The Intergovernmental Panel on Climate Change (IPCC) has made significant strides in refining its sea-level rise projections over the years. However, its inherent conservative bias—stemming from the need for broad consensus among scientists and governments, limitations in ice sheet modelling, and a reluctance to emphasise high-uncertainty scenarios—raises concerns that official estimates may downplay worst-case outcomes. In 2025, climate scientist Professor Tim Lenton highlighted these issues, warning that tipping points, such as the collapse of the West Antarctic Ice Sheet (WAIS), could lead to 2–3 meters of sea-level rise by 2100 under unchecked emissions. This aligns more closely with rapid rates observed in paleoclimate events like Meltwater Pulse 1A (MWP-1A) from about 14,300 years ago (~40 mm/year) than with the IPCC’s median projections. The discrepancy between real-world ice loss—such as the Thwaites Glacier’s retreat—and model outputs, combined with historical evidence, fuels fears of underestimation. Lenton advocates for “immediate, unprecedented action,” including updating Nationally Determined Contributions (NDCs) by September 2025 and rapidly scaling renewable energy, to mitigate risks beyond the IPCC’s baseline scenarios.
Ancient Rapid Sea-Level Rise Events: Meltwater Pulses 1A and 1B
Meltwater Pulse 1A (MWP-1A) and Meltwater Pulse 1B (MWP-1B) represent key episodes of abrupt sea-level rise during the last deglaciation, occurring roughly 14,700–14,300 years ago for MWP-1A and 11,600–11,300 years ago for MWP-1B. These events resulted in sea-level increases of ~10–20 meters over ~100–500 years, primarily driven by the rapid collapse or melting of massive ice sheets, including the Laurentide in North America and possibly parts of the Antarctic Ice Sheet.Research indicates that MWP-1A, in particular, may have involved even faster rates during shorter intervals, with intense pulses embedded within the overall event. While the average rate of 40–80 mm/year is well-constrained, Tahiti coral records suggest peak rates of ~80–100 mm/year over ~200 years, and brief surges possibly exceeding 100 mm/year over ~50–100 years (Deschamps et al., 2012). North Atlantic sediment cores point to a catastrophic phase with rates up to 100–130 mm/year over ~50–100 years, associated with massive Laurentide ice stream surges and calving, as evidenced by Heinrich layers (Clark et al., 2004). Modeling of the Laurentide’s saddle collapse further supports these high-end estimates, simulating 9–12 meters of rise in ~100 years (90–120 mm/year) (Gregoire et al., 2012, Nature).The certainty for MWP-1A’s average rate (40–80 mm/year) is high, backed by consistent coral data from sites like Barbados and Tahiti, along with robust uranium-thorium (U-Th) and radiocarbon dating. Peak rates of 100–130 mm/year over shorter periods carry moderate certainty, limited by dating precision and proxy resolution.
Current Sea-Level Rise and Future Projections
Today’s global sea-level rise, measured at 3.7 mm/year from 1993–2023 via satellite data, is ~10–20 times slower than MWP-1A’s average. Yet, in worst-case scenarios, projections suggest acceleration: studies like Bamber et al. (2019) estimate 2–3 meters by 2100 (equivalent to 20–30 mm/year) if the Antarctic and Greenland ice sheets undergo rapid collapse through mechanisms such as marine ice sheet instability (MISI) or marine ice cliff instability (MICI).In contrast, the IPCC’s Sixth Assessment Report (AR6, 2021, with 2025 updates) projects 0.28–1.01 meters by 2100 under the high-emissions SSP5-8.5 pathway, with a “low-likelihood” upper limit of ~2 meters. MWP-1A’s rates of ~40–100 mm/year imply that models may undervalue dynamic ice loss, such as the Thwaites Glacier’s ongoing retreat at ~1 km/year, particularly if tipping points cascade. If events like WAIS destabilization or permafrost thaw intensify, rates could climb to ~20–30 mm/year by 2100—well above the IPCC median. Mitigation strategies, including a 50% reduction in emissions by 2030, are essential to prevent pulses reminiscent of MWP-1A.
Temperatures During MWP-1A: A Cooler World with Rapid Change
Estimating global average atmospheric temperatures for MWP-1A is complex, as paleoclimate proxies mainly offer regional insights, and global reconstructions depend on sparse data and modeling assumptions. MWP-1A unfolded during the Bølling-Allerød warming, a swift transition from the Last Glacial Maximum (LGM, ~26,000–19,000 years ago) to a warmer interstadial phase.During the LGM, global mean surface temperatures were 4–7°C cooler than the pre-industrial era (13.5°C average from 1850–1900), with estimates ranging from 4–5°C (Shakun et al., 2012, Nature) to 5–6°C (Annan & Hargreaves, 2013, Climate of the Past). By MWP-1A’s onset (14,700 years ago), temperatures had risen notably from LGM lows. Proxy records (e.g., ice cores, pollen) and models indicate global means ~2–3°C below pre-industrial levels, with a warming pulse of 0.5–1.5°C over a few centuries driven by Milankovitch orbital changes, a CO₂ increase of ~50 ppm (from ~190 to ~240 ppm), and methane release. This yields a global average of ~12–12.5°C during MWP-1A, compared to today’s ~14.6°C (2023, ~1.1°C above pre-industrial).Certainty is moderate, relying on proxies like Greenland/Antarctic ice cores and ocean sediment δ¹⁸O, integrated via models such as TraCE-21ka. Uncertainties stem from uneven proxy coverage and CO₂ sensitivity assumptions. The rapid North Atlantic warming (~5–10°C) likely initiated the Laurentide collapse, amplified by ice-albedo feedback and CO₂ release.
MWP-1A in a Warmer Modern World: Global Forcing Meets Regional Extremes
Remarkably, MWP-1A occurred at global temperatures ~3–4°C cooler than today’s ~14.6°C, yet it produced sea-level surges of ~14–20 meters at 40–80 mm/year (with peaks up to ~100–130 mm/year) through Laurentide collapse. Modern anthropogenic warming arises from uniform global forcing via elevated greenhouse gases, resulting in a more distributed average temperature rise compared to MWP-1A’s concentrated deglacial pulses (e.g., ~5–10°C North Atlantic spike over ~100–200 years).However, this global driver manifests as extreme regional amplification, especially in the Arctic, where warming proceeds 3–4 times faster than the global average due to ice-albedo loss, Arctic amplification, and circulation shifts. This makes regional warming more intense than global averages imply, with direct consequences for ice sheet stability.Recent data as of October 2025 illustrate this in Greenland and Alaska, where extremes often surpass earlier model forecasts, underscoring IPCC conservatism in dynamic ice loss estimates.
Greenland’s Amplification:
- May 2025 Heatwave: Eastern Greenland (e.g., Ittoqqortoormiit) reached 14.3°C on May 19—~13°C above the average daily maximum of 0.8°C. This event, intensified by ~3°C and made 40 times more likely by climate change (World Weather Attribution analysis), accelerated melt rates up to 17 times the 1981–2010 average.
- Longer-Term Trends: Annual means rise 1.29–1.53 times faster than global, with northern Greenland warming twice as fast as the south. The 2011–2021 decade in central-north Greenland was ~1.5°C warmer than pre-industrial—the warmest in a millennium.
- 2025 Context: Extremes tied to clear skies, high-pressure blocking, and moisture influx contributed to ~150 billion tons of annual ice loss, signaling tipping points like widespread melt at 1.5–2°C global warming.
These changes echo MWP-1A’s destabilization but at a warmer baseline (~14.6°C vs. ~12–12.5°C).
Alaska’s Amplification:
- June 2025 Heat Advisory: A first for central Alaska (e.g., Fairbanks), with temperatures in the 30s°C, feeling like 43°C due to extended daylight. Peaks neared 36°C, driven by persistent high-pressure ridges.
- Spring 2025: March–May averaged -2.4°C (~2.1°C above normal), the warmest third on record; May was wetter, delaying North Slope snowmelt.
- Winter 2024–2025: Early warmth (2.8–5.6°C above average) caused near-zero snowpack in Anchorage, leading to rain and floods; recent years show 20–30% of record-warm days, with highs outpacing colds 5–30 times since 2014.
- Broader Trends: ~1.5°C warming since 1970 exacerbates permafrost thaw, wildfires, and methane release.
While modern warming’s global driver differs from MWP-1A’s abrupt ocean-circulation pulses (e.g., Atlantic Meridional Overturning shifts), the regional response—polar lands warming faster than oceans—mirrors it. Greenland’s accelerated glacier sliding (e.g., Jakobshavn) and Alaska’s permafrost destabilization could push sea-level contributions to ~2–3 meters by 2100 if tipping points cascade, exceeding IPCC medians.
Parallels Between MWP-1A and Today’s Greenland Ice Sheet
Although Greenland’s ice sheet holds only ~7.4 meters of sea-level equivalent—far less than the Laurentide’s ~20–30 meters—the driving conditions today bear notable resemblances to MWP-1A. Greenland’s extreme regional warming (2–3°C overall, with 2025 anomalies like 13°C) parallels the North Atlantic’s 5–10°C pulse that spurred Laurentide collapse. Shared feedbacks, including ice-albedo loss and methane emissions, along with dynamic processes like outlet glacier acceleration, heighten Greenland’s vulnerability to rapid ice loss.Rates of 40–80 mm/year (up to 100–130 mm/year) during MWP-1A at a cooler global temperature (12–12.5°C vs. today’s 14.6°C) indicate Greenland could generate similar pulses if glaciers destabilize. Models like DeConto & Pollard (2021) project ~0.5–1 meter from Greenland by 2100 under 4–5°C warming, nearing MWP-1A’s lower rates. IPCC models often underrepresent such dynamics (e.g., MISI in Greenland basins, akin to Laurentide sliding), and 2025 heatwaves suggest conditions are nearing MWP-1A thresholds.
In essence, MWP-1A’s triggers—regional warming, feedbacks, and ice stream collapse—are not vastly dissimilar from Greenland’s current state. While size limits total impact, amplified warming could yield pulses of ~20–30 mm/year by 2100 if tipping points activate, underscoring the need for aggressive mitigation to avert ancient-scale risks in our warmer world.
by Diogo de Gusmão-Sørensen, Lisbon, 14 October 2025
Related articles: https://scientinel.com/2025/01/07/scientinel-publishes-sea-levels-app/