̽��ֱ�� of Cambridge - Luke Skinner

New insights into the dynamics of past climate change

sc604 — Wed, 14 Oct 2015 07:05:16 +0000

A new study of the relationship between ocean currents and climate change has found that they are tightly linked, and that changes in the polar regions can affect the ocean and climate on the opposite side of the world within one to two hundred years, far quicker than previously thought.

̽��ֱ��study, by an international team of scientists led by the ̽��ֱ�� of Cambridge, examined how changes in ocean currents in the Atlantic Ocean were related to climate conditions in the northern hemisphere during the last ice age, by examining data from ice cores and fossilised plankton shells. It found that variations in ocean currents and abrupt climate events in the North Atlantic region were tightly linked in the past, and that changes in the polar regions affected the ocean circulation and climate on the opposite side of the world.

̽��ֱ��researchers determined that as large amounts of fresh water were emptied into the North Atlantic as icebergs broke off the North American and Eurasian ice sheets, the deep and shallow currents in the North Atlantic rapidly slowed down, which led to the formation of sea ice around Greenland and the subsequent cooling of the Northern Hemisphere. It also strongly affected conditions in the South Atlantic within a matter of one to two hundred years. ̽��ֱ��results, published in the journal Nature Geoscience, show how climate events in the Northern Hemisphere were tightly coupled with changes in the strength of deep ocean currents in the Atlantic Ocean, and how that may have affected conditions across the globe.

During the last ice age, which took place from 70,000 to 19,000 years ago, the climate in the Northern Hemisphere toggled back and forth between warm and cold states roughly every 1000 to 6000 years. These events, known as Dansgaard-Oeschger events, were first identified in data from Greenland ice cores in the early 1990s, and had far-reaching impacts on the global climate.

̽��ֱ��ocean, which covers 70% of the planet, is a huge reservoir of carbon dioxide and heat. It stores about 60 times more carbon than the atmosphere, and can release or take up carbon on both short and long timescales. As changes happen in the polar regions, they are carried around the world by ocean currents, both at the surface and in the deep ocean. These currents are driven by winds, ocean temperature and salinity differences, and are efficient at distributing heat and carbon around the globe. Ocean currents therefore have a strong influence on whether regions of the world are warm (such as Europe) or whether they are not (such as Antarctica) as they modulate the effects of solar radiation. They also influence whether CO2 is stored in the ocean or the atmosphere, which is very important for global climate variability.

“Other studies have shown that the overturning circulation in the Atlantic has faced a slowdown during the last few decades,” said Dr Julia Gottschalk of Cambridge Department of Earth Sciences, the paper's lead author. “ ̽��ֱ��scientific community is only beginning to understand what it would mean for global climate should this trend continue, as predicted by some climate models.”

Analysing new data from marine sediment cores taken from the deep South Atlantic, between the southern tip of South America and the southern tip of Africa, the researchers discovered that during the last ice age, deep ocean currents in the South Atlantic varied essentially in unison with Greenland ice-core temperatures. “This implies that a very rapid transmission process must have operated, that linked rapid climate change around Greenland with the otherwise sluggish deep Atlantic Ocean circulation,” said Gottschalk, who is a Gates Cambridge Scholar. Best estimates of the delay between these two records suggest that the transmission happened within about 100 to 200 years.

Digging through metres of ocean mud from depths of 3,800 metres, the team studied the dissolution of fossil plankton shells that was closely linked to the chemical signature of different water masses. Water masses originating in the North Atlantic are less corrosive than water masses from the South Atlantic.

“Periods of very intense North Atlantic circulation and higher Northern Hemisphere temperatures increased the preservation of microfossils in the sediment cores, whereas those with slower circulation, when the study site was primarily influenced from the south, were linked with decreased carbonate ion concentrations at our core site which led to partial dissolution,” said co-author Dr Luke Skinner, also from Cambridge's Department of Earth Sciences.

To better understand the physical mechanisms of rapid ocean adjustment, the data was compared with a climate model simulation which covers the same period. “ ̽��ֱ��data of the model simulation was so close to the deep ocean sediment data, that we knew immediately, we were on the right track,” said co-author Dr Laurie Menviel from the ̽��ֱ�� of New South Wales, Australia, who conducted the model simulation.

̽��ֱ��timescales of these large-scale adjustments found in the palaeoceanographic data agree extremely well with those predicted by the model. “Waves between layers of different density in the deep ocean are responsible for quickly transmitting signals from North to South. This is a paradigm shift in our understanding of how the ocean works,” said Axel Timmermann, Professor of Oceanography at the ̽��ֱ�� of Hawaii.

Although conditions at the end of the last ice age were very different to those of today, the findings could shed light on how changing conditions in the polar regions may affect ocean currents. However, much more research is needed in this area. ̽��ֱ��study's findings could help test and improve climate models that are run for both past and future conditions.

̽��ֱ��sediment cores were recovered by Dr Claire Waelbroeck and colleagues aboard the French research vessel Marion Dufresne.

̽��ֱ��research was supported by the Gates Cambridge Trust, the Natural Environmental Research Council of the UK, the Royal Society, the European Research Council, the Australian Research Council and the National Science Foundation of the United States of America.

Reference:
Gottschalk, J et. al. Abrupt changes in the southern extent of North Atlantic Deep Water during Dansgaard-Oeschger events. Nature Geoscience (2015). DOI: 10.1038/ngeo2558

A new study finds that changing climate in the polar regions can affect conditions in the rest of the world far quicker than previously thought.

Other studies have shown that the overturning circulation in the Atlantic has faced a slowdown during the last few decades. ̽��ֱ��scientific community is only beginning to understand what it would mean for global climate should this trend continue, as predicted by some climate models

Julia Gottschalk

Left: Marine sediment core sample from the South Atlantic with fossilised partially dissolved shells of planktonic organisms. Right: Well-preserved plankton shells.

̽��ֱ��text in this work is licensed under a Creative Commons Attribution 4.0 International License. For image use please see separate credits above.

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Ice Age, interrupted

bjb42 — Mon, 09 Jan 2012 08:33:12 +0000

In terms of the ebb and flow of the Earth’s climate over the course of its history, the next Ice Age is starting to look overdue. Periods between recent Ice Ages, or ‘interglacials’, average out to be around 11 thousand years, and it’s currently been 11, 600 since the last multi-millennial winter. Although it is almost impossible to predict exactly when the next Ice Age will occur (if it will at all), it is clear that a global freeze is not on the horizon; the amount of CO₂ emitted by human activity and the enhanced greenhouse effect that results all but preclude it. But what if we weren’t around and CO2 was lower?

In a paper published in Nature Geoscience this week, new research proposes that the next Ice Age would have been kick-started sometime in the next thousand years, just round the corner in the context of the Earth’s lifespan, if CO₂ was sufficiently low.

By looking at the onset of abrupt flip-flops in the temperature contrast between Greenland and Antarctica (extreme climate behaviour that would have only been possible if vast and expanding ice sheets were disrupting ocean circulation), the researchers believe they have been able to identify the fingerprint of an Ice Age activation, or the ‘glacial inception’.

By applying this fingerprinting method to an interglacial period with nearly identical solar radiation, or ‘insolation’, to our own - some 780 thousand years ago - the researchers have been able to determine that glacial inception would indeed be expected to occur sometime soon.

“ ̽��ֱ��mystery of the Ice Ages, which represent the dominant mode of climate change over the past few million years, is that while we can identify the various ingredients that have contributed to them, it’s the arrangement of these ingredients, and how they march to the beat of subtle changes in seasonality, that we lack an understanding of,” says Dr Luke Skinner from the Department of Earth Sciences, who helped to conduct the research with Professor David Hodell and their colleague Professor Chronis Tzedakis from ̽��ֱ�� College London.

Insolation, the seasonal and latitudinal distribution of solar radiation energy, changes over tens of thousands of years due to the variations in the Earth’s orbit around the sun. It has long been apparent that insolation changes have acted as a pace-maker for the Ice Ages. But, like a metronome paces music, it sets the beat of climate change but not its every movement. ̽��ֱ��changing concentrations of greenhouse gases, CO₂ in particular, are evidently what determine when a shift in insolation will trigger climate change.

“From 8,000 years ago, as human civilization flourished, CO₂ reversed its initial downward trend and drifted upwards, accelerating sharply with the industrial revolution,” says Skinner. “Although the contribution of human activities to the pre-industrial drift in CO₂ remains debated, our work suggests that natural insolation will not be cancelling the impacts of man-made global warming.”

Research shows that a new Ice Age could well have been upon us in the next millennium were it not for increases in CO2 due to humans, despite the advantageous trend in solar radiation of our current age.

Our work suggests that natural insolation will not be cancelling the impacts of man-made global warming.

Dr Luke Skinner

Trey Ratcliff from Flickr

Deep into the Patagonia Glacier

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Taking the long view on climate change

bjb42 — Sat, 01 Sep 2007 15:53:42 +0000

̽��ֱ��Earth’s climate has always changed and no doubt always will. However, this alone does not tell us very much about the climate system: we need to be able to say exactly how and why climate can change. Today this requirement has been brought to the fore by the prospect of human-induced global climate change resulting largely from greenhouse gas emissions that arise from our massive and growing appetite for fossil fuels. Thousands of scientists are now striving to predict how a sharp rise in greenhouse gas concentrations will affect the entire climate system, including the ecosystems and societies that it supports. But how can we be sure about our theories of climate change, let alone our theories of ecosystem or market response? Just how important are greenhouse gases in controlling global climate? And what are the timescales and thresholds of climate adjustment? These are just some of the urgent questions that have been raised by the prospect of anthropogenic climate change.

To help answer such questions we can look to the past, at how the Earth’s climate evolved prior to the relative stability that human society has so far enjoyed. Researchers in the Department of Earth Sciences are taking up this challenge, using marine sediments as their lens into the past, and as a guide to the future.

Palaeoclimatology

̽��ֱ��study of past climate change – palaeoclimatology – aims to reconstruct what has happened in the past, in the oceans, on the land, in the atmosphere and in ecosystems, and to infer how the global climate system works ‘as a whole’. In the last 20 years of palaeoclimate research, three major questions have emerged that are particularly relevant to modern climate change. First, how did changes in solar radiation (insolation) and atmospheric carbon dioxide (CO2) conspire to trigger massive global climate upheavals such as the glacial–interglacial (‘ice-age’) climate cycles? Second, what regulates atmospheric CO2 concentrations under changing climatic conditions, and what roles can we ascribe to marine biological productivity or ocean circulation changes in particular? And third, how abruptly can regional climate change and with what repercussions for the rest of the world?

All of these questions are interconnected of course, although each bears on a different aspect of the climate system’s ability to pace and amplify climate perturbations through sensitive ‘feedback’ processes.

Past climate by proxy

A central aspect of palaeoclimate reconstructions is the ‘proxy’ character of our observations. Because scientists cannot measure past ocean temperatures directly, they must measure the impacts of past temperature changes instead, usually based on temperature-sensitive organisms or temperature-sensitive chemical constituents in their shells or skeletons.

As a palaeoceanographer, Dr Luke Skinner specifically makes use of marine sediments as a window into the past. Among the many advantages of using marine sediments are that they can be obtained from nearly two thirds of the Earth’s surface, they generally provide unbroken and often very high-resolution records of past conditions, and they contain a diversity of constituents that can be analysed, from tiny fossil shells to grains of sand dropped by passing icebergs.

To reconstruct past climate change, Dr Skinner collects and studies the fossil calcite shells of foraminifera – single-celled blobs of protoplasm – that have accumulated on the sea floor. Using the shells of these tiny creatures, Dr Skinner has been able to generate detailed records of temperature change, both at the sea surface and in the ocean interior. In combination with ice-rafted debris and oxygen- and carbon-isotope records, these reconstructions have helped to demonstrate that the North Atlantic region experienced very intense and abrupt climate swings in the past, involving massive glacier surges as well as drastic changes in the deep ocean circulation system and the Gulf Stream. It has also been possible to show that these same changes in the Atlantic Ocean’s circulation were accompanied by a ‘see-saw’ in temperatures across the hemispheres, with heat pooling in the South to the extent that it was not efficiently delivered to the North. Based on records such as these it is now clear that global change can be heterogeneous and can occur too suddenly to be presaged by obvious warnings.

Perspectives on the future

Although it is clear that no previous climate period can really serve as a blueprint for the future, important lessons can still be learned from the study of the past. One important example is the use of palaeoclimate data to guide the improvement of our climate simulation models. Because numerical and statistical models provide our only means for predicting future climate, it is imperative that they be as general as possible. Studies like those described here are helping to achieve this, by revealing the feedbacks, thresholds and characteristic timescales for climate adjustment, across a wide range of climatic contexts.

In the future, global CO2 levels will only be stabilised if we either drastically cut our emissions or identify, trigger or create a process that ‘mops up’ exactly as much CO2 as millions of consumers are able to produce each day (the basis of carbon capture). ̽��ֱ��history of climate change tells us that we are going to need as many one-way fluxes out of the atmosphere as we can muster if are going to compete with the ‘leak’ we have created in the Earth’s largest standing carbon reservoir, the solid Earth. We have much to learn about the climate system, both for our own sake and for the sake of knowledge itself.

For more information, please contact the author Dr Luke Skinner (luke00@esc.cam.ac.uk) at the Department of Earth Sciences.

Cambridge Earth Scientists are contributing to our understanding of the climate system by studying the history of climate change recorded in sediments deposited on the sea floor.

Studies like those described here are helping to achieve this, by revealing the feedbacks, thresholds and characteristic timescales for climate adjustment, across a wide range of climatic contexts.

Irargerich from Flickr

Melting is your destiny

Carbon capture

Whenever fossil fuel (coal, oil or gas) is burnt, carbon is released as CO2 into the atmosphere, where it traps the Sun's heat. Can we counteract this build-up by capturing and storing CO2? Any solution would require storage of many millions of tonnes reliably and possibly for up to 10,000 years. Compressing and injecting CO2 into deep geological formations could provide the answer. ̽��ֱ��presence of oil, gas and natural CO2 trapped in reservoirs underground for millions of years demonstrates that storage of CO2 is feasible. At the Sleipner Oil Field in the Norwegian sector of the North Sea, CO2 is already being separated from natural gas and re-injected at about 1 km depth below the sea surface. ̽��ֱ��CO2 rises through the sandy earth before spreading out below a series of thin mudstones beneath the thick overlying mudstone. A collaborative research project between Professor Mike Bickle in the Department of Earth Sciences and Professor Herbert Huppert in the Institute of Theoretical Geophysics has been modelling the spread of these accumulations to work out how much CO2 is trapped and to understand the flow of CO2 in the reservoir. A particular challenge is to predict the behaviour of the stored CO2 over time to determine the safety of long-term CO2 storage in this way. ̽��ֱ��benefits are clear, as Professor Bickle explains: 'CO2 storage is a feasible, politically achievable and relatively inexpensive way for dealing with the problem of increasing atmospheric CO2 levels. For more information, please contact Professor Mike Bickle (mb72@esc.cam.ac.uk) at the Department of Earth Sciences.

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