A storm swell at high tide causes sunny day flooding of the coastline at Haumoana, Hawkes Bay. Source: Hawkes Bay Regional Council Strategic Plan, Adaptation, 2022-2025
Future sea level rise in Aotearoa and Antarctica
Among the most visible effects of human-induced global warming are rising seas around the world. What does this mean for Antarctica, and for Aotearoa New Zealand?
Here, we provide an overview of recent global and local trends, and the importance of understanding the contribution of the Antarctic Ice Sheet to future sea level rise. Research within the Antarctic Science Platform is improving the accuracy of ice sheet models, reducing uncertainty in future projections, and contributing to sea level projection tools here and overseas.
Global and local trends
Since 1880, the global mean sea level (GMSL) has increased by 20cm and the rate of increase has accelerated six-fold since 1990.
There’s no easy way to halt or reverse this change.
The Earth’s oceans and ice sheets respond slowly to changes in the heat they receive from the atmosphere, and they hold onto this heat for decades to centuries. As a result, sea level globally, and around Aotearoa, will continue to rise well beyond the 21st century. Even if warming of the planet is stabilized below the target set by the 2015 Paris climate agreement (2°C above the pre-industrial average), the world cannot avoid an average increase of 50cm by 2100.
This projected sea level rise places one billion people at risk of high tide flooding.
In many coastal settings, even a small increase in baseline sea level can substantially increase the frequency and magnitude of flooding during high tides from storm surges and extreme weather. For Aotearoa, this level is 30-40cm above the 2005 baseline, which will be reached by mid-century under all future warming scenarios. When this happens, the historical 100-year coastal flood will be an annual occurrence.
According to the latest IPCC Report, global mean sea level rise of up to 2m by 2100 can’t be ruled out. This is because, based on the currently implemented mitigation policies, the world is on track for higher levels of global warming (i.e. 3.2°C above pre-industrial).
Antarctic data improves accuracy of sea level rise projections
To pin down the rate of sea level rise beyond 2050 and reduce uncertainty, the ASP’s Antarctic ice sheet dynamics (Project 1) team are working to identify and understand key rate-determining processes of the Antarctic Ice Sheet (AIS).
The AIS is the largest and most uncertain potential contributor to future sea level rise. These ice sheets hold close to 58m of potential global mean sea level rise – but how and when this ice melt might contribute to sea level rise is uncertain. Melting processes and rates are affected by complex interactions between the ice, ocean, atmosphere and solid Earth processes. One possible sequence of changes and consequences, including feedback loops and thresholds, is outlined in this recent article on Antarctic tipping points.
The ASP leads the international SWAIS2C Project (Sensitivity of the West Antarctic Ice Sheet (WAIS) under 2°C of global warming). This coming Antarctic field season, the SWAIS2C team will drill through the WAIS using a bespoke ice and sediment drilling system. Their goal is to determine if this most vulnerable part of the AIS collapsed during the last time the Earth experienced 1.5°C of global warming (125,000 years ago). At that time, the GMSL was known to be at least 5m higher.
Our research is providing critical constraints for assessing the sensitivity and performance of AIS models currently being used in sea-level projections. Key rate-determining processes of the WAIS have been identified and incorporated into our ice sheet model. This model was used in the IPCC’s Sixth Assessment Report and is being used by the international community to understand the Antarctic contribution to global mean sea level rise.
The figure below shows historically constrained AIS sea-level contribution projections and probabilities for AIS sea-level contribution (m) projected by the statistical emulator relative to 2000 for high and low emissions scenarios. This was one of the models used in the IPCC AR6 global sea level projections, and in the New Zealand SeaRise local sea-level projections for Aotearoa. (Source: Lowry et al., 2021)
Sea level rise related hazards in New Zealand
Sea level rise related hazards were identified by the New Zealand Government’s National Adaptation Plan as one of the greatest set of risks facing Aotearoa. These risks include coastal erosion, groundwater rise and flooding, storm surge inundation, and aquifer and soil salinisation. According to the NZ Climate Change Risk Assessment, the exposure of physical assets and people includes five airports, 200km of road, 1500 jetties, 43,000 residential buildings with a replacement cost of $19 billion and population 133,000 impacted, as well as 80% of the coastal marae, unmeasurable taonga and widespread loss of natural habitat for native flora and fauna.
Consequently, a broad range of stakeholders are concerned with anticipating and managing the impacts of sea level change. These includes central government regulators, local government planners, coastal communities, iwi, and the business, agricultural, maritime, infrastructure, finance and insurance sectors.
The ASP’s Antarctic ice sheet model and data have improved the local sea level projection information and tools, developed by the New Zealand SeaRise Programme. This new national set of site-specific sea-level projections are spaced every 2km around Aotearoa’s coastline.
Sea level change in Antarctica
The impacts of sea level and ice sheet change around Antarctica itself are of critical interest for national Antarctic programme operations, tourism and fisheries.
The ASP Project 1 team recently delivered an Information Paper, co-sponsored by the Scientific Committee for Antarctic Research (SCAR) and the Committee of Managers of National Antarctic Programs (COMNAP) to the 45th Antarctic Treaty Consultative Meeting in Helsinki in June 2023. The paper, titled ‘Understanding Future Sea Level Change Around Antarctica’ outlined:
- Challenges in accurately predicting location-specific sea-level change for Antarctica’s coastline
- Risks that future sea level change represents for operations including science support and tourism, coastal infrastructure and assets, heritage sites, specially protected and managed areas, and bioregions
- Future research requirements and recommendations for actions to mitigate the identified risks.
The report was very well-received by the Antarctic Treaty’s Committee for Environmental Projection, who have asked SCAR to deliver a set of site-specific sea level projections for the coast of Antarctica. The ASP will lead this work with international partners, for delivery to the 46th ATCM in 2024.
This photo, above right, shows waves breaking over reverse osmosis water intake at Scott Base. While future sea level rise or fall is dependent on our future emissions pathway, any change will affect the intake unit and associated infrastructure. (Source: Levy et al, 2020.)
The image below shows Antarctic Specially Protected Areas (ASPAs, blue circles), National Antarctic Programme facilities (red triangles), Heritage Sites and Monuments (HSMs, green squares), and Penguin rookeries (yellow diamonds) that are potentially at risk from future sea level change. (Source: Information Paper 95 to 45th ATCM).
