The Groundbreaking Nature Study on Underestimated Coastal Sea Levels

A recent study published in the prestigious journal Nature has sent shockwaves through the climate science community by revealing a critical methodological flaw in how coastal sea levels are measured and projected worldwide. Led by researchers Katharina Seeger from the University of Padova and Philip S.J. Minderhoud from Wageningen University & Research and Deltares, the analysis scrutinized 385 peer-reviewed publications from 2009 to 2025 on sea-level rise (SLR) and coastal hazards. Their findings? Over 99% of these studies mishandled sea-level and elevation data, leading to systematic underestimations of current coastal water heights by an average of 24 to 27 centimeters globally, and up to 1 meter or more in parts of the Indo-Pacific region, including areas near New Zealand.

This "methodological blind spot" arises primarily from relying on global geoid models like EGM96 or EGM2008, which assume a zero-meter sea level baseline without incorporating local measurements from tide gauges or accounting for dynamic factors such as tides, waves, currents, temperature variations, and events like El Niño. In reality, measured mean sea levels (MSL) at the coast often exceed these geoid assumptions, particularly in the Pacific where offsets can surpass 1 meter on low-lying atolls and deltas.

Key Findings: How Much Higher Are Coastal Seas?

The meta-analysis employed four advanced global digital elevation models (DEMs)—CoastalDEM v2.1, FABDEM v1.0, GLL-DTM v2, and DeltaDTM v1—calibrated against the latest mean dynamic topography (MDT) dataset (HYBRID-CNES-CLS2022). Results showed global mean offsets of 0.27 meters for EGM96 (standard deviation 0.76 m) and 0.24 meters for EGM2008 (s.d. 0.52 m). In the Indo-Pacific hotspot, discrepancies balloon to 0.9–1.1 meters on average, with extremes up to 5.5–7.6 meters in data-sparse zones.

For a hypothetical 1-meter relative sea-level rise (RSLR)—a plausible scenario by 2100 under moderate emissions—the corrected baselines reveal 31–37% more land exposed (an additional 287,400–470,700 km²) and 48–68% more people at risk (55–102 million additional individuals), based on WorldPop 2020 and LandScan datasets. In Southeast Asia alone, exposed area surges by 94% (up to 93,800 km² added) and population by 96% (24–47 million). Currently, 22–35 million people live below MSL globally; post-1m SLR, that jumps to 102–132 million, excluding future population growth.

These errors even propagate into major reports like the IPCC AR6, underestimating low-elevation coastal zone (LECZ, <10 m) populations by 8% (896 million vs. 966 million–1.07 billion).

Global and Regional Implications for Coastal Vulnerability

Globally, this underestimation compresses adaptation timelines, as the "starting line" for SLR projections is set too low. Regions like Southeast Asia's Mekong and Ayeyarwady deltas face century-equivalent SLR errors in mere decades, exacerbating subsidence-driven flooding. In the Pacific—relevant to New Zealand's neighbors—low-elevation islands see amplified risks, with sacred sites, graves, and entire communities threatened, as voiced by Pacific activists like Vepaiamele Trief from Vanuatu.

Experts like Anders Levermann from the Potsdam Institute warn of heightened Southeast Asian flooding, while counterpoints from Gonéri Le Cozannet (French Geological Survey) argue local planners intuitively adjust for coastal dynamics. Regardless, the study urges recalibrating workflows with tide-gauge MSL, MDT corrections, and vertical land motion (VLM) data to avoid maladaptation.

New Zealand's Unique Position: Accurate Data Amid Global Shortfalls

While the study spotlights Global South gaps, New Zealand stands out positively. Professor Tim Naish from Victoria University of Wellington's Antarctic Research Centre notes NZ's robust data infrastructure: LiDAR elevation from LINZ (accuracy ±1 mm/year via GPS/satellite), integrated VLM, and NIWA tide gauges. Subsidence affects 70% of the 15,000 km coastline (up to 4 mm/year from tectonics, aquifers, deltas), but tools like NZ SeaRise provide 2 km-resolution projections to 2150 (soon 200 m), stress-testing IPCC AR6 scenarios (SSP1-1.9 to SSP5-8.5).

Dr. Emma Ryan from the University of Auckland emphasizes local datasets like the Coastal Change Database (coastalchange.nz) for erosion rates, underscoring NZ's proactive edge. However, Professor Silvia Serrao-Neumann from the University of Waikato laments political hurdles to using this data for restricting at-risk coastal development.

Photo by Apoorva Nayak on Unsplash

NIWA Projections: What Sea Level Rise Means for Aotearoa

NIWA reports New Zealand's mean sea level rising at 3.5 mm/year, blending thermal expansion, glacier melt, and ice-sheet dynamics. Relative SLR varies: higher in subsiding North Island lowlands (e.g., Auckland 1.5 mm/year subsidence, doubling effective rise), lower in uplifting South Island zones. Extreme coastal flood maps simulate 0–2 m RSLR in 10 cm increments, revealing frequent inundation in Nelson, parts of Auckland (e.g., 2011/2014 events), and river deltas.

By 2150, low-emissions scenarios project 0.3–0.6 m rise; high-emissions up to 1.5–2.5 m nationally, but localized extremes from storms. NZ$180 billion in housing faces flooding risk, per recent reports, with Christchurch subsiding fastest at 3.6 mm/year.

NZ SeaRise: Empowering Coastal Adaptation Nationwide

The NZ SeaRise programme, a collaborative effort by NIWA, GNS Science, and universities, delivers tailored projections every 2 km along the coast to 2300, fusing absolute SLR with VLM from Envisat satellites (2003–2011). Users access timelines via an online toolkit, aiding councils in hazard mapping, infrastructure planning, and regulated setbacks. It addresses "locked-in" risks unavoidable even with net-zero emissions, promoting adaptive pathways over rigid defenses. Explore NZ SeaRise projections for your local coast.

Complementing this, Our Changing Coast guides integrate flooding, erosion, and inundation, urging 100-year planning horizons.

Vulnerable Coastal Hotspots: Auckland, Wellington, Christchurch

Auckland's harbors and isthmus face amplified risks from 1.5 mm/year subsidence plus SLR, with low-lying suburbs like Onehunga prone to king tides. Wellington's subsiding Miramar Peninsula and Te Aro sees VLM doubling rise rates, threatening ports and CBD. Christchurch's 3.6 mm/year subsidence exacerbates post-quake vulnerabilities, with Avon-Heathcote estuary at high inundation risk. NIWA maps highlight 20th-century 220 mm rise already causing episodic floods; future scenarios predict routine high-tide overtopping by mid-century.

• Auckland: Harbor infill, airport runways vulnerable.
• Wellington: Port, rail, and urban reclamation zones.
• Christchurch: Estuaries, new subdivisions on lowlands.

Smaller communities like Nelson and Coromandel face erosion, saltwater intrusion into aquifers, and iwi cultural sites loss.

Expert Perspectives from New Zealand Universities

Dr. Dalila Gharbaoui (University of Canterbury) warns the study's Indo-Pacific focus implies underestimated NZ hazard maps, urging updates for climate finance and Loss & Damage claims. Gabriel Mara (Climate Analytics) stresses compressed adaptation windows for Pacific partners, vital for NZ's regional leadership. University researchers drive innovation: Victoria's Antarctic Centre models ice contributions, Auckland analyzes shoreline change, Waikato plans resilient communities. For those in higher ed jobs in environmental science, opportunities abound in coastal modeling and adaptation research.

Photo by Matthew Stephenson on Unsplash

Solutions and Adaptation Strategies for Kiwi Coasts

New Zealand's adaptive framework shines: Ministry for the Environment's Coastal Hazards Guidance promotes nature-based solutions like dune restoration, wetland buffers, and managed retreat. Councils deploy setbacks (e.g., 20 m from mean high water springs), elevate infrastructure, and use green-gray hybrids. Universities contribute: NIWA-Victoria collaborations forecast storm-tide extremes; Waikato tests resilient agriculture against intrusion. Globally, the study calls for MSL tide-gauge baselines—NZ already leads here. Explore university jobs in NZ climate resilience or career advice for coastal researchers.

NIWA Sea Level Rise Resources offer free tools for planning.

Future Outlook: Research Opportunities and Calls to Action

With SLR accelerating (3.5 mm/year NZ average), locked-in 0.3–0.6 m by 2100 demands urgent upskilling. NZ universities like Otago, Canterbury, and Auckland seek experts in VLM modeling, hydrodynamic simulations, and socio-economic impacts—check faculty positions. Rate professors shaping policy via Rate My Professor. As Pacific risks mount, NZ's research leadership positions it for grants in adaptation science. Act now: Use NZ SeaRise, advocate managed retreat, and support low-carbon pathways to cap high-end scenarios.