Holocene permafrost aggradation in Svalbard

OLE HUMLUM

Department of Physical Geography, Institute of Geosciences, University of Oslo, PO Box 1042 Blindern, N-0316 Oslo and Department of Geology, University Centre in Svalbard, N-9170 Longyearbyen, Norway

Abstract: The distribution and dynamics of permafrost represent a complex problem, confounded by a short research history and a limited number of deep vertical temperature profiles. This lack of knowledge is pronounced for the High Arctic, where most permafrost is found and where amplified responses to various climatic forcing mechanisms are expected. Within the High Arctic, the Svalbard region displays a unique climatic sensitivity and knowledge of Holocene, and modern permafrost dynamics in this region therefore have special interest. This paper reviews knowledge on Holocene permafrost development in Svalbard and the climatic background for this. In Svalbard, modern permafrost thickness ranges from less than 100 m near the coasts to more than 500 m in the highlands. Ground ice is present as rock glaciers, as ice-cored moraines, buried glacial ice, and in pingos and ice wedges in major valleys. Svalbard is characterized by ongoing local-scale twentieth-century permafrost aggradation, even though a distinct temperature increase around 1920 introduced relatively unfavourable climatic conditions for permafrost in Svalbard. Modern permafrost aggradation is to a large extent controlled by wind, solid precipitation and avalanche activity, and exemplifies the complexity of relating climate and permafrost dynamics.

Recent changes in the Arctic atmosphere -ice-ocean system have sparked intense discussions as to whether these changes represent episodic events or long-term shifts in the Arctic environment. Concerns about future climate change stem from the increasing concentration of greenhouse gases in the atmosphere. During the last 15 years, the Arctic has gained a prominent role in the scientific debate regarding global climatic change (Houghton et al. 2001). Global circulation models predict that the present and future global climatic change should be amplified in the polar regions due to feedbacks in which variations in the extent of glaciers, snow, sea ice, permafrost and atmospheric greenhouse gases play key roles. This is the basic reason for a renewed research interest in the Arctic region. Slight changes in mean annual air temperature, wind speed and precipitation have the potential to change the state of large regions of presently frozen ground (Nelson et al. 2001, 2002; Anisimov et al. 2002). Sub-continental-scale analysis of meteorological data obtained during the observation period apparently lends empirical support to the alleged high climatic sensitivity of the Arctic (Giorgi 2002). Polyakov et al. (2002a, b), however, recently presented updated observa tional trends and variations of Arctic climate and sea-ice cover during the twentieth century that question the modelled polar amplification of temperature changes registered by surface stations at lower latitudes.

There is reason therefore to evaluate past and present climate dynamics and their respective impacts on high-latitude permafrost regions such as Svalbard (Fig. 1). Permafrost is widely distributed in the Arctic and underlies as much as 20-25% of the present global land surface, affecting a wide range of ecosystems and landscapes (Pewe 1983; Zhang et al. 2000). In Svalbard, virtually all land areas not covered by glaciers have permafrost (Humlum et al. 2003). The presence of permafrost, with its overlying active layer, is a primary factor distinguishing arctic from temperate watersheds (McCann & Cogley 1972; Woo 1986; Kane & Hinzmann 1988; Woo et al. 1992; Brown et al. 2000). Permafrost is also the thermal background for a suite of permafrost-specific landforms such as ice wedges, pingos, patterned ground, palsas and rock glaciers. Massive bodies of ground ice have implications for construction work or issues relating to slope stability. From an engineering point of view, any temperature change of permafrost is likely to introduce

From: Harris, C. & Murton, J. B. (eds) 2005. Cryospheric Systems: Glaciers and Permafrost. Geological Society, London, Special Publications, 242, 119-130. 0305-8719/05/$ 15.00 © The Geological Society of London 2005.

Permafrost Holocene
Fig. 1. Map of 63,000 km2 Svalbard archipelago. About 60% is covered by glaciers (white). The remaining 25,000 km" (grey) are permafrost areas without permanent ice cover

changes in creep rates of existing foundations such as piles and footings, embankment foundations and variations in adfreeze bond support for pilings. Changes in the active-layer thickness will cause variations in thaw settlement during seasonal thawing, changes in frost-heave forces on pilings and total and differential frost heaving during winter (Pewe et al. 1981; Humlum et al. 2003).

At the beginning of the twenty-first century, mean annual air temperature (MAAT) is about -5°C near sea level in central Spitsbergen, and the mean annual precipitation is around 180 mm water equivalent (w.e.). The warmest period in the twentieth century was the period 19301940, with MAAT varying between —5 and — 4°C (F0rland et al. 1997). The average vertical precipitation gradient on Spitsbergen is 15-20% (per 100 m) in the coastal regions, while it is somewhat smaller (5-10%) in the central part of Spitsbergen (Hagen & Liest0l 1990; Hagen & Lefauconnier 1995; Killingtveit et al. 1996). The coastal-inland contrast in vertical precipitation gradient is assumed to be caused by enhanced orographic effects in the coastal regions, compared to inland areas (Humlum 2002). The amount and distribution of solid precipitation represents an important control on permafrost development in general (e.g., Smith & Riseborough 2002), as well as on a local scale, as will be described in the present paper.

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