A satellite view of volcanoes finds the link between ground deformation and eruption

ESA’s Sentinel satellite, due for launch on April 3rd, should allow scientists to test this link in greater detail and eventually develop a forecast system for all volcanoes, including those that are remote and inaccessible.

Volcano deformation and, in particular, uplift are often considered to be caused by magma moving or pressurizing underground. Magma rising towards the surface could be a sign of an imminent eruption. On the other hand, many other factors influence volcano deformation and, even if magma is rising, it may stop short, rather than erupting.

Dr Juliet Biggs and colleagues in the School of Earth Sciences, with collaborators from Cornell, Oxford and Southern Methodist University, looked at the archive of satellite data covering over 500 volcanoes worldwide, many of which have been systematically observed for over 18 years. Satellite radar (InSAR) can provide high-resolution maps of deformation, allowing the detection of unrest at many volcanoes that might otherwise go unrecognised. Such satellite data is often the only source of information for remote or inaccessible volcanoes.

The researchers applied statistical methods more traditionally used for medical diagnostic testing and found that many deforming volcanoes also erupted (46 per cent). Together with the very high proportion of non-deforming volcanoes that did not erupt (94 per cent), these jointly represent a strong indicator of a volcano’s long-term eruptive potential.

Dr Biggs said: “The findings suggest that satellite radar is the perfect tool to identify volcanic unrest on a regional or global scale and target ground-based monitoring.”

The work was co-funded by the UK Centre for Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET) and STREVA, a research consortium aimed at finding ways to reduce the negative consequences of volcanic activity on people and their assets.

“Improving how we anticipate activity using new technology such as this is an important first step in doing better at forecasting and preparing for volcanic eruptions,” said STREVA Principal Investigator, Dr Jenni Barclay.

Co-author Professor Willy Aspinall added: “Global studies of volcano deformation using satellite data will increasingly play a part in assessing eruption potential at more and more volcanoes, especially in regions with short historical records or limited conventional monitoring.”

However, many factors and processes, some observable, but others not, influence deformation to a greater or lesser extent. These include the type of rock that forms the volcano, its tectonic characteristics and the supply rate and storage depth of magma beneath it. Thus, deformation can have different implications for different types of volcanoes. For volcanoes with short eruption cycles, the satellite record typically spans episodes that include both deformation and eruption, resulting in a high correlation between the two. For volcanoes with long eruption cycles, the satellite record tends to capture either deformation or eruption but rarely both.

In the past, radar images of the majority of the world’s volcanoes were only acquired a few times a year, but seismological data indicate that the duration of unrest before an eruption might be as short as only a few days.

Dr Biggs said: “This study demonstrates what can be achieved with global satellite coverage even with limited acquisitions, so we are looking forward to the step-change in data quantity planned for the next generation of satellites.”

The European Space Agency is planning to launch its next radar mission, Sentinel-1 in early April. This mission is designed for global monitoring and will collect images every six to twelve days. Using this, scientists should be able to test the causal and temporal relationship with deformation on much shorter timescales.

Professor Tim Wright, Director of COMET, added: “This study is particularly exciting because Sentinel-1 will soon give us systematic observations of the ups and downs of every volcano on the planet. For many places, particularly in developing countries, these data could provide the only warning of an impending eruption.”

A satellite view of volcanoes finds the link between ground deformation and eruption

ESA’s Sentinel satellite, due for launch on April 3rd, should allow scientists to test this link in greater detail and eventually develop a forecast system for all volcanoes, including those that are remote and inaccessible.

Volcano deformation and, in particular, uplift are often considered to be caused by magma moving or pressurizing underground. Magma rising towards the surface could be a sign of an imminent eruption. On the other hand, many other factors influence volcano deformation and, even if magma is rising, it may stop short, rather than erupting.

Dr Juliet Biggs and colleagues in the School of Earth Sciences, with collaborators from Cornell, Oxford and Southern Methodist University, looked at the archive of satellite data covering over 500 volcanoes worldwide, many of which have been systematically observed for over 18 years. Satellite radar (InSAR) can provide high-resolution maps of deformation, allowing the detection of unrest at many volcanoes that might otherwise go unrecognised. Such satellite data is often the only source of information for remote or inaccessible volcanoes.

The researchers applied statistical methods more traditionally used for medical diagnostic testing and found that many deforming volcanoes also erupted (46 per cent). Together with the very high proportion of non-deforming volcanoes that did not erupt (94 per cent), these jointly represent a strong indicator of a volcano’s long-term eruptive potential.

Dr Biggs said: “The findings suggest that satellite radar is the perfect tool to identify volcanic unrest on a regional or global scale and target ground-based monitoring.”

The work was co-funded by the UK Centre for Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET) and STREVA, a research consortium aimed at finding ways to reduce the negative consequences of volcanic activity on people and their assets.

“Improving how we anticipate activity using new technology such as this is an important first step in doing better at forecasting and preparing for volcanic eruptions,” said STREVA Principal Investigator, Dr Jenni Barclay.

Co-author Professor Willy Aspinall added: “Global studies of volcano deformation using satellite data will increasingly play a part in assessing eruption potential at more and more volcanoes, especially in regions with short historical records or limited conventional monitoring.”

However, many factors and processes, some observable, but others not, influence deformation to a greater or lesser extent. These include the type of rock that forms the volcano, its tectonic characteristics and the supply rate and storage depth of magma beneath it. Thus, deformation can have different implications for different types of volcanoes. For volcanoes with short eruption cycles, the satellite record typically spans episodes that include both deformation and eruption, resulting in a high correlation between the two. For volcanoes with long eruption cycles, the satellite record tends to capture either deformation or eruption but rarely both.

In the past, radar images of the majority of the world’s volcanoes were only acquired a few times a year, but seismological data indicate that the duration of unrest before an eruption might be as short as only a few days.

Dr Biggs said: “This study demonstrates what can be achieved with global satellite coverage even with limited acquisitions, so we are looking forward to the step-change in data quantity planned for the next generation of satellites.”

The European Space Agency is planning to launch its next radar mission, Sentinel-1 in early April. This mission is designed for global monitoring and will collect images every six to twelve days. Using this, scientists should be able to test the causal and temporal relationship with deformation on much shorter timescales.

Professor Tim Wright, Director of COMET, added: “This study is particularly exciting because Sentinel-1 will soon give us systematic observations of the ups and downs of every volcano on the planet. For many places, particularly in developing countries, these data could provide the only warning of an impending eruption.”

Hot mantle drives elevation, volcanism along mid-ocean ridges

Scientists have found that temperature deep in Earth's mantle controls the expression of mid-ocean ridges, mountain ranges that line the ocean floor. Higher mantle temperatures are associated with higher elevations. The findings help scientists understand how mantle temperature influences the contours of Earth's crust. -  Dalton Lab / Brown University
Scientists have found that temperature deep in Earth’s mantle controls the expression of mid-ocean ridges, mountain ranges that line the ocean floor. Higher mantle temperatures are associated with higher elevations. The findings help scientists understand how mantle temperature influences the contours of Earth’s crust. – Dalton Lab / Brown University

Scientists have shown that temperature differences deep within Earth’s mantle control the elevation and volcanic activity along mid-ocean ridges, the colossal mountain ranges that line the ocean floor. The findings, published April 4 in the journal Science, shed new light on how temperature in the depths of the mantle influences the contours of the Earth’s crust.

Mid-ocean ridges form at the boundaries between tectonic plates, circling the globe like seams on a baseball. As the plates move apart, magma from deep within the Earth rises up to fill the void, creating fresh crust as it cools. The crust formed at these seams is thicker in some places than others, resulting in ridges with widely varying elevations. In some places, the peaks are submerged miles below the ocean surface. In other places – Iceland, for example – the ridge tops are exposed above the water’s surface.

“These variations in ridge depth require an explanation,” said Colleen Dalton, assistant professor of geological sciences at Brown and lead author of the new research. “Something is keeping them either sitting high or sitting low.”

That something, the study found, is the temperature of rocks deep below Earth’s surface.

By analyzing the speeds of seismic waves generated by earthquakes, the researchers show that mantle temperature along the ridges at depths extending below 400 kilometers varies by as much as 250 degrees Celsius. High points on the ridges tend to be associated with higher mantle temperatures, while low points are associated with a cooler mantle. The study also showed that volcanic hot spots along the ridge – volcanoes near Iceland as well as the islands of Ascension, Tristan da Cunha, and elsewhere – all sit above warm spots in Earth’s mantle.

“It is clear from our results that what’s being erupted at the ridges is controlled by temperature deep in the mantle,” Dalton said. “It resolves a long-standing controversy and has not been shown definitively before.”

A CAT scan of the Earth


The mid-ocean ridges provide geologists with a window to the interior of the Earth. The ridges form when mantle material melts, rises into the cracks between tectonic plates, and solidifies again. The characteristics of the ridges provide clues about the properties of the mantle below.

For example, a higher ridge elevation suggests a thicker crust, which in turn suggests that a larger volume of magma was erupted at the surface. This excess molten rock can be caused by very hot temperatures in the mantle. The problem is that hot mantle is not the only way to produce excess magma. The chemical composition of the rocks in Earth’s mantle also controls how much melt is produced. For certain rock compositions, it is possible to generate large volumes of molten rock under cooler conditions. For many decades it has not been clear whether mid-ocean ridge elevations are caused by variations in the temperature of the mantle or variations in the rock composition of the mantle.

To distinguish between these two possibilities, Dalton and her colleagues introduced two additional data sets. One was the chemistry of basalts, the rock that forms from solidification of magma at the mid-ocean ridge. The chemical composition of basalts differs depending upon the temperature and composition of the mantle material from which they’re derived. The authors analyzed the chemistry of nearly 17,000 basalts formed along mid-ocean ridges around the globe.

The other data set was seismic wave tomography. During earthquakes, seismic waves are sent pulsing through the rocks in the crust and mantle. By measuring the velocity of those waves, scientists can gather data about the characteristics of the rocks through which they traveled. “It’s like performing a CAT scan of the inside of the Earth,” Dalton said.

Seismic wave speeds are especially sensitive to the temperature of rocks. In general, waves propagate more quickly in cooler rocks and more slowly in hotter rocks.

Dalton and her colleagues combined the seismic data from hundreds of earthquakes with data on elevation and rock chemistry from the ridges. Correlations among the three data sets revealed that temperature deep in the mantle varied between around 1,300 and 1,550 degrees Celsius underneath about 61,000 kilometers of ridge terrain. “It turned out,” said Dalton, “that seismic tomography was the smoking gun. The only plausible explanation for the seismic wave speeds is a very large temperature range.”

The study showed that as ridge elevation falls, so does mantle temperature. The coolest point beneath the ridges was found near the lowest point, an area of very deep and rugged seafloor known as the Australian-Antarctic discordance in the Indian Ocean. The hottest spot was near Iceland, which is also the ridges’ highest elevation point.

Iceland is also where scientists have long debated whether a mantle plume – a vertical jet of hot rock originating from deep in the Earth – intersects the mid-ocean ridge. This study provides strong support for a mantle plume located beneath Iceland. In fact, this study showed that all regions with above-average temperature are located near volcanic hot spots, which points to mantle plumes as the culprit for the excess volume of magma in these areas.

Understanding a churning planet


Despite being made of solid rock, Earth’s mantle doesn’t sit still. It undergoes convection, a slow churning of material from the depths of the Earth toward the surface and back again.

“Convection is why we have plate tectonics and earthquakes,” Dalton said. “It’s also responsible for almost all volcanism at the surface. So understanding mantle convection is crucial to understanding many fundamental questions about the Earth.”

Two factors influence how that convection works: variations in the composition of the mantle and variations in its temperature. This work, says Dalton, points to temperature as a primary factor in how convection is expressed on the surface.

“We get consistent and coherent temperature measurements from the mantle from three independent datasets,” Dalton said. “All of them suggest that what we see at the surface is due to temperature, and that composition is only a secondary factor. What is surprising is that the data require the temperature variations to exist not only near the surface but also many hundreds of kilometers deep inside the Earth.”

The findings from this study will also be useful in future research using seismic waves, Dalton says. Because the temperature readings as indicated by seismology were backed up by the other datasets, they can be used to calibrate seismic readings for places where geochemical samples aren’t available. This makes it possible to estimate temperature deep in Earth’s mantle all over the globe.

That will help geologists gain a new insights into how processes deep within the Earth mold the ground beneath our feet.

Hot mantle drives elevation, volcanism along mid-ocean ridges

Scientists have found that temperature deep in Earth's mantle controls the expression of mid-ocean ridges, mountain ranges that line the ocean floor. Higher mantle temperatures are associated with higher elevations. The findings help scientists understand how mantle temperature influences the contours of Earth's crust. -  Dalton Lab / Brown University
Scientists have found that temperature deep in Earth’s mantle controls the expression of mid-ocean ridges, mountain ranges that line the ocean floor. Higher mantle temperatures are associated with higher elevations. The findings help scientists understand how mantle temperature influences the contours of Earth’s crust. – Dalton Lab / Brown University

Scientists have shown that temperature differences deep within Earth’s mantle control the elevation and volcanic activity along mid-ocean ridges, the colossal mountain ranges that line the ocean floor. The findings, published April 4 in the journal Science, shed new light on how temperature in the depths of the mantle influences the contours of the Earth’s crust.

Mid-ocean ridges form at the boundaries between tectonic plates, circling the globe like seams on a baseball. As the plates move apart, magma from deep within the Earth rises up to fill the void, creating fresh crust as it cools. The crust formed at these seams is thicker in some places than others, resulting in ridges with widely varying elevations. In some places, the peaks are submerged miles below the ocean surface. In other places – Iceland, for example – the ridge tops are exposed above the water’s surface.

“These variations in ridge depth require an explanation,” said Colleen Dalton, assistant professor of geological sciences at Brown and lead author of the new research. “Something is keeping them either sitting high or sitting low.”

That something, the study found, is the temperature of rocks deep below Earth’s surface.

By analyzing the speeds of seismic waves generated by earthquakes, the researchers show that mantle temperature along the ridges at depths extending below 400 kilometers varies by as much as 250 degrees Celsius. High points on the ridges tend to be associated with higher mantle temperatures, while low points are associated with a cooler mantle. The study also showed that volcanic hot spots along the ridge – volcanoes near Iceland as well as the islands of Ascension, Tristan da Cunha, and elsewhere – all sit above warm spots in Earth’s mantle.

“It is clear from our results that what’s being erupted at the ridges is controlled by temperature deep in the mantle,” Dalton said. “It resolves a long-standing controversy and has not been shown definitively before.”

A CAT scan of the Earth


The mid-ocean ridges provide geologists with a window to the interior of the Earth. The ridges form when mantle material melts, rises into the cracks between tectonic plates, and solidifies again. The characteristics of the ridges provide clues about the properties of the mantle below.

For example, a higher ridge elevation suggests a thicker crust, which in turn suggests that a larger volume of magma was erupted at the surface. This excess molten rock can be caused by very hot temperatures in the mantle. The problem is that hot mantle is not the only way to produce excess magma. The chemical composition of the rocks in Earth’s mantle also controls how much melt is produced. For certain rock compositions, it is possible to generate large volumes of molten rock under cooler conditions. For many decades it has not been clear whether mid-ocean ridge elevations are caused by variations in the temperature of the mantle or variations in the rock composition of the mantle.

To distinguish between these two possibilities, Dalton and her colleagues introduced two additional data sets. One was the chemistry of basalts, the rock that forms from solidification of magma at the mid-ocean ridge. The chemical composition of basalts differs depending upon the temperature and composition of the mantle material from which they’re derived. The authors analyzed the chemistry of nearly 17,000 basalts formed along mid-ocean ridges around the globe.

The other data set was seismic wave tomography. During earthquakes, seismic waves are sent pulsing through the rocks in the crust and mantle. By measuring the velocity of those waves, scientists can gather data about the characteristics of the rocks through which they traveled. “It’s like performing a CAT scan of the inside of the Earth,” Dalton said.

Seismic wave speeds are especially sensitive to the temperature of rocks. In general, waves propagate more quickly in cooler rocks and more slowly in hotter rocks.

Dalton and her colleagues combined the seismic data from hundreds of earthquakes with data on elevation and rock chemistry from the ridges. Correlations among the three data sets revealed that temperature deep in the mantle varied between around 1,300 and 1,550 degrees Celsius underneath about 61,000 kilometers of ridge terrain. “It turned out,” said Dalton, “that seismic tomography was the smoking gun. The only plausible explanation for the seismic wave speeds is a very large temperature range.”

The study showed that as ridge elevation falls, so does mantle temperature. The coolest point beneath the ridges was found near the lowest point, an area of very deep and rugged seafloor known as the Australian-Antarctic discordance in the Indian Ocean. The hottest spot was near Iceland, which is also the ridges’ highest elevation point.

Iceland is also where scientists have long debated whether a mantle plume – a vertical jet of hot rock originating from deep in the Earth – intersects the mid-ocean ridge. This study provides strong support for a mantle plume located beneath Iceland. In fact, this study showed that all regions with above-average temperature are located near volcanic hot spots, which points to mantle plumes as the culprit for the excess volume of magma in these areas.

Understanding a churning planet


Despite being made of solid rock, Earth’s mantle doesn’t sit still. It undergoes convection, a slow churning of material from the depths of the Earth toward the surface and back again.

“Convection is why we have plate tectonics and earthquakes,” Dalton said. “It’s also responsible for almost all volcanism at the surface. So understanding mantle convection is crucial to understanding many fundamental questions about the Earth.”

Two factors influence how that convection works: variations in the composition of the mantle and variations in its temperature. This work, says Dalton, points to temperature as a primary factor in how convection is expressed on the surface.

“We get consistent and coherent temperature measurements from the mantle from three independent datasets,” Dalton said. “All of them suggest that what we see at the surface is due to temperature, and that composition is only a secondary factor. What is surprising is that the data require the temperature variations to exist not only near the surface but also many hundreds of kilometers deep inside the Earth.”

The findings from this study will also be useful in future research using seismic waves, Dalton says. Because the temperature readings as indicated by seismology were backed up by the other datasets, they can be used to calibrate seismic readings for places where geochemical samples aren’t available. This makes it possible to estimate temperature deep in Earth’s mantle all over the globe.

That will help geologists gain a new insights into how processes deep within the Earth mold the ground beneath our feet.

Magnetic anomaly deep within Earth’s crust reveals Africa in North America

Boulder, Colo., USA – The repeated cycles of plate tectonics that have led to collision and assembly of large supercontinents and their breakup and formation of new ocean basins have produced continents that are collages of bits and pieces of other continents. Figuring out the origin and make-up of continental crust formed and modified by these tectonic events is a vital to understanding Earth’s geology and is important for many applied fields, such as oil, gas, and gold exploration.

In many cases, the rocks involved in these collision and pull-apart episodes are still buried deep beneath the Earth’s surface, so geologists must use geophysical measurements to study these features.

This new study by Elias Parker Jr. of the University of Georgia examines a prominent swath of lower-than-normal magnetism — known as the Brunswick Magnetic Anomaly — that stretches from Alabama through Georgia and off shore to the North Carolina coast.

The cause of this magnetic anomaly has been under some debate. Many geologists attribute the Brunswick Magnetic Anomaly to a belt of 200 million year old volcanic rocks that intruded around the time the Atlantic Ocean. In this case, the location of this magnetic anomaly would then mark the initial location where North America split from the rest of Pangea as that ancient supercontinent broke apart. Parker proposes a different source for this anomalous magnetic zone.

Drawing upon other studies that have demonstrated deeply buried metamorphic rocks can also have a coherent magnetic signal, Parker has analyzed the detailed characteristics of the magnetic anomalies from data collected across zones in Georgia and concludes that the Brunswick Magnetic Anomaly has a similar, deeply buried source. The anomalous magnetic signal is consistent with an older tectonic event — the Alleghanian orogeny that formed the Alleghany-Appalachian Mountains when the supercontinent of Pangea was assembled.

Parker’s main conclusion is that the rocks responsible for the Brunswick Magnetic Anomaly mark a major fault-zone that formed as portions of Africa and North America were sheared together roughly 300 million years ago — and that more extensive evidence for this collision are preserved along this zone. One interesting implication is that perhaps a larger portion of what is now Africa was left behind in the American southeast when Pangea later broke up.</P

Magnetic anomaly deep within Earth’s crust reveals Africa in North America

Boulder, Colo., USA – The repeated cycles of plate tectonics that have led to collision and assembly of large supercontinents and their breakup and formation of new ocean basins have produced continents that are collages of bits and pieces of other continents. Figuring out the origin and make-up of continental crust formed and modified by these tectonic events is a vital to understanding Earth’s geology and is important for many applied fields, such as oil, gas, and gold exploration.

In many cases, the rocks involved in these collision and pull-apart episodes are still buried deep beneath the Earth’s surface, so geologists must use geophysical measurements to study these features.

This new study by Elias Parker Jr. of the University of Georgia examines a prominent swath of lower-than-normal magnetism — known as the Brunswick Magnetic Anomaly — that stretches from Alabama through Georgia and off shore to the North Carolina coast.

The cause of this magnetic anomaly has been under some debate. Many geologists attribute the Brunswick Magnetic Anomaly to a belt of 200 million year old volcanic rocks that intruded around the time the Atlantic Ocean. In this case, the location of this magnetic anomaly would then mark the initial location where North America split from the rest of Pangea as that ancient supercontinent broke apart. Parker proposes a different source for this anomalous magnetic zone.

Drawing upon other studies that have demonstrated deeply buried metamorphic rocks can also have a coherent magnetic signal, Parker has analyzed the detailed characteristics of the magnetic anomalies from data collected across zones in Georgia and concludes that the Brunswick Magnetic Anomaly has a similar, deeply buried source. The anomalous magnetic signal is consistent with an older tectonic event — the Alleghanian orogeny that formed the Alleghany-Appalachian Mountains when the supercontinent of Pangea was assembled.

Parker’s main conclusion is that the rocks responsible for the Brunswick Magnetic Anomaly mark a major fault-zone that formed as portions of Africa and North America were sheared together roughly 300 million years ago — and that more extensive evidence for this collision are preserved along this zone. One interesting implication is that perhaps a larger portion of what is now Africa was left behind in the American southeast when Pangea later broke up.</P

The Atlantic Ocean dances with the sun and volcanoes

Imagine a ballroom in which two dancers apparently keep in time to their own individual rhythm. The two partners suddenly find themselves moving to the same rhythm and, after a closer look, it is clear to see which one is leading.

It was an image like this that researchers at Aarhus University were able to see when they compared studies of solar energy release and volcanic activity during the last 450 years, with reconstructions of ocean temperature fluctuations during the same period.

The results actually showed that during the last approximately 250 years – since the period known as the Little Ice Age – a clear correlation can be seen where the external forces, i.e. the Sun’s energy cycle and the impact of volcanic eruptions, are accompanied by a corresponding temperature fluctuation with a time lag of about five years.

In the previous two centuries, i.e. during the Little Ice Age, the link was not as strong, and the temperature of the Atlantic Ocean appears to have followed its own rhythm to a greater extent.

The results were recently published in the scientific journal Nature Communications.

In addition to filling in yet another piece of the puzzle associated with understanding the complex interaction of the natural forces that control the climate, the Danish researchers paved the way for linking the two competing interpretations of the origin of the oscillation phenomenon.

Temperature fluctuations discovered around the turn of the millennium

The climate is defined on the basis of data including mean temperature values recorded over a period of thirty years. Northern Europe thus has a warm and humid climate compared with other regions on the same latitudes. This is due to the North Atlantic Drift (often referred to as the Gulf Stream), an ocean current that transports relatively warm water from the south-west part of the North Atlantic to the sea off the coast of Northern Europe.

Around the turn of the millennium, however, climate researchers became aware that the average temperature of the Atlantic Ocean was not entirely stable, but actually fluctuated at the same rate throughout the North Atlantic. This phenomenon is called the Atlantic Multidecadal Oscillation (AMO), which consists of relatively warm periods lasting thirty to forty years being replaced by cool periods of the same duration.

The researchers were able to read small systematic variations in the water temperature in the North Atlantic in measurements taken by ships during the last 140 years.

Although the temperature fluctuations are small – less than 1°C – there is a general consensus among climate researchers that the AMO phenomenon has had a major impact on the climate in the area around the North Atlantic for thousands of years, but until now there has been doubt about what could cause this slow rhythm in the temperature of the Atlantic Ocean. One model explains the phenomenon as internal variability in the ocean circulation – somewhat like a bathtub sloshing water around in its own rhythm. Another model explains the AMO as being driven by fluctuations in the amount of solar energy received by the Earth, and as being affected by small changes in the energy radiated by the Sun itself and the after-effects of volcanic eruptions. Both these factors are also known as ‘external forces’ that have an impact on the Earth’s radiation balance.

However, there has been considerable scepticism towards the idea that a phenomenon such as an AMO could be driven by external forces at all – a scepticism that the Aarhus researchers now demonstrate as unfounded.

“Our new investigations clearly show that, since the Little Ice Age, there has been a correlation between the known external forces and the temperature fluctuations in the ocean that help control our climate. At the same time, however, the results also show that this can’t be the only driving force behind the AMO, and the explanation must therefore be found in a complex interaction between a number of mechanisms. It should also be pointed out that these fluctuations occur on the basis of evenly increasing ocean temperatures during the last approximately fifty years – an increase connected with global warming,” says Associate Professor Mads Faurschou Knudsen, Department of Geoscience, Aarhus University, who is the main author of the article.

Convincing data from the Earth’s own archives

Researchers have attempted to make computer simulations of the phenomenon ever since the discovery of the AMO, partly to enable a better understanding of the underlying mechanism. However, it is difficult for the computer models to reproduce the actual AMO signal that can be read in the temperature data from the last 140 years.

Associate Professor Knudsen and his colleagues instead combined all available data from the Earth’s own archives, i.e. previous studies of items such as radioactive isotopes and volcanic ash in ice cores. This provides information about solar energy release and volcanic activity during the last 450 years, and the researchers compared the data with reconstructions of the AMO’s temperature rhythm during the same period.

“We’ve only got direct measurements of the Atlantic Ocean temperature for the last 140 years, where it was measured by ships. But how do you measure the water temperature further back in time? Studies of growth rings in trees from the entire North Atlantic region come into the picture here, where ‘good’ and ‘bad’ growth conditions are calibrated to the actual measurements, and the growth rings from trees along the coasts further back in time can therefore act as reserve thermometers,” explains Associate Professor Knudsen.

The results provide a new and very important perspective on the AMO phenomenon because they are based on data and not computer models, which are inherently incomplete. The problem is that the models do not completely describe all the physical correlations and feedbacks in the system, partly because these are not fully understood. And when the models are thus unable to reproduce the actual AMO signal, it is hard to know whether they have captured the essence of the AMO phenomenon.

Impact of the sun and volcanoes

An attempt to simply explain how external forces such as the Sun and volcanoes can control the climate could sound like this: a stronger Sun heats up the ocean, while the ash from volcanic eruptions shields the Sun and cools down the ocean. However, it is hardly as simple as that.

“Fluctuations in ocean temperature have a time lag of about five years in relation to the peaks we can read in the external forces. However, the direct effect of major volcanic eruptions is clearly seen as early as the same year in the mean global atmospheric temperature, i.e. a much shorter delay. The effect we studied is more complex, and it takes time for this effect to spread to the ocean currents,” explains Associate Professor Knudsen.

“An interesting new theory among solar researchers and meteorologists is that the Sun can control climate variations via the very large variations in UV radiation, which are partly seen in connection with changes in sunspot activity during the Sun’s eleven-year cycle. UV radiation heats the stratosphere in particular via increased production of ozone, which can have an impact on wind systems and thereby indirectly on the global ocean currents as well,” says Associate Professor Knudsen. However, he emphasises that researchers have not yet completely understood how a development in the stratosphere can affect the ocean currents on Earth.

Towards a better understanding of the climate

“In our previous study of the climate in the North Atlantic region during the last 8,000 years, we were able to show that the temperature of the Atlantic Ocean was presumably not controlled by the Sun’s activity. Here the temperature fluctuated in its own rhythm for long intervals, with warm and cold periods lasting 25 years. The prevailing pattern was that this climate fluctuation in the ocean was approximately 30󈞔% faster than the fluctuation we’d previously observed in solar activity, which lasted about ninety years. What we can now see is that the Atlantic Ocean would like to – or possibly even prefer to – dance alone. However, under certain circumstances, the external forces interrupt the ocean’s own rhythm and take over the lead, which has been the case during the last 250 years,” says Associate Professor Bo Holm Jacobsen, Department of Geoscience, Aarhus University, who is the co-author of the article.

“It’ll be interesting to see how long the Atlantic Ocean allows itself to be led in this dance. The scientific challenge partly lies in understanding the overall conditions under which the AMO phenomenon is sensitive to fluctuations in solar activity and volcanic eruptions,” he continues.

“During the last century, the AMO has had a strong bearing on significant weather phenomena such as hurricane frequency and droughts – with considerable economic and human consequences. A better understanding of this phenomenon is therefore an important step for efforts to deal with and mitigate the impact of climate variations,” Associate Professor Knudsen concludes.

The Atlantic Ocean dances with the sun and volcanoes

Imagine a ballroom in which two dancers apparently keep in time to their own individual rhythm. The two partners suddenly find themselves moving to the same rhythm and, after a closer look, it is clear to see which one is leading.

It was an image like this that researchers at Aarhus University were able to see when they compared studies of solar energy release and volcanic activity during the last 450 years, with reconstructions of ocean temperature fluctuations during the same period.

The results actually showed that during the last approximately 250 years – since the period known as the Little Ice Age – a clear correlation can be seen where the external forces, i.e. the Sun’s energy cycle and the impact of volcanic eruptions, are accompanied by a corresponding temperature fluctuation with a time lag of about five years.

In the previous two centuries, i.e. during the Little Ice Age, the link was not as strong, and the temperature of the Atlantic Ocean appears to have followed its own rhythm to a greater extent.

The results were recently published in the scientific journal Nature Communications.

In addition to filling in yet another piece of the puzzle associated with understanding the complex interaction of the natural forces that control the climate, the Danish researchers paved the way for linking the two competing interpretations of the origin of the oscillation phenomenon.

Temperature fluctuations discovered around the turn of the millennium

The climate is defined on the basis of data including mean temperature values recorded over a period of thirty years. Northern Europe thus has a warm and humid climate compared with other regions on the same latitudes. This is due to the North Atlantic Drift (often referred to as the Gulf Stream), an ocean current that transports relatively warm water from the south-west part of the North Atlantic to the sea off the coast of Northern Europe.

Around the turn of the millennium, however, climate researchers became aware that the average temperature of the Atlantic Ocean was not entirely stable, but actually fluctuated at the same rate throughout the North Atlantic. This phenomenon is called the Atlantic Multidecadal Oscillation (AMO), which consists of relatively warm periods lasting thirty to forty years being replaced by cool periods of the same duration.

The researchers were able to read small systematic variations in the water temperature in the North Atlantic in measurements taken by ships during the last 140 years.

Although the temperature fluctuations are small – less than 1°C – there is a general consensus among climate researchers that the AMO phenomenon has had a major impact on the climate in the area around the North Atlantic for thousands of years, but until now there has been doubt about what could cause this slow rhythm in the temperature of the Atlantic Ocean. One model explains the phenomenon as internal variability in the ocean circulation – somewhat like a bathtub sloshing water around in its own rhythm. Another model explains the AMO as being driven by fluctuations in the amount of solar energy received by the Earth, and as being affected by small changes in the energy radiated by the Sun itself and the after-effects of volcanic eruptions. Both these factors are also known as ‘external forces’ that have an impact on the Earth’s radiation balance.

However, there has been considerable scepticism towards the idea that a phenomenon such as an AMO could be driven by external forces at all – a scepticism that the Aarhus researchers now demonstrate as unfounded.

“Our new investigations clearly show that, since the Little Ice Age, there has been a correlation between the known external forces and the temperature fluctuations in the ocean that help control our climate. At the same time, however, the results also show that this can’t be the only driving force behind the AMO, and the explanation must therefore be found in a complex interaction between a number of mechanisms. It should also be pointed out that these fluctuations occur on the basis of evenly increasing ocean temperatures during the last approximately fifty years – an increase connected with global warming,” says Associate Professor Mads Faurschou Knudsen, Department of Geoscience, Aarhus University, who is the main author of the article.

Convincing data from the Earth’s own archives

Researchers have attempted to make computer simulations of the phenomenon ever since the discovery of the AMO, partly to enable a better understanding of the underlying mechanism. However, it is difficult for the computer models to reproduce the actual AMO signal that can be read in the temperature data from the last 140 years.

Associate Professor Knudsen and his colleagues instead combined all available data from the Earth’s own archives, i.e. previous studies of items such as radioactive isotopes and volcanic ash in ice cores. This provides information about solar energy release and volcanic activity during the last 450 years, and the researchers compared the data with reconstructions of the AMO’s temperature rhythm during the same period.

“We’ve only got direct measurements of the Atlantic Ocean temperature for the last 140 years, where it was measured by ships. But how do you measure the water temperature further back in time? Studies of growth rings in trees from the entire North Atlantic region come into the picture here, where ‘good’ and ‘bad’ growth conditions are calibrated to the actual measurements, and the growth rings from trees along the coasts further back in time can therefore act as reserve thermometers,” explains Associate Professor Knudsen.

The results provide a new and very important perspective on the AMO phenomenon because they are based on data and not computer models, which are inherently incomplete. The problem is that the models do not completely describe all the physical correlations and feedbacks in the system, partly because these are not fully understood. And when the models are thus unable to reproduce the actual AMO signal, it is hard to know whether they have captured the essence of the AMO phenomenon.

Impact of the sun and volcanoes

An attempt to simply explain how external forces such as the Sun and volcanoes can control the climate could sound like this: a stronger Sun heats up the ocean, while the ash from volcanic eruptions shields the Sun and cools down the ocean. However, it is hardly as simple as that.

“Fluctuations in ocean temperature have a time lag of about five years in relation to the peaks we can read in the external forces. However, the direct effect of major volcanic eruptions is clearly seen as early as the same year in the mean global atmospheric temperature, i.e. a much shorter delay. The effect we studied is more complex, and it takes time for this effect to spread to the ocean currents,” explains Associate Professor Knudsen.

“An interesting new theory among solar researchers and meteorologists is that the Sun can control climate variations via the very large variations in UV radiation, which are partly seen in connection with changes in sunspot activity during the Sun’s eleven-year cycle. UV radiation heats the stratosphere in particular via increased production of ozone, which can have an impact on wind systems and thereby indirectly on the global ocean currents as well,” says Associate Professor Knudsen. However, he emphasises that researchers have not yet completely understood how a development in the stratosphere can affect the ocean currents on Earth.

Towards a better understanding of the climate

“In our previous study of the climate in the North Atlantic region during the last 8,000 years, we were able to show that the temperature of the Atlantic Ocean was presumably not controlled by the Sun’s activity. Here the temperature fluctuated in its own rhythm for long intervals, with warm and cold periods lasting 25 years. The prevailing pattern was that this climate fluctuation in the ocean was approximately 30󈞔% faster than the fluctuation we’d previously observed in solar activity, which lasted about ninety years. What we can now see is that the Atlantic Ocean would like to – or possibly even prefer to – dance alone. However, under certain circumstances, the external forces interrupt the ocean’s own rhythm and take over the lead, which has been the case during the last 250 years,” says Associate Professor Bo Holm Jacobsen, Department of Geoscience, Aarhus University, who is the co-author of the article.

“It’ll be interesting to see how long the Atlantic Ocean allows itself to be led in this dance. The scientific challenge partly lies in understanding the overall conditions under which the AMO phenomenon is sensitive to fluctuations in solar activity and volcanic eruptions,” he continues.

“During the last century, the AMO has had a strong bearing on significant weather phenomena such as hurricane frequency and droughts – with considerable economic and human consequences. A better understanding of this phenomenon is therefore an important step for efforts to deal with and mitigate the impact of climate variations,” Associate Professor Knudsen concludes.