1. Effects of climate change on the ocean around New Zealand

The ocean surrounding Aotearoa New Zealand is deeply connected to the global climate system. As the climate warms, sea-surface temperatures are rising, storms and marine heatwaves are intensifying, and ocean acidification is increasing. These changes are already reshaping the marine environment around New Zealand – and they are happening faster here than in many other parts of the world.

Natural climate oscillations such as the El Niño Southern Oscillation and the Southern Annular Mode continue to influence ocean conditions, and their interactions with climate trends are amplifying impacts. Together, these forces are altering ocean circulation, productivity and species distributions, with cascading effects on ecosystems and fisheries.

This section explores the physical changes occurring in our oceans — warming, acidification, shifting currents and rising sea levels — and what they mean for marine ecosystems.

New Zealand’s oceans are warming faster than the global average

• Human activities have driven rapid increases in atmospheric greenhouse gas concentrations, causing Earth to warm. Globally, oceans have captured 90 percent of the excess heat from greenhouse gas emissions, increasing ocean temperatures (Venegas et al, 2023).
• Average sea-surface temperatures have risen across New Zealand’s four oceanic regions between 1982 and 2023. Their warmest year was recorded in either 2022 or 2023, depending on the region (see indicator: Sea-surface temperature: Data to 2023).
• Between 1982 and 2023, sea-surface temperature in the country’s four oceanic regions increased, on average, 0.16 to 0.26 degrees Celsius per decade (see indicator: Sea-surface temperature: Data to 2023).
• The rate of warming in ocean waters around New Zealand is increasing and is now 34 percent faster than the global average warming rates (Pinkerton et al, 2024). New Zealand’s oceans are warming faster than the global average due to changes in atmospheric circulation and corresponding changes in ocean currents (Trenberth et al, 2025).
• Projections indicate sea-surface temperatures in New Zealand’s oceanic regions will warm 1.0 to 1.5 degrees Celsius by 2050 and 1.0 to 3.0 degrees Celsius by 2100 (relative to 1982–2022), with stronger warming in subtropical waters than subantarctic waters (Behrens et al, 2025).

Temperatures of coastal waters are increasing

• Coastal waters around New Zealand are warming faster than the global average, with rates of increase in the country’s nine coastal regions ranging from 0.19 to 0.34 degrees Celsius per decade (Pinkerton et al, 2024; see indicator: Sea-surface temperature: Data to 2023 and figure 1). Remote sensing shows this warming is most pronounced around the South Island and occurs year-round in most regions, highlighting New Zealand’s heightened sensitivity to ocean change (Pinkerton et al, 2024).
• Studies of long-term records of water temperatures in Pelorus Sound (Marlborough Sounds) and at Leigh (Northland) have shown annual warming trends, but the patterns of seasonal timing of warming varies between these regions. In Pelorus Sound, the warming is less rapid in winter and spring, and Leigh showed warming in autumn and winter, but not in summer and spring (Broekhuizen et al, 2021; Shears et al, 2024).
• Warming ocean trends affect the structure and functioning of marine ecosystems (see section 2).

Figure 1: Trends in coastal sea-surface temperature, 1982–2023

Ocean stratification is increasing, and ocean oxygen content is reducing 

• Ocean stratification is the separation of ocean waters into horizontal layers. As the upper ocean warms, ocean waters separate into more distinct layers, which reduces vertical mixing of heat and nutrients. This also reduces the ability of the ocean to take up carbon dioxide (Holt et al, 2022; Jo et al, 2022; Riebesell et al, 2009; see Our marine environment 2019).
• Ocean stratification reduces the amount of oxygen absorbed by the ocean from the atmosphere and so contributes to deoxygenation, which is increasingly recognised as a major stressor for marine ecosystems (Hollitzer et al, 2024). This will affect ecosystems, biodiversity and fisheries, although individual responses to these changes will vary widely (Breitburg et al, 2018). For example, sponges have made adaptive changes to survive in reduced- or low-oxygen water (Micaroni et al, 2022), but the size of some fish populations that are sensitive to these changes may decrease (Gong et al, 2021).

Ocean acidification is increasing globally and around New Zealand

• Oceans have captured about 26 percent of total human carbon emissions since industrialisation (Friedlingstein et al, 2025). This is making the ocean more acidic (Law et al, 2018). Surface ocean acidity is estimated to have increased almost 30 percent from 1750 to 2000 (Jiang et al, 2023). Ocean acidity increased 8.6 percent in subantarctic surface waters off the coast of Otago between 1998 and 2020 (see indicator: Ocean acidification).

The first shift in the large-scale ocean circulation and state around New Zealand has been observed

• The Subtropical Front is the boundary between cold subantarctic water from the south and warmer subtropical water from the north. It is an important area of biological and economic productivity (NIWA, nd-b).
• The region south of the Chatham Islands has shown strong, full-depth ocean warming since 2006, with surface warming around five times the global rate. The warming is a result of the Subtropical Frontal Zone unexpectedly shifting 120 kilometres west, with additional southward displacement further east (Sutton et al, 2024). These shifts have been driven by reduced Southern Ocean currents, likely arising from changes in the ocean heat content gradient between mid and high latitudes, and from changes in wind.
• This is the first time a shift in large-scale ocean circulation and state around New Zealand has been observed. Global climate models project these same changes will continue, suggesting that this warming will persist and strengthen through to the modelling horizon of 2100 (Fox-Kemper et al, 2023). Changes to other global ocean circulation systems, such as the Atlantic Meridional Overturning Circulation, could also accelerate warming trends in the Southern Hemisphere, potentially affecting New Zealand and the surrounding region (Boers, 2021).

Sea levels are rising at an accelerating rate in many locations

• Increasing ocean temperatures cause sea water to expand (Venegas et al, 2023). Combined with melting glaciers and ice sheets, this contributes to absolute sea-level rise (Lindsey & Dahlman, 2025; Venegas et al, 2023).
• The rate of annual mean coastal sea-level rise (relative to land) is accelerating. At four longer-term monitoring sites around New Zealand, annual mean coastal sea levels rose faster (relative to land) between 1961 and 2020 than during the period between 1901 and 1960. These sites are located at Auckland, Wellington, Lyttelton and Dunedin. Two additional monitoring sites with shorter monitoring terms (since 1961) are located at Moturiki (Mount Maunganui) and New Plymouth. At these sites, annual mean coastal sea levels are also rising (see indicator: Coastal sea-level rise).
• Although land uplift is slowing relative sea-level rise in some parts of New Zealand, land subsidence is accelerating it in others. Even without land movement, sea levels are expected to rise at least 20 to 30 centimetres by 2050, compared with 2005 levels (MfE, 2024). For parts of the country, a 30-centimetre rise is a threshold for coastal flooding – above this level, a coastal storm that previously had a 1 percent chance of occurring each year becomes an annual event (NZ SeaRise, nd).

Marine heatwaves have become more frequent, more intense and longer-lasting

• Marine heatwaves are when the temperature of ocean water is abnormally high (in relation to a temperature threshold) for at least five consecutive days. They can have significant impacts on marine ecosystems and the services these ecosystems provide (Hobday et al, 2016; see indicator: Sea surface temperature: Data to 2023).
• Globally, marine heatwaves have become more frequent, more intense and longer-lasting (Montie et al, 2024; Sun et al, 2023).
• In 2022, New Zealand experienced a record number of marine heatwave days, including the two longest and most intense marine heatwaves on record (Salinger et al, 2023; Shears et al, 2024).
• In 2022, the Western North Island region spent an average of 88.5 percent of the year in marine heatwaves, which was the longest duration of the nine coastal regions (see figure 2). The Tasman Sea region spent an average of 61.1 percent of the year in marine heatwaves, which was the longest duration of the four oceanic regions (see indicator: Seasurface temperature: Data to 2023).

Figure 2: Percentage of year spent in a marine heatwave, by coastal region, 2022

Projections show marine heatwaves will continue to grow in frequency, intensity and duration

• The trends in increasing frequency, intensity and duration of heatwaves are expected to continue, but to varying degrees around New Zealand (Behrens et al, 2022, 2025; Bodeker et al, 2022; Montie et al, 2024; Sun et al, 2023).
• Marine heatwave intensities are projected to increase more strongly around the North Island, but the number of annual marine heatwave days will increase more around the South Island (ie, in Southland, South-East (Rise) and the Subantarctic) (Behrens et al, 2025). Under a high-emissions pathway (SSP3-7.0), marine heatwave conditions could become permanent year-round by the end of the century, relative to present-day conditions (Behrens et al, 2022).
• The New Zealand Earth System Model projects that marine heatwave intensity will increase more in subtropical waters than in subantarctic waters (Behrens et al, 2022).
• The largest changes in annual marine heatwave days are projected south of Australia and in the Tasman Sea within the Subtropical Frontal Zone, off the south-west coast of the South Island (Behrens et al, 2022).

Climate oscillations influence natural climate variability

• Natural climate oscillations (cyclical variations in atmospheric and ocean conditions) influence weather and climate in New Zealand. These include the El Niño Southern Oscillation (ENSO), Interdecadal Pacific Oscillation and Southern Annular Mode (SAM) (see indicators: El Niño Southern Oscillation, Interdecadal Pacific Oscillation and Southern Annular Mode).
• ENSO affects our weather through changes in air pressure, sea temperature and wind direction. ENSO has three phases: neutral, El Niño and La Niña. It influences rainfall, temperature and wind patterns in New Zealand and globally. During La Niña, New Zealand may experience more north-easterly winds and wetter conditions in the north and east, and warmer-than-average air and sea temperatures. During an El Niño phase in summer, westerly winds increase, with more rain in the west and dryness in the east. In winter, El Niño can lead to more frequent, cooler southerly winds (Ummenhofer et al, 2009; see indicator: El Niño Southern Oscillation). However, understanding of how ENSO modulates the climate remains limited.
• The most recent El Niño phase was from July 2015 to April 2016. This was one of the two strongest El Niño phases between 1990 and 2022 – the other was during the period 1997 to 1998. The most recent La Niña phase was from April 2022 to December 2022 (see indicator: El Niño Southern Oscillation).

Climate change is affecting these oscillations, which in turn affect ocean conditionsn and extreme weather events

• Knowledge of the compounding impacts of climate oscillations and climate change is growing. Recent developments include new information on the occurrence of marine heatwaves, new methods for determining the significance of extreme weather conditions, growing knowledge of the role of climate change in amplifying large-scale circulation cycles such as ENSO, and new data on coastal erosion (Aldridge & Bell, 2025; Minobe et al, 2025; Oginni et al, 2025; Salinger et al, 2023, 2024). There is also new information on how climate oscillations influence the health and functioning of coastal ecosystems (Lam-Gordillo et al, 2023).
• Climate models indicate that ENSO variations have increased up to 10 percent since 1960, partly due to rising greenhouse gas concentrations in the atmosphere. One result is that El Niño and La Niña events could become stronger and more frequent (Cai et al, 2023).
• In its ‘positive’ phase, the SAM is associated with higher-than-normal air pressure in the New Zealand region, which tends to bring relatively light winds and tranquil weather conditions (NIWA, nd-a). Over recent decades, the SAM has shown a positive trend (Abram et al, 2014), creating conditions that are more favourable to marine heatwaves (Salinger et al, 2024).
• Evidence indicates that the positive trend in the SAM is partly an indirect response to stratospheric ozone depletion and climate change (Goyal et al, 2021; King et al, 2023; Morgenstern, 2021).
• If ENSO and SAM phases align (La Niña combined with positive SAM), this can have a compounding effect on ocean temperatures, and extreme heat conditions are a potential consequence (Salinger et al, 2024).

Most marine food webs depend on energy from primary producers

• Marine primary productivity describes the growth of primary producers in coastal and ocean waters, such as phytoplankton, seaweeds, seagrasses and periphyton. These organisms provide the energy that supports most marine food webs. Phytoplankton productivity is especially important, because it supports healthy marine ecosystems and sustains commercial fisheries for shellfish and finfish. In addition, phytoplankton play a key role in removing carbon dioxide from the atmosphere (MPI, 2021a; Pinkerton et al, 2023). 
• Marine primary productivity is affected by nutrients, water temperature and the availability of light (see indicator: Marine primary productivity: Data to 2023). Large-scale changes to climate and ocean conditions can lead to changes in phytoplankton growth and primary productivity. 
• In estuaries and coastal waters, unusually high primary productivity can indicate ecological stress. Excess nutrients from land run-off can result in harmful algal blooms, which reduce oxygen levels, disrupt food webs and pose risks to marine life and human health (Gall & Pinkerton, 2024; Pinkerton et al, 2023; Roberts & Hendriks, 2022).

Primary productivity is increasing in some areas and decreasing in others

• It is important to monitor changes in primary productivity to understand when conditions are generally beneficial for supporting marine ecosystems, but also when they are potentially harmful – such as when driven by excess nutrients (Nixon & Buckley, 2002; see indicator: Marine primary productivity: Data to 2023).
• Primary productivity is monitored by using satellites to measure chlorophyll-a (chl-a), which is an indicator of the amount of phytoplankton in ocean water. Greater chl-a concentrations indicate more phytoplankton and more primary production (see indicator: Marine primary productivity: Data to 2023).
• Most coastal regions saw very likely increasing primary productivity trends between 1998 and 2022, with average rates ranging from an increase of 5 to 13 percent a decade. Between 1998 and 2022, the highest average surface chl-a concentrations in coastal waters were around the West Coast and East Coast regions of the South Island. The North Eastern North Island coastal region had the lowest average chl-a concentration, and was the only coastal region to see a very likely decreasing trend between 1998 and 2022 (see indicator: Marine primary productivity: Data to 2023 and Our environment 2025: Technical annex).
• In ocean waters, mixed trends have been observed, with both increases and decreases in primary productivity (see indicator: Marine primary productivity: Data to 2023 and figure 3). Surface ocean chl-a concentrations across New Zealand’s exclusive economic zone show a slight decreasing trend in the Subtropical ocean region, despite patterns of both increasing and decreasing trends in the northern Tasman Sea and southern Subtropical ocean regions (see indicator: Marine primary productivity: Data to 2023). At the exclusive economic zone scale, surface ocean chl-a concentrations in the Subantarctic region have generally been increasing over the period from 1997 to 2023, although a decline has occurred in the most recent years of the time series (Pinkerton et al, 2024).
• Primary productivity is expected to decrease around the North Island and the West Coast of the South Island, but it may increase in the Subantarctic region to the south of New Zealand, as habitable ranges shift in response to increasing sea temperatures (Law et al, 2018; Roberts & Hendriks, 2022). However, an unexpected and rapid decline in chl-a in all New Zealand ocean regions has been observed since mid-2019, which has continued to late 2023. This could potentially have significant ecological implications, but more analyses are needed to fully understand the risk (Pinkerton et al, 2024).
• Suspended solids can carry nutrients and block light in coastal and ocean environments. Total suspended solids showed more complex spatial and temporal patterns, but the trends observed off northern Coromandel Peninsula, the east coast of the North Island, Marlborough Sounds, Cloudy Bay (Cook Strait), the eastern South Island and Fiordland were virtually certain to be increasing. In contrast, virtually certain decreasing trends were observed around Kaipara Harbour over the period 2002 to 2023 (Pinkerton et al, 2023).
• Changes have been observed in the species that make up phytoplankton communities in the Firth of Thames, including increases in the abundance of toxic algae (Pseudo-nitzschia). These changes reflect nutrient enrichment over decades, driven by dissolved inorganic and organic nitrogen inputs from land run-off (Safi et al, 2022).

Figure 3: Trends in oceanic marine primary productivity, 1998–2022

Note: See Our environment 2025: Technical annex for interpretation of Stats NZ trend likelihood.