New study shows 3 abrupt pulse of CO2 during last deglaciation

A new study shows that the rise of atmospheric carbon dioxide that contributed to the end of the last ice age more than 10,000 years ago did not occur gradually, but was characterized by three “pulses” in which C02 rose abruptly.

Scientists are not sure what caused these abrupt increases, during which C02 levels rose about 10-15 parts per million – or about 5 percent per episode – over a period of 1-2 centuries. It likely was a combination of factors, they say, including ocean circulation, changing wind patterns, and terrestrial processes.

The finding is important, however, because it casts new light on the mechanisms that take the Earth in and out of ice age regimes. Results of the study, which was funded by the National Science Foundation, appear this week in the journal Nature.

“We used to think that naturally occurring changes in carbon dioxide took place relatively slowly over the 10,000 years it took to move out of the last ice age,” said Shaun Marcott, lead author on the article who conducted his study as a post-doctoral researcher at Oregon State University. “This abrupt, centennial-scale variability of CO2 appears to be a fundamental part of the global carbon cycle.”

Some previous research has hinted at the possibility that spikes in atmospheric carbon dioxide may have accelerated the last deglaciation, but that hypothesis had not been resolved, the researchers say. The key to the new finding is the analysis of an ice core from the West Antarctic that provided the scientists with an unprecedented glimpse into the past.

Scientists studying past climate have been hampered by the limitations of previous ice cores. Cores from Greenland, for example, provide unique records of rapid climate events going back 120,000 years – but high concentrations of impurities don’t allow researchers to accurately determine atmospheric carbon dioxide records. Antarctic ice cores have fewer impurities, but generally have had lower “temporal resolution,” providing less detailed information about atmospheric CO2.

However, a new core from West Antarctica, drilled to a depth of 3,405 meters in 2011 and spanning the last 68,000 years, has “extraordinary detail,” said Oregon State paleoclimatologist Edward Brook, a co-author on the Nature study and an internationally recognized ice core expert. Because the area where the core was taken gets high annual snowfall, he said, the new ice core provides one of the most detailed records of atmospheric CO2.

“It is a remarkable ice core and it clearly shows distinct pulses of carbon dioxide increase that can be very reliably dated,” Brook said. “These are some of the fastest natural changes in CO2 we have observed, and were probably big enough on their own to impact the Earth’s climate.

“The abrupt events did not end the ice age by themselves,” Brook added. “That might be jumping the gun a bit. But it is fair to say that the natural carbon cycle can change a lot faster than was previously thought – and we don’t know all of the mechanisms that caused that rapid change.”

The researchers say that the increase in atmospheric CO2 from the peak of the last ice age to complete deglaciation was about 80 parts per million, taking place over 10,000 years. Thus, the finding that 30-45 ppm of the increase happened in just a few centuries was significant.

The overall rise of atmospheric carbon dioxide during the last deglaciation was thought to have been triggered by the release of CO2 from the deep ocean – especially the Southern Ocean. However, the researchers say that no obvious ocean mechanism is known that would trigger rises of 10-15 ppm over a time span as short as one to two centuries.

“The oceans are simply not thought to respond that fast,” Brook said. “Either the cause of these pulses is at least part terrestrial, or there is some mechanism in the ocean system we don’t yet know about.”

One reason the researchers are reluctant to pin the end of the last ice age solely on CO2 increases is that other processes were taking place, according to Marcott, who recently joined the faculty of the University of Wisconsin-Madison.

“At the same time CO2 was increasing, the rate of methane in the atmosphere was also increasing at the same or a slightly higher rate,” Marcott said. “We also know that during at least two of these pulses, the Atlantic Meridional Overturning Circulation changed as well. Changes in the ocean circulation would have affected CO2 – and indirectly methane, by impacting global rainfall patterns.”

“The Earth is a big coupled system,” he added, “and there are many pieces to the puzzle. The discovery of these strong, rapid pulses of CO2 is an important piece.”

New study shows 3 abrupt pulse of CO2 during last deglaciation

A new study shows that the rise of atmospheric carbon dioxide that contributed to the end of the last ice age more than 10,000 years ago did not occur gradually, but was characterized by three “pulses” in which C02 rose abruptly.

Scientists are not sure what caused these abrupt increases, during which C02 levels rose about 10-15 parts per million – or about 5 percent per episode – over a period of 1-2 centuries. It likely was a combination of factors, they say, including ocean circulation, changing wind patterns, and terrestrial processes.

The finding is important, however, because it casts new light on the mechanisms that take the Earth in and out of ice age regimes. Results of the study, which was funded by the National Science Foundation, appear this week in the journal Nature.

“We used to think that naturally occurring changes in carbon dioxide took place relatively slowly over the 10,000 years it took to move out of the last ice age,” said Shaun Marcott, lead author on the article who conducted his study as a post-doctoral researcher at Oregon State University. “This abrupt, centennial-scale variability of CO2 appears to be a fundamental part of the global carbon cycle.”

Some previous research has hinted at the possibility that spikes in atmospheric carbon dioxide may have accelerated the last deglaciation, but that hypothesis had not been resolved, the researchers say. The key to the new finding is the analysis of an ice core from the West Antarctic that provided the scientists with an unprecedented glimpse into the past.

Scientists studying past climate have been hampered by the limitations of previous ice cores. Cores from Greenland, for example, provide unique records of rapid climate events going back 120,000 years – but high concentrations of impurities don’t allow researchers to accurately determine atmospheric carbon dioxide records. Antarctic ice cores have fewer impurities, but generally have had lower “temporal resolution,” providing less detailed information about atmospheric CO2.

However, a new core from West Antarctica, drilled to a depth of 3,405 meters in 2011 and spanning the last 68,000 years, has “extraordinary detail,” said Oregon State paleoclimatologist Edward Brook, a co-author on the Nature study and an internationally recognized ice core expert. Because the area where the core was taken gets high annual snowfall, he said, the new ice core provides one of the most detailed records of atmospheric CO2.

“It is a remarkable ice core and it clearly shows distinct pulses of carbon dioxide increase that can be very reliably dated,” Brook said. “These are some of the fastest natural changes in CO2 we have observed, and were probably big enough on their own to impact the Earth’s climate.

“The abrupt events did not end the ice age by themselves,” Brook added. “That might be jumping the gun a bit. But it is fair to say that the natural carbon cycle can change a lot faster than was previously thought – and we don’t know all of the mechanisms that caused that rapid change.”

The researchers say that the increase in atmospheric CO2 from the peak of the last ice age to complete deglaciation was about 80 parts per million, taking place over 10,000 years. Thus, the finding that 30-45 ppm of the increase happened in just a few centuries was significant.

The overall rise of atmospheric carbon dioxide during the last deglaciation was thought to have been triggered by the release of CO2 from the deep ocean – especially the Southern Ocean. However, the researchers say that no obvious ocean mechanism is known that would trigger rises of 10-15 ppm over a time span as short as one to two centuries.

“The oceans are simply not thought to respond that fast,” Brook said. “Either the cause of these pulses is at least part terrestrial, or there is some mechanism in the ocean system we don’t yet know about.”

One reason the researchers are reluctant to pin the end of the last ice age solely on CO2 increases is that other processes were taking place, according to Marcott, who recently joined the faculty of the University of Wisconsin-Madison.

“At the same time CO2 was increasing, the rate of methane in the atmosphere was also increasing at the same or a slightly higher rate,” Marcott said. “We also know that during at least two of these pulses, the Atlantic Meridional Overturning Circulation changed as well. Changes in the ocean circulation would have affected CO2 – and indirectly methane, by impacting global rainfall patterns.”

“The Earth is a big coupled system,” he added, “and there are many pieces to the puzzle. The discovery of these strong, rapid pulses of CO2 is an important piece.”

Past temperature in Greenland adjusted

The revised Greenland temperature history (black curve, grey uncertainties) for the period 18,000 to 10,000 before present. This temperature history is based on temperature interpretation from nitrogen measurements (green curve) and O18 diffusion measurements (red curve). The blue curve is from a previous study, based on nitrogen measurements. -  Niels Bohr Institute
The revised Greenland temperature history (black curve, grey uncertainties) for the period 18,000 to 10,000 before present. This temperature history is based on temperature interpretation from nitrogen measurements (green curve) and O18 diffusion measurements (red curve). The blue curve is from a previous study, based on nitrogen measurements. – Niels Bohr Institute

One of the common perceptions about the climate is that the amount of carbon dioxide in the atmosphere, solar radiation and temperature follow each other – the more solar radiation and the more carbon dioxide, the hotter the temperature. This correlation is also seen in the Greenland ice cores that are drilled through the approximately three kilometer thick ice sheet. But during a period of several thousand years up until the last ice age ended approximately 12,000 years ago, this pattern did not fit and this was a mystery to researchers. Now researchers from the Niels Bohr Institute have solved this mystery using new analytical techniques. The results are published in the prestigious scientific journal Science.

The Greenland ice sheet is an archive of knowledge about the Earth’s climate more than 125,000 years back in time. The ice was formed by the precipitation that fell as snow from the clouds and remained year after year, gradually being compressed into ice. By drilling down through the approximately three kilometer thick ice sheet, the researchers draw up ice cores, which provide detailed knowledge of the climate of the past annual layer after annual layer. By measuring the content of the special oxygen isotope O18 in the ice cores, you can get information about the temperature in the past climate, year by year.

But something didn’t fit. In Greenland, the end of the Ice Age started 15,000 years ago and the temperature rose quickly. Then it became colder again until 12,000 years ago, when there was again a rapid rise in temperature. The first rise in temperature is called the Bølling-Allerød interstadial and the second is called the Holocene interglacial.

Temperatures contrary to expectations

“We could see that the concentration of carbon dioxide and solar radiation was higher during the cold period between the two warm periods compared with the cold period before the first warming 15,000 years ago. But the temperature measurements based on the oxygen isotope O18 showed that the period between the two warm periods was colder than the cold period before the first warming 15,000 years ago. This was the exact opposite of what you would expect,” explains postdoc Vasileios Gkinis, Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen.

The researchers investigated ice cores from three different Greenland ice cores: the NEEM project, the NGRIP project and the GISP 2 project. But amount of the oxygen isotope O18 was not enough to reconstruct period temperatures in detail or their geographic distribution.

To get more detailed temperature data, the researchers used two relatively new methods of investigation, both of which examine the layer of compressed granular snow that is formed between the top layer of soft and fluffy snow and the layer deeper down in the ice sheet, where the compressed snow has been turned into ice. This process of transforming the fluffy snow into hard ice is physical and both the thickness and the movement of the water molecules are dependent on the temperature.

“With the first method, we measured the nitrogen content and by measuring the relationship between the two isotopes of nitrogen, N15 and N14, we could reconstruct the thickness of the compressed snow 19,000 years back in time,” explains Vasileios Gkinis.

The second method involved measuring the spread of air with water molecules with different isotope composition in the layers with the compressed snow. This process of smoothing the original water isotope variations from precipitation is dependent on the temperature, as the water molecules in vapour form are more mobile at warmer temperatures.

Temperatures ‘fall into place’

Data for the spread of the water molecules in the individual annual layers in the Greenland ice cores has thus made it possible to calculate the temperature in the layers with compressed snow 19,000 years back in time.

“What we discovered was that the previous temperature curve, which was only based on the measurements of the oxygen isotope O18, was inaccurate. The oxygen temperature curve said that the climate in central Greenland was colder around 12,000 years ago than around 15,000 years ago, despite the fact that two key climate drivers – carbon dioxide in the atmosphere and solar radiation – would suggest the opposite. With our new, more direct reconstruction, we have been able to show that the climate in central Greenland was actually warmer around 12,000 years ago compared to 15,000 years ago. So the temperatures actually follow the solar radiation and the amount of carbon dioxide in the atmosphere. We estimate that the temperature difference was 2-6 degrees,” says Bo Vinther, Associate Professor at the Centre for Ice and Climate at the Niels Bohr Institute, University of Copenhagen.

Past temperature in Greenland adjusted

The revised Greenland temperature history (black curve, grey uncertainties) for the period 18,000 to 10,000 before present. This temperature history is based on temperature interpretation from nitrogen measurements (green curve) and O18 diffusion measurements (red curve). The blue curve is from a previous study, based on nitrogen measurements. -  Niels Bohr Institute
The revised Greenland temperature history (black curve, grey uncertainties) for the period 18,000 to 10,000 before present. This temperature history is based on temperature interpretation from nitrogen measurements (green curve) and O18 diffusion measurements (red curve). The blue curve is from a previous study, based on nitrogen measurements. – Niels Bohr Institute

One of the common perceptions about the climate is that the amount of carbon dioxide in the atmosphere, solar radiation and temperature follow each other – the more solar radiation and the more carbon dioxide, the hotter the temperature. This correlation is also seen in the Greenland ice cores that are drilled through the approximately three kilometer thick ice sheet. But during a period of several thousand years up until the last ice age ended approximately 12,000 years ago, this pattern did not fit and this was a mystery to researchers. Now researchers from the Niels Bohr Institute have solved this mystery using new analytical techniques. The results are published in the prestigious scientific journal Science.

The Greenland ice sheet is an archive of knowledge about the Earth’s climate more than 125,000 years back in time. The ice was formed by the precipitation that fell as snow from the clouds and remained year after year, gradually being compressed into ice. By drilling down through the approximately three kilometer thick ice sheet, the researchers draw up ice cores, which provide detailed knowledge of the climate of the past annual layer after annual layer. By measuring the content of the special oxygen isotope O18 in the ice cores, you can get information about the temperature in the past climate, year by year.

But something didn’t fit. In Greenland, the end of the Ice Age started 15,000 years ago and the temperature rose quickly. Then it became colder again until 12,000 years ago, when there was again a rapid rise in temperature. The first rise in temperature is called the Bølling-Allerød interstadial and the second is called the Holocene interglacial.

Temperatures contrary to expectations

“We could see that the concentration of carbon dioxide and solar radiation was higher during the cold period between the two warm periods compared with the cold period before the first warming 15,000 years ago. But the temperature measurements based on the oxygen isotope O18 showed that the period between the two warm periods was colder than the cold period before the first warming 15,000 years ago. This was the exact opposite of what you would expect,” explains postdoc Vasileios Gkinis, Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen.

The researchers investigated ice cores from three different Greenland ice cores: the NEEM project, the NGRIP project and the GISP 2 project. But amount of the oxygen isotope O18 was not enough to reconstruct period temperatures in detail or their geographic distribution.

To get more detailed temperature data, the researchers used two relatively new methods of investigation, both of which examine the layer of compressed granular snow that is formed between the top layer of soft and fluffy snow and the layer deeper down in the ice sheet, where the compressed snow has been turned into ice. This process of transforming the fluffy snow into hard ice is physical and both the thickness and the movement of the water molecules are dependent on the temperature.

“With the first method, we measured the nitrogen content and by measuring the relationship between the two isotopes of nitrogen, N15 and N14, we could reconstruct the thickness of the compressed snow 19,000 years back in time,” explains Vasileios Gkinis.

The second method involved measuring the spread of air with water molecules with different isotope composition in the layers with the compressed snow. This process of smoothing the original water isotope variations from precipitation is dependent on the temperature, as the water molecules in vapour form are more mobile at warmer temperatures.

Temperatures ‘fall into place’

Data for the spread of the water molecules in the individual annual layers in the Greenland ice cores has thus made it possible to calculate the temperature in the layers with compressed snow 19,000 years back in time.

“What we discovered was that the previous temperature curve, which was only based on the measurements of the oxygen isotope O18, was inaccurate. The oxygen temperature curve said that the climate in central Greenland was colder around 12,000 years ago than around 15,000 years ago, despite the fact that two key climate drivers – carbon dioxide in the atmosphere and solar radiation – would suggest the opposite. With our new, more direct reconstruction, we have been able to show that the climate in central Greenland was actually warmer around 12,000 years ago compared to 15,000 years ago. So the temperatures actually follow the solar radiation and the amount of carbon dioxide in the atmosphere. We estimate that the temperature difference was 2-6 degrees,” says Bo Vinther, Associate Professor at the Centre for Ice and Climate at the Niels Bohr Institute, University of Copenhagen.

Has the puzzle of rapid climate change in the last ice age been solved?

During the cold stadial periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong northwest winds drove the Arctic sea ice southward, even as far as the French coast. Since the extended ice cover over the North Atlantic prevented the exchange of heat between the atmosphere and the ocean, the strong driving forces for the ocean currents that prevail today were lacking. Ocean circulation, which is a powerful 'conveyor belt' in the world's oceans, was thus much weaker than at present, and consequently transported less heat to northern regions. -  Map: Alfred-Wegener-Institut
During the cold stadial periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong northwest winds drove the Arctic sea ice southward, even as far as the French coast. Since the extended ice cover over the North Atlantic prevented the exchange of heat between the atmosphere and the ocean, the strong driving forces for the ocean currents that prevail today were lacking. Ocean circulation, which is a powerful ‘conveyor belt’ in the world’s oceans, was thus much weaker than at present, and consequently transported less heat to northern regions. – Map: Alfred-Wegener-Institut

During the last ice age a large part of North America was covered with a massive ice sheet up to 3km thick. The water stored in this ice sheet is part of the reason why the sea level was then about 120 meters lower than today. Young Chinese scientist Xu Zhang, lead author of the study who undertook his PhD at the Alfred Wegener Institute, explains. “The rapid climate changes known in the scientific world as Dansgaard-Oeschger events were limited to a period of time from 110,000 to 23,000 years before present. The abrupt climate changes did not take place at the extreme low sea levels, corresponding to the time of maximum glaciation 20,000 years ago, nor at high sea levels such as those prevailing today – they occurred during periods of intermediate ice volume and intermediate sea levels.” The results presented by the AWI researchers can explain the history of climate changes during glacial periods, comparing simulated model data with that retrieved from ice cores and marine sediments.

How rapid temperature changes might have occurred during times when the Northern Hemisphere ice sheets were at intermediate sizes (see schematic depictions on http://bit.ly/1uQoI70).

During the cold stadial periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong westerly winds drove the Arctic sea ice southward, even as far as the French coast. Since the extended ice cover over the North Atlantic prevented the exchange of heat between the atmosphere and the ocean, the strong driving forces for the ocean currents that prevail today were lacking. Ocean circulation, which is a powerful “conveyor belt” in the world’s oceans, was thus much weaker than at present, and consequently transported less heat to northern regions.

During the extended cold phases the ice sheets continued to thicken. When higher ice sheets prevailed over North America, typical in periods of intermediate sea levels, the prevailing westerly winds split into two branches. The major wind field ran to the north of the so-called Laurentide Ice Sheet and ensured that the sea ice boundary off the European coast shifted to the north. Ice-free seas permit heat exchange to take place between the atmosphere and the ocean. At the same time, the southern branch of the northwesterly winds drove warmer water into the ice-free areas of the northeast Atlantic and thus amplified the transportation of heat to the north. The modified conditions stimulated enhanced circulation in the ocean. Consequently, a thicker Laurentide Ice Sheet over North America resulted in increased ocean circulation and therefore greater transportation of heat to the north. The climate in the Northern Hemisphere became dramatically warmer within a few decades until, due to the retreat of the glaciers over North America and the renewed change in wind conditions, it began to cool off again.

“Using the simulations performed with our climate model, we were able to demonstrate that the climate system can respond to small changes with abrupt climate swings,” explains Professor Gerrit Lohmann, leader of the Paleoclimate Dynamics group at the Alfred Wegener Institute, Germany. In doing so he illustrates the new study’s significance with regards to contemporary climate change. “At medium sea levels, powerful forces, such as the dramatic acceleration of polar ice cap melting, are not necessary to result in abrupt climate shifts and associated drastic temperature changes.”

At present, the extent of Arctic sea ice is far less than during the last glacial period. The Laurentide Ice Sheet, the major driving force for ocean circulation during the glacials, has also disappeared. Climate changes following the pattern of the last ice age are therefore not to be anticipated under today’s conditions.

“There are apparently some situations in which the climate system is more resistant to change while in others the system tends toward strong fluctuations,” summarises Gerrit Lohmann. “In terms of the Earth’s history, we are currently in one of the climate system’s more stable phases. The preconditions, which gave rise to rapid temperature changes during the last ice age do not exist today. But this does not mean that sudden climate changes can be excluded in the future.”

Has the puzzle of rapid climate change in the last ice age been solved?

During the cold stadial periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong northwest winds drove the Arctic sea ice southward, even as far as the French coast. Since the extended ice cover over the North Atlantic prevented the exchange of heat between the atmosphere and the ocean, the strong driving forces for the ocean currents that prevail today were lacking. Ocean circulation, which is a powerful 'conveyor belt' in the world's oceans, was thus much weaker than at present, and consequently transported less heat to northern regions. -  Map: Alfred-Wegener-Institut
During the cold stadial periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong northwest winds drove the Arctic sea ice southward, even as far as the French coast. Since the extended ice cover over the North Atlantic prevented the exchange of heat between the atmosphere and the ocean, the strong driving forces for the ocean currents that prevail today were lacking. Ocean circulation, which is a powerful ‘conveyor belt’ in the world’s oceans, was thus much weaker than at present, and consequently transported less heat to northern regions. – Map: Alfred-Wegener-Institut

During the last ice age a large part of North America was covered with a massive ice sheet up to 3km thick. The water stored in this ice sheet is part of the reason why the sea level was then about 120 meters lower than today. Young Chinese scientist Xu Zhang, lead author of the study who undertook his PhD at the Alfred Wegener Institute, explains. “The rapid climate changes known in the scientific world as Dansgaard-Oeschger events were limited to a period of time from 110,000 to 23,000 years before present. The abrupt climate changes did not take place at the extreme low sea levels, corresponding to the time of maximum glaciation 20,000 years ago, nor at high sea levels such as those prevailing today – they occurred during periods of intermediate ice volume and intermediate sea levels.” The results presented by the AWI researchers can explain the history of climate changes during glacial periods, comparing simulated model data with that retrieved from ice cores and marine sediments.

How rapid temperature changes might have occurred during times when the Northern Hemisphere ice sheets were at intermediate sizes (see schematic depictions on http://bit.ly/1uQoI70).

During the cold stadial periods of the last ice age, massive ice sheets covered northern parts of North America and Europe. Strong westerly winds drove the Arctic sea ice southward, even as far as the French coast. Since the extended ice cover over the North Atlantic prevented the exchange of heat between the atmosphere and the ocean, the strong driving forces for the ocean currents that prevail today were lacking. Ocean circulation, which is a powerful “conveyor belt” in the world’s oceans, was thus much weaker than at present, and consequently transported less heat to northern regions.

During the extended cold phases the ice sheets continued to thicken. When higher ice sheets prevailed over North America, typical in periods of intermediate sea levels, the prevailing westerly winds split into two branches. The major wind field ran to the north of the so-called Laurentide Ice Sheet and ensured that the sea ice boundary off the European coast shifted to the north. Ice-free seas permit heat exchange to take place between the atmosphere and the ocean. At the same time, the southern branch of the northwesterly winds drove warmer water into the ice-free areas of the northeast Atlantic and thus amplified the transportation of heat to the north. The modified conditions stimulated enhanced circulation in the ocean. Consequently, a thicker Laurentide Ice Sheet over North America resulted in increased ocean circulation and therefore greater transportation of heat to the north. The climate in the Northern Hemisphere became dramatically warmer within a few decades until, due to the retreat of the glaciers over North America and the renewed change in wind conditions, it began to cool off again.

“Using the simulations performed with our climate model, we were able to demonstrate that the climate system can respond to small changes with abrupt climate swings,” explains Professor Gerrit Lohmann, leader of the Paleoclimate Dynamics group at the Alfred Wegener Institute, Germany. In doing so he illustrates the new study’s significance with regards to contemporary climate change. “At medium sea levels, powerful forces, such as the dramatic acceleration of polar ice cap melting, are not necessary to result in abrupt climate shifts and associated drastic temperature changes.”

At present, the extent of Arctic sea ice is far less than during the last glacial period. The Laurentide Ice Sheet, the major driving force for ocean circulation during the glacials, has also disappeared. Climate changes following the pattern of the last ice age are therefore not to be anticipated under today’s conditions.

“There are apparently some situations in which the climate system is more resistant to change while in others the system tends toward strong fluctuations,” summarises Gerrit Lohmann. “In terms of the Earth’s history, we are currently in one of the climate system’s more stable phases. The preconditions, which gave rise to rapid temperature changes during the last ice age do not exist today. But this does not mean that sudden climate changes can be excluded in the future.”

Rewriting the history of volcanic forcing during the past 2,000 years

Locations of Antarctic ice core sites used for volcanic sulfate aerosol deposition reconstruction (right); a  DRI scientist examines a freshly drilled ice core in the field before ice cores are analyzed in DRI's ultra-trace ice core analytical laboratory. -  M. Sigl
Locations of Antarctic ice core sites used for volcanic sulfate aerosol deposition reconstruction (right); a DRI scientist examines a freshly drilled ice core in the field before ice cores are analyzed in DRI’s ultra-trace ice core analytical laboratory. – M. Sigl

A team of scientists led by Michael Sigl and Joe McConnell of Nevada’s Desert Research Institute (DRI) has completed the most accurate and precise reconstruction to date of historic volcanic sulfate emissions in the Southern Hemisphere.

The new record, described in a manuscript published today in the online edition of Nature Climate Change, is derived from a large number of individual ice cores collected at various locations across Antarctica and is the first annually resolved record extending through the Common Era (the last 2,000 years of human history).

“This record provides the basis for a dramatic improvement in existing reconstructions of volcanic emissions during recent centuries and millennia,” said the report’s lead author Michael Sigl, a postdoctoral fellow and specialist in DRI’s unique ultra-trace ice core analytical laboratory, located on the Institute’s campus in Reno, Nevada.

These reconstructions are critical to accurate model simulations used to assess past natural and anthropogenic climate forcing. Such model simulations underpin environmental policy decisions including those aimed at regulating greenhouse gas and aerosol emissions to mitigate projected global warming.

Powerful volcanic eruptions are one of the most significant causes of climate variability in the past because of the large amounts of sulfur dioxide they emit, leading to formation of microscopic particles known as volcanic sulfate aerosols. These aerosols reflect more of the sun’s radiation back to space, cooling the Earth. Past volcanic events are measured through sulfate deposition records found in ice cores and have been linked to short-term global and regional cooling.

This effort brought together the most extensive array of ice core sulfate data in the world, including the West Antarctic Ice Sheet (WAIS) Divide ice core – arguably the most detailed record of volcanic sulfate in the Southern Hemisphere. In total, the study incorporated 26 precisely synchronized ice core records collected in an array of 19 sites from across Antarctica.

“This work is the culmination of more than a decade of collaborative ice core collection and analysis in our lab here at DRI,” said Joe McConnell, a DRI research professor who developed the continuous-flow analysis system used to analyze the ice cores.

McConnell, a member of several research teams that collected the cores (including the 2007-2009 Norwegian-American Scientific Traverse of East Antarctica and the WAIS Divide project that reached a depth of 3,405 meters in 2011), added, “The new record identifies 116 individual volcanic events during the last 2000 years.”

“Our new record completes the period from years 1 to 500 AD, for which there were no reconstructions previously, and significantly improves the record for years 500 to 1500 AD,” Sigl added. This new record also builds on DRI’s previous work as part of the international Past Global Changes (PAGES) effort to help reconstruct an accurate 2,000-year-long global temperature for individual continents.

This study involved collaborating researchers from the United States, Japan, Germany, Norway, Australia, and Italy. International collaborators contributed ice core samples for analysis at DRI as well as ice core measurements and climate modeling.

According to Yuko Motizuki from RIKEN (Japan’s largest comprehensive research institution), “The collaboration between DRI, National Institute of Polar Research (NIPR), and RIKEN just started in the last year, and we were very happy to be able to use the two newly obtained ice core records taken from Dome Fuji, where the volcanic signals are clearly visible. This is because precipitation on the site mainly contains stratospheric components.” Dr. Motizuki analyzed the samples collected by the Japanese Antarctic Research Expedition.

Simulations of volcanic sulfate transport performed with a coupled aerosol-climate model were compared to the ice core observations and used to investigate spatial patterns of sulfate deposition to Antarctica.

“Both observations and model results show that not all eruptions lead to the same spatial pattern of sulfate deposition,” said Matthew Toohey from the German institute GEOMAR Helmholtz Centre for Ocean Research Kiel. He added, “Spatial variability in sulfate deposition means that the accuracy of volcanic sulfate reconstructions depends strongly on having a sufficient number of ice core records from as many different regions of Antarctica as possible.”

With such an accurately synchronized and robust array, Sigl and his colleagues were able to revise reconstructions of past volcanic aerosol loading that are widely used today in climate model simulations. Most notably, the research found that the two largest volcanic eruptions in recent Earth history (Samalas in 1257 and Kuwae in 1458) deposited 30 to 35 percent less sulfate in Antarctica, suggesting that these events had a weaker cooling effect on global climate than previously thought.

Rewriting the history of volcanic forcing during the past 2,000 years

Locations of Antarctic ice core sites used for volcanic sulfate aerosol deposition reconstruction (right); a  DRI scientist examines a freshly drilled ice core in the field before ice cores are analyzed in DRI's ultra-trace ice core analytical laboratory. -  M. Sigl
Locations of Antarctic ice core sites used for volcanic sulfate aerosol deposition reconstruction (right); a DRI scientist examines a freshly drilled ice core in the field before ice cores are analyzed in DRI’s ultra-trace ice core analytical laboratory. – M. Sigl

A team of scientists led by Michael Sigl and Joe McConnell of Nevada’s Desert Research Institute (DRI) has completed the most accurate and precise reconstruction to date of historic volcanic sulfate emissions in the Southern Hemisphere.

The new record, described in a manuscript published today in the online edition of Nature Climate Change, is derived from a large number of individual ice cores collected at various locations across Antarctica and is the first annually resolved record extending through the Common Era (the last 2,000 years of human history).

“This record provides the basis for a dramatic improvement in existing reconstructions of volcanic emissions during recent centuries and millennia,” said the report’s lead author Michael Sigl, a postdoctoral fellow and specialist in DRI’s unique ultra-trace ice core analytical laboratory, located on the Institute’s campus in Reno, Nevada.

These reconstructions are critical to accurate model simulations used to assess past natural and anthropogenic climate forcing. Such model simulations underpin environmental policy decisions including those aimed at regulating greenhouse gas and aerosol emissions to mitigate projected global warming.

Powerful volcanic eruptions are one of the most significant causes of climate variability in the past because of the large amounts of sulfur dioxide they emit, leading to formation of microscopic particles known as volcanic sulfate aerosols. These aerosols reflect more of the sun’s radiation back to space, cooling the Earth. Past volcanic events are measured through sulfate deposition records found in ice cores and have been linked to short-term global and regional cooling.

This effort brought together the most extensive array of ice core sulfate data in the world, including the West Antarctic Ice Sheet (WAIS) Divide ice core – arguably the most detailed record of volcanic sulfate in the Southern Hemisphere. In total, the study incorporated 26 precisely synchronized ice core records collected in an array of 19 sites from across Antarctica.

“This work is the culmination of more than a decade of collaborative ice core collection and analysis in our lab here at DRI,” said Joe McConnell, a DRI research professor who developed the continuous-flow analysis system used to analyze the ice cores.

McConnell, a member of several research teams that collected the cores (including the 2007-2009 Norwegian-American Scientific Traverse of East Antarctica and the WAIS Divide project that reached a depth of 3,405 meters in 2011), added, “The new record identifies 116 individual volcanic events during the last 2000 years.”

“Our new record completes the period from years 1 to 500 AD, for which there were no reconstructions previously, and significantly improves the record for years 500 to 1500 AD,” Sigl added. This new record also builds on DRI’s previous work as part of the international Past Global Changes (PAGES) effort to help reconstruct an accurate 2,000-year-long global temperature for individual continents.

This study involved collaborating researchers from the United States, Japan, Germany, Norway, Australia, and Italy. International collaborators contributed ice core samples for analysis at DRI as well as ice core measurements and climate modeling.

According to Yuko Motizuki from RIKEN (Japan’s largest comprehensive research institution), “The collaboration between DRI, National Institute of Polar Research (NIPR), and RIKEN just started in the last year, and we were very happy to be able to use the two newly obtained ice core records taken from Dome Fuji, where the volcanic signals are clearly visible. This is because precipitation on the site mainly contains stratospheric components.” Dr. Motizuki analyzed the samples collected by the Japanese Antarctic Research Expedition.

Simulations of volcanic sulfate transport performed with a coupled aerosol-climate model were compared to the ice core observations and used to investigate spatial patterns of sulfate deposition to Antarctica.

“Both observations and model results show that not all eruptions lead to the same spatial pattern of sulfate deposition,” said Matthew Toohey from the German institute GEOMAR Helmholtz Centre for Ocean Research Kiel. He added, “Spatial variability in sulfate deposition means that the accuracy of volcanic sulfate reconstructions depends strongly on having a sufficient number of ice core records from as many different regions of Antarctica as possible.”

With such an accurately synchronized and robust array, Sigl and his colleagues were able to revise reconstructions of past volcanic aerosol loading that are widely used today in climate model simulations. Most notably, the research found that the two largest volcanic eruptions in recent Earth history (Samalas in 1257 and Kuwae in 1458) deposited 30 to 35 percent less sulfate in Antarctica, suggesting that these events had a weaker cooling effect on global climate than previously thought.

Solving the puzzle of ice age climates

The paleoclimate record for the last ice age – a time 21,000 years ago called the “Last Glacial Maximum” (LGM) – tells of a cold Earth whose northern continents were covered by vast ice sheets. Chemical traces from plankton fossils in deep-sea sediments reveal rearranged ocean water masses, as well as extended sea ice coverage off Antarctica. Air bubbles in ice cores show that carbon dioxide in the atmosphere was far below levels seen before the Industrial Revolution.

While ice ages are set into motion by Earth’s slow wobbles in its transit around the sun, researchers agree that the solar-energy decrease alone wasn’t enough to cause this glacial state. Paleoclimatologists have been trying to explain the actual mechanism behind these changes for 200 years.

“We have all these scattered pieces of information about changes in the ocean, atmosphere, and ice cover,” says Raffaele Ferrari, the Breene M. Kerr Professor of Physical Oceanography in MIT’s Department of Earth, Atmospheric and Planetary Sciences, “and what we really want to see is how they all fit together.”

Researchers have always suspected that the answer must lie somewhere in the oceans. Powerful regulators of Earth’s climate, the oceans store vast amounts of organic carbon for thousands of years, keeping it from escaping into the atmosphere as CO2. Seawater also takes up CO2 from the atmosphere via photosynthesizing microbes at the surface, and via circulation patterns.

In a new application of ocean physics, Ferrari, along with Malte Jansen PhD ’12 of Princeton University and others at the California Institute of Technology, have found a new approach to the puzzle, which they detail in this week’s Proceedings of the National Academy of Sciences.

Lung of the ocean


The researchers focused on the Southern Ocean, which encircles Antarctica – a critical part of the carbon cycle because it provides a connection between the atmosphere and the deep ocean abyss. Ruffled by the winds whipping around Antarctica, the Southern Ocean is one of the only places where the deepest carbon-rich waters ever rise to the surface, to “breathe” CO2 in and out.

The modern-day Southern Ocean has a lot of room to breathe: Deeper, carbon-rich waters are constantly mixing into the waters above, a process enhanced by turbulence as water runs over jagged, deep-ocean ridges.

But during the LGM, permanent sea ice covered much more of the Southern Ocean’s surface. Ferrari and colleagues decided to explore how that extended sea ice would have affected the Southern Ocean’s ability to exchange CO2 with the atmosphere.

Shock to the system


This question demanded the use of the field’s accumulated knowledge of ocean physics. Using a mathematical equation that describes the wind-driven ocean circulation patterns around Antarctica, the researchers calculated the amount of water that was trapped under the sea ice by currents in the LGM. They found that the shock to the entire Earth from this added ice cover was massive: The ice covered the only spot where the deep ocean ever got to breathe. Since the sea ice capped these deep waters, the Southern Ocean’s CO2 was never exhaled to the atmosphere.

The researchers then saw a link between the sea ice change and the massive rearrangement of ocean waters that is evident in the paleoclimate record. Under the expanded sea ice, a greater amount of upwelled deep water sank back downward. Southern Ocean abyssal water eventually filled a greater volume of the entire midlevel and lower ocean – lifting the interface between upper and lower waters to a shallower depth, such that the deep, carbon-rich waters lost contact with the upper ocean. Breathing less, the ocean could store a lot more carbon.

A Southern Ocean suffocated by sea ice, the researchers say, helps explain the big drop in atmospheric CO2 during the LGM.

Dependent relationship


The study suggests a dynamic link between sea-ice expansion and the increase of ocean water insulated from the atmosphere, which the field has long treated as independent events. This insight takes on extra relevance in light of the fact that paleoclimatologists need to explain not just the very low levels of atmospheric CO2 during the last ice age, but also the fact that this happened during each of the last four glacial periods, as the paleoclimate record reveals.

Ferrari says that it never made sense to argue that independent changes drew down CO2 by the exact same amount in every ice age. “To me, that means that all the events that co-occurred must be incredibly tightly linked, without much freedom to drift beyond a narrow margin,” he says. “If there is a causality effect among the events at the start of an ice age, then they could happen in the same ratio.”

Solving the puzzle of ice age climates

The paleoclimate record for the last ice age – a time 21,000 years ago called the “Last Glacial Maximum” (LGM) – tells of a cold Earth whose northern continents were covered by vast ice sheets. Chemical traces from plankton fossils in deep-sea sediments reveal rearranged ocean water masses, as well as extended sea ice coverage off Antarctica. Air bubbles in ice cores show that carbon dioxide in the atmosphere was far below levels seen before the Industrial Revolution.

While ice ages are set into motion by Earth’s slow wobbles in its transit around the sun, researchers agree that the solar-energy decrease alone wasn’t enough to cause this glacial state. Paleoclimatologists have been trying to explain the actual mechanism behind these changes for 200 years.

“We have all these scattered pieces of information about changes in the ocean, atmosphere, and ice cover,” says Raffaele Ferrari, the Breene M. Kerr Professor of Physical Oceanography in MIT’s Department of Earth, Atmospheric and Planetary Sciences, “and what we really want to see is how they all fit together.”

Researchers have always suspected that the answer must lie somewhere in the oceans. Powerful regulators of Earth’s climate, the oceans store vast amounts of organic carbon for thousands of years, keeping it from escaping into the atmosphere as CO2. Seawater also takes up CO2 from the atmosphere via photosynthesizing microbes at the surface, and via circulation patterns.

In a new application of ocean physics, Ferrari, along with Malte Jansen PhD ’12 of Princeton University and others at the California Institute of Technology, have found a new approach to the puzzle, which they detail in this week’s Proceedings of the National Academy of Sciences.

Lung of the ocean


The researchers focused on the Southern Ocean, which encircles Antarctica – a critical part of the carbon cycle because it provides a connection between the atmosphere and the deep ocean abyss. Ruffled by the winds whipping around Antarctica, the Southern Ocean is one of the only places where the deepest carbon-rich waters ever rise to the surface, to “breathe” CO2 in and out.

The modern-day Southern Ocean has a lot of room to breathe: Deeper, carbon-rich waters are constantly mixing into the waters above, a process enhanced by turbulence as water runs over jagged, deep-ocean ridges.

But during the LGM, permanent sea ice covered much more of the Southern Ocean’s surface. Ferrari and colleagues decided to explore how that extended sea ice would have affected the Southern Ocean’s ability to exchange CO2 with the atmosphere.

Shock to the system


This question demanded the use of the field’s accumulated knowledge of ocean physics. Using a mathematical equation that describes the wind-driven ocean circulation patterns around Antarctica, the researchers calculated the amount of water that was trapped under the sea ice by currents in the LGM. They found that the shock to the entire Earth from this added ice cover was massive: The ice covered the only spot where the deep ocean ever got to breathe. Since the sea ice capped these deep waters, the Southern Ocean’s CO2 was never exhaled to the atmosphere.

The researchers then saw a link between the sea ice change and the massive rearrangement of ocean waters that is evident in the paleoclimate record. Under the expanded sea ice, a greater amount of upwelled deep water sank back downward. Southern Ocean abyssal water eventually filled a greater volume of the entire midlevel and lower ocean – lifting the interface between upper and lower waters to a shallower depth, such that the deep, carbon-rich waters lost contact with the upper ocean. Breathing less, the ocean could store a lot more carbon.

A Southern Ocean suffocated by sea ice, the researchers say, helps explain the big drop in atmospheric CO2 during the LGM.

Dependent relationship


The study suggests a dynamic link between sea-ice expansion and the increase of ocean water insulated from the atmosphere, which the field has long treated as independent events. This insight takes on extra relevance in light of the fact that paleoclimatologists need to explain not just the very low levels of atmospheric CO2 during the last ice age, but also the fact that this happened during each of the last four glacial periods, as the paleoclimate record reveals.

Ferrari says that it never made sense to argue that independent changes drew down CO2 by the exact same amount in every ice age. “To me, that means that all the events that co-occurred must be incredibly tightly linked, without much freedom to drift beyond a narrow margin,” he says. “If there is a causality effect among the events at the start of an ice age, then they could happen in the same ratio.”