September 2008
Interpreting our past climatic conditions helps us to understand the current situation and provides a tool with which to predict the future.
The key to understanding many current situations, including enhanced global warming and their future implications, is being able to compare the past trends with today.
Recent research has revealed that CO2 levels in the atmosphere now stand at 387 parts per million (ppm), up almost 40% since the Industrial Revolution and the highest for at least the last 650 000 years (The Guardian 13 May 2008).
The only way we can compare the present with the past is through research into the earth’s geological history. One particular area of study has been into the history of the British Isles from about 2million years ago up to the present: the Quaternary Period. Looking at ice core data has provided an insight into the past climatic conditions and the trends of the Quaternary Period.
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Past environmental changes and implications for the future: Conference on the Quaternary Period of the British Isles, 8-10 January 2008
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The Quaternary
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Climate change: lessons from the past
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A2 practise question
Conference on the Quaternary Period of the British Isles, 8-10 January 2008
The Royal Geographical Society hosted the Annual Discussion Meeting of the Quaternary Research Association (QRA) from 8 – 10 January. The conference, entitled Quaternary of the British Isles and adjoining seas, attracted over 200 delegates to hear 34 lectures and to view about 70 poster presentations over three days. The topics covered were all broadly related to the environmental and climatic history of the British Isles from about 2 million years ago up to the present: the time span known to geologists as the Quaternary Period.
This period of geological time has long been associated with the ‘Ice Age’ when Britain (along with much of Europe and North America) was under the grip of ice. At their maximum during the Quaternary, ice sheets covered about three times the global area that they do today. However, research over recent decades has shown that the pattern of climate and environmental change over this time has been far more varied and complex than many people realise: while the Quaternary Period is a cold time compared with most of Earth’s geological history, it has also been characterised by dramatic fluctuations in climate and sea-level along with numerous episodes of glacier expansion and retreat.
It was evident at the conference that studying these changes is important, and not just for those interested in Earth’s past. Much research into the Quaternary has direct relevance for understanding how the Earth’s climate may change in the future - both naturally and due to the actions of humans. Study of the Quaternary Period also makes it possible to place recent environmental and climatic changes (e.g. in temperature, ice cover and sea-level) within a longer-term context. Several of the speakers offered insights into what the interpretation of past climates implies for Britain’s environment in the future.
Several presentations at the conference Quaternary of the British Isles and adjoining seas (8-10 January) addressed aspects of climate change:
Summary table of the QRA-RGS conference presentations related to climate change
Jim Rose (University of London, Royal Holloway) considered how our understanding of past climate has improved over the past 30 years. There has been a great expansion in the range of evidence used for reconstructing past climates; and our knowledge of the timing, rates, and magnitudes of climatic changes is much more precise. Recent research has also shown that climate has been more variable during our current interglacial (the Holocene) than previously believed.
Nick Golledge (British Geological Survey) discussed the complex relationships between climate change and glacier advance and retreat. He also referred to an important article from the Proceedings of the National Academy of Sciences (2007) by Firestone and co-authors discussing evidence for a comet-type object striking Earth about 12,900 years ago. In the article it is suggested that this extra-terrestrial impact in North America may have increased melting of the Laurentide ice sheet leading to freshening of the North Atlantic and the dramatic cooling at the onset of the Younger Dryas phase recorded in the Greenland ice cores.
Sune Rasmussen (University of Copenhagen) reviewed the latest evidence from the Greenland ice cores. He discussed how different ice cores have been correlated with each other to create a common timescale for the various data sets obtained from Greenland ice. He also discussed the next major ice coring initiative in Greenland: to obtain ice dating back to the previous interglacial over 115,000 years ago.
Edouard Bard (College de France, CEREGE) gave an account of the history of research into past climates, discussing the contributions of the French scientist Adhémar to the development of the ‘astronomical theory’. Most of his talk focused on the reconstruction of past sea-levels from fossil corals; however, he also considered the relationship between sea level change and climate change.
Dan Charman (University of Plymouth) addressed the nature of climatic changes in recent millennia. Over the past 30 years there has been a fundamental shift in the way we view the climate history of our current interglacial – the Holocene. Instead of being inherently ‘stable’ (with any change being slow and gradual), we now know that there have been many climate fluctuations on decade to century timescales. A key area of climate research is to better understand the magnitude of these changes (e.g. temperature, precipitation, seasonality) as well as their causes.
Russell Coope (University of London, Royal Holloway) discussed the different types of beetles that have been found in deposits dating back to various interglacials. During some interglacials, notably the one dating between 115,000 and 130,000 years ago (immediately before the last glacial phase) there were warm-loving beetles in Britain that cannot survive in Britain today. During this time the climate of Britain was warmer and global sea level was higher than today.
Keith Barber (University of Southampton) discussed the use of peat bog deposits for reconstructing past climate changes. This type of evidence has proven particularly useful for studying past precipitation and temperature changes that have occurred in Britain over recent millennia.
Danny McCarroll (University of Wales, Swansea) talked about the importance of assessing the quality of general circulation models used to predict future climate by testing their ability to reproduce climate changes that have happened in the past. In order to test climate models in the most useful way, efforts need to be focused on simulating changes that have occurred over the last thousand years. This is because the ‘climate boundary conditions’ have been similar to the present, and because our knowledge of past climate change over this time period is much more detailed and precise than for earlier times.
Mark Maslin (University College London) gave the final lecture of the conference. His theme was the importance of studying past climates for understanding how climate might change in the future and what this could mean for the British Isles. He drew some key inferences pertaining to the issue of climate change:
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The predictions based on computer models (general circulation models) are inherently conservative in the inbuilt assumptions and their estimates.
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Past climate records tell us that climate transitions are rarely, if ever, smooth.
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Understanding past climate changes is essential for putting the modern climate into context.
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There is still much we don’t understand about natural causes of climate change over century to millennium timescales.
While the future is uncertain, there is widespread consensus that human activity has caused warming over the past century and is continuing to alter the climate in ways that society will need to adapt to in the coming century.
This conference was organised to mark the 40th anniversary of the founding of the QRA and to provide a timely update on the state of knowledge and research in this area. The last conference to consider the British Quaternary record comprehensively was the 10th INQUA (International Union for Quaternary Research) Congress held in Birmingham in 1977, and since then, many important advances have been made. The theme of the conference was also chosen as the QRA’s contribution to the United Nations designated ‘International Year of Planet Earth’ 2008.
The following article sections look specifically at areas of sixth-form geography that are related to Quaternary studies and discuss how aspects of Quaternary science fit into various A-level topics.
The Quaternary is a subdivision of geological time approximating the last two million years or so of Earth’s history. It, in turn, is subdivided into two further time units: the Pleistocene and the Holocene.
In geological terminology, the Quaternary has the status of a ‘period’, and the Pleistocene and Holocene are known as ‘epochs’ or ‘series’. The classification of geological time works by naming and defining very long periods of time and by slicing up these long time units into shorter sub-units which are further divided into their own sub-units, and so on.
The main classifications in ascending hierarchy are ‘epoch’, ‘period’, ‘era’, and ‘eon’; and as the time line below shows, we are now living during the Holocene Epoch, of the Quaternary Period, of the Cenozoic Era, of the Phanerozoic Eon!
The term ‘Quaternary’ itself comes from a classification scheme dating back to the early 19th century in which the term was used to refer to the fourth, and most recent, span of geological time. While the terms ‘primary’ and ‘secondary’ are not used today in a geological context, the term ‘Tertiary’ has also survived from earlier classification schemes. In modern usage it refers to the period before the Quaternary, and together the Tertiary and Quaternary comprise the Cenozoic Era.
Attempts to subdivide or categorise aspects of the natural world often generate debate and disagreement because of differences in how scientists interpret the evidence; and the geological time scale is no exception. In fact, the status and definition of the Quaternary within the formalised geological time scale is a particularly contentious issue at the moment, with scientists holding different views about where to draw the lower boundary of the period, and even whether the Quaternary should keep its status as a ‘period’ or become a sub-unit within a much longer ‘Neogene Period’. While the finer points of these debates need not concern us here, it is worth summarising the current status quo and some of the broader issues that are being considered by the geological community at the moment.
It is widely accepted that the Quaternary represents a time of unusually cold and fluctuating climatic conditions in comparison with most other times in Earth’s past. The planet was significantly warmer on average during the Tertiary, and the dominant trend since about 50 million years ago has been one of cooling, with initiation of ice sheets in Antarctica about 38 million years ago and in the Northern Hemisphere about 3 million years ago. When viewed against hundreds of millions of years, the Quaternary can be seen as the latest time when the Earth has been in an ‘ice-house’, rather than ‘greenhouse’ state – meaning that a significant fraction of the world’s water is locked up in glaciers and ice sheets. (During ‘greenhouse’ times there was little to no permanent ice, the concentration of greenhouse gases in the atmosphere was higher, and temperatures were higher.) The last time glaciers were as extensive as they have been during the past 2 million years was during the Carboniferous and Permian periods extending up to about 250 million years ago.
Hence, the association of Quaternary time with the most recent ‘Ice Age’ is well established: the difficulty arises in establishing a date for when the Quaternary began. In 1984 at a meeting of the International Geological Conference it was decided to draw the lower boundary of the Pleistocene (and hence the boundary between the warmer Tertiary and colder Quaternary) at a point where there is an appearance of cold water mollusc and foraminifera species preserved in a section of marine sediments located at Vrica in Calabria, Italy. The transition in fossils seen in the sedimentary sequence at Vrica is dated to 1.81 million years ago; and, at the time of writing, this boundary is still the officially recognized ‘Global Stratotype Section and Point’ (GSSP) for the start of the Quaternary Period and Pleistocene Epoch.
While any attempt to define an exact time when climatic conditions changed enough to warrant recognition of a new geological period is bound to be subjective, arbitrary, and open to debate; there are, nonetheless, grounds for reviewing the status quo represented by the Vrica section. Recent research over the past couple decades, particularly on deep-sea sediment cores, suggests that a more significant change towards a colder global climate occurred between 3 and 2.5 million years ago, than at 1.8 million years ago, and a majority of scientists are of the opinion that marking the base of the Quaternary at 2.6 million years ago has more logic, and is more easily applied world-wide, than continuing with the 1.8 million year GSSP. The International Commission on Stratigraphy is currently reviewing the status and timing of the Quaternary within the geological time scale and is likely to come to a decision later this year. The International Union for Quaternary Research (INQUA) has made the case that the Quaternary should retain the status of a geological period, that its onset should be pushed back to 2.6 million years ago, and that the base of the Pleistocene Epoch should be extended with the base of the Quaternary so that the onset of the two chronological units remains time-equivalent.
Whatever is decided for the future, it is important for scientists to work with a common chronology for the Quaternary; not least because research into the environmental changes that have occurred during this time span is being seen as increasingly relevant for understanding how climates and environments may change in the future due to human activities. There are a range of issues studied by geographers that are being advanced greatly from a knowledge of Quaternary time scale processes: these include climate change, sea-level change, changes in ice sheets and glaciers, desertification and dust storms, and variation of ecosystems and biodiversity. Moreover, it is nigh on impossible to evaluate human impact on the environment fully without a knowledge of the degree of natural fluctuation that has occurred over recent geological time. The Quaternary Period is also of particular interest as the time span during which humans have evolved and spread across the Earth.
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The next section in this article looks at a selection of topics within A-level geography that have strong connections with research into the Quaternary Period, and in each one there are examples of how the latest Quaternary research findings are furthering our understanding of important environmental issues.
Studies of the changes in climate that have occurred during the Quaternary Period have contributed a great deal to our understanding of how the climate system works and the ways in which climate may respond to increases in greenhouse gases. Findings of particular importance include:
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The relationship between the concentration of greenhouse gases and air temperature extending back to 800,000 years ago
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The ways in which changes in ocean currents influence climate, both globally and regionally
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The existence of abrupt and large-scale climatic changes in the Earth’s recent geological past
These insights (which have come from the study of ice cores, marine sediments, and other types of deposits) are unlikely to have been reached from theoretical studies and climate modelling alone: in fact, the evidence of abrupt changes in the ice cores shows that climate can change much faster than anyone had previously believed possible.
Greenhouse gases and climate from the ice core record
Over the past few decades, several long cores of ice (between 3 and 4 km long) have been pulled up from the Greenland and Antarctic ice sheets to investigate long-term climatic change. As the glacier ice making up an ice sheet forms from successive layers of snowfall, by sampling and analyzing layers contained within an ice sheet it is possible to examine snowfall that fell in the past, as well as bubbles of ancient air that became trapped as the snow was compressed.
Such analyses can yield a remarkable amount of information. For instance, the ratios of oxygen isotopes (18O/16O) and hydrogen isotopes (2H/1H) making up the ice are directly related to the temperature of the air when the original snowfall formed. The ice can also be examined for concentrations of dust, volcanic ash and various chemicals – all giving evidence of what was in the atmosphere at various times in the past. The pockets of air trapped within the ice enable scientists to reconstruct the gaseous composition of the atmosphere in the past, notably the concentrations of carbon dioxide (CO2) and methane (CH4).
More remarkable still, this information can be plotted at a very high time resolution. From the Greenland ice cores it has been possible to count annual layers of ice extending from the present back to over 40,000 years ago, providing a precisely dated, year-by-year record of changes. Although it becomes increasingly difficult to identify annual layers in older ice, estimates have been made that extend the Greenland ice core record back to around 110,000 years ago.
Much of our knowledge about the changes in climate influencing Greenland over the past 100,000 years comes from the GRIP(Greenland Ice Core Project ) and GISP2 (Greenland Ice Sheet Project Two) cores (drilled from the summit area of central Greenland in the early 1990s) and the more recently drilled NorthGRIP core.
In Antarctica it has been possible to analyze much older ice. The EPICA project (European Project for Ice Coring in Antarctica) has extended the Antarctic record from the present back to 800,000 years ago. It is this record that is particularly useful for putting the current greenhouse gas concentration into a longer-term context. The key findings are:
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Current levels of CO2 (383 ppm) and CH4 in the atmosphere are higher than at any time in the last 800,000 years
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There is a close relationship between greenhouse gases and air temperature through time
The first point shows that we are entering uncharted territory as far as greenhouse gases are concerned. While levels of greenhouse gases have been much higher than now in the distant geological past (and the Earth’s climate much warmer), they are currently far above normal for the Quaternary Period. Levels of CO2 are typically around 200 ppm during glacial times and 280 ppm during interglacial times.
Figure 1 ice core data
The second point is not as easy to interpret: while elevated greenhouse gases and warm times go together, the extent to which greenhouse gases are driving the changes seen in the ice core record, or simply responding to warming caused by other factors, is less clear. Interestingly, the ice cores show that the temperature increase after each glacial cycle starts a little bit before the increases in CO2 and CH4. (The timing of the glacial cycles seen in the record is paced by subtle variations in Earth’s orbit and tilt.) However, even if increases in greenhouse gases are not triggering the periods of warming seen in the ice cores, most scientists believe that they are nonetheless playing an important role in amplifying that initial warming.
Links between ocean and atmosphere
A challenging aspect of climate theory and modelling concerns the ways in which the oceans and atmosphere interact, and studies of marine sediments stretching back through Quaternary time have made major contributions in this area. One of the best known examples of the ocean’s influence on climate is the effect of the Gulf Stream and North Atlantic Drift currents on western Europe. Without the warm North Atlantic Drift current, the British Isles would experience much colder winters – like northern Ontario, Canada, which is at a similar latitude.
It used to be thought that ocean currents were fairly stable features that take a long time to change. Instead, analyses of sediments from the sea floor of the Atlantic Ocean have shown that the North Atlantic Drift current (which is part of a much larger system called the thermohaline circulation) has altered dramatically and quickly in the past. It has been found that this current is particularly sensitive to inputs of freshwater (e.g. from melting ice and emptying of glacial lakes) in northern parts of the North Atlantic region. By correlating marine-based data with ice core data it has been found that:
In climate change, the idea of a ‘threshold’ process is one in which change is slight or non-existent until some ‘forcing’ factor (i.e. amount of freshwater in the case of the Atlantic) reaches a critical level. Suddenly the system can no longer cope and ‘flips’ to a different mode of operation.
The transport of warm water towards the north-west European coastlines relies upon sea water in the Greenland-Iceland-Norwegian Sea becoming sufficiently salty to be denser than surrounding water so sinking to the deep ocean. The sinking of this water, and its return to lower latitudes as deep water flow, keeps surface water flowing north to replace it. The formation of this North Atlantic Deep Water (NADW) is an important part of the whole ‘great ocean conveyor’ system, and changes in its strength have effects far outside of the North Atlantic region.
Because Quaternary studies have shown that NADW formation has shut-down in the past when melting ice has reduced sea surface salinity, much attention has been focused on the possibility that global warming could, paradoxically, cause cooling in western Europe: the idea being that increased melt water from the Greenland ice sheet, along with increased snow melt, rainfall and river discharge from northern Canada and northern Europe, could freshen the North Atlantic enough to disrupt the system. This idea is exaggerated to the point of triggering the next ice age in the popular Hollywood disaster movie The Day After Tomorrow released in 2004.
Today, most climate scientists believe it is very unlikely that human-caused climate warming this century will add enough freshwater fast enough to tip the system over the threshold and shut-down NADW formation. However, there is still uncertainty about how sensitive the system is; and, if worst-case scenarios for melting of the Greenland ice sheet happen, then a major change in Atlantic circulation is a possibility in the future.
Abrupt changes
Closely linked with the threshold behaviour exhibited by Atlantic Ocean circulation is the phenomenon of abrupt climate change. As the thermohaline circulation has flipped between strong, weak, and shut-down modes, the regional climate has also swung dramatically between warm and cold. This is best seen in the Greenland ice core records which show that over the past 100,000 years there have been many swings in mean annual air temperature over Greenland of as much as 15ºC occurring in just decades. At the transition from glacial to interglacial conditions about 11,500 years ago (onset of the Holocene) climate warming over Greenland was remarkably rapid, on the order of 7ºC in just a matter of years.
Figure 2 Greenland ice core data
Such abrupt changes were not restricted to Greenland: studies in many parts of Europe, notably of changes between cold and warm-loving beetle fauna represented in British deposits, also show large-scale and rapid climate changes that correlate with the ice cores. Climate changes experienced in Britain from the last glacial maximum (LGM) through to the start of our present interglacial (the Holocene) can be summarised as follows:
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British ice sheet at its maximum extent around 20,000 years ago
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Limited climate warming from 20,000 to about 17,000 years ago
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Rapid climate warming at 14,700 years ago (onset of Bølling warm phase)
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Brief colder phase from about 14,000 years ago called the Older Dryas
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Recovery of warmth and onset of the Allerød warm phase
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Rapid cooling from about 12,800 years ago with return to glacial conditions (Younger Dryas cold phase)
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Rapid warming from about 11,500 years ago at onset of the Holocene
While the sequence described above, and shown in Fig.2, provides a good example of abrupt climate change, it’s important to be aware that the geological record has revealed many other examples that have occurred at various times in the past. In addition to dramatic temperature swings, there have also been abrupt shifts in precipitation levels in various parts of the world. Furthermore, abrupt changes can occur due to many factors besides just variation in ocean circulation as described here. Often, abrupt changes result from a combination of causes. Given what we know of abrupt climate changes of the past, those attempting to model and make predictions of future climate must take into account the idea of ‘thresholds’ in the climate system which, if crossed, can result in large and rapid changes that seem disproportionate to the initial triggering factor. This is why many scientists are concerned that if total greenhouse gas emissions are not controlled sufficiently to keep further global warming within about 2ºC, various effects could take hold to strongly amplify the changes initiated by human activity.
Write an account of the long term climatic changes that have taken place in the British Isles including the phenomenon known as recent global warming. (AQA Specification B (Jan 07) A2 Unit 4 “Global Change”)
In your account:
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describe the fluctuations in temperature and precipitation in the British Isles that have taken place since the Pleistocene Ice Age
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outline the evidence for such fluctuations in temperature and precipitation
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discuss the reasons for recent global warming, and its possible effects on the British Isles.
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This question requires an essay style response which places the issue of global warming within a longer-term context. The essay could start with an introduction paragraph that sets the scene; for example, by commenting on how the climate of the British Isles has been changing constantly over a range of timescales due to natural causes; but that only recently have humans themselves become a cause of climate change through increased emissions of greenhouse gases.
The main body of the essay could then be structured according to the bullet points given. The contrast could be drawn between the most recent glacial phase (end of the Pleistocene when Britain was much colder and much precipitation fell as snow) and the onset of our current interglacial (the Holocene) when the climate warmed naturally. Detail could also be given of natural changes in climate that have affected Britain over the last several thousand years and in recent history, such as the ‘Medieval Warm Period’ and the ‘Little Ice Age’.
After outlining the main climate fluctuations since the Pleistocene, it is necessary to comment on the evidence that has enabled reconstruction of these past climatic changes. There are many techniques that could be discussed, including the analysis of ancient plant remains and pollen in lake and peat sediments, the study of tree rings, and the study of marine sediments and ice cores.
The transition can now be made to the most recent part of the climate record for which instrumental records exist. The trend of warming through the last century (and particularly since the 1970s) should be described and should be correlated with the rise in greenhouse gases due to human activities. It is worth noting, however, that there is still debate over the extent to which the recent warming trend can be explained by human factors as opposed to natural ones. Effects on the British Isles can include a possible increase in climate extremes (summer heat waves, droughts and floods) and problems associated with sea-level rise and the invasion of warmer-adapted weeds, pests, and diseases. The essay could finish with a conclusion about the degree of human impact on climate as well as some comment about how future climate will depend on the interaction between natural and human factors.