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56 Scientific American, July 2011 © 2011 Scientific American Last
C L I M AT E C H A N G E THE Surprising new evidence suggests the pace of the earth?s most abrupt
prehistoric warm-up paled in comparison to what we face today.
The episode has lessons for our future
By Lee R. Kump
July 2011, ScientificAmerican.com 57 Illustration by Ron Miller © 2011 Scientific American P Lee R. Kump is a professor of geosciences at Pennsylvania State
University and co-author of the book Dire Predictions: Understanding
Global Warming (DK Adult, 2008). Planetary fevers are his specialty. OLAR BEARS DRAW MOST VISITORS TO
Spitsbergen, the largest island in
Norway?s Svalbard archipelago. For
me, rocks were the allure. My colleagues and I, all geologists and climate scientists, flew to this remote
Arctic island in the summer of 2007
to find definitive evidence of what was then considered the
most abrupt global warming episode of all time. Getting to the
rocky outcrops that might entomb these clues meant a rugged,
two-hour hike from our old bunkhouse in the former coalmining village of Longyearbyen, so we set out early after a
night?s rest. As we trudged over slippery pockets of snow and
stunted plants, I imagined a time when palm trees, ferns and
alligators probably inhabited this area.
Back then, around 56 million years ago, I would have been
drenched with sweat rather than fighting off a chill. Research
had indicated that in the course of a few thousand years?a mere
instant in geologic time?global temperatures rose five degrees
Celsius, marking a planetary fever known to scientists as the
Paleocene-Eocene Thermal Maximum, or PETM. Climate zones
shifted toward the poles, on land and at sea, forcing plants and
animals to migrate, adapt or die. Some of the deepest realms of
the ocean became acidified and oxygen-starved, killing off many
of the organisms living there. It took nearly 200,000 years for the
earth?s natural buffers to bring the fever down.
The PETM bears some striking resemblances to the humancaused climate change unfolding today. Most notably, the culprit
behind it was a massive injection of heat-trapping greenhouse
gases into the atmosphere and oceans, comparable in volume to
what our persistent burning of fossil fuels could deliver in coming centuries. Knowledge of exactly what went on during the
PETM could help us foresee what our future will be like. Until recently, though, open questions about the event have made predictions speculative at best. New answers provide sobering clarity. They suggest the consequences of the planet?s last great global warming paled in comparison to what lies
ahead, and they add new support for predictions that humanity will suffer if our
course remains unaltered.
GREENHOUSE CONSPIRACY TODAY INVESTIGATORS think the PETM unfolded something like this: As is true of
our current climate crisis, the PETM began, in a sense, with the burning of fossil
fuels. At the time the supercontinent Pangaea was in the final stages of breaking
up, and the earth?s crust was ripping apart, forming the northeastern Atlantic Ocean. As a result, huge volumes of molten rock
and intense heat rose up through the landmass that encompassed Europe and Greenland, baking carbon-rich sediments
and perhaps even some coal and oil near the surface. The baking
sediments, in turn, released large doses of two strong greenhouse
gases, carbon dioxide and methane. Judging by the enormous
volume of the eruptions, the volcanoes probably accounted for
an initial buildup of greenhouse gases on the order of a few hundred petagrams of carbon, enough to raise global temperature by
a couple of degrees. But most analyses, including ours, suggest it
took something more to propel the PETM to its hottest point.
A second, more intense warming phase began when the volcano-induced heat set other types of gas release into motion.
Natural stirring of the oceans ferried warmth to the cold seabed,
where it apparently destabilized vast stores of frozen methane
hydrate deposits buried within. As the hydrates thawed, methane gas bubbled up to the surface, adding more carbon into the
atmosphere. Methane in the atmosphere traps heat much more
effectively than CO2 does, but it converts quickly to CO2. Still, as
long as the methane release continued, elevated concentrations IN BRIEF Global temperature rose - That intense gas release The speed of today?s rise - 58 Scientific American, July 2011 © 2011 Scientific American PETM curve)
CLIMATIC CHANGE, of that gas would have persisted, strongly amplifying the greenhouse effect and the resulting temperature rise.
A cascade of other positive feedbacks probably ensued at the
same time as the peak of the hydrate-induced warming, releasing yet more carbon from reservoirs on land. The drying, baking
or burning of any material that is (or once was) living emits
greenhouse gases. Droughts that would have resulted in many
parts of the planet, including the western U.S. and western Europe, most likely exposed forests and peat lands to desiccation
and, in some cases, widespread wildfires, releasing even more
CO2 to the atmosphere. Fires smoldering in peat and coal seams,
which have been known to last for centuries in modern times,
could have kept the discharge going strong.
Thawing permafrost in polar regions probably exacerbated
the situation as well. Permanently frozen ground that locks
away dead plants for millions of years, permafrost is like frozen
hamburger in the freezer. Put that meat on the kitchen counter,
and it rots. Likewise, when permafrost defrosts, microbes consume the thawing remains, burping up lots of methane. Scientists worry that methane belches from the thawing Arctic could
greatly augment today?s fossil-fuel-induced warming. The potential contribution of thawing permafrost during the PETM
was even more dramatic. The planet was warmer then, so even
before the PETM, Antarctica lacked the ice sheets that cover the
frozen land today. But that continent would still have had permafrost?all essentially ?left on the counter? to thaw.
When the gas releases began, the oceans absorbed much of
the CO2 (and the methane later converted to CO2). This natural
carbon sequestration helped to offset warming at first. Eventually, though, so much of the gas seeped into the deep ocean that it
created a surplus of carbonic acid, a process known as acidification. Moreover, as the deep sea warmed, its oxygen content dwindled (warmer water cannot hold as much of this life-sustaining
gas as cold water can). These changes spelled disaster for certain
microscopic organisms called foraminifera, which lived on the
seafloor and within its sediments. The fossil record reveals their
inability to cope: 30 to 50 percent of those species went extinct.
CORE KNOWLEDGE THAT A SPECTACULAR RELEASE of greenhouse gases fueled the PETM
has been clear since 1990, when a pair of California-based re- searchers first identified the event in a multimillion-year climate
record from a sediment core drilled out of the seabed near Antarctica. Less apparent were the details, including exactly how
much gas was released, which gas predominated, how long the
spewing lasted and what prompted it.
In the years following that discovery, myriad scientists analyzed hundreds of other deep-sea sediment cores to look for answers. As sediments are laid down slowly, layer by layer, they trap
minerals?including the skeletal remains of sea life?that retain
signatures of the composition of the surrounding oceans or atmosphere as well as life-forms present at the time of deposition. The
mix of different forms, or isotopes, of oxygen atoms in the skeletal
remains revealed the temperature of the water, for instance.
When well preserved, such cores offer a beautiful record of climate history. But many of those that included the PETM were not
in good shape. Parts were missing, and those left behind had been
degraded by the passage of time. Seafloor sediment is typically
rich in the mineral calcium carbonate, the same chemical compound in antacid tablets. During the PETM, ocean acidification
dissolved away much of the carbonate in the sediments in exactly
the layers where the most extreme conditions of the PETM era
should have been represented.
It is for this reason that my colleagues and I met up in Spitsbergen in 2007 with a group of researchers from England, Norway and the Netherlands, under the auspices of the Worldwide
Universities Network. We had reason to believe that rocks from
this part of the Arctic, composed almost entirely of mud and clay,
could provide a more complete record?and finally resolve some
of the unanswered questions about that ancient warming event.
Actually we intended to pluck our samples from an eroded plateau, not from underneath the sea. The sediments we sought
were settled into an ancient ocean basin, and tectonic forces at
play since the PETM had thrust that region up above sea level,
where ice age glaciers later sculpted it into Spitsbergen?s spectacular range of steep mountains and wide valleys.
After that first scouting trip from Longyearbyen, while devising plans for fieldwork and rock sampling, we made a discovery
that saved much heavy lifting. We learned from a forward-thinking local geologist that a Norwegian mining company he worked
for had cored through sediment layers covering the PETM era
years earlier. He had taken it on himself to preserve kilometers Now and Then
How fast the world warms depends on
how fast greenhouse gases build in the
atmosphere. Projections anticipate a
warm-up of about eight degrees Celsius
by 2400 if fossil-fuel burning and carbon
sequestration go unaltered. The projected
carbon release, about 5,000 petagrams,
is similar in volume to what fueled the
Paleocene-Eocene Thermal Maximum,
or PETM, but the past rate, once thought
to be rapid, was slower than today?s. Global temperature is rising much more quickly today than it did during the PETM
Modern: Fueled by high emission rates
(up to 25 petagrams of carbon a year),
global temperature is rising quickly and 8 Temperature Rise
(degrees Celsius) 2 SURPRISING FINDING PETM: Slow but steady emissions
(up to 1.7 petagrams of carbon a year)
resulted in a more gradual heating of
the planet some 56 million years ago 4 Where we
Greenhouse gas release begins 10,000
Duration (years) 20,000 July 2011, ScientificAmerican.com 59 Graphic by Jen Christiansen © 2011 Scientific American STRETCHING TIME OUR ARCTIC CORES turned out to be quite special. The first to record the full duration of the PETM warm-up and recovery, they
provided a much more complete snapshot of the period when
greenhouse gases were being released to the atmosphere. We
suspected that the unprecedented fidelity of these climate records would ultimately provide the most definitive answers to
date about the amount, source and duration of gas release. But
to get those results, we had to go beyond extrapolations from
the composition and concentration of materials in the cores.
We asked Ying Cui, my graduate student at Pennsylvania State
University, to run a sophisticated computer model that simulated the warming based on what we knew about the changes in
the carbon isotope signatures from the Arctic cores and the degree of dissolution of seafloor carbonate from deep-sea cores.
Cui tried different scenarios, each one taking a month of computer time to play out the full PETM story. Some assumed greater
contributions from methane hydrates, for instance; others assumed more from CO2 sources. The scenario that best fit the physical evidence required the addition of between 3,000 and 10,000
petagrams of carbon into the atmosphere and ocean, more than
the volcanoes or methane hydrates could provide; permafrost or
peat and coal must have been involved. This estimate falls on the
high side of those made previously based on isotope signatures
from other cores and computer models. But what surprised us
most was that this gas release was spread out over approximately
20,000 years?a time span between twice and 20 times as long as
anyone has projected previously. That lengthy duration implies
that the rate of injection during the PETM was less than two petagrams a year?a mere fraction of the rate at which the burning of
fossil fuels is delivering greenhouse gases into the air today. Indeed, CO2 concentrations are rising probably 10 times faster now
than they did during the PETM.
This new realization has profound implications for the future. The fossil record tells us that the speed of climate change
has more impact on how life-forms and ecosystems fare than
does the extent of the change. Just as you would prefer a hug
from a friend to a punch in the stomach, life responds more favorably to slow changes than to abrupt ones. Such was the case
during an extreme shift to a hothouse climate during the Cretaceous period (which ended 65 million years ago, when an asteroid impact killed the dinosaurs). The total magnitude of green- I M P L I C AT I O N S Mild Planetary fevers that come on suddenly?
such as the scenario unfolding today?are
much harder on life than the slower ones are.
The fossil record shows that the slow shift to
a hothouse from 120 million to 90 million
years ago, during the Cretaceous period, was
innocuous relative to the PETM, which was
1,000 times more abrupt. The latter episode
has long been analyzed for clues to how our
own warming trend will play out, but today?s
much faster temperature change suggests
that the consequences for life on earth will be
harsher than anything that has come before. Moderate Severe Lessons from
Harm to Life of that core on the off chance that scientists would one day find
them useful. He led us to a large metal shed on the outskirts of
town where the core is now housed, since cut into 1.5-meter-long
cylinders stored in hundreds of flat wood boxes. Our efforts for
the rest of that trip, and during a second visit in 2008, were directed at obtaining samples from selected parts of that long core.
Back in the lab, over several years, we extracted from those
samples the specific chemical signatures that could tell us about
the state of the earth as it passed into and out of the PETM. To understand more about the greenhouse gas content of the air, we
studied the changing mix of carbon isotopes, which we gleaned
mostly from traces of organic matter preserved in the clay. By
making extractions and analyses for more than 200 layers of the
core, we could piece together how these factors changed over
time. As we suspected, the isotope signature of carbon shifted dramatically in the layers we knew to be about 56 million years old. 146 Millions of Years Ago (mya) C R E T house warming during the Cretaceous was similar to that of the
PETM, but that former episode unfolded over millions, rather
than thousands, of years. No notable extinctions occurred; the
planet and its inhabitants had plenty of time to adjust.
For years scientists considered the PETM to be the supreme
example of the opposite extreme: the fastest climate shift ever
known, rivaling the gloomiest projections for the future. In that
light, the PETM?s outcomes did not seem so bad. Aside from the
unlucky foraminifera in the deep sea, all animals and plants apparently survived the heat wave?even if they had to make some
serious adaptations to do so. Some organisms shrank. In particular, mammals of the PETM are smaller than both their predecessors and descendants. They evolved this way presumably because smaller bodies are better at dissipating heat than larger
ones. Burrowing insects and worms, too, dwarfed.
A great poleward migration saved other creatures. Some even
thrived in their expanded territories. At sea, the dinoflagellate
Apectodinium, usually a denizen of the subtropics, spread to the
Arctic Ocean. On land, many animals that had been confined to
the tropics made their way into North America and Europe for the
first time, including turtles and hoofed mammals. In the case of
mammals, this expansion opened up myriad opportunities to
evolve and fill new niches, with profound implications for human
beings: this grand diversification included the origin of primates.
TOO FAST? NOW THAT WE KNOW the pace of the PETM was moderate at worst
and not really so fast, those who have invoked its rather innocuous biological consequences to justify impenitence about fossil-fuel combustion need to think again. By comparison, the 60 Scientific American, July 2011 © 2011 Scientific American A Cretaceous Hothouse (Slow) PETM (Moderately fast) degree Celsius Rate of heating: Rate of heating: Main underlying cause: Volcanic eruptions
Environmental change: Oceans absorbed
carbon dioxide slowly so did not acidify
Life?s response: Nearly all creatures had time
to adapt or migrate C E O U S Modern Warming (Fast)
Rate of heating: 1 to 4 ° Duration: Thousands of years
Overall warming: 5 °C
Main underlying cause: Volcanoes; methane
bubbling up from the ocean bottom; peat and Duration: Millions of years
Overall warming: 5 °C 65 mya ° Environmental change:
but most life on land adapted or migrated Duration: Decades to hundreds of years
Main underlying cause: Fossil-fuel burning
Environmental change: Acidifying oceans; more
extreme weather, glacier melting; sea-level rise
Life?s response: Poleward movement of many
species; habitat loss; coral bleaching; extinctions 56 mya PALEOCENE E O C E N E climate shift currently under way is happening at breakneck
speed. In a matter of decades, deforestation and the cars and
coal-fired power plants of the industrial revolution have increased CO2 by more than 30 percent, and we are now pumping
nine petagrams of carbon into the atmosphere every year. Projections that account for population growth and increased industrialization of developing nations indicate that rate may
reach 25 petagrams a year before all fossil-fuel reserves are
Scientists and policy makers grappling with the potential effects of climate change usually focus on end products: How
much ice will melt? How high will sea level rise? The new lesson
from PETM research is that they should also ask: How fast will
these changes occur? And will the earth?s inhabitants have time
to adjust? If change occurs too fast or if barriers to migration or
adaptation loom large, life loses: animals and plants go extinct,
and the complexion of the world is changed for millennia.
Because we are in the early interval of the current planetary
fever, it is difficult to predict what lies ahead. But already we
know a few things. As summarized in recent reports from the
Intergovernmental Panel on Climate Change, ecosystems have
been responding sensitively to the warming. There is clear evidence of surface-water acidification and resulting stress on sea
life [see ?Threatening Ocean Life from the Inside Out,? by Marah
J. Hardt and Carl Safina; SCIENTIFIC AMERICAN, August 2010].
Species extinctions are on the rise, and shifting climate zones
have already put surviving plants and animals on the move, often with the disease-bearing pests and other invasive species
winning out in their new territories. Unlike those of the PETM,
modern plants and animals now have roads, railways, dams, cit- Today ies and towns blocking their migratory paths to more suitable
climate. These days most large animals are already penned into
tiny areas by surrounding habitat loss; their chances of moving
to new latitudes to survive will in many cases be nil.
Furthermore, glaciers and ice sheets are melting and driving
sea-level rise; coral reefs are increasingly subject to disease and
heat stress; and episodes of drought and flooding are becoming
more common. Indeed, shifts in rainfall patterns and rising
shorelines as polar ice melts may contribute to mass human migrations on a scale never before seen. Some have already begun
[see ?Casualties of Climate Change,? by Alex de Sherbinin, Koko
Warner and Charles Ehrhart; SCIENTIFIC AMERICAN, January].
Current global warming is on a path to vastly exceed the
PETM, but it may not be too late to avoid the calamity that
awaits us. To do so requires immediate action by all the nations
of the world to reduce the buildup of atmospheric carbon dioxide?and to ensure that the Paleocene-Eocene Thermal Maximum remains the last great global warming. The Paleocene-Eocene Thermal Maximum: A Perturbation of Carbon Cycle, Climate,
and Biosphere with Implications for the Future.
Annual Review of Earth and Planetary Sciences,
America?s Climate Choices.
Slow Release of Fossil Carbon during the Palaeocene-Eocene Thermal Maximum.
SCIENTIFIC AMERICAN ONLINE July 2011, ScientificAmerican.com 61 Illustrations by Ron Miller, Graphic by Emily Cooper © 2011 Scientific American
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