Timing of Little Ice Age maxima and subsequent glacier retreat in northern Troms and western Finnmark, northern Norway

Abstract

Glaciers are important indicators of climate change, and recent observations worldwide document increasing rates of mountain glacier recession. Here we present approximately 200 years of change in mountain glacier extent in northern Troms and western Finnmark, northern Norway. This was achieved through (1) mapping and lichenometric dating of major moraine systems within a subset of the main study area (the Rotsund Valley) and (2) mapping recent (post-1980s) changes in ice extent from remotely sensed data. Lichenometric dating reveals that the Little Ice Age (LIA) maximum occurred approximately 1814 (±41 years), which is before the early twentieth-century LIA maximum proposed on the nearby Lyngen Peninsula but younger than LIA maximum limits in southern and central Norway (mid-eighteenth century). Between LIA maximum and 1989, a small sample of measured glaciers (n = 15) shrank a total of 3.9 km 2 (39 percent), and those that shrank by more than 50 percent are fronted by proglacial lakes. Between 1989 and 2018, the total area of glaciers within the study area (n = 219 in 1989) shrank by approximately 35 km 2. Very small glaciers (<0.5 km 2) show the highest relative rates of shrinkage, and 90 percent of mapped glaciers within the study area were less than 0.5 km 2 in 2018.

Glacial and periglacial geomorphology of central Troms and Finnmark county, Arctic Norway

“…Mapping of glaciers and lakes was conducted using multispectral satellite imagery and a semi-automated process, to ensure speed and to provide the most current outlines. We used a semi-automated mapping approach utilising a band ratio method to generate initial outlines, followed by manual correction of mapped units to account for glaciers and lakes which were poorly or erroneously mapped (for further details on semi-automated mapping see Andreassen et al, 2012;Du et al, 2016;Leigh et al, 2019Leigh et al, , 2020Nagy & Andreassen, 2019;Raup et al, 2007;Song et al, 2017;Watson et al, 2018). Glaciers have also been divided into individual units based on drainage divides provided by Andreassen et al (2012).…”

“…Minor moraines (e.g. those <4 m wide), mostly found within likely Little Ice Age glacier limits (see Leigh et al, 2020), are mapped as lines which are drawn along their crests (Figure 2).…”

“…Mapping of glacier outlines was guided by the prior outlines of Andreassen et al (2012) and Leigh et al (2020). Glaciers are easily identifiable on the multispectral imagery, especially when using a false colour composite of bands 5-4-3 as Red-Green-Blue, whereby glaciers appear as bright, fluorescent blue (e.g.…”

“…The multitude of large scale, and sometimes rapid, glacier fluctuations has produced a complex landscape signature of glacial and periglacial features and surficial materials (Olsen et al, 2013). However, the most recent glacier advance during the Little Ice Age (typically within the past ∼200 years: Ballantyne, 1990; Leigh et al, 2020) is thought to have been the most extensive neoglacial advance and has likely overridden glacial deposits formed during the mid-to late-Holocene (cf. Matthews et al, 2000.…”

“…Bakke et al, 2005;Ballantyne, 1990), the Bergsfjord Peninsula (e.g. Wittmeier et al, 2015), the Rotsund Valley (Leigh et al, 2020), and the island of Arnøya (Wittmeier et al, 2020). Most of the more isolated mountain and plateau regions of central Troms and Finnmark county have received little or no attention.…”