Offsetting is no substitute for emissions cuts

Key points

• Reaching net-zero emissions and limiting human-caused warming in line with the Paris Agreement will require removing some carbon from the atmosphere. 
• However, some governments are placing big bets on meeting their climate goals by relying on large-scale carbon removal in the future to counterbalance, or offset, emissions. 
• This poses significant risks, as offsetting via removing carbon is not an exact substitute for reducing emissions. 
• It is not proven that removing one tonne of carbon has the equal and opposite effect as releasing one tonne of carbon. Earth system models suggest that this would result in a slightly higher CO2 concentration than before.
• Uptake and storage of carbon, particularly in nature-based approaches, do not often occur in the same stable forms for million-year timescales as carbon stored in fossil fuels.
• Often, removals can take longer or will be delayed until further in the century than emissions reductions, resulting in higher total emissions and worsened associated climate impacts.
• In the first case of its kind, a government is being challenged over its over-reliance on carbon offsetting in official climate plans. The Environmental Law Initiative and Lawyers for Climate Action NZ argue that carbon offsets and emissions cuts are not equivalent, meaning the government’s plans lack credibility, among other issues.
• Climate litigation is a growing field, and cases like these can help set precedents that hold governments accountable for action on climate change.

Research shows the non-equivalence of carbon offsets and emissions reductions

Under the Paris Agreement, countries have agreed to limit warming to well below 2°C above pre-industrial levels and to pursue efforts to limit temperature increase to 1.5°C. Countries have also acknowledged that this requires reaching a balance between sources and sinks of CO2 emissions, known as reaching net zero, by around mid-century. 

Due to the delayed implementation of climate action, the different possible pathways to limit warming to 1.5°C all rely on some degree of removing carbon from the atmosphere to balance out, or offset, the high levels of emissions. This is because we are already on track to overshoot the 1.5°C warming threshold, meaning we need to remove carbon to bring us back below 1.5°C. Countries plan to remove carbon to achieve net zero and meet their climate targets. 

However, cutting emissions and offsetting these emissions via removing carbon are not interchangeable. Treating them as equivalent measures in national climate plans raises significant issues. 

Uncertainties raise risks of using carbon offsetting for net zero

The concept of net zero is dependent on one major assumption – that removing one tonne of CO2 is exactly opposite and equivalent to the impact of adding one tonne of CO2. 

However, this has not been proven, and in practice, it might not be the case. Basing net zero on this assumption may pose huge unseen risks. 

A 2021 paper published in Nature Climate Change used an earth system model to assess the impacts of adding and removing equal amounts of CO2, finding that the impacts were not symmetrical. The researchers modelled what happened to the atmosphere if they added in and took away the same amount of CO2 and found these actions did not cancel each other out. Instead, slightly more CO2 remained in the atmosphere than when they started. 
This asymmetry increases the more CO2 is in play. Adding and removing 100 gigatonnes (Gt) of CO2 resulted in a 3% increase in CO2 relative to the initial concentration after 100 years. Increasing the amount to 500 Gt CO2 – similar to the amount of emissions and removals included in climate scenarios – resulted in an 18% higher concentration.

As they found no equivalency in releasing emissions and removing them, the authors highlight that the degree to which removing CO2 will contribute to net zero is unclear. Offsetting refers to carbon removal that does not result in overall negative emissions because it is used to balance out other emissions that remain. Thus, there is uncertainty in the outcome of relying on removals to offset emissions. Preventing emissions in the first place removes the risk associated with this uncertainty.

Carbon would need to be locked away permanently for removal to contribute to net zero

Even if one tonne of carbon removed equalled one emitted, the carbon removed would need to be stored permanently for it to be equivalent, as assumed in the study above.

When fossil fuels are burnt, they release CO2 into the atmosphere that had previously been locked away beneath the earth in a stable form for millions of years. Once released, these emissions contribute to warming for centuries. For carbon removal to counteract these emissions, they should also stably store carbon for comparable time scales. 

Carbon removals are often facilitated through the sale of carbon offsets. The most commonly issued types of carbon offsets are for forestry and land use projects, which aim to capture carbon through natural sequestration. Changes in land use, such as planting more trees, account for almost all of the carbon removed so far, and almost all of the carbon removal featured in climate pathways use land, including bioenergy with carbon capture and storage (BECCS). 

However, the carbon taken up by the forest will – at best – be stored for decades, until the trees and plant matter begin to degrade naturally. 

Some types of carbon offset, particularly nature-based ones, also risk the premature release of carbon back into the atmosphere due to natural or human-caused events, such as wildfires or the clearing of forests. Satellite imagery used to track offsets based on forest preservation projects in Cambodia showed that half of these areas had been cleared four years later. Elsewhere, a forest carbon offset project in California continued to sell credits even after many trees were burned in wildfires, the Los Angeles Times reported in 2021.

So while storing carbon in nature could help to delay warming in the short term, nature-based carbon removals cannot be treated as an equivalent reduction of fossil fuel emissions.
Using carbon offsetting, particularly via nature-based approaches, also carries additional risks that have been shown to reduce its effectiveness, for example, by overstating emissions reductions or through ‘leakage’, where emissions reductions in one area unintentionally result in an increase in emissions elsewhere, such as a forest carbon offsetting project causing an uptick in logging in adjacent unprotected areas. This increases overall emissions, widening the gap between carbon removals claimed and the amount of carbon actually remaining in the atmosphere.

Offsets and reductions are not equal routes to net zero

Carbon reductions and offsets via removals are often not temporally equivalent, even when they are treated as interchangeable in net zero accounting. Direct emissions reductions prevent CO2 from entering the atmosphere immediately from the source, while carbon offsetting typically relies on future or gradual removals that occur years or decades after the emissions they are intended to counterbalance.

Some projects, particularly those that rely on planting trees, can also take years to take up carbon. For example, afforestation, reforestation and revegetation projects often require 3 to 7 years before the first offset credits are issued, depending on tree growth rates and methodological requirements. Carbon uptake varies greatly throughout the lifespan of a tree, peaking at 10 to 30 years. 

The temporal mismatch between the emissions released now and the future offsetting means there are more years with higher emissions levels. This could result in higher cumulative emissions, even if the year that net zero is achieved remains unchanged.

From a climate system perspective, this distinction is critical. Warming is driven by cumulative emissions over time, not by whether emissions are eventually balanced out at a future time. Even if significant levels of carbon are removed in future decades to lower these cumulative emissions and overall warming, the impacts of a warmer world would already have affected billions of people. These emissions also increase the possibility of surpassing irreversible climate tipping points. 

Additionally, prioritising future carbon removal over immediate carbon reductions to reach net zero can delay decarbonisation, locking in more emissions down the line. It also locks in greater reliance on carbon sequestration, which becomes increasingly difficult as resilience declines. When emissions cuts are delayed, this can create further uncertainty about whether they will occur, potentially increasing future reliance on carbon removal.

Why does removing and reducing emissions not being equivalent matter in practice?

A number of countries are relying heavily on offsets to meet climate targets, at levels that are unrealistic and not aligned with science. Current government climate pledges require 1 billion hectares for land-based carbon removal – an area larger than China. Some countries have pledged areas of land to deliver land-based carbon removals that are larger than the land area of the entire country.¹Dominica and Equatorial Guinea’s climate pledges would require 256.7% and 178.8%, respectively, of their national land area.

Favouring carbon removal above emission reduction risks missing global climate targets and worsening climate impacts for billions of people. Academics and legal experts argue that this is not aligned with countries’ due diligence to address their climate-harming actions. But so far, there have been no repercussions for countries that rely too heavily on carbon offsetting to meet their climate targets. 

However, a surge in climate litigation has laid a strong foundation for further cases as people turn to the courts to hold countries accountable. Crucially, in its landmark 2025 advisory opinion, the International Court of Justice (ICJ) confirmed that the 1.5°C temperature target under the Paris Agreement is legally binding. Based on this, the ICJ said that all states “must take ambitious mitigation measures in line with the best available science” and that developed countries are obligated to “take the lead in combating climate change”.

When does offsetting make sense?

It is not possible to limit the harmful levels of warming without some amount of carbon removal. However, the capacity to remove and store carbon in plants and other natural materials, in oceans, and in geological storage underground is limited. 

As warming and climate impacts intensify, it becomes increasingly difficult to rely on larger future carbon sequestration, as natural processes become less suitable. For example, wildfires risk is anticipated to increase, making mitigation scenarios that rely on expanding forest areas unrealistic if this risk is not considered. Additionally, as the ocean warms and holds more carbon, it becomes less able to sequester additional carbon. 

Physical and other sustainability limits to carbon removal mean we have a finite amount we can feasibly use. 
Carbon removal is now required to counterbalance historical emissions to bring us back to 1.5°C or lower after a temporary temperature overshoot. In line with IPCC guidance, carbon removals should be prioritised for this purpose (to counterbalance the few residual emissions that are hard to abate), not to compensate for avoidable emissions.

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  Dominica and Equatorial Guinea’s climate pledges would require 256.7% and 178.8%, respectively, of their national land area.