Laura Larocca

Lake sediments: Accumulated sediments at the bottom of lakes are invaluable archives of past climate and environmental change. These sediments contain a variety of physical, geochemical, and biological proxy indicators that can be used to gain a multi-faceted understanding of past conditions, and generally offer continuous records of the climate and environment over thousands of years or longer. For example, northern high-latitude records from glacier-fed lakes may be used to infer continuous changes in the relative size of mountain glaciers, and to constrain past climate variability over the Holocene epoch. Although lakes are complex environmental systems and lake sediment proxies can be challenging to interpret, lake-derived proxies have provided critical context for human-driven climate change in regions like the Arctic where instrumental observations are sparse and short in duration.

Historical observations & glacial geomorphology: Owing to their remote and logistically challenging setting, direct field measurements on Arctic glaciers are very rare. Rather, most studies have relied on satellite imagery and other space–based observations to estimate glacier area, volume, or length change, but are inherently restricted to the satellite era, which began in the late 1970s. Glacial geomorphic evidence (e.g., moraines and trim lines) and historical observations (e.g., air photos and declassified photographic survalliance imagery) enable an extension of the limited time frame of satellite-based observational records of glacier change. For example, geomorphic evidence of past glacier extent mapped on high-resolution imagery can be utilized to model historical glacier surfaces and their associated equilibrium-line altitudes (ELAs), a parameter modulated by glacier mass balance. Changes in glacier ELAs provide the basis for quantitative reconstructions of past climate conditions, including regional-scale temperature and precipitation patterns, as well as shifts in ocean and atmospheric circulation, including in associated synoptic patterns and modes of variability. In addition, air photos collected during twentieth-century mapping expeditions of Greenland’s coastline have been used to document the response of the island's glaciers prior to the launch of Earth observing satellites. These data help constrain future projections of glacier mass loss and provide enhanced confidence that recent changes are exceptional on a century timescale.

Satellite remote sensing & geographic information systems: Glaciers distinct from Earth’s ice sheets are rapidly receding with wide ranging effects on water resources, regional hydrology, natural hazards, and sea–level. Over the past two decades, their mass loss has constituted 21% of the observed sea–level rise. Roughly a quarter of Earth’s glaciers, which account for ~60% of the global total glacierized area, lie in the Arctic⎯a region that has warmed almost four times faster than the globe since 1979. Owing to their broadly dispersed, remote, and logistically challenging settings, direct field observations on glaciers are sparse. Although glaciological mass–balance observations from a few hundred glaciers have been collected and are available at the World Glacier Monitoring Service (WGMS), only ~42 glaciers worldwide have continuous records spanning more than 30 years. Where field–based measurements are lacking, long–operational satellite missions, such as the Landsat program, may be utilized alongside geographic information systems (GIS) to observe specific glacier characteristics, such as the snow covered area, which are indicative of glacier health. The development of automated methods will also aid in measuring glacier health metrics metrics over extended periods and across large spatial scales. This will be particularly useful in regions where multi-decade observations are notably scarce, such as many areas of the Arctic.

Synthesis of paleoclimate and environmental data: Paleoclimate data are often disparate, stored in separate systems or file formats, frequently lacking compatibility with one another. Such fragmentation can result in data silos, making it challenging to make comparisons across studies and difficult to access and consolidate information effectively to study past regional or global changes. The assembly and synthesis of datasets of proxy climate and environmental records using FAIR data principles are crucial as they enable recent global changes to be placed within the context of natural variability. For example, Holocene glacial lake sediment records often use several geochemical and physical properties of sediment to infer glacier size over time, values of which are lake system-specific and difficult to compare between studies. Additionally, the interpretation of individual glacial lake records is dependent upon the configuration of glaciers within the catchments. In this case, the most common and robust evidence—glacier presence versus absence (or smaller than present)—can be used to summarize all records and assess the regional and broad Arctic trends in Holocene glacier fluctuations that may be gleaned from these records. Importantly, this particular synthesis strongly reinforced that relatively modest summer warming (compared with projections of larger future climate change) drove major environmental changes across the Arctic including the widespread loss of small mountain glaciers.