THEY call it “the gloop”. It came up out of the Pacific Ocean floor looking
rather like a Mr Whippy soft ice cream, all white and squishy. But the gloop is
actually a mush of tiny planktonic shells that landed on extinct marine
volcanoes 60 million years ago. And it carries a record of an atmosphere that
was very hot, very rich in carbon dioxide and uncannily like the greenhouse
world some climatologists predict for the 22nd century.
As researchers attempt to probe the make-up of the atmosphere thousands and
millions of years ago, many have wondered if plate tectonics, changing ocean
currents or the movements of the Sun and Moon could be the true determinants of
climate on the Earth’s surface. But, says Paul Pearson of the University of
Bristol—the proud owner of the gloop scooped from the depths of the
Pacific—those tiny shells hold vital clues to the truth. They show that of
all the factors that seem to be involved, CO2 appears to be the
indispensable main player in the drama of the Earth’s changing climate.
It was in 1896 that the Swedish chemist Svante Arrhenius first suggested that
CO2acted as a planetary temperature control. He said that the early
Cenozoic—the warm era after the fall of the dinosaurs, 65 million years
ago—would have had high CO2 levels in the air. And he pointed out
that CO2 was accumulating in the atmosphere because of burning fossil
fuels, and that it could trigger warming by trapping infrared radiation close to
the Earth’s surface.
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His ideas were largely ignored at the time. Nobody had a way of measuring
CO2 levels from the past—or much interest in tracking current
levels, which seemed hopelessly variable. It was another half-century before the
first serious attempt was made. In the mid-1950s, Charles David Keeling, a young
student at the Scripps Institution of Oceanography in La Jolla, California,
began painstakingly measuring CO2. He did this first in the
bear-infested hills of the state’s Yosemite National Park and later in the clean
air 4200 metres up on top of Mauna Loa volcano in Hawaii.Very soon Keeling had
established a background CO2 level of 315 parts per million (ppm) in
the global atmosphere. And before long he started noticing that year-on-year
atmospheric CO2 levels were rising, a trend that he has assiduously
plotted to this day and which has become known as “Keeling’s curve”.
The implications of Keeling’s curve were profound. The rising CO2
levels followed increases in the burning of fossil fuels, and thanks to the
curve, Arrhenius’s ideas were rescued from the dustbin of scientific history. It
seemed he was right that people were tampering with the planetary thermostat.
Climatologists soon discarded their predictions of an end to the world’s era of
warm interglacial climate. The fear of a new ice age was replaced by one of
global warming. But to prove their new theories required concrete evidence from
the ancient past that warmer worlds really did have more CO2 in the
air.
The most direct approach was to measure the CO2 in some ancient air.
And where better to look than in the world’s ice sheets, where falling
snowflakes hundreds of thousands of years ago trapped bubbles of air in the ice.
Here the heroes proved to be Soviet drilling engineers, collaborating with
French scientists at Vostok, a remote research station in Antarctica. Starting
in 1980, the Soviets began to drill a tube 10 centimetres across into the ice
and backwards in time. Over the next 18 years, they drilled down 3.6
kilometres—the deepest ice core in the world
(91av, 29 January, p 40).
The results of this endeavour were published last year in Nature
(vol 399, p 429). The message was the same all the way to the bottom of the
core, where the air was 420 000 years or three ice ages old: CO2 levels
and temperature in the bubbles were irrevocably connected in matching sawtooth
patterns. As the world went into its ice ages, both CO2 and temperature
gradually fell; as the ice retreated the two measures abruptly soared in
unison.
Climatologists had already concluded that the main force driving the ice ages
is wobbles in the Earth’s orbit, named in the 1920s after their Serbian
discoverer, Milutin Milankovitch. These wobbles change the intensity and angle
of solar radiation hitting the Earth, altering the amount reaching the critical
polar regions where ice caps form. A 100 000-year Milankovitch cycle fits very
well with the timing of the major glaciations.
So where does the CO2 fit in? Jean-Robert Petit of the Laboratory of
Glaciology and Geophysics of the Environment in Grenoble, France, and colleagues
on the Vostok team concluded in their Nature paper that the wobbles are
the initial driving force behind the oscillating climate, but that Earthly
processes somehow amplify the solar changes. And the strongest process is the
greenhouse effect of CO2. “Greenhouse gases contributed about half of
the temperature change,” says Petit.
So the Milankovitch wobbles must have triggered a change in levels of CO2
in the atmosphere, which in turn amplified the change in temperature.
Gases belching from the Earth in volcanic eruptions are the main source of
additional CO2 in the atmosphere, and it is not thought that
Milankovitch wobbles can trigger volcanic eruptions. Researchers believe the
wobbles must instead alter the distribution of CO2 at the Earth’s
surface—particularly between the oceans and atmosphere.
The most obvious way for a planetary wobble to cool the planet would be for
it to make the oceans more biologically productive, drawing extra CO2
out of the atmosphere into the waters where it is absorbed by plankton and other
organisms. This can be a very dynamic and fast-acting biological “pump” for
CO2. When the organisms die, their shells and skeletons fall to the
ocean floor and are buried. If a wobble both cooled the planet and at the same
time triggered an intensification of the biological pump, emptying the
atmosphere of CO2, then this strong positive feedback could help
explain the severity of the glaciations, says Petit.
The role of CO2 in amplifying the influence of the Milankovitch
cycles and pushing the planet into and out of glaciations now seems clear,
thanks to the Vostok core. But has CO2 played a similar role in eras
without glaciations and over timescales not involving orbital wobbles?
Air trapped in ice is no help here. “Before the ice ages began 2 million
years ago, there is no direct measurement. We have to explore other ways,” says
Martin Palmer, a chemist from Imperial College, London. Palmer’s approach is to
study the isotopic composition of marine organisms living in ancient times, and
now found in mud on the ocean floor.
In recent years the field has become crowded. Researchers have analysed the
isotopes of carbon in the shells of marine organisms buried in the ocean
sediments, with mixed results, says Nick Shackleton of the University of
Cambridge, one of the pioneers of this approach. “Many workers in this field get
fouled up because they don’t know about the organisms they are dealing with.”
Not all organisms live in the critical surface layers where the water reflects
atmospheric conditions. Not all organisms absorb carbon without altering it in
ways that confuse the analysis.
Some researchers have looked for other, better proxies. Palmer, along with
Pearson, who is a specialist in micro-plankton, has led the way in finding
planktons that measure past acidity of the oceans. CO2 dissolved in
water forms carbonic acid, so the acidity of the surface waters of the ocean is
an effective proxy for CO2 levels in the water. And—at least in
undisturbed areas of ocean—for atmospheric CO2.
But how to measure past pH? Easy, says Pearson. There is a
ready-made isotopic measure in boron. Boron is common in the ocean and some
plankton, such as the ubiquitous foraminifera, take it up by accident while
seeking carbon to build their shells. And, depending on how acidic the waters
are, foraminifera take up different proportions of the two available isotopes,
boron-10 and boron-11. Measure the ratio between the two and you have the pH
of the water in which the organism lived.
And here’s where the gloop comes in. Numerous sediment cores have been
punched out from the floor of the Pacific Ocean during the international Ocean
Drilling Program over the past decade. Much of Pearson and Palmer’s gloop fell
as a biological snowstorm onto ancient sunken atolls that pepper the ocean
floor. These flat-topped mounds are closer to the surface than the deep sea
floor, and easier to reach with a drill. Their layers of old shells can be up to
200 metres thick—a well-preserved record of the marine organisms that
lived in the waters above. “The gloop is 95 per cent made up of calcium
carbonate—just a big pile of shells,” says Pearson.
In a paper published last June in Science (vol 284, p 1824), Pearson
and Palmer revealed analysis of CO2 levels from the gloop going back 43
million years. It caused a flurry among climate modellers. There just did not
seem to be enough CO2 in the atmosphere. “The world then was about 5
°C warmer than today. Yet the pH of the surface water was at 8.05,
suggesting that CO2 levels in the air were not substantially different
from what they are today: 400 ppm at most, compared to 365 ppm today,” says
Palmer.
There were two possibilities. One, that temperature is even more sensitive to
CO2 levels than anyone thought. The second, thought to be much more
likely, was that CO2 wasn’t really driving temperature at all back
then. Scientists began to worry that the CO2-temperature connection
might be crumbling. “Researchers will be looking at other factors to explain the
long-term chill of the past 50 million years,” noted Science
correspondent Richard Kerr.
Interesting times
One such factor could be a realignment of ocean currents, triggered by
shifting continents, which had perhaps allowed the ocean to grab a greater share
of the heat available on the planet’s surface, cooling the atmosphere. There is
no evidence that this has happened over recent decades, or that this is what
drove the world into and out of ice ages, but over timescales of millions of
years, such geologically driven events could play a role.
This was an interesting moment for Palmer and Pearson. “We couldn’t say
anything then, but by the time our paper was published, we had new analyses from
10 million years earlier that suggested CO2 was important,” says
Pearson. The new data showed that there had been a major change involving both
temperature and CO2 just before the period covered by their Science paper.
The details of their findings are now under consideration at Nature,
but Palmer revealed the outline at a meeting of the American Geophysical Union
in San Francisco last December. He showed that between 53 and 59 million years
ago—when temperatures were much higher than today—the surface waters
of the oceans were much more acidic than at any time since. They had a pH
of around 7.4, compared to around 8.1 today. “CO2 levels were
far higher than anything seen since—an estimated 3000 to 3500 ppm, or 8 to
10 times current levels,” said Palmer.
Soon after, the pH record reveals that CO2 levels crashed
to 700 ppm within a million years, staying low till 47 million years ago,
bumping up and then coming down again to near present levels, although the
seamount record irritatingly gives out between 40 and 26 million years ago.
Their work, once published, is bound to put CO2 right back in the
driving seat of climate change. Shackleton believes the results are very
important. “I am very impressed with Pearson and Palmer’s work. What they are
doing here is getting a real understanding of one of the main problems in the
surface history of the Earth—the role of CO2 in climate
Բ.”
So where did all that CO2 in the atmosphere 55 million years ago
come from? It might have bubbled up from the ocean after some cataclysm in the
depths. But a more likely source is volcanic eruptions. Around the time of the
high CO2 levels, there was widespread volcanic activity in the North
Atlantic which ended quite suddenly around 53 million years ago.
And where did the extra CO2 go? Recent isotope research by Adina
Paytan at Stanford University suggests that the oceans were very biologically
active at that time, so the biological pump lowered the atmospheric CO2
levels. “There is evidence too of the large-scale burial of organic matter on
land and in coastal regions, with swamps and coal formation,” says Palmer. “This
too would have drawn CO2 out of the atmosphere.” Humans have spent the
last two centuries digging up that coal and releasing its CO2 back into
the atmosphere.
All this is tentative, Pearson and Palmer admit. And the sequence of events
between 40 and 50 million years ago is still open to interpretation. But the big
story seems to be the emergence of CO2 as a major player in climate
change on the grand scale of tens of millions of years. Equally important, the
two researchers may have come up with an effective proxy measure of CO2
that can go right back into the early years of evolution on Earth. “We have
plans to reconstruct CO2 levels going back to the mid-Cretaceous era,
100 million years ago, using sediment cores taken from off the Florida coast,”
says Pearson. “After that, well, in principle we could go back to the
Precambrian, 600 million years ago.”
From yearly changes to geological timescales, from the top of Mauna Loa to
the depths of the oceans, CO2 is emerging as the driving force behind
changing climate. It should give us pause as we dig up and burn fossilised
carbon that was buried out of harm’s way tens of millions of years ago, when the
world was hotter than a greenhouse. That burial cooled the planet to a
temperature we find convenient. If we insist on releasing the carbon back
into the atmosphere again, we will have to live with the consequences.