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56 Scientific American, July 2011 © 2011 Scientific American Last

 

Great

 

Global

 

Warm?ng

 

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)

 

modern curve

 

NATURE GEOSCIENCE

 

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

 

are today

 

0

 

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

 

Past Warmings

 

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:

 

Life?s response:

 

but most life on land adapted or migrated Duration: Decades to hundreds of years

 

Overall warming:

 

°

 

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

 

exhausted.

 

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.

 

Nature Geoscience

 

SCIENTIFIC AMERICAN ONLINE July 2011, ScientificAmerican.com 61 Illustrations by Ron Miller, Graphic by Emily Cooper © 2011 Scientific American

 







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