The world’s oceans are a large carbon sink, with almost all marine habitats having some role in capturing and storing carbon. This role as a carbon sink means the oceans play a major role in determining the concentrations of carbon dioxide in the atmosphere. The world’s oceans have absorbed about one-quarter the total amount of carbon dioxide emitted by human activities since pre-industrial times. This has significantly reduced greenhouse gas levels in the atmosphere and minimised some of the impacts of global warming (IPCC, 2013).
Our oceans are becoming more acidic
The uptake of carbon dioxide by the oceans is a natural process. However, high concentrations of carbon dioxide in the atmosphere have caused carbon dioxide to be absorbed by oceans at a faster rate. As the world’s oceans have absorbed carbon dioxide emitted by human activities, they have increased in acidity.
The longest record of acidity of New Zealand’s oceans is from measurements in the subantarctic ocean off the Otago coast. Acidity is measured by seawater pH – a reduction in pH indicates increased acidity. Since 1998, the pH in the east subantarctic ocean had an average decrease of 0.0015 units a year (see figure 3).
Note: The decreasing pH over an extended time period indicates the ocean has become more acidic. The fluctuations from year to year reflect strong natural variability due to a myriad of complex biological, physical, and geological processes.
This graph shows how the pH of subantarctic waters east of New Zealand has fluctuated between 1998 and 2014. Visit the MfE data service for the full breakdown of the data.
This increase in New Zealand’s ocean acidity is a statistically significant amount, and is consistent with changes measured elsewhere in the world (Bates et al, 2014). The average pH of global ocean surface waters has fallen by about 0.1 units since the beginning of the industrial era (IPCC, 2013). While this may not seem much, the pH scale is logarithmic – so a decrease of 0.1 pH units is equivalent to a 26 percent increase in acidity (IPCC, 2013).
Ocean acidification will continue for centuries if emissions continue
In 2014, the Intergovernmental Panel on Climate Change (IPCC) concluded, with high confidence, that ocean acidification will increase for centuries if substantial carbon dioxide emissions from human activities continue, and will strongly affect marine ecosystems (IPCC, 2014). Scientists who wrote the oceans chapter within the same IPCC assessment round were less conservative. They concluded that ocean acidification is effectively irreversible in any reasonable human time scale as it will take tens of thousands of years to reverse this chemical change to the world’s oceans (Hoegh-Guldberg et al, 2014).
Ocean acidification may cause widespread harm to ecosystems
While there is uncertainty about the full implications of ocean acidification for New Zealand’s marine environment, ocean acidification could cause widespread changes to marine ecosystems. In a 2012 assessment of human-based threats to New Zealand’s marine habitats, marine experts ranked ocean acidification as the most serious threat to New Zealand’s marine habitats (MacDiarmid et al, 2012a).
Ocean researchers around the world are investigating the impacts of ocean acidification and are treating the issue as a matter of urgency (Secretariat of the Convention on Biological Diversity, 2014). Of particular concern is the growth and reproduction of organisms with shells composed of calcium carbonate, because a more acidic ocean makes it harder for them to build their shells. This covers a diverse range of organisms, including plankton, corals, crustaceans, and molluscs such as shellfish. Plankton form the base of the food chain and are a direct or indirect source of food for almost all marine animals, so any disruption to these organisms may have widespread effects on marine ecosystems (Fabry et al, 2008).
Many overseas studies show that acidification reduces the growth and survival rates of corals, molluscs, and echinoderms (such as sea urchins and starfish), although the responses of different species are variable. Studies also show acidification changes the sensory systems and behaviour of some fish and invertebrates (Secretariat of the Convention on Biological Diversity, 2014).
In New Zealand, ocean acidification may affect some of the species we harvest for customary, commercial, or recreational purposes, such as pāua, mussels, and oysters.
For more detail see Environmental indicators Te taiao Aotearoa: Ocean acidification.
Our oceans are warming
Alongside their role as a carbon sink, oceans also have a critical role in regulating global temperature. The vast mass of the world’s oceans can store huge amounts of heat. In addition, regional climates are strongly influenced by huge ocean currents that transport enormous amounts of heat around the world.
High concentrations of greenhouse gases in the atmosphere are leading to warmer oceans around the world, although this warming is neither constant nor uniform. More than 90 percent of the heat associated with global warming from 1971 to 2010 has been absorbed by the world’s oceans (IPCC, 2013).
Natural variability in sea-surface temperature
New Zealand’s annual average sea-surface temperatures measured by satellite have not shown a statistically significant trend over the past 20 years. This is not surprising given the short time series of these data and expected natural variability of sea-surface temperatures.
Sea-surface temperatures naturally fluctuate with the seasons and from decade to decade, in part due to climate oscillations – persistent atmospheric patterns and variations in ocean currents. During certain phases, climate oscillations can lead to warmer seas or more stormy weather.
The El Niño Southern Oscillation is one of the main climate oscillations over the Pacific region, and influences weather patterns across New Zealand. The warmer El Niño phase can lead to more winds from the south in the winter, causing colder climate patterns in New Zealand and lower sea-surface temperatures in the surrounding ocean (Salinger & Mullan, 1999).
New Zealand’s long-term data indicate rising sea-surface temperatures
Other long-term New Zealand data show a statistically significant trend. Long-term sea-surface temperature measurements taken around New Zealand’s coastline using ships, buoys, and satellites indicate an increase of about 0.71 degrees Celsius over the period 1909–2009 (Mullan et al, 2010). This is consistent with a widely accepted record of global average sea surface temperature showing an increase of 0.067 degrees Celsius per decade for the period 1909–2012 (Hartmann et al, 2013).
The huge heat capacity of the world’s oceans means they will be slow to adjust to any changes in greenhouse gas concentrations. Scientists project that even if greenhouse gas concentrations could be held at present levels, the deep-ocean temperature will continue to warm for centuries to millennia (IPCC, 2013).
The full implications of ocean warming for marine ecosystems are not clear
Change and variability in ocean temperatures, along with other climate changes, can affect ocean currents, marine processes, habitats, and species.
Depending on their tolerance for changing environmental conditions, some species may find it hard to survive in areas where waters are warming. Other species are expected to extend their range as ocean temperatures increase (Willis et al, 2007; Molinos et al, 2015). For example, climate change may strengthen the East Auckland ocean current in northern New Zealand, promoting the establishment of tropical or subtropical species that currently occur as occasional visitors (vagrants) in warm La Niña years (Willis et al, 2007).
These shifts in the range of some species may change the distribution of wild fisheries and aquaculture species (Norman-Lopez et al, 2011), with both challenges and opportunities for fishing and aquaculture industries. There is no conclusive evidence of climate change impact on fish abundance in New Zealand waters (Reisinger, 2014).
In a 2012 assessment of human-based threats to New Zealand’s marine habitats, marine experts ranked warming sea temperature as the second-greatest serious threat to New Zealand’s marine habitats, after ocean acidification (MacDiarmid et al, 2012a).
Sea level is rising
Anthropogenic (human-induced) climate change is causing sea levels to rise around the world. During the 20th century, sea-level rise was mainly due to melting glaciers releasing water into the oceans, and the expansion of warming sea water (IPCC, 2013).
Between 1900 and 2013, sea levels rose (relative to land) between 1.31 millimetres and 2.14 millimetres a year on average at observation sites around New Zealand (see figure 4). These changes are consistent with sea-level rise observed worldwide (Church & White, 2011). Globally, sea level has risen an average of about 20 centimetres since the beginning of the 20th century (IPCC, 2013).
Sea-level rise associated with climate change is not uniform around the world, and is unlikely to be uniform along New Zealand’s coastline. This is because sea level on any coastline is influenced by a range of factors, including the rising or sinking of land relative to the sea, and regional variations in ocean temperatures and circulation.
Note: No information is available for New Plymouth before 1955.
This graph shows sea level-rise at five monitored sites around New Zealand (Auckland, New Plymouth, Wellington, Lyttelton and Dunedin) between 1899 and 2013. Visit the MfE data service for the full breakdown of the data.
Sea-level rise is already having an impact on coastlines and will continue to do so
Sea-level rise is set to continue for many centuries, even if global greenhouse gas emissions are stabilised. The amount and rate of rise depends on future emissions. The IPCC (2013) projected that by the end of the century, likely global average sea-level rise will be between 26 and 98 centimetres, based on scenarios of varying global responses to climate change. While New Zealand’s sea-level rise has been in line with the global average so far, at least one study projected that our sea-level may rise a little faster than the global average in the future (Ackerley et al, 2013).
In New Zealand, an 80-centimetre sea-level rise will mean the current 1-in-100-year high-tide level will be exceeded almost daily (Royal Society of New Zealand, 2016). Rising sea levels and increased heavy rainfall associated with climate change are projected to increase coastal flooding and erosion, which may in turn cause damage to coastal ecosystems and infrastructure (Reisinger et al, 2014). Extreme coastal flooding, usually due to storm surges coinciding with very high tides, already causes disruption and damage in some places around New Zealand’s coastline. Rising sea levels will mean these once very rare flooding events will occur more often.
The implications of sea-level rise for New Zealand are discussed in the Parliamentary Commissioner for the Environment’s 2015 report, Preparing New Zealand for rising seas: Certainty and uncertainty.
For more detail see Environmental indicators Te taiao Aotearoa: Coastal sea-level rise.
Ocean storms and extreme waves are highly variable
Ocean storms and extreme wave events can damage marine ecosystems and affect coastal infrastructure, marine-based industries, and other human activities. At a global level, we have a poor understanding of the likely impact of climate change on ocean storms. Climate change could alter the frequency of storms, with variability between regions. Our knowledge of the impact climate change has on wave height is limited (Hoegh-Guldberg et al, 2014).
Data on the number of days when ocean wind speeds exceed gale force and storm force in New Zealand waters are available from 1979, and show high variability around the country and from year to year. Data on extreme waves are only available from 2008. We need a much longer time series for both measures to assess any statistically significant trend.
Coastal erosion, such as that pictured here in Haumoana, Hawke’s Bay, occurs when waves, wind, or storms erode the coastal environment. Coastal erosion will become more widespread as sea levels rise this century. (Photo: Alan Blacklock, NIWA)