The scientists say that for a relatively brief period around 55 million
years ago, after the extinction of the
dinosaurs but long before the onset of
today's pattern of periodic ice ages,
the temperature of the earth's surface in northern latitudes, and of the
deep ocean, soared by some 9 to 12
This is substantially more than the 5 to 9 degrees
the world has warmed since the
depths of the last ice age 18,000 to
20,000 years ago.
So great was the effect of the
warming, experts say, that it wiped
out many species of marine life and
created climatic conditions that led
to an explosive expansion in the number and variety of mammal species.
It was at this time that the primates,
from which Homo sapiens eventually
evolved, first appeared.
The analogy to today's conditions
is far from perfect, and many questions remain to be answered, but
those who have investigated the ancient warming spike say it reinforces
a belief that has lately been growing
among climate scientists: that a
gradual warming of the climate can
abruptly soar to new heights once a
certain threshold is reached.
what scientists believe happened in
the case of the warming 55 million
years ago -- and perhaps what could
happen again, in certain conditions.
The chief greenhouse gas was and
is carbon dioxide, and since the start
of the Industrial Revolution in the
18th century, its atmospheric concentration has gradually increased
by nearly 30 percent. The average
global surface temperature has risen
by about 1 degree or a little more in
the last century.
Mainstream scientists believe, based on computerized
simulations of the climate's workings, that the temperature will rise
by about another 3.5 degrees by the
year 2100 if greenhouse gas emissions continue at the present rate.
A gradual warming of unknown
cause preceded the sharp upward
spike in temperature 55 million
years ago, said Miriam E. Katz, a
paleooceanographer at Rutgers University, who is the leading author of a
report on the ancient phenomenon in
the current issue of the journal Science.
But at some point, the warming
crossed a threshold that abruptly
kicked the temperature up to a new
level, said another author of the Science paper, Dr. Gerald R. Dickens of
James Cook University in Australia.
He compared it to the stretching of a
rubber band: "You gradually pull at
both ends and, at some instant, the
rubber band suddenly breaks."
What caused the climate to snap
and send the temperature soaring,
according to a hypothesis formulated by Dr. Dickens, was a sudden
release of methane locked in the
ocean floor, touched off by the previous, more gradual warming.
Methane is a greenhouse gas in its own
right, and when released from ocean
sediments it also combines with oxygen to form carbon dioxide that eventually percolates to the atmosphere.
Dr. Dorothy K. Pak of the University of California at Santa Barbara
and Dr. Kenneth G. Miller of Rutgers
are the other authors of the report in
Science, which describes chemical
and geological evidence of the ancient rush of greenhouse gases and
the way it happened. The evidence
was contained in corings and ultrasound readings of sediments on a
subsea promontory called the Blake
Nose, northeast of Cape Canaveral,
The researchers believe that the
original, gradual warming, beginning about 60 million years ago,
caused a change in ocean circulation
currents that pushed warm surface
waters down into the deep sea.
deep-sea warming converted icelike
solid methane locked in crystalline
structures in the sea-floor sediments
into gaseous form.
This gas then
blasted upward through the sediment, starting mudslides that freed
the methane and allowed it to escape
into the water and eventually to the
atmosphere. On the way, it reacted
with oxygen to produce globe-warming carbon dioxide.
One effect of the warming spike,
Ms. Katz says, was to transform the
environment of the deep ocean, as
evidenced by the extinction of more
than half of all species of microscopic bottom-dwelling animals.
warming is also believed to have
enabled that era's relatively small
array of mammals to spread out into
formerly frozen regions to colonize
other continents, where they proliferated in many evolutionary directions.
Among these were the ancestors of horses, apes and humans.
Dr. Dickens has calculated that
the sudden influx of carbon associated with the sharp spike of global
warming 55 million years ago
amounted to at least a billion billion
At present rates of carbon-dioxide emission from global
sources, about two-thirds of that
amount would be added to the atmosphere by 2100.
Some scientists say it
is possible that before then, some
threshold could again be surpassed,
resulting in an abrupt but unknown
change in climate.
Many questions remain. The cause
of the gradual warming that preceded the spike 55 million years ago is
Nor do scientists know the
magnitude of the atmospheric concentrations before the gradual
warming trend and the spike, frustrating comparisons with today.
Moreover, it is not clear how much of
the ancient warming resulted from
the influx of greenhouse gas and how
much from accompanying changes
in ocean circulation; Dr. Dickens believes both were involved.
A further complication, says Dr.
Dickens, is that the influx of greenhouse gases was spewed initially into
the ocean 55 million years ago, but is
going directly into the atmosphere
That difference could affect
the rate of the consequent warming,
since the ocean's inertia might slow
the migration of carbon dioxide into
the air, making the ancient warming
spike less abrupt than otherwise.
The evidence drawn from ocean
sediments in the new study was not
fine-grained enough to determine
just how sharp the ancient warming
was, Dr. Pak said, though it took
place within a few thousand years at
Other large, abrupt climatic
changes of the more recent past --
during the transition out of the last
ice age, for example -- have taken
place within a human lifetime or less
and sometimes within a decade, according to recent evidence.
All of these complications muddy
the possible comparison between
what happened in the transformational climatic event 55 million years
ago and what is happening today.
Nevertheless, says Dr. Dickens, the
ancient transformation provides a
new and continuing opportunity to
explore the possible effects of growing concentrations of greenhouse
gases without resorting to computer-assisted simulations of the climate.
And, he says, it teaches that "the
earth can, for natural reasons, suddenly change dramatically."