Ancient Fossil Evidence Supports Carbon Dioxide As Driver Of Global Warming

A team of American and Canadian scientists has devised a new way to study Earth’s past climate by analyzing the chemical composition of ancient marine fossils. The first published tests with the method further support the view that atmospheric CO2 has contributed to dramatic climate variations in the past, and strengthen projections that human CO2 emissions could cause global warming.

In the current issue of the journal Nature, geologists and environmental scientists from the California Institute of Technology, the University of Ottawa, the Memorial University of Newfoundland, Brock University, and the Waquoit Bay National Estuarine Research Reserve report the results of a new method for determining the growth temperatures of carbonate fossils such as shells and corals. This method looks at the percentage of rare isotopes of oxygen and carbon that bond with each other rather than being randomly distributed through their mineral lattices.

Because these bonds between oxygen-18 and carbon-13 form in greater abundance at low temperatures and lesser abundance at higher temperatures, a precise measurement of their concentration in a carbonate fossil can quantify the temperature of seawater in which the organisms lived. By comparing this record of temperature change with previous estimates of past atmospheric CO2 concentrations, the study demonstrates a strong coupling of atmospheric temperatures and carbon dioxide concentrations across one of Earth’s major environmental shifts.

According to Rosemarie Came, a postdoctoral scholar in geochemistry at Caltech and lead author of the article, only about 60 parts per million of the carbonate molecular groups that make up the mineral structures of carbonate fossils are a combination of both oxygen-18 and carbon-13, but the amount varies predictably with temperature. Therefore, knowing the age of the sample and how much of these exotic carbonate groups are present allows one to create a record of the planet’s temperature through time.

“This clumped-isotope method has an advantage over previous approaches because we’re looking at the distribution of rare isotopes inside a single shell or coral,” Came says. “All the information needed to study the surface temperature at the time the animal lived is stored in the fossil itself.”

In this way, the method contrasts with previous approaches that require knowledge of the chemistry of seawater in the distant past–something that is poorly known.

The study contrasts the growth temperatures of fossils from two times in the distant geological past. The Silurian period, approximately 400 million years ago, is thought to have been a time of highly elevated atmospheric CO2 (more than 10 times the modern concentration), and was found by the researchers to be a time of exceptionally warm shallow-ocean temperatures–nearly 35 degrees C. In contrast, the Carboniferous period, roughly 300 million years ago, appears to have been characterized by far lower levels of atmospheric carbon dioxide (similar to modern values) and had shallow marine temperatures similar to or slightly cooler than today-about 25 degrees C. Thus, the draw-down of atmospheric CO2 coincided with strong global cooling.

“This is a huge change in temperature,” says John Eiler, a professor of geochemistry at Caltech and a coauthor of the study. “It shows that carbon dioxide really has been a powerful driver of climate change in Earth’s past.”

The title of the Nature paper is “Coupling of surface temperatures and atmospheric CO2 concentrations during the Paleozoic era.” The other authors are Jan Veizer of the University of Ottawa, Karem Azmy of Memorial University of Newfoundland, Uwe Brand of Brock University, and Christopher R. Weidman of the Waquoit National Estuarine Research Reserve, Massachusetts.

Climate research gives clues to human expansion out of tropical Africa

New research that involves a University of Nebraska-Lincoln scientist has shed light on an important, but previously little-understood period in Africa’s climate history that has implications for understanding human evolution and the expansion of Homo sapiens out of tropical Africa.

In a paper published this week in the online version of the Proceedings of the National Academy of Sciences, a team headed by Andrew Cohen of the University Arizona and including UNL’s Jeffery Stone as the second author, reported findings from sediment cores recovered from one of the world’s deepest lakes, Lake Malawi in East Africa’s Great Rift Valley. Cohen, Stone and colleagues reported finding evidence of two extended periods of extreme aridity between 135,000 and 70,000 years ago, an important time in human pre-history.

“Prior to this research, there was not a really good terrestrial record of climate that stretched back through the period of human development and migration from the tropical region of Africa,” said Stone, an adjunct faculty member in the UNL Department of Geosciences who also has a research appointment at Arizona.

“Most of the previous records basically stretch back to the last glacial maximum, maybe 20,000 years. They show some really dry conditions and it’s assumed that that had a huge impact on human populations in Africa, but nobody really had a sense of what was going on before that.”

The scientists studied a variety of fossils and other sediments that settled to the lake bottom over the millennia and used them as proxies to interpret the climate at various times during the last 140,000 years. For example, diatoms, Stone’s specialty, are one-celled organisms having a silica skeleton that fossilizes readily. Different species of diatoms flourish or fail during different climatic conditions, so their relative abundance or absence provides a good indication of contemporary climate conditions.

What the diatoms and other proxies indicate is a Lake Malawi basin between 135,000 and 70,000 years ago that looked a lot different from the lush conditions found there today. Modern Lake Malawi has a surface area of 29,500 square kilometers (more than 11,000 square miles) and reaches a depth of 706 meters (more than 2,300 feet). Annual rainfall in its watershed varies from 800 to 2,400 millimeters a year (31-93 inches; for comparison, Lincoln averages 27.8 inches of rainfall per year).

But records from the sediment cores reveal two periods of megadrought, from 135,000 to 127,000 and 115,000 to 95,000 years ago, when the level of Lake Malawi fell 550 to 600 meters (1,800-1,970 feet) below present-day levels. The surrounding watershed was a semidesert that received less than 400 millimeters (16 inches) of rain per year, creating much drier conditions than occurred during the last glacial maximum, 35,000 to 15,000 years ago, when Lake Malawi’s level fell by only 30 to 200 meters.

There is little archaeological evidence of human habitation in tropical Africa during the megadroughts, a period that coincides with the earliest evidence of humans outside the region — about 125,000 years ago in North Africa and the Middle East.

The research by Stone and colleagues, however, indicates that tropical Africa’s climate became wetter after 70,000 years ago and reached conditions comparable to today by about 60,000 years ago. That period coincided with increased evidence of human habitation in the area, and closely coincided with increased aridity in other parts of the continent.

Cohen said the new finding provides an ecological explanation for the “Out-of-Africa” hypothesis that suggests that all humans descended from just a few people living in Africa sometime between 150,000 and 70,000 years ago. He said it’s possible that the human population crashed during the megadroughts, but rebounded when the climate became more hospitable. The growing human population eventually expanded down the Nile valley and dispersed around the globe.

The PNAS paper concluded that this timing is “consistent with the idea that the earlier (approximately 125,000 years ago) documented occurrence of modern humans in North Africa and the Levant represents ultimately unsuccessful ‘excursions’ out of Africa.”

The article is scheduled for publication in the Oct. 16 print edition of PNAS. The National Science Foundation, the International Continental Drilling Program and the Smithsonian Institution funded the research.

Carbon Dioxide Did Not End The Last Ice Age, Study Says

Lowell Stott, professor of earth sciences at the University of Southern California, examines a sediment core. (Credit: Dietmar Quistorf)
Lowell Stott, professor of earth sciences at the University of Southern California, examines a sediment core. (Credit: Dietmar Quistorf)

Carbon dioxide did not cause the end of the last ice age, a new study in Science suggests, contrary to past inferences from ice core records.

“There has been this continual reference to the correspondence between CO2 and climate change as reflected in ice core records as justification for the role of CO2 in climate change,” said USC geologist Lowell Stott, lead author of the study, slated for advance online publication Sept. 27 in Science Express.

“You can no longer argue that CO2 alone caused the end of the ice ages.”

Deep-sea temperatures warmed about 1,300 years before the tropical surface ocean and well before the rise in atmospheric CO2, the study found. The finding suggests the rise in greenhouse gas was likely a result of warming and may have accelerated the meltdown — but was not its main cause.

The study does not question the fact that CO2 plays a key role in climate.

“I don’t want anyone to leave thinking that this is evidence that CO2 doesn’t affect climate,” Stott cautioned. “It does, but the important point is that CO2 is not the beginning and end of climate change.”

While an increase in atmospheric CO2 and the end of the ice ages occurred at roughly the same time, scientists have debated whether CO2 caused the warming or was released later by an already warming sea.

The best estimate from other studies of when CO2 began to rise is no earlier than 18,000 years ago. Yet this study shows that the deep sea, which reflects oceanic temperature trends, started warming about 19,000 years ago.

“What this means is that a lot of energy went into the ocean long before the rise in atmospheric CO2,” Stott said.

But where did this energy come from” Evidence pointed southward.

Water’s salinity and temperature are properties that can be used to trace its origin — and the warming deep water appeared to come from the Antarctic Ocean, the scientists wrote.

This water then was transported northward over 1,000 years via well-known deep-sea currents, a conclusion supported by carbon-dating evidence.

In addition, the researchers noted that deep-sea temperature increases coincided with the retreat of Antarctic sea ice, both occurring 19,000 years ago, before the northern hemisphere’s ice retreat began.

Finally, Stott and colleagues found a correlation between melting Antarctic sea ice and increased springtime solar radiation over Antarctica, suggesting this might be the energy source.

As the sun pumped in heat, the warming accelerated because of sea-ice albedo feedbacks, in which retreating ice exposes ocean water that reflects less light and absorbs more heat, much like a dark T-shirt on a hot day.

In addition, the authors’ model showed how changed ocean conditions may have been responsible for the release of CO2 from the ocean into the atmosphere, also accelerating the warming.

The link between the sun and ice age cycles is not new. The theory of Milankovitch cycles states that periodic changes in Earth’s orbit cause increased summertime sun radiation in the northern hemisphere, which controls ice size.

However, this study suggests that the pace-keeper of ice sheet growth and retreat lies in the southern hemisphere’s spring rather than the northern hemisphere’s summer.

The conclusions also underscore the importance of regional climate dynamics, Stott said. “Here is an example of how a regional climate response translated into a global climate change,” he explained.

Stott and colleagues arrived at their results by studying a unique sediment core from the western Pacific composed of fossilized surface-dwelling (planktonic) and bottom-dwelling (benthic) organisms.

These organisms — foraminifera — incorporate different isotopes of oxygen from ocean water into their calcite shells, depending on the temperature. By measuring the change in these isotopes in shells of different ages, it is possible to reconstruct how the deep and surface ocean temperatures changed through time.

If CO2 caused the warming, one would expect surface temperatures to increase before deep-sea temperatures, since the heat slowly would spread from top to bottom. Instead, carbon-dating showed that the water used by the bottom-dwelling organisms began warming about 1,300 years before the water used by surface-dwelling ones, suggesting that the warming spread bottom-up instead.

“The climate dynamic is much more complex than simply saying that CO2 rises and the temperature warms,” Stott said. The complexities “have to be understood in order to appreciate how the climate system has changed in the past and how it will change in the future.”

Stott’s collaborators were Axel Timmermann of the University of Hawaii and Robert Thunell of the University of South Carolina. Stott was supported by the National Science Foundation and Timmerman by the International Pacific Research Center.

Stott is an expert in paleoclimatology and was a reviewer for the Intergovernmental Panel on Climate Change. He also recently co-authored a paper in Geophysical Research Letters tracing a 900-year history of monsoon variability in India.

The study, which analyzed isotopes in cave stalagmites, found correlations between recorded famines and monsoon failures, and found that some past monsoon failures appear to have lasted much longer than those that occurred during recorded history. The ongoing research is aimed at shedding light on the monsoon’s poorly understood but vital role in Earth’s climate.

Cave Records Provide Clues To Climate Change

A close up of one of the stalagmites analyzed in the study. (Credit: Jud Partin)
A close up of one of the stalagmites analyzed in the study. (Credit: Jud Partin)

When Georgia Tech Assistant Professor Kim Cobb and graduate student Jud Partin wanted to understand the mechanisms that drove the abrupt climate change events that occurred thousands of years ago, they didn’t drill for ice cores from the glaciers of Greenland or the icy plains of Antarctica, as is customary for paleoclimatolgists. Instead, they went underground.

Growing inside the caves of the tropical Pacific island of Borneo are some of the keys to understanding how the Earth’s climate suddenly changed – several times – over the last 25,000 years. By analyzing stalagmites, the pilar-like rock formations that stem from the ground in caves, they were able to produce a high-resolution and continuous record of the climate over this equatorial rainforest.

“These stalagmites are, in essence, tropical ice cores forming over thousands of years,” said Partin. “Each layer of the rock contains important chemical traces that help us determine what was going on in the climate thousands of years ago, much like the ice cores drilled from Greenland or Antarctica.”

The tropical Pacific currently plays a powerful role in shaping year-to-year climate variations around the globe (as evidenced by the number of weather patterns influenced by the Pacific’s El Nino), but its role in past climate change is less understood. Partin and Cobb’s results suggest that the tropical Pacific played a much more active role in some of the abrupt climate change events of Earth’s past than was once thought and may even have played a leading role in some of these changes.

Polar ice cores reveal that the Northern Hemisphere and the Southern Hemisphere each have their own distinct patterns of abrupt climate change; the tropical Pacific may provide the mechanistic link between the two systems. Understanding how the climate changes occurred and what they looked like is important to helping scientists put into context the current trends in today’s climate.

The research team collected stalagmites from the Gunung Buda cave system in Borneo in 2003, 2005 and 2006. Analyzing three stalagmites from two separate caves allowed the pair to create a near-continuous record of the climate from 25,000 years ago to the present. While this study is not the first to use stalagmites to examine climate over this time period, it is the first to do so in the tropical Pacific. Typically, in these types of studies, only one stalagmite is analyzed, but Partin and Cobb compared their three stalagmite records to isolate shared climate-related signals.

Stalagmites are formed as rain water, mixed with calcium carbonate and other elements, makes its way through the ground and onto the cave floor. As this solution drips over time, it hardens in layers, creating a column of rock.

Partin and Cobb cut open each stalagmite and took 1,300 measurements of their chemical content to determine the relative moisture of the climate at various periods in history starting from the oldest layers at the bottom to the present at the top. They dated the rocks by analyzing the radioactive decay of uranium and thorium, and determined the amount of precipitation at given times by measuring the ratio of oxygen isotopes.

“Our records contain signatures of both Northern and Southern Hemisphere climate influences as the Earth emerged from the last ice age, which makes sense given its equatorial location,” said Cobb. “However, tropical Pacific climate was not a simple linear combination of high-latitude climate events. It reflects the complexity of mechanisms linking high and low latitude climate.”

For example, Partin and Cobb’s records suggest that the tropical Pacific began drying about 20,000 years ago and that this trend may have pre-conditioned the North Atlantic for an abrupt climate change event that occurred about 16,500 years ago, known as the Heinrich 1 event.

“In addition, the Borneo records indicate that the tropical Pacific began to get wetter before the North Atlantic recovered from the Heinrich 1 event 14,000 years ago. Perhaps the tropical Pacific is again driving that trend,” said Partin.

“Currently our knowledge of how these dramatic climate changes occurred comes from just a few sites,” said Cobb. “As more studies are done from caves around the world, hopefully we’ll be able to piece together a more complete picture of these changes. Understanding how the dominoes fell is very important to our understanding of our current warming trend.”

These findings are published in the Sept 27, 2007 issue of the journal Nature.

Studying Evidence From Ice Age Lakes

Northern Dvina starts as the confluence of Yug River (on left) and Sukhona River (on top) near Velikiy Ustyug
Northern Dvina starts as the confluence of Yug River (on left) and Sukhona River (on top) near Velikiy Ustyug

During the last Ice Age, the ice dammed enormous lakes in Russia. The drainage system was reversed several times and the rivers flowed southwards. A group of geologists is now investigating what took place when the ice melted and the lakes released huge volumes of fresh water into the Arctic Ocean.

“The ice-dammed lakes in Russia were larger than the largest lakes we know today,” Eiliv Larsen, a geologist at the Geological Survey of Norway (NGU), tells me. He is in charge of the important SciencePub International Polar Year project that is studying natural climate changes in the Arctic and the ways in which man has adapted to them.

Moving glaciers

“The entire drainage system in Russia has been reversed several times during the past 130 000 years. The heavy ice cap covering the land area in the north dammed up lakes and forced the large rivers, the Dvina, Mezen, Pechora and Vychegda, to flow southwards to the Caspian Sea, the Black Sea and on to the Mediterranean,” Eiliv Larsen continues.

However, the ice margin in the north shifted; the ice cap varied in extent, sometimes suddenly advancing eastwards, sometimes retreating. When the plug was released as the ice melted, the water poured from the huge lakes into the Kara Sea and the White Sea, causing the sea level to rise.

The sum total of all these dramatic changes greatly influenced the climate and the circulation in the seas in the Barents Region. SciencePub scientists are trying to find answers to where, when and how often this took place.

Digging down

“We drive vehicles and boats along the large valleys and rivers,” Maria Jensen, a group leader and geologist at NGU, tells me. “Where sediments are exposed along the coast and rivers, we study the sequence of their deposition. It’s here, in the transportation of clay, silt and sand out into the marine system, that we find evidence of the major events,” she points out.

In a cutting beside the River Vychegda, a few kilometres from the village of Ust Nem in the Komi Republic, geologists from Norway, Denmark, Russia and Germany are digging their way down through layers that were deposited during the last Ice Age.

Astrid Lyså, another NGU geologist, explains: “We remove the outermost layer to get at undisturbed sediments. Then we clean the section, measure it, and photograph, draw and describe the structures in every single layer. We also take samples to date the beds using luminescence, a technique that reveals, for example, when a sand grain was last exposed to the light.”

Reading the history

By investigating a given section, geologists can, for instance, find out what types of deposits are present in the area. They may be moraine, peat, mire, or sediments deposited in the sea, rivers and ice-dammed lakes. The scientists can also find out in which environment these sediments were deposited; was it deep, still or shallow water, beneath the ice, or were the sediments deposited by the wind?

It is thus actually possible to read the history of an ice-dammed lake by studying the succession of layers dug out with a spade, a scraper, a knife and a trowel.

That is precisely what they are doing here at Ust Nem. There seems to have been two ice-dammed lakes here. The patterns of patches of coloured clay and the remains of crushed clay in the sediments suggest, for example, a strong increase in pore pressure and that this particular lake was tapped extremely rapidly.

A big jigsaw puzzle

The scientists are also investigating the landscape around the river, searching for old, dry valleys and small depressions that may explain the drainage system. After driving for just a couple of hours and walking a bit in the forest, they find small valleys which drained into the present course of the river.

The pieces in the jigsaw puzzle are thus gradually falling into place.

“When we fit everything together, we can see the details in and around the enormous lakes which the Russian rivers drained southwards on several occasions and at other times emptied into the Arctic Ocean when the ice melted,” Eiliv Larsen explains.

Glaciologist Looks To Ice For Clues About Global Warming

Glaciologists extract ice cores, analyze them and determine changes in climate over time.
Glaciologists extract ice cores, analyze them and determine changes in climate over time.

Once or twice a year Keith Mountain, chair of the Department of Geography and Geosciences at the University of Louisville, and colleagues from the Byrd Polar Research Center at Ohio State University spend months hunting for a disappearing treasure: ice.

They travel to a glacier in the mountains of Bolivia, Peru, China, Antarctica or Tanzania. The conditions can be brutal at elevations as high as 20,000 feet, but what they find might help save the planet.

Glacier ice contains thousands of years of the Earth’s climate history. It also provides clues as to how and why global warming is happening today, Mountain said.

The researchers use a portable drilling system to extract from the glaciers cylindrical ice cores about 13 centimeters in diameter and hundreds of meters long. They cut each core into 1-meter segments, then mark and pack it for later analyses.

Like the rings of trees, these ice cores are time capsules which can span tens of thousands of years.

Through mathematical models and other methods, including preliminary test drillings, the team can determine the amount of ice compression and come up with reliable ways to interpret these ice cores, he explained.

“We can reconstruct atmospheric temperatures and ascertain precipitation rates and how much dust there was in the atmosphere,” Mountain said. “We can find out the chemical composition of the atmosphere. You can pick up things like various nitrates-sea salt, for example-and figure out wind directions and the sources of moisture and how those may have changed over time.”

The team has determined from interpreting the ice records over time that, “yes, climate change is global and real,” he said.

Rising temperatures worldwide and local decreases in precipitation are contributing to the decline of the glaciers — which makes their work a race against time.

“When I started out in this field in the late ’70s I never would have thought that the photographs we took of this big ice sheet in southern Peru would become archival records of something lost,” Mountain said. “We photograph the changes there every year now, and the changes are occurring quickly. It’s retreating on the order of 50 meters a year.”

Ice is retreating not only in Peru, but worldwide. The famed white cap of Mt. Kilimanjaro in Tanzania soon will be no more. At its current rate of melting, he said, it will disappear by about 2015.

Glaciologists are just one scientific sector contributing to the mass of evidence on the reality of global climate change, Mountain said, noting that 95 percent of the scientific community believes that global climate change is happening and that humans are a significant causal factor.

Yet somehow, he said, a 95 to 5 percent ratio becomes a yes-no, either-or vote: “Some are trying to turn this into a debate, but there is no debate.”

In his native Australia where years of drought have led to bush fires and dying cattle, global warming is a real issue, Mountain said. That’s also true in other parts of the world.

“For the people in Peru, as the glaciers melt they are losing their irrigation water for farming. In Tibet, as the glaciers recede streams are evaporating and leaving big salt deposits that make the remaining water undrinkable,” he said.

The jury is no longer out on global warming, Mountain said.

“At some point, the jury has to come back and make a decision. What kind of policies are we going to develop to deal with this?”