The impact of recent climate change on the evolution of Bridge Glacier- Since the Little Ice Age

Introduction

This is the first of a  three-part blog series examining the impact of climate change on the evolution of Bridge Glacier. We begin our journey by examining the rate of retreat of Bridge Glacier and the resulting changes in the morphology of the glacier forefield since the Little Ice Age (LIA). Next, we will take a closer look at changes over the last 10 years to gain a better insight into how ongoing climate change is impacting the dynamics of the glacier. Last, we will investigate trends observed at the glacier during the 2021 summer field season, as well as explore the future of Bridge Glacier and the downstream impacts of ongoing retreat. 

We start our journey 147 km south of Bridge Glacier in the city of Vancouver. From here, it is a 6-hour drive up mostly dirt road to the Tyax Lodge and Adventure tourism operation outside of Goldbridge, BC at a crossroads of the unceded traditional Lil’wat and St’at’imc territories. At Tyax, we are surrounded by groups of mountain bikers, and hikers catching flights into the backcountry, we load our supplies up into a floatplane and take off from Tyaughton Lake. On our floatplane flight to the glacier, we see summer cottages, logging operations, and as we fly deeper into the valley Chilcotin mountains rise around us, the Bridge River meandering through. Suddenly we see it! Bridge Glacier, descending from the high peaks and terminating in a turquoise lake where small, calved icebergs glisten as they drift across the water. As we exit the plane, we see the striking contrast between the sparkling white ice and the grey almost “barren” till, we hear the rushing sounds of glacial streams, and feel the cold katabatic wind descending down the glacier surface. We have arrived…

The Field Site

Bridge Glacier (50◦48′11′′ N, 123◦38′40′′ W) is an outlet of the Lillooet Icefield, located in Southwestern British Columbia, nestled in the Pacific Range of the Coast Mountains. From head to toe the glacier loses approximately 1510m  in elevation.  In 2013, the glacier covered an area of approximately  83 km2 but has retreated significantly since then. Bridge Glacier is currently a lake-calving glacier (i.e., one of the two arms of the terminus ends in a proglacial lake).  (Chernos et. al 2016). The reduced stress on the calving terminus  (i.e. its’ buoyancy in water) generally leads to more rapid retreat compared to completely land terminating glaciers (Benn et al., 2007). Calving glaciers are unique in that they usually retreat in a step-like pattern (i.e periods of relative stability followed by rapid retreat) that typically coincides with the flotation of the terminus (Warren and Kirkbride, 2003; Boyce et al., 2007; Dykes et al., 2011).  

Bridge Glacier is not only a breathtaking sight, but it plays a major role in the local hydrological cycle, along with other alpine glaciers in the region. Any changes in glacier mass balance impact downstream water resources, influencing water quality, aquatic habitat, and agriculture in the Bridge River watershed, and in particular the large Bridge River BCHydro hydroelectric project which produces 6-8% of the province’s energy needs (Barry, 2006; Chernos et al., 2016; Radić & Hock, 2011; Zemp et al., 2015). Alpine glaciers generally respond more rapidly to increases in temperature compared to large ice sheets and are more likely to contribute to surrounding hydrological systems (Maurer et al., 2019). Alpine glaciers that cover areas as small as 1-2% of their respective catchments, have been found to contribute to changes in seasonal streamflow; these glacial contributions are particularly important in warm, dry periods such as the late summer months when there is little precipitation  (Stahl & Moore, 2006). With the intensification of climate change, it has only become more critical to track the evolution of alpine glaciers in the region. Bridge glacier hence serves as a proxy for changes in lake-terminating glaciers throughout the PNW Coast Mountains (Chernos et al., 2016; Moyer et al., 2016).

The History of Bridge Glacier Since the little ice age

In this first, blog we will be looking a Bridge Glacier, since the Little Ice Age. So, what even is the ‘Little Ice Age’? Throughout the Holocene there have been periods of abrupt climatic variation; during these cycles, global average temperature falls and rises cyclically (Lockwood, 2001; Dahl et al., 2002). In the Northern Hemisphere the last of these temperature declines was about 0.2 °C and lasted from approximately 1300 CE to 1850 CE, this period is often referred to as the ‘Little Ice Age’ (LIA) although there is some argument among experts on when it exactly began  but it is fairly well established that the LIA ended at the start the industrial revolution. Global average temperature has been generally moving in an upward trajectory post the LIA (Dahl et al., 2002; Matthews & Briffa, 2005). It is estimated that due to this warming, glaciers globally have lost 25% of their area in the 20th century (Luckman, 1995). 

A 2002-2003 study approximated the terminus of Bridge glacier using dendroglaciological dating, a technique using radiocarbon dating of subfossil wood buried in glacial till and lichenometric dating, a technique that establishes a minimum surface date of rocks using measured lichen thallus diameter to suggest how long a particular rock has been exposed to the surface. The study indicates that the glacier expanded throughout the LIA with the maximum down valley extension occurring around 1367 CE this was roughly 3,000m down valley from the 2003 terminus (Allen & Smith, 2007). The LIA maximum extent is marked by a distinct trimline and lateral moraine on the side of the valley this is demarcated by the tree line.  

From 1367- 1949 CE Bridge only retreated approximately 1,000m but from 1949-2003 it has retreated over 2,000m indicative of its current rapid retreat due to climate change (Allen & Smith, 2007). From 2004-2013 the glacier retreated nearly 3,000 m at a rate of 250 m/year and by aerial photo analysis in 2021 the glacier had retreated approximately another 1600 m. 

There have also been changes in the morphology of the glacier due to its rapid retreat. A 1991 study utilizing 14C dating, tree ring count, and historic air photographs suggest that Bridge glacier previously impounded ice-marginal lakes (i.e lakes that are dammed by ice or moraine of a glacier), by 1970 CE they were completely drained due to reduction in ice mass (Ryder, 1991). Evidence of said lakes can be seen in the glacial outwash plain. The initiation of these lakes occurred around 1405 CE post the LIA maximum and although carbon dating suggests that glacier retreat began in around 1860 CE with the start of the industrial revolution, the lakes remained at their original level until around 1935 CE. The loss of said marginal lakes was due to failure of ice and moraine dams caused by a series of floods initiated by rapid glacial melt flow, this further solidifies evidence for rapid retreat and morphological change throughout the 20th century into the 21st (Ryder, 1991). 

Along with the direct morphological changes at the glacier attributed to rapid retreat, there is also potential for ecological changes. There are not currently any peer-reviewed papers published on the ecology of Bridge Glacier post-LIA but the removal of ice from the valley allowing for the colonization of previously ice-covered regions by alpine flora has been observed during fieldwork. A study of several alpine glaciers in Norway found that the first species began to appear on newly exposed till approximately five years after deglaciation (Prach & Rachlewicz, 2012). Changes in ecology could impact everything from the stability of the till to what large fauna could arrive in the area so it is important to consider when discussing the evolution of Bridge Glacier. Another study of alpine glaciers in the Alps suggested that vegetation in glacial forelands is dependent on both seed arrival paired with timing of suitable environment for growth, this indicates that to obtain estimates of where and how rapidly vegetation is occurring at Bridge Glacier a specific study of the area is needed (Erschbamer et al., 2008). 

In summary, we have established that since the end of the Little Ice Age Bridge Glacier has been in steady retreat which has intensified exponentially over the last 70 years and this trajectory will likely continue. The loss of so much ice mass has changed the morphology as well as the surrounding environment of the glacier and the magnitude of these changes will also likely increase for the foreseeable future. In our next blog, we will dive deeper into what has been occurring at the glacier over the last 10 years and begin to piece together the future implications for climate change-triggered retreat at Bridge. Thanks for joining us on our journey and we hope to see you back for the next part of the series!