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Understanding Negative Emissions
Negative emissions technologies are methods that remove greenhouse gases, mainly carbon dioxide, from the atmosphere and store them for long periods. They are different from standard mitigation measures, which focus on avoiding new emissions. Negative emissions aim to lower the existing concentration of greenhouse gases already in the air.
These technologies appear in most scientific pathways that try to limit global warming to 1.5°C or well below 2°C. The reason is that many sectors still have emissions that are hard or very costly to remove completely, such as certain industrial processes, aviation, and agriculture. Negative emissions can balance these remaining emissions, and they can also help correct for past delays in reducing emissions.
However, negative emissions are not a substitute for rapid emission cuts. They are a complement. Heavy reliance on them in future scenarios can be risky if the technologies do not perform at the required scale, prove too expensive, or cause new environmental and social problems.
Why Negative Emissions Appear in Climate Pathways
Climate models that explore pathways to net zero consider how fast emissions can realistically be reduced in different sectors, how energy demand might change, and how land and technology can be used. Many of these pathways eventually require a period in which global emissions become net negative, which means that more carbon dioxide is removed than emitted each year.
This requirement comes from several facts. First, the world has already emitted a large share of the total carbon budget associated with 1.5°C or 2°C. Second, some sectors are expected to continue emitting even under ambitious policies. Third, if the world overshoots a temperature goal, negative emissions would be one of the few ways to help bring temperatures back down in the long term.
Because of this, negative emissions technologies are often framed as a tool for both reaching net zero and potentially going beyond net zero later this century. At the same time, every tonne of carbon dioxide not emitted today reduces the need for future removals.
Main Types of Negative Emissions Approaches
Negative emissions approaches can be grouped into land based methods, technology intensive methods, and hybrid approaches that use elements of both.
Land based approaches rely on biological processes to remove carbon dioxide from the atmosphere. Plants use photosynthesis to convert carbon dioxide into biomass. In practices such as afforestation, reforestation, and improved forest or soil management, this carbon can be stored for years or decades. Wetland restoration and agroforestry are other examples.
Technology intensive approaches tend to use physical or chemical processes. Direct air carbon dioxide capture and storage, often called DACCS, uses sorbents or solvents to capture carbon dioxide directly from ambient air, concentrate it, and then store it in geological formations. Enhanced weathering spreads certain minerals that chemically react with carbon dioxide and form stable carbonates. Ocean based approaches seek to increase the ocean’s ability to absorb and store carbon, for example through alkalinity enhancement, but most of these methods are still in early research stages.
Hybrid approaches combine biological uptake with technological storage or use. Bioenergy with carbon capture and storage, often called BECCS, grows biomass that absorbs carbon dioxide, uses that biomass for energy, and captures and stores the carbon dioxide from combustion or processing. Biochar production converts biomass into a stable carbon rich material that can be added to soils, where it can remain for long periods and may also improve soil properties.
Removal, Storage, and “Net” Emissions
To count as negative emissions, an activity must both remove carbon dioxide from the atmosphere and keep it stored beyond typical human time scales. A tree that grows and is then cut, burned, and not replaced does not provide durable negative emissions, because the carbon returns to the air.
It is useful to think in terms of a simple balance. If $E_{removal}$ is the amount of carbon dioxide removed and $E_{leakage}$ is the amount later released from the storage pool, the net removal can be written as
$$E_{net} = E_{removal} - E_{leakage}.$$
If $E_{net}$ is positive, the activity provides net negative emissions. In practice, leakage can occur through deforestation, fires, soil disturbance, poor storage site design, or equipment failure, so long term monitoring and management are essential.
To deliver genuine negative emissions, a system must:
- Remove carbon dioxide from the atmosphere, not just avoid new emissions.
- Store the removed carbon securely for long periods.
- Account for all leaks and indirect effects to ensure net removal.
This distinction between gross and net removal also applies to the full life cycle of a negative emissions technology. The energy used to run machinery, capture equipment, or transport biomass can create emissions that reduce the overall benefit. A robust assessment therefore looks at the complete chain from removal to storage.
Potential, Limits, and Risks
Different negative emissions options have very different potentials, costs, and risks. Land based approaches such as afforestation and reforestation can often be started relatively quickly and at moderate cost. However, they are limited by land availability and can create trade offs with food production, biodiversity, and local communities. They are also vulnerable to climate impacts such as drought, pests, and wildfires, which can reverse the carbon storage.
Technology intensive methods such as DACCS do not require fertile land and can be sited in locations with suitable geology for storage. They are still expensive and energy intensive, but costs could decrease with innovation and scale. Enhanced weathering and ocean methods have large theoretical potential, but many practical, environmental, and governance questions remain.
These differences mean that there is no single universal negative emissions solution. Portfolios of approaches, chosen according to local conditions and social priorities, are more likely to be sustainable. It also means that counting on very large amounts of future negative emissions can be dangerous if those expectations drive weaker near term emission reductions.
One specific risk is called mitigation deterrence, where the promise of future removals encourages delay in present day mitigation. Another is double counting, where the same tonne of removed carbon is claimed by several actors or policies. Clear rules, transparent accounting, and conservative assumptions can help limit these problems.
Negative Emissions and Net-Zero Strategies
Net zero targets rely on the balance between remaining emissions and removals. In a net zero framework, hard to abate emissions from sectors such as aviation, agriculture, or heavy industry can be counterbalanced by negative emissions from land use or engineered removals.
At the national level, this requires coherent strategies. These strategies typically include rapid emission reductions in all sectors, protection and enhancement of natural carbon sinks, support for research and demonstration of engineered removal technologies, and development of regulations for storage, monitoring, and liability. Over time, the relative share of natural and engineered removals may change, especially as climate impacts alter ecosystems and as technologies mature.
At the corporate level, negative emissions are sometimes used in claims about carbon neutrality or net zero. A responsible strategy prioritizes real cuts in value chain emissions and uses removals for residual emissions that are hard to eliminate. It also distinguishes between short lived storage in biological systems and more permanent storage in geological or mineral forms.
Governance, Ethics, and Equity
The role of negative emissions technologies raises significant governance and ethical questions. Large scale land based removals can affect land rights, local livelihoods, and food prices. Engineered removals involve long term storage responsibilities and possible environmental risks. Ocean based approaches may affect marine ecosystems in ways that are not yet well understood.
Equity issues arise because the historical responsibility for climate change lies mainly with high income countries, while many of the best locations for some negative emissions approaches may be in low and middle income regions. Fair benefit sharing, respect for local and Indigenous rights, and inclusive decision processes are therefore central concerns.
Internationally, the way removals are counted in national inventories and carbon markets affects incentives and trust. Robust standards for measurement, reporting, and verification, as well as safeguards for biodiversity and communities, are essential elements of any large scale deployment.
A Complement, Not a Shortcut
Negative emissions technologies play an important role in many visions of a climate safe future. They can balance unavoidable emissions, help restore carbon budgets, and create new options for managing climate risks. At the same time, they cannot replace the need for deep and rapid emission reductions across energy, transport, industry, and land use.
A balanced approach recognizes both the promise and the limits of negative emissions. It pursues early and sustained mitigation, invests in a diverse set of removal options, protects natural ecosystems, and builds strong institutions to oversee deployment. In this way, negative emissions can contribute to long term climate stability without becoming a justification for continued high fossil fuel use.