Most people understand carbon sequestration as trees absorbing CO2. That is true but it is only one part of a much more interesting story, and the part that surprises most people is what happens underground.

I measured soil carbon flux directly during my field research. I spent two growing seasons at an open-air experimental site in Finland, measuring CO2 efflux from soil under silver birch, Betula pendula, plots using a LICOR 6400-09 gas analyser. We applied four treatments: control, elevated temperature of plus 0.9 degrees Celsius, elevated ozone at 33.4 parts per billion, and a combined treatment. What I found was that a temperature increase of less than one degree increased soil CO2 efflux by 36 percent in one birch genotype and 24 percent in the other.

That single finding reframed how I think about carbon sequestration. The soil is not just a passive store. It is an active system that releases carbon back to the atmosphere at a rate that responds sensitively to temperature. Understanding that is essential to understanding why carbon sequestration is both important and complicated.

 

The Problem Carbon Sequestration Is Trying to Solve

Atmospheric CO2 has been rising since the industrial revolution as fossil fuel combustion returns carbon to the atmosphere that took millions of years to accumulate underground. The rate of release significantly exceeds the rate at which natural sinks can absorb it.

The consequence is a shift in atmospheric chemistry that drives temperature increases, which in turn affects living systems in measurable ways. I saw this directly in my field data. A sub-degree warming shifted soil respiration rates substantially. Scale that up to whole ecosystems across decades and the carbon balance of forests and soils becomes a significant variable in how the atmosphere evolves.

Carbon sequestration addresses this by finding ways to move carbon out of the atmosphere and into long-term storage, whether that is in soil organic matter, plant biomass, geological formations, or mineral compounds.

 

How Natural Sequestration Works

Plants fix atmospheric CO2 through photosynthesis, building it into organic compounds in leaves, stems, and roots. This is carbon moving from the atmosphere into living biomass.

When plant material dies and decomposes, some of that carbon moves into the soil organic matter pool. Soil microbes respire part of it back to CO2, but a fraction becomes stabilised in forms that resist decomposition, humic substances bound to mineral surfaces, charcoal-like pyrogenic carbon from natural fires, and deep soil carbon that cycles on timescales of centuries.

Forests hold significant carbon in both biomass and soil. My biogeochemistry training covered how carbon and nitrogen cycle through ecosystems, and one thing that stays with me is how the soil carbon pool globally is larger than the carbon held in all living vegetation plus all atmospheric CO2 combined. That makes soil carbon management one of the most consequential variables in climate science.

Oceans absorb CO2 through both physical dissolution and biological processes. Marine phytoplankton fix carbon through photosynthesis just as land plants do, and when they die a fraction of that carbon sinks to depth where it stays out of the atmosphere for centuries.

 

 

Why Soil Warming Is a Problem for Sequestration

Here is the part my field data made concrete for me. Carbon sequestration depends on soil carbon staying in the soil. But soil respiration, the process by which microbes break down organic matter and release CO2, speeds up as temperature rises.

In my experiment that relationship was measurable and significant. A warming of 0.9 degrees Celsius, which is smaller than most climate projections for coming decades, increased soil CO2 efflux by 24 to 36 percent depending on the birch genotype. The soil was releasing carbon faster under even modest warming.

This creates what climate scientists call a positive feedback loop. Rising temperatures increase soil respiration, which releases more CO2, which raises temperatures further. The natural sequestration capacity of soils does not simply expand to compensate. In some scenarios warming soils could shift from being net carbon sinks to net carbon sources.

This does not mean natural sequestration is worthless. It means the relationship between temperature and soil carbon flux is something that climate models and land management decisions need to take seriously.

 

How Artificial Carbon Sequestration Works

Because natural sinks have limits and are under pressure from warming, engineered approaches to carbon sequestration have developed alongside them.

Carbon capture and storage, CCS, captures CO2 from concentrated emission sources, typically power stations or industrial facilities, before it enters the atmosphere. The CO2 is then compressed and injected into geological formations, often depleted oil and gas reservoirs or deep saline aquifers, where it is stored under pressure. The chemistry of mineralisation means that over time the CO2 reacts with surrounding rock to form stable carbonate minerals, which is a more permanent form of storage than simple physical trapping.

Direct air capture, DAC, uses chemical processes to extract CO2 directly from ambient air. The concentration of CO2 in ambient air is much lower than in industrial flue gases, around 420 parts per million versus thousands of parts per million in a power station exhaust, which makes direct air capture significantly more energy-intensive per tonne of CO2 captured. The chemistry typically involves passing air over a sorbent material that binds CO2 selectively, then heating the sorbent to release the CO2 in a concentrated form for storage or use.

Both technologies are real and operational at various scales, but both are currently expensive relative to the cost of emitting CO2 in the first place. That economic gap is one of the main obstacles to large-scale deployment.

 

Soil Carbon Farming as a Practical Bridge

Between the large-scale engineered approaches and the passive natural sinks sits a category of land management practices that can increase soil carbon storage meaningfully.

No-till agriculture reduces soil disturbance, which slows the oxidation of soil organic matter and reduces CO2 release. Cover cropping adds organic matter inputs. Composting returns carbon to soil rather than sending it to landfill where decomposition releases it as methane and CO2.

Biochar, produced by heating organic material in low-oxygen conditions, is particularly interesting from a soil chemistry perspective. It is a highly stable form of carbon that resists microbial decomposition on timescales of centuries to millennia. Added to soil it also improves water retention and nutrient availability.

These approaches will not solve the atmospheric CO2 problem alone, but they represent sequestration that is scalable, low-cost, and achievable with existing knowledge and practice.

 

The Limits of Carbon Sequestration

I think it is worth being clear about what carbon sequestration can and cannot do.

It cannot substitute for reducing emissions. The rate at which we are adding CO2 to the atmosphere currently exceeds the realistic capacity of all natural and engineered sequestration approaches combined. Sequestration buys time and reduces peak concentrations, but emission reduction remains the primary lever.

Another important issue is how long the carbon stays stored. Carbon stored in forests and soils can return to the atmosphere because of fires, droughts, diseases, land-use changes, or, as my field data showed, increased soil respiration caused by warming. Carbon stored underground is usually more permanent, but there is still a risk of leakage, so it needs long-term monitoring.

Carbon sequestration is a genuinely important part of the climate response toolkit. I believe that from my field experience and my biogeochemistry training. But it works alongside reduced emissions, not instead of them.

 

Common Questions

What is carbon sequestration in simple terms?

It is the process of moving CO2 from the atmosphere into a form of storage, whether that is soil organic matter, plant biomass, geological formations, or stable mineral compounds, where it stays out of the atmosphere for an extended period.

How does soil store carbon?

Soil microbes break down plant material and some of the resulting compounds stabilise onto mineral surfaces or form chemically resistant structures that resist further decomposition. These stable forms cycle on timescales of decades to centuries.

Does warming reduce soil carbon storage?

Yes. Warmer temperatures increase the rate at which soil microbes respire organic matter, releasing CO2. In my field research a temperature increase of 0.9 degrees Celsius increased soil CO2 efflux by 24 to 36 percent, showing how sensitively soil carbon flux responds to temperature.

What is the difference between CCS and DAC?

Carbon capture and storage, CCS, captures CO2 at concentrated emission sources like power stations before it reaches the atmosphere. Direct air capture, DAC, extracts CO2 from ambient air. DAC is more energy-intensive because CO2 is far more dilute in ambient air than in industrial exhaust.

Is biochar a reliable carbon store?

Yes, relatively. Biochar resists microbial decomposition on timescales of centuries to millennia, making it one of the more stable forms of biological carbon sequestration. It also improves soil water retention and nutrient availability.

Can planting trees solve climate change?

Trees sequester carbon in biomass and contribute to soil carbon, but the scale of current CO2 emissions exceeds what reforestation alone can absorb. Planting trees is genuinely useful but works alongside emission reductions, not instead of them.

What is a carbon feedback loop?

A process where an initial change triggers responses that amplify the original change. Warming increases soil respiration, which releases more CO2, which causes more warming. This is a positive feedback loop in climate science, meaning it reinforces the original trend.

How permanent is geological carbon storage?

CO2 injected into deep geological formations under pressure is physically trapped initially. Over time it dissolves into formation water and eventually reacts with minerals to form stable carbonates. Mineralisation is considered very long-term storage, though monitoring for leakage is still required.

What role does the ocean play in carbon sequestration?

Oceans absorb CO2 through physical dissolution and through photosynthesis by marine phytoplankton. When phytoplankton die, a fraction of the carbon they fixed sinks to depth where it stays out of the atmosphere for centuries. Ocean uptake currently absorbs roughly a quarter of annual human CO2 emissions.