Thwaites Glacier is an unusually broad and vast Antarctic glacier located east of Mount Murphy, on the Walgreen Coast of Marie Byrd Land. It was initially sighted by polar researchers in 1940, mapped in 1959–1966 and officially named in 1967, after the late American glaciologist Fredrik T. Thwaites. The glacier flows into Pine Island Bay, part of the Amundsen Sea, at surface speeds which exceed per year near its grounding line. Its fastest-flowing grounded ice is centered between east of Mount Murphy. Like many other parts of the cryosphere, it has been adversely affected by climate change, and provides one of the more notable examples of the retreat of glaciers since 1850.
Thwaites Glacier is closely monitored for its potential to elevate sea levels. Since the 1980s, Thwaites and Pine Island Glacier have been described as part of the "weak underbelly" of the West Antarctic Ice Sheet, in part because they seem vulnerable to irreversible retreat and collapse even under relatively little warming, but mainly because if they go, the entire ice sheet is likely to eventually follow. This hypothesis is based on both theoretical studies of the stability of marine ice sheets and observations of large changes on these two glaciers. In recent years, the flow of both of these glaciers has accelerated, their surfaces have lowered, and their grounding lines have retreated. They are believed very likely to eventually collapse without any further warming. The outsized danger Thwaites poses has led to some reporters nicknaming it the Doomsday Glacier, although this nickname is controversial among scientists.
The Thwaites Ice Shelf, a floating ice shelf which braces and restrains the eastern portion of Thwaites Glacier, is likely to collapse within a decade from 2021. The glacier's outflow is likely to accelerate substantially after the shelf's disappearance; while the outflow currently accounts for 4% of global sea level rise, it would quickly reach 5%, before accelerating further. The amount of ice from Thwaites likely to be lost in this century will only amount to several centimetres of sea level rise, but its breakdown will rapidly accelerate in the 22nd and 23rd centuries, and the volume of ice contained in the entire glacier can ultimately contribute to global sea level rise, which is more than twice the total sea level rise to date. Some researchers have proposed engineering interventions to stabilize the glacier, but they are costly and their success remains uncertain.
Location and features
Thwaites Glacier is located at the northern edge of the West Antarctic Ice Sheet, next to Pine Island Glacier. Both glaciers continually shed ice from their grounding line into Pine Island Bay, which is part of the Amundsen Sea. The fastest flows of ice occur between east of Mount Murphy, where they can exceed per year. At in width, Thwaites Glacier is the single widest glacier in the world, and it has an area of . This makes it larger than the American state of Florida (), and a little smaller than the entire island of Great Britain (). It is also very tall, with ice thickness from bedrock to surface measuring between and . Due to this immense size, enormous mass is shed when the repeated ice calving events occur at the glacier's marine terminus—the point where grounding line is in contact with water. The largest events, on the glacier's more vulnerable western side, are seismically detectable at ranges up to .
The third Antarctic expedition of Richard E. Byrd in 1940 is believed to be first official sighting of the coastline of Thwaites. Detailed mapping of the glacier's surface took place between 1959 and 1966. In 1967, it was officially named by the Advisory Committee on Antarctic Names after Fredrik T. Thwaites (1883–1961), who had never personally visited the glacier, but was a renowned glacial geologist, geomorphologist and professor emeritus at the University of Wisconsin–Madison. McMurdo Station is used by researchers studying the glacier, such as the International Thwaites Glacier Collaboration (ITGC).
Thwaites Glacier Tongue and Thwaites Iceberg Tongue
The Thwaites Glacier Tongue, or Western Glacier Tongue () was a narrow, floating part of the glacier, located about east of Mount Murphy. It was the first part of the glacier to be mapped, based on 65,000 aerial photographs collected during Operation Highjump in 1947. Back then, it was about long and wide. By the time updated mapping took place during Operation Deepfreeze in 1967, the glacier tongue had advanced up to further north, and had also experienced massive ice calving events which had produced Thwaites Iceberg Tongue (), a loose collection of icebergs occupying an area as large as long and wide at the time. After breaking off from Thwaites Glacier Tongue, those icebergs ran aground in the Amundsen Sea, about northeast of Bear Peninsula. Initially, their southern extent was north of Thwaites Glacier Tongue, but as parts of the iceberg tongue continued to calve, it diminished in size (to long and wide). By 1986, the iceberg tongue had rotated to the side and started to drift away, travelling west between 1986 and 1992.
Post-2010 break-up and current state Thwaites Glacier Tongue had also experienced destructive changes, eventually shortening to long and wide. By 2012, it went from an ice tongue firmly attached to the rest of the glacier to a series of icebergs floating next to each other, each no larger than in width and only held in place by sea ice. The final remainder of the old glacier tongue, with an area of , disintegrated in 2016. This "melange" of icebergs is still referred to by its old name, as it continues to occupy a substantial amount of area and may retain a stabilizing effect on the glacier. However, future retreat of the surrounding sea ice is likely to trigger disintegration of ever-larger sections, like during the 2019 disintegration of icebergs on its western margin. In 2023, scientists found that ice tongue retreat rates are subject to wide fluctuations after its break-up: over six years of observations, annual retreat accelerated by as much as 40% (from around to per year) twice, before slowing back down. These researchers have also repurposed a machine learning algorithm normally used in microbiology to identify crevasses in the remains of the ice tongue and project how they may affect its stability.
Iceberg B-22a
On 15 March 2002, a notable calving event took place, when the National Ice Center reported that an iceberg named B-22 broke off. This iceberg was about long by wide, with a total area of some , comparable to Rhode Island. While most of the iceberg broke up quickly, the largest piece, B-22A, with an area of around or "twice the size of Houston, Texas", drifted in the vicinity of the glacier even as the rest of the glacier tongue continued to break up. In 2012, it got stuck on seafloor, away from the ice tongue, where its presence had some stabilizing impact on the rest of the glacier. In October 2022, it started moving again, rapidly drifting to the northwest. It is likely to be one of the longest-lived icebergs in history.
Thwaites Ice Shelf
Glaciers in Antarctica commonly have ice shelves, which are large bodies of sea ice that are permanently floating just offshore, and whose presence helps to stabilize the glacier. Though the Thwaites Ice Shelf has a width of and a vertical thickness of at least ), it is relatively light for its size, and is stabilized by partially resting on an underwater mountain offshore. While it only shields the eastern part of the glacier (with the western formerly covered by the Ice Tongue), its presence is sufficient to counter large calving events on that side of the glacier. Under the hypothesis of marine ice cliff instability, ice cliffs at the edge of the glacier would be unsustainably tall once this ice shelf fails and no longer buttresses them, leading to a chain reaction of collapse over centuries. However, the accuracy of this hypothesis has been disputed in multiple papers, and some research suggests that the loss of the ice shelf would result in almost no change to the glacier's trajectory.
Subglacial features
Swamp-like canal areas and streams underlie the glacier. The upstream swamp canals feed streams, while the dry areas between those streams slow the flow of the glacier. Due to this friction, the glacier is considered stable in the short term. As warming progresses, these streams expand and form larger structures underneath the glacier. The largest one to date was discovered by NASA researchers in 2019—an underwater cavity formed mostly in the previous three years, nearly tall and wide, with an area two-thirds the size of New York's Manhattan island.
In 2014, heat flow from geothermal activity in the area underneath Thwaites Glacier was found to be nearly twice the global average, and about 3.5 times larger in hotspots. By 2017, scientists had mapped 138 volcanoes beneath the West Antarctic Ice Sheet, with 91 of them previously unknown. Marie Byrd Land, the location of Thwaites and Pine Island Glaciers, was found to harbor around one volcano per every of area. This density is relatively high, though not as high as in other global hotspots such as the East African Rift (one per ) or Antarctica's own central rift (one per ). The heat from magma flows beneath these volcanoes can affect melting, and as more ice is lost, isostatic rebound increases the risk of volcano eruptions. At the same time, both Marie Byrd Land and the central rift also contain the majority of West Antarctica's 29 volcanoes whose height exceeds , which remain covered by ice. This massive size is likely to make them into significant roadblocks to ice flows, and thus gives them the potential to delay glacier retreat in its advanced stages.
Importance
Between 1992 and 2017, Thwaites Glacier retreated at between and annually, depending on the sector, and experienced a resulting net loss of over 600 billion tons of ice. This loss had caused about 4% of the global sea level rise over that period. If all of the ice contained within Thwaites Glacier melted (which is expected to take place over multiple centuries), that would raise the global sea level by . This is more than twice as large as all of the sea level rise which occurred between 1901 and 2018 (estimated at ), though only a fraction of the total sea level rise which would be seen in the future, particularly under high warming.IPCC, 2021: Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, US, pp. 3−32, doi:10.1017/9781009157896.001.