Stephan Harrison

Climate change and geomorphological hazards in the eastern European Alps

Climate and environmental changes associated with anthropogenic global warming are being increasingly identified in the European Alps, as seen by changes in long-term high-alpine temperature, precipitation, glacier cover and permafrost. In turn, these changes impact on land-surface stability, and lead to increased frequency and magnitude of natural mountain hazards, including rock falls, debris flows, landslides, avalanches and floods. These hazards also impact on infrastructure, and socio-economic and cultural activities in mountain regions. This paper presents two case studies (2003 heatwave, 2005 floods) that demonstrate some of the interlinkages between physical processes and human activity in climatically sensitive alpine regions that are responding to ongoing climate change. Based on this evidence, we outline future implications of climate change on mountain environments and its impact on hazards and hazard management in paraglacial mountain systems.

Characteristics of tide-water calving at Glaciar San Rafael, Chile

Glacial calving is a poorly understood process. This study tests the influence of local environmental variables on the magnitude and frequency distributions of calving behaviour at Glaciar San Rafael, Chile. Near the terminus of the glacier, surface speeds average 17 m d −1 in summer and calving is profuse and continual. The size, location and characteristics of over 7000 calving events were recorded during 32 d in 1991 and 1992, together with meteorological, bathymetric and oceanographic data. Mean daily calving exceeds 400 events per day and the mean calving flux is more than 2 Mm 3 d −1 . Mean annual calving speed and calving flux are about 4500 m a −1 and 2.0 km 3 a −1 , respectively. This calving speed is higher than that predicted by the established empirical relationship between tide-water calving speed and water depth. This is surprising, given the low salinity of Laguna San Rafael and that fresh-water calving speeds are commonly much lower than those in tide water. Daily patterns of calving frequency and flux correlate poorly or not at all with meteorological variables, but tidal stage may have some control over the timing of large submarine calving events. Submarine calving produced the largest bergs. However, the relatively small total flux recorded from the submerged part of the ice cliff may imply unusually rapid melt rates.

Global sea-level contribution from the Patagonian Icefields since the Little Ice Age maximum

The melting of mountain glaciers and ice caps is expected to contribute significantly to sea-level rise in the twenty-first century(1-)3, although the magnitude of this contribution is not fully constrained. Glaciers in the Patagonian Icefields of South America are thought to have contributed about 10% of the total sea-level rise attributable to mountain glaciers in the past 50 years(3). However, it is unclear whether recent rates of glacier recession in Patagonia are unusual relative to the past few centuries. Here we reconstruct the recession of these glaciers using remote sensing and field determinations of trimline and terminal moraine location. We estimate that the North Patagonian Icefield has lost 103 +/- 20.7 km(3) of ice since its late Holocene peak extent in AD 1870 and that the South Patagonian Icefield has lost 503 +/- 101.1 km(3) since its peak in AD 1650. This equates to a sea-level contribution of 0.0018 +/- 0.0004 mm yr(-1) since 1870 from the north and 0.0034 +/- 0.0007 mm yr(-1) since 1650 from the south. The centennial rates of sea-level contribution we derive are one order of magnitude lower than estimates of melting over the past 50 years(3), even when we account for possible thinning above the trimline. We conclude that the melt rate and sea-level contribution of the Patagonian Icefields increased markedly in the twentieth century.

Dendrochronology and lichenometry : colonization, growth rates and dating of geomorphological events on the east side of the North Patagonian Icefield, Chile

This paper highlights the importance for dating accuracy of initial studies of delay before colonization for both trees and lichens and tree age below core height, particularly in recently deglaciated terrain where colonization and growth rates may vary widely due to differences in micro-environment. It demonstrates, for the first time, how dendrochronology and lichenometry can be used together in an assessment of each others colonization and growth rates, and then cross-correlated to provide a supportive dating framework. The method described for estimating tree age below core height is also new. The results show that on the east side of the North Patagonian Icefield in the Arco and Colonia valleys, Nothofagus age below a core height of 112 cm can vary from 5 to 41 years and delay before colonization may range from a maximum of 22 years near water to a minimum of 93 years on the exposed flanks of the Arenales and Colonia Glaciers. Tree age plus colonization delay supplied a maximum growth rate of 4.7 mm/year for the lichen Placopsis perrugosa and lichen colonization is estimated to take from 2.5 to approximately 13 years. A minimum lichenometric date of 1883 was estimated for an ice-formed trimline at the junction of the Arenales and Colonia glaciers and a maximum dendrochronological date of 1881 for a water-formed trimline in the Arco valley. Tree and lichen ages around the valley suggest that a glacial outburst drained the 1881 high level lake releasing approximately 265 million cubic metres of water. Repeated flooding, with a minimum of 38 high lake levels, is suggested by horizontal sediment lines on the Arco valley walls and moraine flanks. Dating confirmed diminishing flood levels with a last minor flood in 1963. The wider significance of the work is that it should produce more accurate dating of recent glacier fluctuations around the North Patagonia Icefield, an area where dated reference surfaces are extremely scarce.

On reductionism and emergence in geomorphology

Much geomorphological enquiry has been devoted to the understanding of landscapes via the construction of models based on the relationships between process and form. This paper examines the philosophical, theoretical and practical problems involved in bridging the gap between studies of geomorphological processes and explanations of landscape development. It argues that process geomorphology is essentially reductionist and discusses the practical and logical limitations of such an approach to science. It suggests that landscapes are emergent phenomena and, by drawing from the philosophical and practical lessons derived from the physics of non-linear systems, demonstrates that they are not amenable to reductionist explanations.

Nineteenth- and Twentieth-Century Glacier Fluctuations and Climatic Implications in the Arco and Colonia Valleys, Hielo Patagonico Norte, Chile

Dendrochronology, lichenometry, and analysis of aerial photographs taken in 1944, 1979, and 1983 were used to date the 19th- and 20th-century fluctuations of the Arco, Colonia, and Arenales glaciers on the eastern side of the Hielo Patagónico Norte in southern Chile. This work has demonstrated that the glaciers retreated from their Little Ice Age maximum positions between 1850 and 1880, with retreat rates increasing during the 1940s and with surface thinning of at least 30 m since 1980. Comparison with the fluctuation behavior of other outlet glaciers of the icefield suggests a degree of synchrony in the timing of their variations and therefore argues for a common climatic control for these movements.

Recent oscillations of the San Quintin and San Rafael Glaciers, Patagonian Chile

Earlier reports, maps, aerial photographs, and tree-ring dates for moraines are used to investigate and compare the oscillations of the land-based San Quintin Glacier and the calving, tidewater San...

Buoyancy-driven lacustrine calving, Glaciar Nef, Chilean Patagonia

Glaciar Nef, a 164 km 2 eastern outlet of Hielo Patagonico Norte (the northern Patagonia icefield), terminates in a proglacial lake that has formed in conjunction with 20th-century glacier retreat. The terminus is inferred to be transiently afloat. A hinge-calving mechanism is proposed in which buoyant forces impose a torque on the glacier tongue, resulting in the release of coherent sections of the glacier tongue as “tabular” icebergs. A simple model shows how torque and tensile stress reach a maximum at the up-glacier limit of the buoyant zone, and that glacier thinning causes this point to migrate up-glacier. Empirical evidence supporting this model includes elevated thermo-erosional notches ≤6.5 m above lake level, and the ubiquitous presence since 1975 of “tabular” icebergs with surface areas ≤0.3 km 2 . Flow speeds of 1.2–1.3 m d −1 were measured near the terminus in February 1998. Extrapolations from these short-term data yield a calving rate of 785–835 m a −1 and a calving flux of 232 × 10 6 m 3 a −1 or 0.2 km 3 a −1 . The calculated mean water depth at the terminus is 190 m. This calving rate is higher than at grounded temperate glaciers calving in fresh water, but is nevertheless almost an order of magnitude less than calving rates at both grounded and floating tidewater glaciers.

Historical fluctuations of the Gualas and Reicher Glaciers, North Patagonian Icefield, Chile

Dendrochronology was used to date historical fluctuations of the little-visited Gualas and Reicher Glaciers on the North Patagonian Icefield in southern Chile. Vegetation trimlines dating to ad 1876, 1909 and 1954 show that glacier downwasting and retreat mirrored the patterns found at the neighbouring San Rafael and San Quintin Glaciers. Intermediate stages of recession of the Gualas and Reicher Glaciers dating to the early 1920s, mid-1930s and 1960s are also reflected by similar stages in the San Rafael and San Quintin Glaciers. We propose that the common responses shown by these four glaciers argue for climatic control of their movements and that glacier dynamics in this region are principally controlled by changes in precipitation rather than temperature since annual mean temperatures at Cabo Raper, the nearest meteorological station, have shown no trend since the early 1940s whereas precipitation, peaking in the winter months, has varied widely since the 1920s.

Recent Retreat Glaciar Nef, Chilean Patagonia, Dated by Lichenometry and Dendrochronology

This paper presents the results of a lichenometric and dendrochronological study of the recent retreat history of Glaciar Nef, an eastern outlet glacier of the Hielo Patagónico Norte. A 600-yr tree regeneration time, based on maximum tree age in the ancient forest, suggests that the forest-clad lateral moraines in the valley, southeast of the 19th century terminal moraine system, were formed some time before A.D. 1370. Dating estimates suggest that retreat from a 19th century maximum began around 1863, a decade or two earlier than the date established for other glaciers in the region, with glacier thinning near the ice front averaging 1.11 m yr−1 between 1863–1881. After 1884, retreat seems to have slowed, with glacier thinning averaging 0.09 m yr−1. Lichen and tree dating suggests that the glacier had retreated approximately 500 m by 1938; this estimate is supported by an aerial photograph showing a proglacial lake just beginning to form in 1944. Recent glacier movements around the Hielo Patagónico Norte are discussed and it is concluded that the general trend of glacier retreat around the icefield, beginning in the 1860s to 1870s, is consistent with Northern Hemisphere trends.