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Department of Geography, University of Exeter, Cornwall Campus, Penryn, Cornwall TR10 9EZ, UK
* Corresponding author (e-mail: j.knight{at}exeter.ac.uk)
Periglacial and paraglacial (cold-climate) environments, located outside the margins of past and present ice sheets but responding to similar climate forcings, are key to identifying climate change effects upon the Earth system (Warburton 2007). These environments are relicts of cold Earth processes and thus are most sensitive to climate change that took place during the last glacial–interglacial transition, and at the present time under enhanced global climate warming. These effects include changes in humidity/aridity and radiation balance, which are most significant in the higher latitudes and at high elevations where periglacial and paraglacial environments are most common and where these environments occur near their climatic limits (Harris 1994; Matsuoka 2001). Variations in humidity and radiation balance have implications for heat budgets, water balance, land surface stability, downslope sediment supply, biodiversity and biogeochemical cycling (e.g. Schneider et al. 1999; Scott et al. 2008). The dynamics of cold-climate environments are, therefore, strongly controlled by external climatic forcing; and hence periglacial and paraglacial processes (and the landforms and sediments that result from them) can be considered as a transient response to the landscape disturbance and land surface instability that accompanies climatic change (Hewitt et al. 2002).
This view of a transient landscape responding to environmental disturbance is significant because it underpins influential deterministic and steady-state models in cold-climate science (Church & Slaymaker 1989; André 2003; Warburton 2007). These models predict a rapid increase in sediment yield (which results from land surface disturbance) associated with initial climate forcing, followed by exponential decay of sediment yield towards background rates which are achieved as land surfaces are stabilized (Church & Ryder 1972; Ballantyne 2002). Such a view of climatic causality is useful because it can be used to consider the magnitude and longevity of landscape impacts of past and future climate changes, respectively.
These views of land surface response to deglaciation are based on the premise that the processes and climates associated with glaciation are related to an increase in sediment generation (by glacial processes themselves, and by enhanced weathering) (Kirkby 1995). In reality, landscape responses are more subtle and strongly conditioned by local-scale geological and topographical factors that lie outside of these models.
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The limitations of uniformitarianism |
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Much of our understanding of past periglacial and paraglacial processes and environments comes from a synthesis of observations drawn from contemporary environments and from preserved geological evidence from the last glacial–interglacial transition (and into the early Holocene). Very little is known about the extent, dynamics and evolution of periglacial and paraglacial environments associated with older glacial cycles. This is probably owing to low preservation potential in areas that were overridden by ice in later glaciations. In addition, as interglacials progress, pre-existing periglacial and paraglacial sediments and structures are probably destroyed by plant growth and soil development. These limitations suggest that little is known about past macroscale dynamics of periglacial and paraglacial environments, and that the principle of uniformitarianism is not always appropriate to apply.
Interglaciations, including the present Holocene, are characterized by low continental ice volume and land surfaces dominated by plants. As interglacials develop, therefore, the geographical zones in which periglacial and paraglacial processes operate retreat towards core high-latitude and high-altitude areas. This means that these processes are time transgressive across the landscape as climate belts shift, and that their environments become smaller and more geographically isolated over time. As interglacials develop, geological records from these environments reflect local-scale controls rather than regional-scale climate, and there are associated problems of correlation. Interglacial records are therefore sparser and their interpretations limited. The present situation of anthropogenically enhanced climate changes (global warming), superimposed upon the already warm Holocene, is unprecedented. The net future climatic effects (in both precipitation and in air and ground temperature) are uncertain (Rosenzweig et al. 2008; Scott et al. 2008). This poses many questions as to how periglacial and paraglacial processes and environments will respond, and how quickly (Warburton 2007), under climatic contexts for which there is no preserved analogue. This clearly illustrates the limitations of uniformitarianism as a tool to understand future changes in periglacial and paraglacial environments.
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Discussion and outlook to the future |
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A probable effect of anthropogenic climate warming is that the present interglacial is extended beyond the timescale determined solely by Milankovitch forcing (Mitchell 1972; Mörner 1972), which has been largely responsible for controlled interglacial length in the past (e.g. Tziperman et al. 2006). As periglacial and paraglacial processes are, on the macroscale, determined by climate, it is to be anticipated that sediment generation and supply will decrease over time as the land area under these favourable climates decreases also. This follows the paraglacial sediment exhaustion model of Ballantyne (2002). Under an extended (and warmer) interglacial, it is probable that sediment fluxes from the headwaters of mid-latitude glaciated basins will decrease dramatically, leading to sediment starvation and, eventually, cannibalization of river lowlands and coastal fringes. In high-latitude areas, permafrost melt and reduced sea ice protection is already leading to enhanced coastal erosion and sediment supply (Lawrence et al. 2008). Global warming, therefore, is already leading to a decrease in the continuity and interconnectedness of permafrost and associated periglacial processes (Lemke et al. 2007). A sediment budget approach (e.g. Syvitski et al. 2003; Phillips & Slattery 2006) can help monitor the progression of this breakup.
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Imperatives in the understanding of periglacial and paraglacial environments |
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In considering how periglacial and paraglacial environments are going to respond to future climate changes, two key questions present themselves. First, given that renewed paraglaciation will accompany future glacier retreat and decreased extent of periglacial environments under global warming, how will we accommodate such geomorphological instability into our models of economic and social use of both mountain and lowland cold-climate regions? Second, how far can models of paraglaciation, and periglacial slope processes, be used to interpret the geomorphic evolution of these landscapes under future climate scenarios? These questions, and related issues, are explored in this volume in an inter- and multidisciplinary framework, through case studies from both contemporary and Quaternary periglacial and paraglacial settings.
This volume is organized into three sections. The first section focuses on periglacial processes and environments. The paper by André sets periglacial studies into a wider and historical context. Periglacial weathering and palsa processes are examined in the papers by Nicholson, and Seppälä & Kujala, respectively. The papers by Waller et al. and Slaymaker discuss the interrelationships between periglacial and glacial processes and environments.
The second section of the book focuses on paraglacial environments and processes in the British Isles. The paper by Whalley discusses how periglacial and paraglacial sediments and structures can be used to reconstruct past climate changes. The succeeding papers by Jarman, Wilson, Passmore & Waddington and Knight provide evidence for paraglacial processes and environmental change during the late Quaternary and early Holocene, using examples from upland areas of Britain and Ireland.
The final section of the book examines paraglacial processes, climate change and related issues of sediment supply using examples from Europe, North America and Asia. The paper by Curry et al. considers the geotechnical and geomorphic implications on ongoing paraglaciation. Specific examples of paraglacial landscape responses from British Columbia are shown in the papers by Wilkie & Clague and Friele & Clague. The paper by Hewitt considers paraglaciation in Pakistan as a transient landscape response to climatic disturbance. The final paper in the volume, by Harrison, addresses the sensitivity of periglacial and paraglacial geomorphic systems to climatic forcing, which is particularly important when one considers that these environments are most at threat from future climate change.
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Acknowledgments |
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The editors wish to thank the authors for their contributions, and acknowledge the following reviewers: N. Betts, J. Boelhouwers, J. Catt, J. Carrivick, P. Christoffersen, M. Clark, J. Desloges, J. Dixon, B. Etzelmuller, A. Findlayson, D. Giles, S. Gurney, K. Hall, P. Hughes, K. Huntington, O. Humlum, J. Kemp, M. Konen, O. Korup, W. Mitchell, A. Nesje, S. Payette, R. Pine, B. Rea, A. Strom, D. Swift, R. Tipping, F. Tweed, C. Whiteman and C. Zangerl.
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