Global glacier volume is shrinking. This loss of Earth’s land ice is of international concern. Rising seas, to which melting ice is a key contributor, are expected to displace millions of people within the lifetime of many of today’s children. But the problems of glacier loss do not stop at sea level rise; glaciers are also crucial water sources, integral parts of Earth’s air and water circulation systems, nutrient and shelter suppliers for flora and fauna, and unique landscapes for contemplation or exploration.
Finding that ice sheets can respond to climate change on subannual to decadal time scales, glaciology research surged in the early 21st century. Scientists now study the world’s glaciers at many scales, from centimeter-scale in situ measurements to worldwide satellite-based monitoring campaigns.
Field studies facilitate detailed spatiotemporal sampling and deployment of coincident measurement suites, such as Global Positioning System (GPS), weather stations, seismometers, time-lapse cameras, and radar instruments. The results elucidate glacier hydrology, subsurface environments, and glacier dynamics on time scales of minutes to months. Aerial surveys cover inaccessible regions such as crevassed zones and even help with data acquisition between satellite missions.
To cover regions as large as ice sheets, space-borne satellite data are indispensable. Gravity-measuring satellites help estimate ice volume variations, altimetry satellites detect changing surface elevations, and optical and radar imaging satellites measure ice motion, monitor glacier advance and retreat, and observe surface properties, including melt. Growing in situ and satellite archives, along with glacier and ice sheet reconstruction efforts, are beginning to provide the longer records needed to separate glacier “climate” from glacier “weather” (1). The results from this surge in data and scientific effort point clearly to rapid and largely irreversible ice loss.
Disappearing Before Our Eyes
One of the most visible worldwide trends is glacier retreat. Photographs and aerial and satellite images of glaciers show consistent, substantial, and anomalous retreat from the Antarctic Peninsula through Patagonia, Kilimanjaro, and the Himalayas to Greenland and the Arctic. Iconic glaciers—such as many in Glacier National Park, Montana—have already disappeared. Modeling efforts suggest that this is only the beginning; studies project that 52% of all small glaciers in Switzerland will disappear in the next 25 years (2), western Canada will lose 70 ± 10% of its total glacier volume by 2100 (3), and glacier mass losses in coming decades will be substantial for most parts of the Himalaya (4). Some glaciers are bucking the retreat trend, but these responses are commonly due to cyclical glacier-sediment interactions that are independent of climate (5) or are consistent with environmental changes resulting from climate change, such as increased precipitation (6).
Measurements of total glacier ice mass change are no less disturbing. Synthesizing results from several distinct methods for measuring ice mass provides conclusive evidence that the Greenland and West Antarctic Ice Sheets are shrinking at accelerating rates (7). The step change in Greenland ice loss likely began in the early 1990s (8); along the Antarctic Peninsula, signs of destabilization have marched south over the past one to two decades (9). Some of the most sobering observations, though, come from the Thwaites and Pine Island Glaciers region of West Antarctica.
Containing ∼5 m of potential sea level rise, the West Antarctic Ice Sheet is particularly vulnerable because it rests on bedrock well below sea level and is exposed to warm ocean waters at depth. This setting is the key ingredient for triggering an amplifying loop of ice loss called the marine ice sheet instability, in which retreat, thinning, and speedup at the ice sheet edge produce runaway ice loss. Multiple studies indicate that this irreversible West Antarctic collapse is under way (10, 11). The safety band of ice that protects more rapid ice loss around other parts of the Antarctic coast, especially in West Antarctica, is also showing signs of weakening (12).
The risks and impacts of a 1- or 2-m sea level rise differ substantially for coastal cities and island nations. But perhaps even more important for planning is whether that flooding occurs in 2050 or 2150. This question can only be addressed by constraining the rates of ice loss, which must be a top research priority. Determining ice loss rates requires continued development of monitoring capacity, a sustained focus on process studies, and further integration of observations and modeling efforts.
Recognition of the value of glacier and ice sheet monitoring has increased in the past decade, but challenges remain. Limited satellite imaging has been a roadblock in assessing multidecadal ice-sheet–wide change. Much of what we know today came from glaciologists thoughtfully pulling data from satellites for which ice sheet monitoring was not a primary mission directive. The result has been data that are not consistently sampled across space or time, supporting important insights but falling short of providing a complete picture of global changes. Thankfully, ice research is becoming a greater priority. For example, the planned NASA–Indian Space Research Organisation (ISRO) Synthetic Aperture Radar (SAR) mission (NISAR) satellite will be the first radar imaging satellite for which monitoring Earth’s ice sheets is a primary mission directive. Securing funding support for long-term on-the-ground monitoring efforts, however, remains difficult.
Process studies, which are often more easily accommodated by commonly 3-year grant cycles, continue to be key to understanding the mechanisms that control glacier behavior. Efforts to determine the role of hydrology in ice motion (13) and understand glacier and ice-sheet surface-darkening (14) are examples of important and necessary research paths. Integration of observations and models is also crucial; an example is the increasing partnership between the Ice Sheet Model Intercomparison (currently ISMIP6) and Greenland Ice Sheet–Ocean Interactions (GRISO) projects. Better connections across observation and modeling research can improve model parameterizations, increase confidence in future forecasts, and pinpoint areas where additional in situ and remote sensing studies are needed.
Combining tools should also help with another challenge: determining methods and best practices to merge studies with disparate spatial and temporal scales. The next frontier in glaciology includes better integrating scales important for critical glacier processes (tens to hundreds of meters) with efforts to understand and predict ice-sheet–wide changes (hundreds to thousands of kilometers). Progress on this front will require substantial cross-disciplinary collaboration.
Beyond Sea Level Rise
Given the wide-ranging effects of glacier ice loss, it is imperative that glaciologists increase efforts to reach across disciplines, applying a systems science approach to elucidate how ice loss is both affected by and affects other elements of the hydrosphere, biosphere, anthroposphere, and beyond. Although sea level rise is the most common alarm bell from the glaciology community, the impacts of losing ice encompass much more, from reducing water security (4) to altering ecosystems (15). Glaciologists need to capture this breadth in research, data dissemination, and outreach efforts. Indeed, disseminating findings beyond the science community is an activity that must be encouraged, supported, and recognized by universities, research institutions, and funding agencies.
The evidence is overwhelming: Earth is losing its ice. Much of this loss is irreversible and the result of human-caused climate change (1). Unless substantial climate response action is taken and the trend of global temperature rise is reversed, we will continue to see Miami streets swallowed by the sea and glacier freshwater reservoirs melt into mud. And we can expect this pattern to continue for decades, centuries, and indeed, millennia. As scientists, we must make this reality clear and help to ensure that action is taken to minimize impacts globally.
This article and images was originally posted on [Science current issue] May 11, 2017 at 07:11AM
by Twila Moon