A NIWA-led study has found New Zealand’s native forests are absorbing more carbon dioxide (CO2) than previously thought.
Study leader, NIWA atmospheric scientist Dr Beata Bukosa, says the findings could have implications for New Zealand’s greenhouse gas reporting, carbon credit costs, and climate and land-use policies.
She says forests – both native and exotic – play a vital role in absorbing CO₂ through photosynthesis, but previous studies may have underestimated the amount of carbon taken up by New Zealand’s mature indigenous forests, which were thought to be roughly carbon neutral.
Using advanced modelling and NIWA’s supercomputer, the researchers examined a decade of atmospheric data, from 2011 to 2020, to better estimate the amount of CO₂ absorbed by New Zealand’s land ecosystems. The NIWA team worked with collaborators at GNS Science and Manaaki Whenua as well as other New Zealand and overseas universities and institutes.
The team used an inverse modelling technique – this combines atmospheric greenhouse gases with a model showing how air is transported through the atmosphere to identify CO2 sources and sinks – and compared the results against New Zealand’s Greenhouse Gas Inventory as well as ‘bottom-up’ models. While the Inventory applies a combination of field inventory, modelling, and remote sensing to quantify forest carbon stocks and stock changes, the ‘bottom-up’ models use calculations based on ecosystem processes, land use and climate across the country, says Dr Bukosa.
“It was thought that some areas and land use types were in a near-balance state with the absorption and release of CO₂. Earlier estimates of how much carbon was removed by New Zealand land ecosystems ranged from a net 24 to 118 million tonnes a year. Our research found that New Zealand’s natural environment absorbed approximately 171 million tonnes of CO₂ annually.” – Dr Bukosa
She says the largest differences between earlier estimates and the new findings came in the South Island.
“This was especially in areas dominated by mature native forests and certain grazing lands. We also found seasonal variation, as during autumn and winter, less CO₂ is released into the atmosphere than earlier estimates suggested.”
“That study was based on only three years of data, and we weren’t sure
if it was just a transient effect related to the climatic conditions, or
if the effect was confined to Fiordland. Our new study shows the carbon
sink is more widespread than we thought, particularly across the South
Island, with greater uptake of CO₂ extending up the West Coast.
“With improvements in our modelling techniques, and data coverage, we’ve
now shown the extra carbon uptake has persisted for at least a decade.
More research could help us understand exactly why our method has shown
such a difference in the carbon source and sink balance compared with
other methods.”
Inverse modelling provides an independent estimate of emissions that can complement inventory-based approaches for emissions reporting, she says.
“New Zealand was the first country to develop the capability to infer
national CO₂ emissions from atmospheric data and has since supported
other countries to develop similar capability.”
Dr Andrea Brandon, a Ministry for the Environment principal scientist who co-authored the study, said the findings help build a clearer picture of the role New Zealand’s natural systems play in absorbing emissions from the atmosphere. However, further work will be needed before they can be included in official emissions reporting.
“We continually improve the Inventory – New
Zealand’s annual record of emissions and removals – as new science and
evidence comes to light. This ensures we have robust information so that
we continue to meet our international reporting obligations.
“The findings from this study indicate there
may be additional carbon uptake somewhere in the system that we are
currently not tracking. We need to identify what we are missing so that
we can further refine our Inventory methods to capture it,” – Dr Andrea Brandon
Dr Bukosa says the results, due to be published shortly in the journal Atmospheric Chemistry and Physics and available here in preprint, have important implications for New Zealand’s tracking of carbon emissions and climate policies:
“We need to better understand why our native forests are absorbing more
CO₂ than expected, and what this could mean for our efforts to reduce
greenhouse gas emissions and achieve our domestic and international
targets.”
The research was part of a NIWA-led, MBIE-funded Endeavour programme called CarbonWatch NZ, which ended last year. NIWA principal scientist Dr Sara Mikaloff-Fletcher led CarbonWatch NZ and says the team is now looking to extend this work to definitively solve the puzzle of the difference in carbon between inventory methods and atmospheric measurements.
“This research suggests that we could make the most of opportunities to
slow climate change through changes to land management. Projections
suggest New Zealand will need 84 million tonnes of emissions reductions
on top of what can be done at home to meet its 2030 international
commitments under the Paris Agreement. In addition to reducing the need
for overseas offsets, better management of our native forests and other
lands could enable New Zealand to be long-term stewards of our carbon
sinks and offer magnificent biodiversity co-benefits.” – Dr
Mikaloff-Fletcher
When people arrived on the shores of Aotearoa New Zealand and began to turn the land to their needs, they set in motion great changes.
The landscape of today bears little resemblance to that of a mere thousand years ago. More than 70% of forest cover has been lost since human arrival. Native bush has been replaced by tussocks, scrublands and, most of all, open agricultural land.
These changes affected our birdlife dramatically. Some species, like the moa, were simply hunted to extinction. Others fell directly to mammalian predators. Many species were victims of severe habitat destruction. The loss of suitable habitat remains a key conservation challenge to this day.
However, a changing distribution of plants is not a uniquely modern feature. New Zealand has seen equally radical shifts in habitat before – during the Ice Age, which lasted 2.6 million years and ended about 12,000 years ago.
This reconstruction shows the extend of glaciers during the height of the last Ice Age some 20,000 years ago.Shulmeister et al, 2019, CC BY-SA
At its height, parts of the country were up to 6°C colder than today, and glacial ice sheets spread wide fingers across the Southern Alps. The dry, cold climate resulted in widespread grass and scrubland. Forest cover became patchy everywhere except for the northern North Island.
Our new research tracks how bird life responded to these changes – in particular how exotic species took advantage of the shifting landscapes to make New Zealand home.
Ice Age invaders
Native birds responded to the Ice Age in a variety of ways. Kiwi populations became so isolated in forest patches they split into new lineages. Several moa species moved across the landscape, following their shifting habitat.
Some groups adapted, spreading into novel environments. Kea split off from their relatives the kākā, becoming more generalised. This is known as in situ adaptation; an existing group changing its habits or character to deal with new environments.
But where new ecological opportunities arise, species from elsewhere will also come to take advantage of them. Our research uncovers a pulse of colonisation by exotic bird species that coincides with the reduction of forest cover and the expansion of grasslands at the start of the Ice Age some 2.6 million years ago.
Many endemic New Zealand birds belong to young lineages that date back to landscape changes during the last Ice Age.Wikimedia Commons, Te Papa by Paul Martinson, CC BY-SA
These species were primarily generalists, able to take advantage of a variety of habitats. But there was also an influx of birds pre-adapted to more open conditions, such as the ancestors of Haast’s eagle, pūtangitangi (paradise shelduck) and pīhoihoi (pipit).
Where did these “invaders” come from? Principally, from Australia. For millions of years, they have ridden the winds across the Tasman Sea and, occasionally, established breeding colonies on our shores.
Over a long enough time, those new populations evolved to become distinct, endemic New Zealand species found nowhere else on earth. Pīwakawaka (fantail), ruru (morepork), weweia (dabchick) and kakī (black stilt), to name a few, are all descended from Ice Age Australian ancestors.
They arrived in a New Zealand characterised by scrub, tussock and grass during cold glacial periods, followed by slowly expanding forests during warmer interglacials.
History repeats itself
Today, open vistas once again dominate the landscape. This time they were sculpted by humans rather than a cooling climate. The changing environment means new ecological opportunities – and vacancies – have been left by the great number of species that have gone extinct.
The open landscapes of today mirror the impacts of the Ice Age. Forest cover is reduced, grass and scrub cover the North and South islands.Lubbe et al, 2025, CC BY-SA
Silvereyes have been here longer, first reported during the 1850s, while glossy ibis and barn owl only started breeding here this century. All likely flew across the Tasman to settle here.
Some arrivals seem to serve as ecological replacements of a kind. The kāhu (swamp harrier) is a stand-in for the now-extinct Eyles’ harrier and Haast’s eagle. The poaka (pied stilt) is a common sight where kakī once dominated. And Australian coots proliferate where New Zealand coots once waded.
Native habitats for native birds
These birds are following ancient patterns and processes. Where new opportunities appear, new organisms will rise to fill them. Our highly modified ecosystems are responding in the only way open to them, with exotic species expanding their range to take advantage of empty ecological niches – job vacancies in the ecosystem.
Indeed, these invasions are likely to become more frequent as species distributions shift in a warming climate. As our native species decline under threats of habitat loss and predation by mammalian pests, they will be ecologically replaced by other species.
Left to their own devices, Aotearoa’s plants and animals will look different in the future. The unique species that have called these islands home for millions of years will increasingly be replaced by more generalist species from elsewhere.
The route to protecting our native species in a fast changing world remains as clear as ever – protect and restore native habitat and eradicate mammalian predators.
Non-native species appear to be better able to resist extreme weather, threatening native plants and animals and potentially creating more favourable conditions for invasive species under climate change. That’s the conclusion of a new study in the scientific journal Nature Ecology and Evolution.
Wildfires, droughts, heavy rainfall and storms are all increasing, and predicted to become more frequent throughout the next century due to human-driven climate change.
Invasive species introduced by humans often possess traits that help them survive or even thrive when ecosystems are disturbed (perhaps by wildfire, a storm or human buildings).
Invasive plants are generally fast-growing, for instance, allowing them to quickly fill gaps before native species can recover from disturbances. They are also often very good at dispersing their seeds, allowing them to quickly colonise disturbed areas.
This is why scientists have long suspected that extreme weather and the success of non-native species could be linked.
If extreme weather removes native plants and animals, that increases the availability of resources such as water and space. Non-native species can then capitalise on these new resources to establish themselves.
Even more concerning is the potential for extreme weather and non-native species to interact, exacerbating their effect on native biodiversity. For instance, in a recent field experiment in the US, scientists deliberately started a fire which killed about 10% of the longleaf pine trees in the area studied.
But in areas where an invasive grass – cogongrass, an Asian native – was allowed to establish itself alongside the pines, the fires had more fuel and were larger, hotter and burned for longer.
Where the scientists had added rain shelters to simulate drought conditions, the grass dried out further and the fires became much more lethal. A combination of drought and the invasive species meant longleaf pine mortality soared to 44%.
Similarly, on the small Macquarie Island in the south west Pacific, a combination of extreme rainfall and the presence of invasive European rabbits reduced the breeding success of nesting black-browed albatrosses. Heavy grazing by the invasive rabbits reduced plant cover, exposing the albatross chicks to the harsh weather conditions.
Macquarie’s grass makes excellent bird nests – and rabbit food.BMJ / shutterstock
This relationship between extreme weather and invasive species – two human-driven drivers of global change – threatens native plants and animals and could cost countries billions of dollars in coming decades. Ecologists must identify priority areas and species that can be targeted in efforts to minimise costs and prevent the loss of native biodiversity.
Bad weather, good for non-natives
To better understand how native and non-native species respond to extreme weather events, the scientists behind the new study reanalysed information from 443 peer-reviewed studies on how species responded to wildfires, droughts and storms. In all, they gathered data on 187 non-native species and 1,852 native species from all major animal groups.
Their results suggest that native and non-native species may indeed respond differently to extreme weather. Across all studies, a total of 24.8% of non-native species benefited from extreme weather events compared to only 12.7% of native species.
Japanese knotweed is an invasive species in much of Europe and North America. Like many invasive plants, it grows quickly and can handle extreme weather.Leon_Brouwer / shutterstock
For example, while native species in both freshwater and land-based ecosystems were harmed by droughts, their non-native counterparts showed no significant response. Notably, marine ecosystems were comparatively more resistant to extreme weather events, with fewer differences between native and non-native species.
The authors did find marine heatwaves harmed native coral species, however, a relationship that has been documented in other scientific studies.
Identifying global hotspots
The authors took this information and combined it with known global hotspots of extreme weather, to identify areas where native species may be particularly vulnerable to the combination of extreme weather and invasive species.
They found high latitude areas such as northern US and Europe, for instance, are both vulnerable to extreme cold spells and possess non-native species that benefit from cold spells. Alternatively, areas of the western Amazon in Brazil and east Asia were identified as vulnerable to flooding and possessing flood-resistant non-native species.
In these regions, non-native species could benefit from increasing cold spells or flooding respectively, posing a greater threat to native plants and animals.
Studies like this are very useful. Regions that are identified as vulnerable can be targeted with early preventative measures to stop the spread of invasive species, or with measures to help native biodiversity cope with climate change.
In my new research, I argue the time has come for the dairy sector to adopt a “just transition” framework to achieve a fair and more sustainable food future and to navigate the disruptions from alternative protein industries.
The concept of a just transition is typically applied to the energy sector in shifting from fossil fuels to renewable energy sources.
But a growing body of research and advocacy is calling for the same principles to be applied to food systems, especially for shifting away from intensive animal agriculture.
Aotearoa New Zealand’s dairy sector is an exemplary case study for examining the possibilities of a just transition because it is so interconnected in the global production and trade of dairy, with 95% of domestic milk production exported as whole-milk powder to more than 130 countries.
Environmental and economic challenges
New Zealand’s dairy sector faces significant threats. This includes environmental challenges such as alarming levels of nitrate pollution in waterways caused by intensive agriculture.
This means livestock farmers, agricultural processors, fertiliser importers and manufacturers won’t have to pay for on-farm emissions. Instead, the government intends to implement a pricing system outside the Emissions Trading Scheme by 2030. To meet emissions targets, it relies on the development of technologies such as methane inhibitors.
The development of plant-based and fermentation proteins poses another threat to the dairy sector.Getty Images
In addition to environmental challenges, global growth and domestic initiatives in the development of alternative dairy products are changing the future of milk production and consumption.
New Zealand dairy giant Fonterra is pursuing the growth of alternative dairy with significant investments in a partnership with Dutch multinational corporation Royal-DSM. This supports precision fermentation start-up Vivici, which already has market-ready products such as whey protein powder and protein water.
Fonterra’s annual report states it anticipates a rise in customer preference towards dairy alternatives (plant-based or precision-fermentation dairy) due to climate-related concerns. The company says these shifting preferences could pose significant business risks for future dairy production if sustainability expectations cannot be met.
Pathways to a just transition for dairy
What happens when one the pillars of the economy becomes a major contributor to environmental degradation and undermines its own sustainability? Nitrate pollution and methane emissions threaten the quality of the land and waterways the dairy sector depends on.
In my recent study which draws on interviews with people across New Zealand’s dairy sector, three key transition pathways are identified, which address future challenges and opportunities.
Deintensification: reducing the number of dairy cows per farm.
Diversification: introducing a broader range of farming practices, landuse options and market opportunities.
Dairy alternatives: government and industry support to help farmers
participate in emerging plant-based and precision-fermentation industries.
While the pathways are not mutually exclusive, they highlight the socioeconomic and environmental implications of rural change which require active participation and engagement between the farming community and policy makers.
The Ministry of Business, Innovation and Employment recently published a guide to just transitions. It maps out general principles such as social justice and job security.
For the dairy transition to be fair and sustainable, we need buy-in from leadership and support from government, the dairy sector and the emerging alternative dairy industry to help primary producers and rural communities. This needs to be specific to different regions and farming methods.
The future of New Zealand’s dairy industry depends on its ability to adapt. Climate adaptation demands balancing social license, sustainable practices and disruptions from novel protein technologies.
With the failure of a global plastics treaty—oil-rich nations and the petrochemical industry putting up the strongest opposition—the following article should give food for thought, especially as every mouthful of seafood contains microplastics.
We’ve all seen the impact of our plastic addiction. It’s hard to miss the devastating images of whales and sea birds that have died with their stomachs full of solidified fossil fuels. The recent discovery of a plastic bag in the Mariana Trench, at over 10,000 metres below sea level, reminds us of the depth of our problem. Now, the breadth is increasing too. New research suggests that chemicals leaching from the bags and bottles that pepper our seas are harming tiny marine organisms that are central to sustained human existence.
Once plastic waste is out in the open, waves, wind and sunlight cause it to break down into smaller pieces. This fragmentation process releases chemical additives, originally added to imbue useful qualities such as rigidity, flexibility, resistance to flames or bacteria, or a simple splash of colour. Research has shown that the presence of these chemicals in fresh water and drinking water can have grave effects, ranging from reduced reproduction rates and egg hatching in fish, to hormone imbalances, reduced fertility or infertility, cardiovascular diseases, diabetes and cancer in humans.
But very little research has looked at how these additives might affect life in our oceans. To find out, researchers at Macquarie University prepared seawater contaminated with differing concentrations of chemicals leached from plastic bags and PVC, two of the most common plastics in the world. They then measured how living in such water affected the most abundant photosynthesising organism on Earth – Prochlorococcus. As well as being a critical foundation of the oceanic food chain, they produce 10% of the world’s oxygen.
Prochlorococcus are miniscule, but there are as many of them in the oceans as there are atoms in a ton of gold.Chisholm Lab/Flickr
The results indicate that the scale and potential impacts of plastic pollution may be far greater than most of us had imagined. They showed that the chemical-contaminated seawater severely reduced the bacteria’s rate of growth and oxygen production. In most cases, bacteria populations actually declined.
What can be done?
Given the importance of oxygen levels to the rate of global heating, and the vital role these phytoplankton play in ensuring thriving marine ecosystems, it is essential that we now conduct research outside of the laboratory into the effects of plastic additives on bacteria in the open seas. In the meantime, we need to take active steps to reduce the risks of chemical plastic pollution.
The clear first step is to reduce the amount of plastic entering the ocean. Recent EU and UK bans on single-use plastics are a start, but much more radical policies are needed now to reduce the role plastic plays in our lives as well as to stop the plastic we do use being released into waterways and dramatically improve appallingly low recycling rates.
At an international level, we must make addressing the waste produced by the fishing industry a priority. Broken fishing nets alone account for almost half of the plastic in the Great Pacific Garbage Patch – and lost or discarded fishing gear accounts for one-third of the plastic litter in European seas. EU incentives announced in 2019 to tackle this waste do not go far enough.
Discarded fishing nets and other fishing gear make up a significant proportion of the plastic in our oceans.Aqua Images/Shutterstock
Legislation is also urgently needed to limit the industrial use of harmful chemical additives to a level that is absolutely necessary. As an example, bisphenol A, found in myriad products ranging from receipt paper to rubber ducks, is now listed as a “substance of very high concern” due to its hormone-disrupting effects. But as yet the few existing laws regulating the chemical do not cover the majority of industrial use. This needs to change – as quickly as possible.
Of course, even if we can completely stop new chemicals from reaching the oceans, we will still have a legacy of plastic and associated chemical pollution to deal with. At the moment, we have no idea whether we’ve already done irreversible damage, or if marine ecosystems are resilient to current levels of plastic pollution in the open oceans. But the health of our oceans is not something we can risk. So, in addition to physical removal schemes such as The Ocean Clean Up, we need to invest in chemical removal technologies as well.
In salty ocean environments, such technologies are under-researched. We are currently in the early stages of developing a floating device that uses a small electric circuit to transform BPA into easily retrievable solid matter, but our work alone is not enough. Scientists and governments need to ramp up their efforts to both understand and eliminate the problem of chemical contamination of our oceans, before it’s too late.
While ocean bacteria may seem far removed from our daily lives, we are dependent on these tiny organisms to maintain the balance of our ecosystems. We ignore their plight at our peril.
Global temperature records are expected to exceed the 1.5 °C threshold for the first time this year. This has happened much sooner than predicted. So can life on the planet adapt quickly enough?
In our new research, published today in Nature, we explored the ability of tiny marine organisms called plankton to adapt to global warming. Our conclusion: some plankton are less able to adapt now than they were in the past.
Plankton live in the top few metres of ocean. These algae (phytoplankton) and animals (zooplankton) are transported by ocean currents as they do not actively swim.
Some data suggest that current climate change have already altered the marine plankton dramatically. Models project a shift of plankton towards both poles (where ocean temperatures are cooler), and losses to zooplankton in the tropics but might not predict the patterns we see in data. Satellite data for plankton biomass are still too short term to determine trends through time.
To overcome these problems, we have compared how plankton responded to past environmental change and modelled how they could respond to future climate changes. As the scientist Charles Lyell said, “the past is the key to the present”.
We explored one of the best fossil records from a group of marine plankton with hard shells called Foraminifera. This comprehensive database of current and past distributions, compiled by researchers at the University of Bremen, has been collected by hundreds of scientists from the seafloor across the globe since the 1960s. We compared data from the last ice age, around 21,000 years ago, and modern records to see what happened when the world has previously warmed.
We used computational models, which combine climate trends with traits of marine plankton and their effect on marine plankton, to simulate the oceanic ecosystems from the last ice age to the pre-industrial age. Comparing the model with the data from the fossil record is giving us support that the model simulated the rules determining plankton growth and distribution.
We found that some subtropical and tropical species’ optimum temperature for peak growth and reproduction could deal with seawater warming in the past, supported by both fossil data and model. Colder water species of plankton managed to drift to flourish under more favourable water temperatures.
Our analysis shows that Foraminifera could handle the natural climate change, even without the need to adapt via evolution. But could they deal with the current warming and future changes in ocean conditions, such as temperature?
Future of the food chain
We used this model to predict the future under four different degrees of warming from 1.5 to 4 °C. Unfortunately, this type of plankton’s ability to deal with climate change is much more limited than it was during past warming. Our study highlights the difference between faster human-induced and slower-paced geological warming for marine plankton. Current climate change is too rapid and is reducing food supply due to ocean stratification, both making plankton difficult to adapt to this time.
Phytoplankton produce around 50% of the world’s oxygen. So every second breath we take comes from marine algae, while the rest comes from plants on land. Some plankton eat other plankton. That in turn gets eaten by fish and then marine mammals, so energy transfers further up the food chain. As it photosynthesises, phytoplankton is also a natural carbon fixation machine, storing 45 times more carbon than the atmosphere.
Around the world, many people depend heavily on food from the ocean as their primary protein sources. When climate change threatens marine plankton, this has huge knock-on effects throughout the rest of the marine food web. Plankton-eating marine mammals like whales won’t have enough food to prey on and there’ll be fewer fish to eat for predators (and people). Reducing warming magnitude and slowing down the warming rate are necessary to protect ocean health.
Recent assessments suggest the ocean current known as Atlantic Meridional Overturning Circulation (AMOC) is slowing down, with collapse a real possibility this century.
The AMOC is a globally important current in the Atlantic Ocean, where surface water moves northward as part of the Gulf Stream and transports warm water towards the Arctic. There it cools and sinks to return southward as a deep ocean current.
The Atlantic meridional overturning current (AMOC) transfers heat to the North Atlantic. Recent trends indicate this current may be slowing.Ruijian Gou, CC BY-ND
However, because Earth’s climate system is interconnected, these impacts could have a global reach. Our new research shows past changes in AMOC have had significant impact on temperatures in New Zealand and across the southern hemisphere. These results imply that future collapse of AMOC may accelerate ongoing warming trends.
Lessons from the past
Between 20,000 and 10,000 years ago, Earth transitioned from peak ice-age conditions to a climate more like today’s. This interval featured rising global temperatures, melting ice sheets and climbing sea levels – all phenomena associated with present-day climate change.
Using this interval as a natural experiment, we have undertaken research to learn more about how AMOC variability can affect climate in New Zealand.
Evidence preserved in the landscape shows cooling and glacier growth in New Zealand coincided with a strengthening AMOC 14,500 years ago.Huw Horgan, Shaun Eaves, CC BY-ND.
To reconstruct how air temperature changed in New Zealand, we examined the past extent of mountain glaciers using evidence preserved in the landscape. Glaciers grow and shrink primarily in response to changing air temperature, which affects the annual balance of snowfall and snow or ice melt. As glaciers change in size, they deposit moraines (rock debris) in the landscape, which can persist for tens of thousands of years.
The analysis of microfossils in marine sediment cores allows scientists to reconstruct past changes in sea-surface temperature.Jenni Hopkins, CC BY-ND
We combined these land-based observations with reconstructions of sea-surface temperature in the Tasman Sea, which we derived from microfossils (smaller than one millimetre in size) known as foraminifera. These microfossils come in a wide range of species and each has a preferred water temperature.
We quantified changes in foraminifera species in a core of marine sediment to trace how local temperature in the Tasman Sea has varied through time.
Global climate connections
Our results show that changes in air and sea-surface temperature followed a similar pattern in the New Zealand region as Earth warmed following the last ice age.
Warming began in both air and sea at about 18,000 years ago, followed by a cooling event at about 14,500 years ago – the Antarctic Cold Reversal. The timing of these changes matches past changes in the AMOC, as recorded in geological climate records from the North Atlantic region.
We examined computer simulations to test the physical connection between changes in the AMOC and New Zealand’s climate. These simulations used a physics-based climate model that captures atmospheric and ocean circulation and their interaction.
Climate model experiments show the impact of past AMOC variability on surface temperature in the Southern Hemisphere.Shaun Eaves, CC BY-ND
The model simulations support our geological evidence, showing air and sea surface temperatures in New Zealand respond sensitively to changes in AMOC intensity. When the AMOC weakens and Europe cools, New Zealand and the southern mid-latitudes undergo warming, and vice versa.
The models also indicate changes in the AMOC are transported rapidly, within decades, to New Zealand via shifting global wind systems. Changes in the AMOC disrupt the temperature gradient between the hemispheres, which is a key control on the strength of westerly wind belts in the southern hemisphere, between the latitudes of 40°S and 60°S where New Zealand is.
Stronger winds over New Zealand bring regional cooling, as more storms track over the country and warm ocean currents are diverted away from the Tasman Sea into the south Pacific. In contrast, when the AMOC weakens, New Zealand has clearer skies and the Tasman Sea receives more tropical water masses, causing regional warming.
Future implications
Scientists have identified several “tipping points” in Earth’s climate system that may be triggered by human-caused climate change. Once these thresholds are crossed, the consequences cannot be easily undone.
Climbing greenhouse gas concentrations have raised air temperatures in New Zealand, and globally, by about 1.1°C since the late 19th century. Projections suggest New Zealand may end this century 1°C to 3°C warmer than now. However, these estimates do not include the potential impacts of a future AMOC collapse.
Our insights from the recent geological past show this AMOC tipping point has global reach, and could accelerate future warming in New Zealand.
Coastal wetlands don’t cover much global area but they punch well above their carbon weight by sequestering the most atmospheric carbon dioxide of all natural ecosystems.
Termed “blue carbon ecosystems” by virtue of their connection to the sea, the salty, oxygen-depleted soils in which wetlands grow are ideal for burying and storing organic carbon.
In our research, published in Nature, we found that carbon storage by coastal wetlands is linked to sea-level rise. Our findings suggest as sea levels rise, these wetlands can help mitigate climate change.
Sea-level rise benefits coastal wetlands
We looked at how changing sea levels over the past few millennia has affected coastal wetlands (mostly mangroves and saltmarshes). We found they adapt to rising sea levels by increasing the height of their soil layers, capturing mineral sediment and accumulating dense root material. Much of this is carbon-rich material, which means rising sea levels prompt the wetlands to store even more carbon.
We investigated how saltmarshes have responded to variations in “relative sea level” over the past few millennia. (Relative sea level is the position of the water’s edge in relation to the land rather than the total volume of water within the ocean, which is called the eustatic sea level.)
What does past sea-level rise tell us?
Global variation in the rate of sea-level rise over the past 6,000 years is largely related to the proximity of coastlines to ice sheets that extended over high northern latitudes during the last glacial period, some 26,000 years ago.
As ice sheets melted, northern continents slowly adjusted elevation in relation to the ocean due to flexure of the Earth’s mantle.
For much of North America and Europe, this has resulted in a gradual rise in relative sea level over the past few thousand years. By contrast, the southern continents of Australia, South America and Africa were less affected by glacial ice sheets, and sea-level history on these coastlines more closely reflects ocean surface “eustatic” trends, which stabilised over this period.
Our analysis of carbon stored in more than 300 saltmarshes across six continents showed that coastlines subject to consistent relative sea-level rise over the past 6,000 years had, on average, two to four times more carbon in the upper 20cm of sediment, and five to nine times more carbon in the lower 50-100cm of sediment, compared with saltmarshes on coastlines where sea level was more stable over the same period.
In other words, on coastlines where sea level is rising, organic carbon is more efficiently buried as the wetland grows and carbon is stored safely below the surface.
Give wetlands more space
We propose that the difference in saltmarsh carbon storage in wetlands of the southern hemisphere and the North Atlantic is related to “accommodation space”: the space available for a wetland to store mineral and organic sediments.
Coastal wetlands live within the upper portion of the intertidal zone, roughly between mean sea level and the upper limit of high tide.
These tidal boundaries define where coastal wetlands can store mineral and organic material. As mineral and organic material accumulates within this zone it creates layers, raising the ground of the wetlands.
The coastal wetlands of Broome, Western Australia.Shutterstock
New accommodation space for storage of carbon is therefore created when the sea is rising, as has happened on many shorelines of the North Atlantic Ocean over the past 6,000 years.
To confirm this theory we analysed changes in carbon storage within a unique wetland that has experienced rapid relative sea-level rise over the past 30 years.
When underground mine supports were removed from a coal mine under Lake Macquarie in southeastern Australia in the 1980s, the shoreline subsided a metre in a matter of months, causing a relative rise in sea level.
Following this the rate of mineral accumulation doubled, and the rate of organic accumulation increased fourfold, with much of the organic material being carbon. The result suggests that sea-level rise over the coming decades might transform our relatively low-carbon southern hemisphere marshes into carbon sequestration hot-spots.
How to help coastal wetlands
The coastlines of Africa, Australia, China and South America, where stable sea levels over the past few millennia have constrained accommodation space, contain about half of the world’s saltmarshes.
Saltmarsh on the shores of Westernport Bay in Victoria.Author provided
A doubling of carbon sequestration in these wetlands, we’ve estimated, could remove an extra 5 million tonnes of CO₂ from the atmosphere per year. However, this potential benefit is compromised by the ongoing clearance and reclamation of these wetlands.
Preserving coastal wetlands is critical. Some coastal areas around the world have been cut off from tides to lessen floods, but restoring this connection will promote coastal wetlands – which also reduce the effects of floods – and carbon capture, as well as increase biodiversity and fisheries production.
In some cases, planning for future wetland expansion will mean restricting coastal developments, however these decisions will provide returns in terms of avoided nuisance flooding as the sea rises.
Finally, the increased carbon storage will help mitigate climate change. Wetlands store flood water, buffer the coast from storms, cycle nutrients through the ecosystem and provided vital sea and land habitat. They are precious, and worth protecting.
The authors would like to acknowledge the contribution of their colleagues, Janine Adams, Lisa Schile-Beers and Colin Woodroffe.
The situation in Antarctica, both what’s currently being observed and the latest research, and the consequences are succinctly explained by Prof. Nerilie Abrahm from the Australian National University. Further consequences are discussed in the second half of the video.
