Editor's Note: The research (preprint ID egusphere-2025-2315) was led by King's College London and the British Antarctic Survey and is currently open for discussion and undergoing peer review for The Cryosphere .

How the 20% Figure Was Calculated

The study used a sophisticated data assimilation method called ensemble Kalman inversion to simulate PIG's behavior over the entire 20th century . By running models both with and without human-induced climate forcing, the team could isolate the effect of greenhouse-gas emissions for the first time .

The key finding: even without human influence, natural climate variability would have driven about 80% of the observed retreat . However, anthropogenic greenhouse-gas emissions warmed the Southern Ocean and pushed the glacier approximately 18–20% further back than nature alone would have .

"The impact of the human-warmed ocean was analyzed to have significantly amplified the retreat of the glacier," researchers stated, describing it as "the first study to quantify the human impact on the retreat of large Antarctic glaciers" .

The 1940s Trigger: A Pulse of Warm Water

Multiple independent lines of evidence converge on the 1940s as the moment PIG's modern retreat began . Sediment cores recovered from beneath the PIG ice shelf tell a clear story: incursion of marine water beyond the crest of a subglacial ridge, forming an ocean cavity beneath the ice shelf, occurred in 1945 (±12 years) . Final ungrounding of the ice shelf from that ridge followed in 1970 (±4 years) .

This initial trigger appears to have been natural — a pulse of unusually warm ocean conditions linked to El Niño variability at the time . The retreat didn't stop even after ocean temperatures in the region returned to normal .

Geological evidence also shows this is not the first time PIG has responded dramatically to external forcing. Glacial-geological and geochronological data demonstrate that PIG experienced rapid thinning during the early Holocene, around 8,000 years ago, at rates comparable to today — confirming the glacier is highly sensitive to external oceanographic and atmospheric drivers .

Irreversible Retreat: Marine Ice-Sheet Instability

Once triggered, PIG entered a self-sustaining phase of retreat driven by marine ice-sheet instability (MISI). Because the glacier sits on a retrograde (reverse-sloping) bed well below sea level, retreat becomes self-perpetuating once it begins .

A 2014 study in Nature Climate Change using three independent ice-flow models found PIG's grounding line was "probably engaged in an unstable 40 km retreat," with simulated mass loss increasing from an average of 20 Gt/yr (observed 1992–2011) to over 100 Gt/yr — equivalent to 3.5–10 mm of sea-level rise .

In 2023, researchers from Northumbria University and Bangor University found that PIG passed an irreversible tipping point within the last 80 years. A temporary increase in melting under its ice shelf was enough to push the glacier past a threshold from which it could not recover, even during subsequent cooler periods . The accelerated phase lasted through the 1970s–1990s .

Crucially, the retreat of PIG and neighboring Thwaites Glacier occurred synchronously, linked to external oceanographic and atmospheric forcings modulated by climate variability — not internal dynamics unique to each glacier .

What This Means for Future Sea-Level Rise

The projected sea-level contribution from PIG alone is up to ~5.1 cm over the next two centuries under anticipated ocean warming, assuming its ice shelf does not collapse from other causes .

The broader Amundsen Sea Embayment (including Thwaites) dominates projections:

• Antarctic Ice Sheet could contribute up to 30 cm by 2100 and 72 cm by 2200 (95th percentile), with the Amundsen Sea sector responsible for up to 25 cm by 2100 and 48 cm by 2200 .
• Under a high-emissions scenario (RCP8.5), total Antarctic contributions could reach ~0.3 m by 2100 and more than 3 m by 2200, including notable contributions from East Antarctica .

A Temporary Pause, Then More Retreat

There is a nuanced detail in the projections. Some modeling indicates that the central grounding line of PIG could reach a temporary steady state within ~150 years on a stabilizing section of the bed .

However, this stabilization is brief. Under continued warming, the glacier eventually resumes retreat as it passes the stabilizing pinning point and re-enters reverse-sloping terrain . The overall trajectory is one of resumed retreat — not recovery — even if short-lived pauses occur at bedrock ridges that temporarily slow the grounding-line migration .

As one modeling study noted, even 25 years of cold forcing after the initial trigger made "no impact on the ice thickness, speed, or grounding-line position of PIG" . The glacier's retreat is now irreversible on human timescales.

Editor's Note: The research (preprint ID egusphere-2025-2315) was led by King's College London and the British Antarctic Survey and is currently open for discussion and undergoing peer review for The Cryosphere .

How the 20% Figure Was Calculated

The study used a sophisticated data assimilation method called ensemble Kalman inversion to simulate PIG's behavior over the entire 20th century . By running models both with and without human-induced climate forcing, the team could isolate the effect of greenhouse-gas emissions for the first time .

The key finding: even without human influence, natural climate variability would have driven about 80% of the observed retreat . However, anthropogenic greenhouse-gas emissions warmed the Southern Ocean and pushed the glacier approximately 18–20% further back than nature alone would have .

"The impact of the human-warmed ocean was analyzed to have significantly amplified the retreat of the glacier," researchers stated, describing it as "the first study to quantify the human impact on the retreat of large Antarctic glaciers" .

The 1940s Trigger: A Pulse of Warm Water

Multiple independent lines of evidence converge on the 1940s as the moment PIG's modern retreat began . Sediment cores recovered from beneath the PIG ice shelf tell a clear story: incursion of marine water beyond the crest of a subglacial ridge, forming an ocean cavity beneath the ice shelf, occurred in 1945 (±12 years) . Final ungrounding of the ice shelf from that ridge followed in 1970 (±4 years) .

This initial trigger appears to have been natural — a pulse of unusually warm ocean conditions linked to El Niño variability at the time . The retreat didn't stop even after ocean temperatures in the region returned to normal .

Geological evidence also shows this is not the first time PIG has responded dramatically to external forcing. Glacial-geological and geochronological data demonstrate that PIG experienced rapid thinning during the early Holocene, around 8,000 years ago, at rates comparable to today — confirming the glacier is highly sensitive to external oceanographic and atmospheric drivers .

Irreversible Retreat: Marine Ice-Sheet Instability

Once triggered, PIG entered a self-sustaining phase of retreat driven by marine ice-sheet instability (MISI). Because the glacier sits on a retrograde (reverse-sloping) bed well below sea level, retreat becomes self-perpetuating once it begins .

A 2014 study in Nature Climate Change using three independent ice-flow models found PIG's grounding line was "probably engaged in an unstable 40 km retreat," with simulated mass loss increasing from an average of 20 Gt/yr (observed 1992–2011) to over 100 Gt/yr — equivalent to 3.5–10 mm of sea-level rise .

In 2023, researchers from Northumbria University and Bangor University found that PIG passed an irreversible tipping point within the last 80 years. A temporary increase in melting under its ice shelf was enough to push the glacier past a threshold from which it could not recover, even during subsequent cooler periods . The accelerated phase lasted through the 1970s–1990s .

Crucially, the retreat of PIG and neighboring Thwaites Glacier occurred synchronously, linked to external oceanographic and atmospheric forcings modulated by climate variability — not internal dynamics unique to each glacier .

What This Means for Future Sea-Level Rise

The projected sea-level contribution from PIG alone is up to ~5.1 cm over the next two centuries under anticipated ocean warming, assuming its ice shelf does not collapse from other causes .

The broader Amundsen Sea Embayment (including Thwaites) dominates projections:

• Antarctic Ice Sheet could contribute up to 30 cm by 2100 and 72 cm by 2200 (95th percentile), with the Amundsen Sea sector responsible for up to 25 cm by 2100 and 48 cm by 2200 .
• Under a high-emissions scenario (RCP8.5), total Antarctic contributions could reach ~0.3 m by 2100 and more than 3 m by 2200, including notable contributions from East Antarctica .

A Temporary Pause, Then More Retreat

There is a nuanced detail in the projections. Some modeling indicates that the central grounding line of PIG could reach a temporary steady state within ~150 years on a stabilizing section of the bed .

However, this stabilization is brief. Under continued warming, the glacier eventually resumes retreat as it passes the stabilizing pinning point and re-enters reverse-sloping terrain . The overall trajectory is one of resumed retreat — not recovery — even if short-lived pauses occur at bedrock ridges that temporarily slow the grounding-line migration .

As one modeling study noted, even 25 years of cold forcing after the initial trigger made "no impact on the ice thickness, speed, or grounding-line position of PIG" . The glacier's retreat is now irreversible on human timescales.