Understanding Atmospheric CO₂ - From 280 to 425 ppm

Atmospheric CO₂ concentration is currently approximately 425 parts per million (ppm), up from pre-industrial levels of around 280 ppm. This 52% increase in atmospheric CO₂ has driven global temperature increases and fundamentally altered the Earth's carbon cycle. Yet this same molecule—considered a pollutant when released to the atmosphere—is an essential nutrient for plant growth. Understanding this paradox helps clarify why greenhouse CO₂ enrichment can be both agriculturally beneficial and climate-relevant.

The Atmospheric Context: Why 425 ppm Matters: Atmospheric CO₂ concentration has been rising at approximately 2.5ppm per year over the past decade, driven primarily by fossil fuel combustion, deforestation, and land-use changes. The current level of 425 ppm represents the highest atmospheric CO₂ concentration in at least 800,000 years, based on ice core records. More concerning, the rate of increase is accelerating compared to historical averages. This rising baseline atmospheric CO₂ has direct implications for greenhouse operations. When greenhouse operators target 600-1200 ppm CO₂ for crop enrichment, they're elevating concentrations to 1.4-2.8 times current atmospheric levels. Maintaining these elevated concentrations requires continuous CO₂ supply because greenhouse structures inevitably exchange air with the outside environment through both intentional ventilation and unintentional leakage. The greater the difference between greenhouse target concentration and outdoor ambient levels, the more CO₂ must be supplied to offset losses.[3]

The Greenhouse as a Carbon Cycle Microcosm - A commercial greenhouse operates as an intensified, controlled version of the natural carbon cycle. In natural ecosystems, plants absorb CO₂ during photosynthesis and release it through respiration and decomposition. Greenhouses accelerate this cycle dramatically. A single hectare of greenhouse tomatoes can absorb 60-440 tons of CO₂ annually depending on target enrichment levels—far exceeding per-hectare uptake in natural forests. However, this absorption is temporary and cyclical. When crops are harvested and consumed, the carbon fixed during photosynthesis is released back to the atmosphere through human respiration, food decomposition, or waste processing. From a carbon accounting perspective,CO₂ used for greenhouse crop production is not permanently sequestered—it's borrowed from the atmosphere (or another source) for weeks to months, then returned. This distinguishes greenhouse CO₂ use from carbon dioxide removal(CDR) approaches that aim for permanent storage.

Carbon Dioxide Removal vs.Carbon Capture and Utilization - The climate policy community distinguishes between Carbon Dioxide Removal (CDR) and Carbon Capture and Utilization (CCU). CDR involves capturing CO₂ from the atmosphere or preventing its release, then storing it permanently in geological formations, mineralized forms, or long-lived products. The goal is a net reduction in atmospheric CO₂concentration. Projects like Climeworks' Orca and Mammoth facility in Iceland capture atmospheric CO₂ via Direct Air Capture and inject it underground for mineralization—a process that removes CO₂ from the carbon cycle for geological timescales.

CCU, in contrast, captures CO₂ and uses it in products or processes where it will eventually return to the atmosphere. Greenhouse crop production is a form of CCU: CO₂ is captured (whether from the atmosphere, industrial sources, or combustion) and temporarily incorporated into plant biomass, then released when the food is consumed. Other CCU applications include carbonated beverages, chemical synthesis, and construction materials. While CCU can reduce fossil fuel demand or displace emissions-intensive processes, it doesn't provide the same climate benefit as permanent CDR.

Does the Source of CO₂Matter? From a plant physiology perspective, CO₂ molecules are identical regardless of their source. A CO₂ molecule captured from the atmosphere via Direct Air Capture is chemically indistinguishable from one produced by natural gas combustion or extracted from industrial flue gas. Plants photosynthesize equally effectively with any pure CO₂ source. However, from a climate and lifecycle emissions perspective, the source matters enormously. Consider three greenhouse CO₂ supply scenarios.

In Scenario A, a greenhouse burns natural gas in a CHP system, producing CO₂ from fossil carbon that was previously sequestered underground. This adds new CO₂ to the active carbon cycle and creates a carbon tax liability. In Scenario B, the greenhouse purchases CO₂ captured from an ammonia plant's industrial process—CO₂ that would otherwise vent to atmosphere. This prevents an emission but doesn't remove CO₂ from the atmosphere. In Scenario C, the greenhouse uses Direct Air Capture to extract CO₂ from ambient air at 425 ppm. This creates a closed carbon loop: atmospheric CO₂ is absorbed by plants, released when food is consumed, and can be recaptured via DAC. Only Scenario C creates a fully closed carbon cycle with zero net fossil emissions. Scenario A adds fossil carbon to the atmosphere (even though plants temporarily absorb it). Scenario B prevents industrial emissions but still relies on processes that generate CO₂ from fossil or process sources. The distinction becomes critical as carbon pricing rises and regulatory frameworks differentiate between fossil and atmospheric carbon sources.

The Climate Math of Greenhouse Operations - To understand the climate implications of greenhouse CO₂ supply, consider a 10-hectare greenhouse operation targeting 600ppm CO₂ enrichment. Based on typical utilization efficiency of approximately 24% at this target concentration, the operation requires roughly 2,820 tons of CO₂ per year (282 tons per hectare annually). This CO₂ is absorbed by crops, incorporated into tomatoes or cucumbers, and shipped to consumers. If this CO₂ comes from natural gas combustion via CHP, the operation burns approximately 1,410 tons of natural gas (using typical conversion factors), producing 2,820 tons of CO₂ emissions and consuming fossil fuel. At €80 per ton carbon pricing, this creates a €225,600 annual carbon cost. At projected 2030 prices of €126 per ton, the cost rises to€355,320 annually. If the same CO₂ comes from Direct Air Capture powered by renewable electricity, the greenhouse captures atmospheric CO₂,feeds it to plants, and releases it when food is consumed—back to the atmosphere where it originated. No fossil carbon is extracted. No new atmospheric CO₂ is added. The carbon tax liability is zero because no fossil emissions occurred. The operation still consumes energy, but if sourced from renewables, there are no associated emissions.

Temporal Dynamics: Short-Cycle vs. Long-Term Storage - The residence time of carbon in greenhouse crops is measured in weeks to months. A tomato plant grows for 8-10 months, with fruit harvested continuously and consumed within days to weeks of picking. This short carbon residence time means greenhouse operations cycle massive quantities of CO₂ annually but never accumulate a standing stock of sequestered carbon. Compare this to forestry CDR, where carbon absorbed by trees remains stored for decades to centuries in wood biomass. A forest planted for carbon sequestration might accumulate 5-10 tons of CO₂ per hectare per year and hold that carbon for 50-100 years. The cumulative sequestration—and climate benefit—builds over time. Greenhouse crops, by contrast, hit a steady state where annual CO₂ uptake equals annual release through consumption. No cumulative removal occurs. This doesn't mean greenhouse CO₂ use is climate-negative, but it does mean the climate benefit depends entirely on the source of CO₂ and the energy used to supply it. Using atmospheric CO₂ captured via renewably-powered DAC creates a closed loop. Using fossil-derived CO₂ perpetuates emissions.

The Role of Greenhouses in Future Food Systems - As global population approaches 10 billion by 2050 and climate change disrupts traditional agriculture, protected horticulture becomes increasingly important for food security. Greenhouses allow year-round production in regions with harsh climates, use water more efficiently than open-field agriculture (often 90% less per kilogram of produce), and can be located near urban consumption centers to reduce food miles. However, the climate credentials of greenhouse production depend on operational choices. A greenhouse powered by renewable electricity, heated by electric heat pumps, and enriched with CO₂ from Direct Air Capture can produce food with near-zero lifecycle emissions. A greenhouse burning natural gas for heat and CO₂ may have higher emissions per kilogram of produce than some open-field alternatives, despite higher yields.

The Netherlands, which operates approximately 10,000 hectares of high-tech greenhouses and is a leading global exporter of greenhouse-grown vegetables, faces this transition directly. Dutch greenhouse operators have historically relied on natural gas CHP for combined heat, power, and CO₂ supply. As carbon prices rise and the EU tightens emissions regulations, the sector is actively exploring geothermal heat, renewable electricity, and alternative CO₂ sources to maintain competitiveness while meeting climate targets.

The Path Forward: Closing the Carbon Loop - The fundamental challenge for greenhouse operators is transitioning from linear carbon flows (fossil fuel → combustion →crops → atmosphere) to circular flows (atmosphere → DAC → crops → atmosphere). This transition is both technically feasible and increasingly economically viable as Direct Air Capture costs decline and carbon prices rise.