recovering
What unleashed the PETM is unclear. Most fingers of
blame point to volcanic eruptions that disgorged
gigatonnes of carbon dioxide, or coastal reservoirs of
methane gas, sealed by icy soil, that were breached by
warmer temperatures or receding seas.
The huge temperature rise may have occurred within
just few thousand years, but as Nunes and Norris point
out, the effects were enduring and the lesson for
mankind today is clear.
"Modern CO2 input to the biosphere from fossil fuel
sources is approaching that estimated for the PETM,
raising concerns about future climate and circulation
change," they warn.
"The PETM example shows that anthropogenic (man-made)
forcing may have lasting effects not only in global
climate but in deep-ocean circulation as well."
The study, which appears on Thursday in the British
journal Nature, comes on the heels of research
published in November which suggests that global
warming is slowing the Atlantic current that gives
western Europe its mild climate.
The suspected reason for this is an inrush of
freshwater into the northern Atlantic, caused by
melting glaciers in Greenland and melting sea ice, and
higher flow into the Arctic from Siberian rivers
caused by greater rainfall.
The influx brakes the conveyor belt in which warm
surface water is taken up to the northeastern Atlantic
from the tropics before returning down to the southern
hemisphere as cool, deep-sea water.
In 2001, the UN's top scientific authority on global
warming, the Intergovernmental Panel on Climate Change
(IPCC), estimated that there would be a temperature
rise of 1.4 to 5.8 C (2.5 to 10.4 F) from 1990-2100.
The increase was predicted according to scenarios of
atmospheric carbon dioxide (CO2), ranging from 540 to
970 parts per million (ppm).
That compares with 280ppm for pre-industrial times and
around 380ppm today, which is already the highest
concentration of CO2 for 650,000 years.
The higher the level, the greater the risk that a
vicious circle of global warming could be unleashed,
inflicting potentially irreversible damage to Earth's
climate system, scientists say.