Natural geological processes have been regulating Earth’s climate for millions of years.
Accelerated versions of these processes are now being promoted as technologies to draw down carbon from the atmosphere – and some are rapidly moving from concept to real-world deployments.
Two such technologies are known as enhanced weathering, which speeds up the chemical breakdown of certain rocks, and ocean alkalinity enhancement, which increases the ocean’s natural ability to remove carbon dioxide from the air.
Startups backed by tech companies including Google and Microsoft are already applying these technologies in field trials. Investment in the sector is rising rapidly, with large-scale trials underway and carbon credits beginning to appear on voluntary markets.
But as our new assessment published in Science highlights, some estimates of carbon removal through these technologies may be too optimistic.
Current models assume carbon captured on land or in coastal waters will reliably make its way into long-term storage in the ocean. However, these models don’t replicate all Earth processes.
In reality, part of the engineered capture of carbon can be reversed as water moves through soils, rivers, estuaries and coastal environments. Dissolved elements can become trapped again in new minerals such as clays, reducing how much carbon ultimately remains stored over long timescales.
The true additional carbon removed from the atmosphere may be smaller than headline estimates suggest.
How enhanced weathering is supposed to work
Enhanced weathering works by accelerating chemical reactions that already occur naturally between rocks, water and carbon dioxide.
When rainwater mixes with carbon dioxide held in the atmosphere and soil, it forms an acid that slowly dissolves rocks that contain the minerals calcium and magnesium. This includes volcanic rocks such as basalt and ultramafic rocks such as dunite.
In nature, the dissolved minerals increase the capactiy of water to store carbon dioxide and these chemical products can then be transported by rivers to the ocean, where the carbon may remain stored for thousands of years.
Enhanced weathering attempts to speed up this natural process. Finely crushed rocks and minerals are spread across landscapes such as agricultural soils, increasing the surface area available for reactions.
Ocean alkalinity enhancement uses similar principles, but aims to increase the ocean’s ability to absorb and store atmospheric carbon dioxide directly.
Carbon losses along the way
Many enhanced weathering assessments assume that once minerals dissolve, the resulting alkalinity and carbon will eventually make their way into the ocean for long-term storage.
However, different materials dissolve at different rates. Climate, rainfall, soil chemistry and biological activity also influence how quickly reactions occur. This means carbon removal can vary enormously between environments.
Earth systems also contain many opportunities for the flow of carbon to weaken before it ever reaches the open ocean.
As alkalinity moves through the environment, dissolved elements released during weathering can become trapped again in new minerals. These reactions can consume alkalinity and reduce the amount of carbon ultimately stored long term.
These challenges are not limited to enhanced weathering on land. Ocean alkalinity enhancement may also experience losses as dissolved elements interact with sediments and seawater chemistry, recycling alkalinity back into solid minerals before it contributes to long-term storage.
The challenge of durable carbon removal
In natural systems, weathering, transport and mineral formation are tightly linked parts of a much larger Earth-system cycle.
While naturally occurring warm and wet environments may accelerate weathering, using a rapid-dissolution model to replicate this does not necessarily guarantee durable carbon storage.
There is also another problem: some enhanced weathering and alkalinity approaches may interfere with natural carbon removal pathways that would have occurred anyway.
For example, increasing alkalinity in one part of the Earth system may reduce natural dissolution or weathering processes elsewhere. This means the amount of truly additional carbon removed from the atmosphere may be smaller.
Many field trials focus on changes occurring at the application site itself, but much of the long-term carbon storage depends on what happens downstream – across entire catchments, rivers and coastal oceans.
As enhanced weathering and ocean alkalinity enhancement move toward larger-scale deployment, the central question is how much carbon remains removed from the atmosphere over decades to centuries – and whether that removal is truly additional.
None of this means these technologies don’t contribute to climate mitigation.
The challenge is whether Earth systems can keep the captured carbon stored or whether we are simply moving carbon across time and space instead of durably removing it from the atmosphere.
New Zealand may offer an opportunity to better understand these questions because volcanic rocks, high rainfall and strong land-to-sea connectivity create ideal conditions for tracking how alkalinity and carbon move through the Earth system.
If these approaches are going to play a major role in future carbon removal strategies – and generate carbon credits at global scale – we need to understand not only how quickly minerals dissolve, but whether carbon is stored durably without weakening natural carbon removal pathways at the same time.
