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
Under the landmark 2015 Paris Agreement on climate change, humanity is seeking to reduce greenhouse gas emissions and keep planetary heating to no more than 1.5°C above the pre-industrial average. In 2024, temperatures on Earth surpassed that limit.
This was not enough to declare the Paris threshold had been crossed, because the temperature goals under the agreement are measured over several decades, rather than short excursions over the 1.5°C mark.
But the two papers just released use a different measure. Both examined historical climate data to determine whether very hot years in the recent past were a sign that a future, long-term warming threshold would be breached.
The answer, alarmingly, was yes. The researchers say the record-hot 2024 indicates Earth is passing the 1.5°C limit, beyond which scientists predict catastrophic harm to the natural systems that support life on Earth.
2024: the first year of many above 1.5°C
Climate organisations around the world agree last year was the hottest on record. The global average temperature in 2024 was about 1.6°C above the average temperatures in the late-19th century, before humans started burning fossil fuels at large scale.
Earth has also recently experienced individual days and months above the 1.5°C warming mark.
But the global temperature varies from one year to the next. For example, the 2024 temperature spike, while in large part due to climate change, was also driven by a natural El Niño pattern early in the year. That pattern has dissipated for now, and 2025 is forecast to be a little cooler.
These year-to-year fluctuations mean climate scientists don’t view a single year exceeding the 1.5°C mark as a failure to meet the Paris Agreement.
However, the new studies published today in Nature Climate Change suggest even a single month or year at 1.5°C global warming may signify Earth is entering a long-term breach of that vital threshold.
What the studies found
The studies were conducted independently by researchers in Europe and Canada. They tackled the same basic question: is a year above 1.5°C global warming a warning sign that we’re already crossing the Paris Agreement threshold?
Both studies used observations and climate model simulations to address this question, with slightly different approaches.
In the European paper, the researchers looked at historical warming trends. They found when Earth’s average temperature reached a certain threshold, the following 20-year period also reached that threshold.
This pattern suggests that, given Earth reached 1.5°C warming last year, we may have entered a 20-year warming period when average temperatures will also reach 1.5°C.
The Canadian paper involved month-to-month data. June last year was the 12th consecutive month of temperatures above the 1.5°C warming level. The researcher found 12 consecutive months above a climate threshold indicates the threshold will be reached over the long term.
Both studies also demonstrate that even if stringent emissions reduction begins now, Earth is still likely to be crossing the 1.5°C threshold.
Heading in the wrong direction
Given these findings, what humanity does next is crucial.
For decades, climate scientists have warned burning fossil fuels for energy releases carbon dioxide and other gases that are warming the planet.
But humanity’s greenhouse gas emissions have continued to increase. Since the Intergovernmental Panel on Climate Change released its first report in 1990, the world’s annual carbon dioxide emissions have risen about 50%.
Put simply, we are not even moving in the right direction, let alone at the required pace.
The science shows greenhouse gas emissions must reach net-zero to end global warming. Even then, some aspects of the climate will continue to change for many centuries, because some regional warming, especially in the oceans, is already locked in and irreversible.
If Earth has indeed already crossed the 1.5°C mark, and humanity wants to get below the threshold again, we will need to cool the planet by reaching “net-negative emissions” – removing more greenhouse gases from the atmosphere than we emit. This would be a highly challenging task.
Feeling the heat
The damaging effects of climate change are already being felt across the globe. The harm will be even worse for future generations.
Australia has already experienced 1.5°C of warming, on average, since 1910.
Our unique ecosystems, such as the Great Barrier Reef, are already suffering because of this warming. Our oceans are hotter and seas are rising, hammering our coastlines and threatening marine life.
These studies are a sobering reminder of how far short humanity is falling in tackling climate change.
They show we must urgently adapt to further global warming. Among the suite of changes needed, richer nations must support the poorer countries set to bear the most severe climate harms. While some progress has been made in this regard, far more is needed.
A major shift is also needed to decarbonise our societies and economies. There is still room for hope, but we must not delay action. Otherwise, humanity will keep warming the planet and causing further damage.
It’s now official. Last year was the warmest year on record globally and the first to exceed 1.5°C above pre-industrial levels. This doesn’t mean it’s too late to rein in further warming, but the ambition required rises with each delay in action.
New Zealand is no exception. Current climate policies are no longer a sufficient contribution to the global effort to keep warming at 1.5°C, according to the Climate Change Commission’s first review of the country’s 2050 climate target.
New Zealand’s current 2050 target has two components. Methane emissions from livestock must be cut by 24% to 47% below 2017 levels and emissions of all other greenhouse gases must reach net zero. But the commission has made three main recommendations to raise ambition:
a net negative target for emissions of long-lived gases (carbon dioxide and nitrous oxide) by removing 20 million tonnes more from the atmosphere than is released each year
a higher target range for biogenic methane emissions to reach at least 35% to 47% below 2017 levels
and the inclusion of emissions from international shipping and aviation.
The commission says these changes would bring New Zealand closer to “net zero for all gases”, in line with what is needed to achieve the goals of the Paris Agreement.
The 2050 target review was the last effort for the commission’s outgoing founding chair, Rod Carr, who has become a significant voice for climate action. In his closing words to parliament, he said:
Those who continue to promote the combustion of fossil fuels in the open air without permanent carbon capture and storage are, in my view, committing a crime against humanity.
The threshold for recommending a change is high. The commission must consider nine key areas and find “significant” developments that justify recommending a different target.
It found three significant changes occurred since the current target was set in 2019.
1. Global action is ahead of New Zealand
While other countries’ current policies, pledges or targets are not sufficient to keep temperature rise at 1.5°C, many countries now have more ambitious targets than New Zealand.
Australia, Japan, US, Canada, EU and Ireland all adopted full net-zero targets in 2021. Finland and Germany have or are considering net negative targets. Among countries with high biogenic methane emissions, several now have full net-zero targets.
2. Scientific understanding of climate change has changed
Climate impacts are appearing sooner and with more severity than the scientific community understood when the target was set in 2019.
3. The burden shifts to future generations
The increased risks and impacts of climate change have implications for inter-generational equity. Delaying action shifts costs and risks to future generations.
The commission’s report also explores New Zealand’s reliance on large-scale commercial exotic afforestation to meet its climate targets. This is one reason why Climate Action Tracker rates New Zealand’s response as highly insufficient and commensurate with a 4°C world.
Carbon in trees is part of the biosphere and will never be stored as permanently as fossil carbon. To take a case in point, Cyclone Gabrielle in 2023 (made worse by climate change) damaged forests, farms and infrastructure, and removed the social licence for forestry in the region.
How the recommended target was set
The commission’s work is tightly prescribed by law. It looked at four possible ways of sharing the global 1.5°C task: equal per capita emissions, national capacity, responsibility for historic warming and the right of all peoples to sustainable development.
New Zealand’s current target does not meet any of these standards, but the commission says the new target would at least meet the “national capacity” criterion and would be feasible and acceptable. However, it would still see New Zealand contributing two to three times its share of global warming this century.
The commission’s assessment is independent of any global warming metrics such as GWP100 (currently the UN standard). Instead, the commission computed New Zealand’s historical and future contribution to temperature rise directly. Both commonly used historical baselines, 1850 and 1990, yield similar results.
New Zealand’s government is currently particularly at odds with the commission’s recommendation on biogenic methane. It appointed a separate advisory panel last year which put forward a target consistent with causing “no additional warming” to the planet from agricultural methane emissions.
This graph shows the contribution to warming from emissions in New Zealand (1850–2100) under the current 2050 target.Climate Change Commission, CC BY-SA
But the commission rejects this idea, finding that unless the rest of New Zealand’s target were to be strengthened significantly, this would not be consistent with the Paris Agreement or the country’s own climate law.
International aviation and shipping emissions
In a quirk of climate diplomacy, international aviation and shipping emissions were excluded from the original 2050 target. But as the commission points out, they most definitely contribute to global warming and are covered by the temperature target of the Paris Agreement.
Other countries are moving in these areas and the International Civil Aviation and maritime organisations have net zero 2050 goals in place. Air New Zealand and the global shipping giant Maersk both support including these emissions in the 2050 target, which the commission finds to be achievable under multiple different pathways.
New Zealand’s dependence on shipping and air transport is a challenge. The commission puts the combined emissions from these sectors at 6.7 megatonnes – 20% of total CO₂ emissions and close to all industrial or all passenger car emissions. The aviation industry in particular is planning for growth, which, unless addressed, will blow the 1.5°C carbon budget both for New Zealand and globally.
Drawing on “net zero pathways” prepared by the international aviation and shipping industries, the commission finds that including these sources in New Zealand’s revised 2050 target would be achievable. The sectors would not necessarily have to enter the Emissions Trading Scheme, but the status quo (under which these sectors do not attract GST, fuel tax or a carbon charge) is inequitable with other sources of economic activity.
The author acknowledges the assistance and contribution by Paul Callister.
Negotiators at the COP29 climate conference in Baku have struck a landmark agreement on rules governing the global trade of carbon credits, bringing to a close almost a decade of debate over the controversial scheme.
The deal paves the way for a system in which countries or companies buy credits for removing or reducing greenhouse gas emissions elsewhere in the world, then count the reductions as part of their own climate efforts.
Some have argued the agreement provides crucial certainty to countries and companies trying to reach net-zero through carbon trading, and will harness billions of dollars for environmental projects.
However, the rules contain several serious flaws that years of debate have failed to fix. It means the system may essentially give countries and companies permissions to keep polluting.
What is carbon offsetting?
Carbon trading is a system where countries, companies or other entities buy or sell “credits”, or permits, that allow the buyer to offset the greenhouse gas emissions they produce.
For example, an energy company in Australia that produces carbon emissions by burning coal may, in theory, offset their impact by buying credits from a company in Indonesia that removes carbon by planting trees.
Other carbon removal activities include renewable energy projects, and projects that retain vegetation rather than cutting it down.
The relevant part of the deal is known as “Article 6”. It sets the rules for a global carbon market, supervised by the United Nations, which would be open to companies as well as countries. Article 6 also includes trade of carbon credits directly between countries, which has begun operating even while rules were still being finalised.
Rules for carbon trading are notoriously complex and difficult to negotiate. But they are important to ensure a scheme reduces greenhouse gas emissions in reality, not just on paper.
A long history of debate
Over the past few years, annual COP meetings made some progress on advancing the carbon trading rules.
For example, COP26 in Glasgow, held in 2021, established an independent supervisory body. It was also tasked with other responsibilities such as recommending standards for carbon removal and methods to guide the issuing, reporting and monitoring of carbon credits.
But the recommendations were rejected at COP meetings in 2022 and 2023 because many countries viewed them as weak and lacking a scientific basis.
At a meeting in October this year, the supervisory body published its recommendations as “internal standards” and so bypassed the COP approval process.
At this year’s COP in Baku, the Azerbaijani hosts rushed through adoption of the standards on day one, prompting claims proper process had not been followed
For the remaining two weeks of the conference, negotiators worked to further develop the rules. A final decision was adopted over the weekend, but has attracted criticism.
For example, the Climate Land Ambition and Rights Alliance says the rules risk “double counting” – which means two carbon credits are issued for only one unit of emissions reduction. It also claims the rules fail to prevent harm to communities – which can occur when, say, Indigenous Peoples are prevented from accessing land where tree-planting or other carbon-storage projects are occurring.
Getting to grips with carbon removal
The new agreement, known formally as the Paris Agreement Trading Mechanism, is fraught with other problems. Most obvious is the detail around carbon removals.
Take, for example, the earlier scenario of a coal-burning company in Australia offsetting emissions by buying credits from a tree-planting company in Indonesia. For the climate to benefit, the carbon stored in the trees should remain there as long as the emissions produced from the company’s burning of coal remains in the atmosphere.
But, carbon storage in soils and forests is considered temporary. To be considered permanent, carbon must be stored geologically (injected into underground rock formations).
The final rules agreed to at Baku, however, fail to stipulate the time periods or minimum standards for “durable” carbon storage.
Temporary carbon removal into land and forests should not be used to offset fossil fuel emissions, which stay in the atmosphere for millennia. Yet governments are already over-relying on such methods to achieve their Paris commitments. The weak new rules only exacerbate this problem.
To make matters worse, in 2023, almost no carbon was absorbed by Earth’s forests or soils, because the warming climate increased the intensity of drought and wildfires.
This trend raises questions about schemes that depend on these natural systems to capture and store carbon.
Temporary carbon removal into land and forests should not be used to offset fossil fuel emissions.
What next?
Countries already can, and do, trade carbon credits under the Paris Agreement. Centralised trading will occur under the new scheme once the United Nations sets up a registry, expected next year.
Under the new scheme, Australia should rule out buying credits for land-based offsets (such as in forests and soil) to compensate for long-lasting emissions from the energy and industry sectors.
Australia should also revise its national carbon trading scheme along the same lines.
We could also set a precedent by establishing a framework that treats carbon removals as a complement — not a substitute — for emissions reduction.
The world is striving to reach net-zero emissions as we try to ward off dangerous global warming. But will getting to net-zero actually avert climate instability, as many assume?
Our new study examined that question. Alarmingly, we found reaching net-zero in the next few decades will not bring an immediate end to the global heating problem. Earth’s climate will change for many centuries to come.
And this continuing climate change will not be evenly spread. Australia would keep warming more than almost any other land area. For example if net-zero emissions are reached by 2060, the Australian city of Melbourne is still predicted to warm by 1°C after that point.
But that’s not to say the world shouldn’t push to reach net-zero emissions as quickly as possible. The sooner we get there, the less damaging change the planet will experience in the long run.
Analysis suggests emissions may peak in the next couple of years then start to fall. But as long as emissions remain substantial, the planet will keep warming.
Most of the world’s nations, including Australia, have signed up to the Paris climate agreement. The deal aims to keep global warming well below 2°C, and requires major emitters to reach net-zero as soon as possible. Australia, along with many other nations, is aiming to reach the goal by 2050.
Getting to net-zero essentially means nations must reduce human-caused greenhouse gas emissions as much as possible, and compensate for remaining emissions by removing greenhouse gases from the atmosphere elsewhere. Methods for doing this include planting additional vegetation to draw down and store carbon, or using technology to suck carbon out of the air.
Getting to net-zero is widely considered the point at which global warming will stop. But is that assumption correct? And does it mean warming would stop everywhere across the planet? Our research sought to find out.
Such models are like lab experiments for climate scientists to test ideas. Models are fed with information about greenhouse gas emissions. They then use equations to predict how those emissions would affect the movement of air and the ocean, and the transfer of carbon and heat, across Earth over time.
We wanted to see what would happen once the world hit net-zero carbon dioxide at various points in time, and maintained it for 1,000 years.
We ran seven simulations from different start points in the 21st century, at five-year increments from 2030 to 2060. These staggered simulations allowed us to measure the effect of various delays in reaching net-zero.
We found Earth’s climate would continue to evolve under all simulations, even if net-zero emissions was maintained for 1,000 years. But importantly, the later net-zero is reached, the larger the climate changes Earth would experience.
We found this temperature would continue to rise slowly under net-zero emissions – albeit at a much slower rate than we see today. Most warming would take place on the ocean surface; average temperature on land would only change a little.
We also looked at temperatures below the ocean surface. There, the ocean would warm strongly even under net-zero emissions – and this continues for many centuries. This is because seawater absorbs a lot of energy before warming up, which means some ocean warming is inevitable even after emissions fall.
Over the last few decades of high greenhouse gas emissions, sea ice extent fell in the Arctic – and more recently, around Antarctica. Under net-zero emissions, we anticipate Arctic sea ice extent would stabilise but not recover.
In contrast, Antarctic sea ice extent is projected to fall under net-zero emissions for many centuries. This is associated with continued slow warming of the Southern Ocean around Antarctica.
Importantly, we found long-term impacts on the climate worsen the later we reach net-zero emissions. Even just a five-year delay would affect on the projected climate 1,000 years later.
Delaying net-zero by five years results in a higher global average surface temperature, a much warmer ocean and reduced sea ice extent for many centuries.
Australia’s evolving climate
The effect on the climate of reaching net-zero emissions differs across the world.
For example, Australia is close to the Southern Ocean, which is projected to continue warming for many centuries even under net-zero emissions. This warming to Australia’s south means even under a net-zero emissions pathway, we expect the continent to continue to warm more than almost all other land areas on Earth.
For example, the models predict Melbourne would experience 1°C of warming over centuries if net-zero was reached in 2060.
Net-zero would also lead to changes in rainfall in Australia. Winter rainfall across the continent would increase – a trend in contrast to drying currently underway in parts of Australia, particularly in the southwest and southeast.
Knowns and unknowns
There is much more to discover about how the climate might behave under net-zero.
But our analysis provides some clues about what climate changes to expect if humanity struggles to achieve large-scale “net-negative” emissions – that is, removing carbon from the atmosphere at a greater rate than it is emitted.
Experiments with more models will help improve scientists’ understanding of climate change beyond net-zero emissions. These simulations may include scenarios in which carbon removal methods are so successful, Earth actually cools and some climate changes are reversed.
Despite the unknowns, one thing is very clear: there is a pressing need to push for net-zero emissions as fast as possible.
This 05 August webinar shares information about the Climate Change Commission’s first annual emissions reduction monitoring report, released in July 2024. The report provides an evidence-based, impartial view of whether the country is on course to reach its goals of reducing and removing greenhouse gas emissions. It provides insight into the progress made, challenges experienced, and opportunities and risks that need to be considered.
The following quote from Dr Rod Carr towards the end of the webinar, paints a realistic picture of what Aotearoa can expect in term of the economic our global standing and the risks. (Pages on this website explain Nationally Determined Contributions, the Paris Accord, and the Emissions Trading Scheme.)
Webinar question: What would happen if New Zealand wasn’t able or didn’t comply with our Nationally Determined Contributions (NDCs)? What are the implications for us?
Answers:
As time it goes on, meeting our NDCs is getting increasingly more difficult and expensive because of delay.
Not meeting the NDCs: we would certainly expect to see greater scrutiny of our actions from our trading partners particularly where we have free trade agreements (FTAs) and particularly with those strong climate elements within them like the EU FTA.
Not meeting them is also likely to come with a loss of influence and on the global scale in relation to climate change, which may mean we are in worse position to advocate for a response that takes into account our national circumstances.
The final thing is that global consumers and customers are increasingly scrutinising their supply chains and looking for products that are reducing emissions, and so we do increase risks around loss of the global markets.
– Jo Hendy CE: Video, The Climate Change Commission 2024 emissions reduction monitoring report, August 2024
When the rest of the world looks at New Zealand, if we haven’t
met our national determine contributions—we won’t know on the 31st of
December 2030 as it takes a couple of years for inventories and count
up— but when the partners that we care about look at our behaviour and
go, ‘Did you do all that you said you would? Did you do all that you
said you would? And did you do all the things you could have done?’
That’s going to inform whether it’s ‘that you tried hard but missed’ or
‘you didn’t try’.
So foreign countries who are in incurring very real economic
costs to reduce their emissions today— and that includes the Europeans,
the Brits, and the Americans (there’s half a trillion U.S. dollars of
taxpayers money being made available to reduce their emissions so the
idea they’re not doing anything; that’s just wrong)—so when those
countries look at NZ in the early 2030s and they look back to 2020, they
go, ‘Well you could have made a better effort to, for example,
decarbonized ground transport there were known technologies that were
available, but you just chose to buy cheap high polluting cars. You
could have chosen to stop burning as much coal and fossil gas to make
electricity by investing more sooner in renewables, but you chose not
to.’ I think that’s going to influence what the world thinks about New
Zealand ‘s behaviour more than whether we did or did hit the exact
number of tonnes for this decade.
And the rest of the world looks at New Zealand and says,
‘You didn’t try. You didn’t take up the known technologies. You are
short sighted, selfish, and reckless in your use of the climate for
profit.’ I think their attitudes to us will be very different than if we
had tried hard and done all we could but things didn’t turn out well.
– Dr Rod Carr, Video, The Climate Change Commission 2024 emissions reduction monitoring report, August 2024
New Zealand is one of the worst countries in the world in terms of meeting its commitments to keep temperatures under 1.5C. (Image: Climate Action Tracker)
New Zealand is also subsidising high greenhouse gas emissions industries by giving the agricultural sector a 100% discount (Image: Nature journal)
Producing concrete blocks with captured carbon has both economic and climate benefits.
Capturing carbon dioxide from the air or industries and recycling it can sound like a win-win climate solution. The greenhouse gas stays out of the atmosphere where it can warm the planet, and it avoids the use of more fossil fuels.
But not all carbon-capture projects offer the same economic and environmental benefits. In fact, some can actually worsen climate change.
I lead the Global CO₂ Initiative at the University of Michigan, where my colleagues and I study how to put captured carbon dioxide (CO₂) to use in ways that help protect the climate. To help figure out which projects will pay off and make these choices easier, we mapped out the pros and cons of the most common carbon sources and uses.
Replacing fossil fuels with captured carbon
Carbon plays a crucial role in many parts of our lives. Materials such as fertilizer, aviation fuel, textiles, detergents and much more depend on it. But years of research and the climate changes the world is already experiencing have made abundantly clear that humanity needs to urgently end the use of fossil fuels and remove the excess CO₂ from the atmosphere and oceans that have resulted from their use.
Balancing the environmental carbon budget is complex, and active carbon management is necessary to stabilize the climate.University of Michigan, CC BY-ND
Some carbon materials can be replaced with carbon-free alternatives, such as using renewable energy to produce electricity. However, for other uses, such as aviation fuel or plastics, carbon will be harder to replace. For these, technologies are being developed to capture and recycle carbon.
Capturing excess CO₂ – from the oceans, atmosphere or industry – and using it for new purposes is called carbon capture, utilization and sequestration, or CCUS. Of all the options to handle captured CO₂, my colleagues and I favor using it to make products, but let’s examine all of them.
CCUS best and worst cases
With each method, the combination of the source of the CO₂ and its end use, or disposition, determines its environmental and economic consequences.
In the best cases, the process will leave less CO₂ in the environment than before. A strong example of this is using captured CO₂ to produce construction materials, such as concrete. It seals away the captured carbon and creates a product that has economic value.
A few methods are carbon-neutral, meaning they add no new CO₂ to the environment. For example, when using CO₂ captured from the air or oceans and turning it into fuel or food, the carbon returns to the atmosphere, but the use of captured carbon avoids the need for new carbon from fossil fuels.
Other combinations, however, are harmful because they increase the amount of excess CO₂ in the environment. One of the most common underground storage methods – enhanced oil recovery – is a prime example.
Underground carbon storage pros and cons
Projects for years have been capturing excess CO₂ and storing it underground in natural structures of porous rock, such as deep saline reservoirs, basalt or depleted oil or gas wells. This is called carbon capture and sequestration (CCS). If done right, geologic storage can durably remove large amounts of CO₂ from the atmosphere.
When the CO₂ is captured from air, water or biomass, this creates a carbon-negative process – less carbon is in the air afterward. However, if the CO₂ instead comes from new fossil fuel emissions, such as from a coal- or gas-fired power plant, carbon neutrality isn’t possible. No carbon-capture technology works at 100% efficiency, and some CO₂ will always escape into the air.
One way to lower the cost is to sell the captured CO₂ for enhanced oil recovery – a common practice that pumps captured CO₂ into oil fields to push more oil out of the ground. While most of the CO₂ is expected to stay underground, the result is more fossil fuels that will eventually send more carbon dioxide into the atmosphere, eliminating the environmental benefit.
Using captured carbon for food and fuel
Short-lived materials made from CO₂ include aviation fuels, food, drugs and working fluids used in machining metals. These items aren’t particularly durable and will soon decompose, releasing CO₂ again. But the sale of the products yields economic value, helping pay for the process.
This CO₂ can be captured from the air again and used to make a future generation of products, which would create a sustainable, essentially circular carbon economy. However, this only works if the CO₂ is captured from the air or oceans. If the CO₂ comes from fossil fuel sources instead, this is new CO₂ that will be added to the environment when the products decompose. So even if it is captured again, it will worsen climate change.
Storing carbon in materials, such as concrete
Some minerals and waste materials can convert CO₂ to limestone or other rock materials. The long-lived materials created this way can be very durable, with lifetimes of longer than 100 years
A good example is concrete. CO₂ can react with particles in concrete, causing it to mineralize into solid form. The result is a useful product that can be sold instead of being stored underground. Other durable products include aggregates used in road construction, carbon fiber used in automotive, aerospace and defense ]applications and some polymers.
Volker Sick, director of the Global CO₂ Initiative at the University of Michigan and author of this article, discusses why carbon capture and its use has been slow to gain attention.
These materials provide the best combination of environmental impact and economic benefit when they are made with CO₂ captured from the atmosphere rather than new fossil fuel emissions.
Choose your carbon projects wisely
CCUS can be a useful solution, and governments have started pouring billions of dollars into its development. It must be closely monitored to ensure that carbon-capture technologies will not delay fossil fuel phaseout. It is an all-hands-on-deck effort to take the best combinations of CO₂ sources and disposition to achieve rapid scaling at an affordable cost to society.
Because climate change is such a complex problem that is harming people throughout the world, as well as future generations, I believe it is imperative that actions are not only fast, but also well thought out and based in evidence.
Authors:
Fred Mason, Gerry Stokes, Susan Fancy and Stephen McCord of the Global CO₂ Initiative contributed to this article.
The COP28 climate summit in Dubai has adjourned. The result is “The UAE consensus” on fossil fuels.
This text, agreed upon by delegates from nearly 200 countries, calls for the world to move “away from fossil fuels in energy systems in a just, orderly and equitable manner”. Stronger demands to “phase out” fossil fuels were ultimately unsuccessful.
The agreement also acknowledges the need to phase down “unabated” coal burning and transition towards energy systems consistent with net zero emissions by 2050, while accelerating action in “the critical decade” of the 2020s.
As engineers and scientists who research the necessary changes to pull off this energy system transition, we believe this agreement falls short in addressing the use of fossil fuels at the heart of the climate crisis.
Such an approach is inconsistent with the scientific consensus on the urgency of drastically reducing fossil fuel consumption to limit global warming to 1.5°C.
‘Abated’ v ‘unabated’
The combustion of coal, oil and gas accounts for 75% of all global warming to date – and 90% of CO₂ emissions.
So what does the text actually ask countries to do with these fuels – and what loopholes might they exploit to continue using them well into the future?
Those countries advocating for the ongoing use of fossil fuels made every effort to add the term “unabated” whenever a fossil fuel phase-down or phase-out was proposed during negotiations.
“Abatement” in this context typically means using capture capture and storage technology to stop CO₂ emissions from engines and furnaces reaching the atmosphere.
However, there is no clear definition of what abatement would entail in the text. This ambiguity allows for a broad and and easily abused interpretation of what constitutes “abated” fossil fuel use.
Will capturing 30% or 60% of CO₂ emissions from burning a quantity of coal, oil or gas be sufficient? Or will fossil fuel use only be considered “abated” if 90% or more of these emissions are captured and stored permanently along with low leakage of “fugitive” emissions of the potent greenhouse gas methane, which can escape from oil and gas infrastructure?
This is important. Despite the agreement supposedly honouring “the science” on climate change, low capture rates with high residual and fugitive emissions are inconsistent with what research has shown is necessary to limit global warming to the internationally agreed guardrails of 1.5°C and 2°C above pre-industrial temperatures.
In a 2022 report, the Intergovernmental Panel on Climate Change (IPCC) indicated that almost all coal emissions and 33%-66% of natural gas emissions must be captured to be compatible with the 2015 Paris agreement.
That’s assuming that the world will have substantial means of sucking carbon (at least several billion tonnes a year) from the air in future decades. If these miracle machines fail to materialise, our research indicates that carbon capture would need to be near total on all fuels.
The fact that the distinction between “abated” and “unabated” fossil fuels has not been clarified is a missed opportunity to ensure the effectiveness of the Dubai agreement. This lack of clarity can prolong fossil fuel dependence under the guise of “abated” usage.
This would cause wider harm to the transition by allowing continued investment in fossil fuel infrastructure – new coal plants, for instance, as long as some of the carbon they emit is captured (abated) – thereby diverting resources from more sustainable power sources. This could hobble COP28’s other goal: to triple renewable energy capacity by 2030.
By not explicitly defining these terms, COP28 missed the chance to set a firm, scientifically-backed benchmark for future fossil fuel use.
The coming age of carbon dioxide removal
Since the world is increasingly likely to overshoot the temperature goals of the Paris agreement, we must actively remove more CO₂ from the atmosphere – with reforestation and direct air capture (DAC), among other methods – than is emitted in future.
Some carbon removal technologies, such as DAC, are very early in their development and scaling them up to remove the necessary quantity of CO₂ will be difficult. And this effort should not detract from the urgent need to reduce emissions in the first place. This balanced approach is vital to not only halt but reverse the trajectory of warming, aligning with the ambitious goals of the Paris agreement.
There has only really been one unambiguously successful UN climate summit: Paris 2015, when negotiations for a top-down agreement were ended and the era of collectively and voluntarily raising emissions cuts began.
A common commitment to “phase down and then out” clearly defined unabated fossil fuels was not reached at COP28, but it came close with many parties strongly in favour of it. It would not be surprising if coalitions of like-minded governments proceed with climate clubs to implement it.
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.
