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NORFOLK AND RECENT CLIMATE CHANGE INTRODUCTION You have probably read a lot in the press recently about climate change, and the effect that changes in temperature and weather could have on our lives. As Norfolk is relatively flat and low-lying, with an extensive coastline, it could bear the brunt of any changes. Many present coastal areas are so low that they are below estimated future sea levels. There is also the potential threat of increased coastal erosion, and salt-water contamination of the region’s drinking water and freshwater habitats such as The Broads. Climate change is often portrayed as being a recent trend caused by human activity producing lots of greenhouse gases and toppling the Earth’s natural balance. Many recent scientific studies provide evidence to support this claim. However, natural cycles of climate change, both warming and cooling, are known to have taken place throughout the Earth’s history, over thousands of millions of years. Geologists divide up geological time into periods, with each period being partly defined by a certain type of climate. Geological periods last for however long the environmental conditions remain the ‘same’. The present geological period, covers the last 2.6 million years of Earth history, and is called the Quaternary. The Quaternary is quite different to other periods, as it defines the length of time that the Earth has been experiencing a changeable climate with cycles of warm and cold temperatures, with some cycles lasting longer, and having warmer or colder temperatures, than others. Evidence shows that although there has been a progressive long-term trend of global cooling during the Quaternary, there have also been several different ice ages, or glaciations, separated by 1 episodes of global warming where the climate was much warmer or more temperate than the present day. Norfolk hasn’t escaped these climate changes. In fact, the geological record of the region, tells a story of episodes when temperatures were much warmer than the present day, and the region resembled the ‘Costa del Cromer’! At other times, the Norfolk landscape was covered by glacier ice and the climate would probably have resembled that of present-day Iceland. These climatic changes had a major effect on the local geography, influencing sea level and the position of the coastline, the behaviour of rivers, as well as the existence of early humans and flora and fauna. HOW HAS CLIMATE CHANGE AFFECTED NORFOLK? Today, Norfolk is situated on the western side of the North Sea. The North Sea is shallow and Norfolk is relatively low lying. Unfortunately, Norfolk has no solid high cliffs like the White Cliffs of Dover to keep the sea out! Consequently, it is vulnerable to rising sea levels that may result from global warming. On the other hand, during a cold episode, the world’s glaciers would grow; taking the water that is presently in the seas. This could cause global sea level to fall by say 30 m. The North Sea would shrink in size, and Norfolk’s coast would move northwards and eastwards. Norfolk would increase in size! But in contrast, a 30 m global sea level rise during a warm episode, when glaciers melt into the seas, would cause the coastline in Norfolk to move westwards, swallowing up the present day coastal areas. To explain climate change during the Quaternary, it is helpful to divide it up into two parts, the first, covering 2.6 million to 900,000 years ago, and the second, 2 from 900,000 years to the present day. It is worth bearing in mind that the last 900,000 years is a short period of time, compared to the Earth’s total age of 4.6 billion years! From 2.6 million to 900,000 years Between about 2.6 million and 900,000 years ago, the coastline of ‘East Anglia’ lay much further inland (Figure 1). Southern England was joined to France, at the Straits of Dover, which now links the North Sea to the English Channel. This meant a ‘land-bridge’ existed between Britain and continental Europe, which enabled animals to migrate between the two areas. Figure 1 Geography of the United Kingdom and the North Sea Basin during the early Quaternary (c. 1,600,000 years ago) showing the position of the major rivers and coastline. The climate is believed to have been mostly warmer than at present, with a trend to slight cooling. Numerous small-scale oscillations from warmer to cooler episodes occurred. The effect of these small climatic oscillations was to drive frequent but relatively small falls and rises in sea level. In the Southern North Sea area and Norfolk, this would have created a fluctuating coastline. In many ways, the landscape would have looked very similar to parts of the present 3 Norfolk coastline, with salt marshes, tidal creeks, mud and sand flats, river estuaries, and pebbly beaches (Figure 2). In Norfolk, the sands, gravels and muds that were laid down during this time, in this coastal environment, are known as ‘crags’. Figure 2 An ancient beach deposit at Norton Subcourse in southern Norfolk. These sands and gravels are called the Wroxham Crag. Inland, Norfolk would have consisted of a low-lying vegetated coastal plain, dissected by three major rivers that drained eastwards across central and eastern England. Evidence of their existence is represented by extensive sand and gravel deposits, which are exposed at the surface, or buried beneath younger Quaternary deposits. The relatively stable climate, and the vegetated land surface which protected the land from too much erosion, would have meant that the rivers were relatively low energy, transporting mainly small grain material such as sands, silts and clays through their catchments and into the North Sea. From 900,000 years ago to the present day Around 900,000 years ago, a marked climatic shift occurred. The effects of this move to a climate dominated by extremes have been marked, both globally and locally. 4 Over 20 major climatic shifts from ‘warm stages’, (or interglacials) to ‘cold stages’, (or glacials), have been recognised globally within the last 900,000 years, each lasting several tens of thousands of years. Although evidence of only some of these has been found in the UK, several of them have been identified in East Anglia. Fossil plants and animals, such as beetles and small vertebrates, suggest that the climate during many of these ‘warm stages’ was broadly similar to that of the present day. Also, on several separate occasions, the Norfolk region was located either near the margins of, or buried beneath the British Ice Sheet that flowed southwards from Scotland, and down the present coast of eastern England, into Norfolk. During the maximum extent of glaciation, ice covered the whole of the British Isles north of Bristol and London. The thickness of ice over Norfolk is estimated to have been between 2 and 3 km. How many glaciations there were is still unclear but at least three major glaciations have been recognised: the earliest of these is called the Happisburgh Glaciation and started around 640,000 years ago. The second, and largest glaciation is called the Anglian Glaciation and occurred around 450,000 years ago. The third glaciation, which didn’t extend as far into Norfolk, is known as the Devensian glaciation which was at its coldest around 25,000 years ago. Each of the glaciations dramatically modified the geography of Norfolk and deposited vast quantities of sediment. Glaciers pick up material of all sizes as they progress over the land, and carry it under the base of the ice. They smear this onto the land surface as they travel over it, transforming the lie of the land, or geomorphology. This material, which is generally very clayey, but can 5 include all grain sizes, is called till, and it is this material that makes many of Norfolk’s fields clayey (Figure 3). The water released from glaciers as they melt, deposits huge amounts of sands and gravels as the glacier retreats. These sands and gravels are also found in Norfolk, interbedded with the tills, and often lie at the surface making some fields much sandier. Figure 3 The cliffs at Happisburgh, north Norfolk, showing deposits laid down during the Happisburgh Glaciation. Three layers of till are separated and overlain by sands that were deposited within a glacial lake. Large changes in climate resulted in major changes in global sea level with sea levels rising and falling between climatic episodes by several tens to hundreds of metres, having a huge effect on the size and extent of the North Sea. During periods of global warming, the position of the coastline may have been similar or further ‘inland’ than at present. Shallow marine and coastal sediments (Figure 4) above present sea level show that in the past, sea levels have been higher. In contrast, in glacial stages sea level would have been much lower and the coastline 6 would have lain considerably further to the east in the present offshore region (Figure 5). Figure 4 Rippled fine sands and muds at Sidestrand, north Norfolk, deposited on an ancient tidal flat. Figure 5 The United Kingdom and the North Sea Basin prior to glaciation during the middle part of the Quaternary (c. 800,000 years ago). We know that climate change is a major driver of sea-level change. Likewise, the activity of rivers also responded to the changeable regime. All of the rivers began to transport higher quantities of material from further afield. The glaciations eroded and buried the pre-glacial landscape, but due to post-glacial erosion, much of the modern-day drainage network of Norfolk is superimposed upon an ancient drainage pattern. 7 DRIVERS OF GLOBAL CLIMATE CHANGE 1. The distribution of land and water There are several theories to explain the glacial – interglacial cycles of the Quaternary, but many scientists now believe that the distribution of the land and water on the Earth’s surface is crucial. Because the oceans are linked this allows large-scale flows of water between them. Land in the Earth’s mid and high-latitude areas allows the growth of glaciers on them. This flow of heat and moisture around the globe is called ‘thermohaline circulation’. 2. Solar Radiation During the Quaternary, as well as this distribution of land and water, an important driver of long-term climate change was temperature. Temperature is controlled by the amount of solar radiation that we receive, which in turn is controlled by our proximity to the sun. At longer time scales, typically thousands to millions of years, the shape of the Earth’s orbit around the sun is known to vary. Variations in the Earth’s orbit occur in cycles, and have the effect of changing the amount of, and distribution of, solar radiation over the Earth’s surface. These cycles are called Milankovitch Cycles after Milutin Milankovitch, the Serbian astrophysicist who recognised them. He discovered three primary cycles (Figure 1) listed below. 1. Orbital shape or eccentricity: The Earth’s orbit varies from being nearly circular (called low eccentricity) to being slightly elliptical (high eccentricity) over a 100,000-year cycle. During periods when the orbit is elliptical, the Earth passes closer to the sun, and the amount of heat received can be as much as 23% greater than when circular. 8 2. Axial tilt or obliquity: The Earth’s spin can wobble between 22.1 and 24.5° over 41,000-year cycles, changing the tilt in the Earth’s axis. When there is a large tilt, this makes winters colder and summers warmer. 3. Axial orientation or precession: The change in the direction of the Earth’s axis of rotation relative to that of the sun over 21,000-year cycles. When the distance to the sun is shorter and the Earth’s axis is pointing towards the sun, one polar hemisphere will have warmer summers and colder winters, whilst the other will be notably milder. These orbital changes, combined with the distribution of land and sea, have driven episodes of global warming and cooling. Over 100 such climatic changes have been recognised, affecting global sea levels and the extent of the worlds ice sheets and glaciers (Figure 2). Global cooling events known as ‘cold stages’ or ‘glacials’, are indicated by the left pointing peaks and labelled with even numbers. They are characterised by a lowering of global sea level, as the water is being drawn away by the growth of ice sheets and glaciers. Global warming events, called ‘warm stages’ or ‘interglacials’, are depicted with the right pointing peaks and odd numbers. Interglacials are characterised by rising global sea levels, and the decay of ice sheets and glaciers. All figures featured are BGS NERC 2006 9 Figure 1 The Milankovitch theory of climate change, showing how changes in the Earth’s orbit around the sun over long time periods affects the amount and distribution of heat over the Earth’s surface. 10 Figure 2 Quaternary climate change. The left hand side of the diagram shows the global pattern of climate change calibrated with a numerical and geomagnetic timescale. Evidence for known temperate, glacial and periglacial climate episodes in East Anglia is shown. The controlling Milankovitch drivers are shown on the right hand side of the figure. 11 Discussion points / homework topics 1. The Industrial Revolution and climate change – what affect have humans had on climate since the Industrial Revolution? How much of the climate we are experiencing today is natural and how much is anthropogenic (caused by man)? Relevant books: 1) The Holocene – An Environmental History by Neil Roberts 2) The Great Ice Age – climate change and life by Wilson, Drury & Chapman 3) Understanding Earth by Press, Siever, Grotzinger and Jordan (Editor). 2. What other activities other than those discussed affect climate? a. Volcanic eruptions – cause short-term variations in climate. Enormous quantities of fine ash and dust released into the atmosphere lead to localised reductions in temperature. As well as this, the sulphur volatiles also released convert into sulphuric acid in the atmosphere and results in cooling due to back scattering of incoming long wave radiation, e.g. the Mount Pinatubo (Philippines) eruption in 1991, when northern hemisphere temperatures dropped by about 1oC. b. Sunspots and solar winds – changes in radiative output from the sun. Sunspots are dark areas of the Sun’s surface. Observations of sunspot occurrence suggest that they have an eleven-year periodicity. High levels of solar activity cause high solar winds, which deflect cosmic rays. 12 3. Research and summarise the findings of the two Greenland ice cores; Greenland Ice Core Project, known as GRIP, (1989-1992) and Greenland Ice Sheet Project 2, known as GISP2 (1989-1993). What were the main results and what is the advantage of drilling two so close to each other? References: 1) The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future. Richard B. Alley (2002 Paperback). This book is written by one of the leading scientists involved in the interpretation of the GISP2 Ice Core. It is highly readable, accessible and informative, explaining many of the main findings of GISP2. Provides many more references that are useful when investigating Ice Cores. 2) Dansgaard et al. (1993). Evidence of general instability of past climate from a 250-kyr ice-core record. Nature 364, 218–220. This paper outlines GRIPs main findings. The advantage of drilling so close to each other, GRIP at the Greenland Summit and GISP2 20km to the West, is the reproducibility of the results. The two cores are identical for the last 110,000 years, making the results and interpretations more reliable. 4. Many will assume the last 2.6 million years (The Quaternary) is a long time, but in geological time it isn’t. The Quaternary is relatively short and recent; try thinking of how short 2.6 million years is compared to 4.6 billion years using analogues, e.g. a roll of toilet paper represents 4.6 billion years, and the last 2.6 million years is represented by the last 2.6cm! 13 Assumptions are as follows: The toilet roll is 230 sheets long. The Which Report (Januray 2006) gives the number of sheets in various rolls. The average is around 230 – 240 therefore a sheet represents: (4600 million divided by 230) 20 million years assume that a sheet is 20 cm long (an easy number to work with but probably an overestimate) so 1 cm is 1 million years so 2.6 cm is 2.6million years. 14