Chương trình bù đắp carbon: Chúng có hiệu quả không?

How Carbon Offsets Work

A carbon offset is a reduction, avoidance, or removal of greenhouse gas emissions elsewhere that a purchaser claims to compensate for their own emissions. When you buy a carbon offset for your flight, you are not reducing the CO₂ emitted by the aircraft — those emissions still occur. Instead, you are funding a project that, in theory, either prevents the same quantity of CO₂ from being released elsewhere (an "avoided emissions" credit) or actively removes CO₂ from the atmosphere (a "removal" credit).

The offset market operates through a chain of verification and certification. A project developer — say, a company that protects a threatened forest in Peru — applies to a standard body like Verra's Verified Carbon Standard (VCS) or Gold Standard. Independent third-party auditors assess whether the project genuinely reduces emissions, quantify how much, and verify that the reductions would not have happened without the carbon finance (a concept called "additionality"). If certified, the project generates carbon credits, typically denominated in metric tonnes of CO₂ equivalent (tCO₂e). These credits are sold to airlines, corporations, or individuals seeking to offset their emissions. Once retired (used for an offset claim), the credit is cancelled in a registry so it cannot be double-counted.

Prices for voluntary carbon offsets vary enormously by project type and quality. Nature-based offsets (forests, wetlands) trade from $3–$30 per tonne in voluntary markets. High-quality removal credits from biochar or enhanced weathering fetch $50–$200 per tonne. Direct Air Capture removal credits from Climeworks or Carbon Engineering cost $400–$1,000 per tonne. Airline-offered offsets, which are purchased at retail and often include service costs, typically range from $10–$40 per tonne when the price per tonne is disclosed.

Types of Offset Projects

The type of offset project matters enormously for climate integrity. Projects vary in permanence, additionality, and measurability — three criteria that determine whether the claimed climate benefit is real and durable.

Forestry and Reforestation

Forestry offsets — including REDD+ (Reducing Emissions from Deforestation and Forest Degradation) avoided deforestation projects and reforestation/afforestation credits — are the largest single category of voluntary offset supply. A 2023 investigation by The Guardian and researchers at the University of West Göttingen found that over 90% of Verra-certified REDD+ rainforest offsets purchased by airlines including Delta, Gucci, and others showed little or no actual deforestation reduction compared to control areas. The study, which analyzed satellite data from 2008–2020, estimated that fewer than 6 out of 29 investigated REDD+ projects showed real emissions reductions.

Reforestation credits, where new trees are planted, carry different risks: permanence. Trees planted today can be harvested, burned, or killed by disease or drought in decades. The lifetime of a tree — typically 50–100 years for a forest — is vastly shorter than the atmospheric lifetime of CO₂ (which persists for centuries to millennia). A 100-year forest credit does not permanently offset a molecule of CO₂ that will be warming the atmosphere for 1,000 years.

Despite these limitations, high-quality forestry projects verified with rigorous baselines, robust monitoring (satellite plus ground truth), and buffer pools (extra credits set aside to cover for project failures) do provide meaningful climate benefit. Investors seeking forest-based offsets should look for certification under Architecture for REDD+ Transactions (ART/TREES) or independently verified projects under premium standards that have updated their methodologies post-2023 investigation.

Renewable Energy Projects

Renewable energy offsets — crediting the construction of solar or wind farms in developing countries — were among the earliest and most popular offset types but have fallen sharply in credibility. The core additionality problem: renewable energy in many markets is now cost-competitive with or cheaper than fossil fuels. A solar farm in India that would have been built anyway, because it is economically viable, does not provide genuine additional emissions reduction when credited as an offset. Gold Standard and Verra have significantly tightened their renewable energy methodologies, requiring more rigorous additionality demonstration, but many critics argue renewable energy offsets in most markets are no longer credible.

Community-scale renewable energy projects in genuinely remote areas — replacing diesel generators on islands, or providing clean energy to communities that would otherwise burn biomass — retain stronger additionality arguments. These projects also deliver co-benefits including energy access and reduced indoor air pollution, which are valued by many offset buyers.

Direct Air Capture

Direct Air Capture (DAC) is the gold standard of carbon removal offsets, and also the most expensive. DAC facilities use giant fans and chemical sorbents to pull CO₂ directly from ambient air, producing concentrated CO₂ that is then either permanently mineralized underground (as Climeworks does at its Mammoth facility in Iceland, turning CO₂ into rock via the CarbFix process) or used in industrial processes. The resulting removal is permanent, measurable to high precision, and highly additional — no DAC facility has ever been built without carbon credit revenue.

Climeworks' Mammoth plant, which opened in Iceland in 2024, has a nameplate capacity of 36,000 tonnes of CO₂ per year — equivalent to offsetting roughly 20,000 long-haul return flights. The cost per tonne is approximately $400–$1,000, meaning a return transatlantic economy flight offset costs $224–$560 via DAC. Airlines offering DAC offsets at $10–$20 per ticket are not purchasing DAC credits; the math does not work at those prices. Microsoft, which has purchased DAC credits from Climeworks and others, pays a reported $400+ per tonne for removal contracts.

Airline Offset Programs Compared

Major airlines offer offset programs of widely varying quality and transparency. Evaluating them requires looking at which project types are funded, how much per tonne is actually paid, and whether the program is independently audited.

Delta Air Lines made headlines in 2020 by committing $1 billion over ten years to carbon neutrality, primarily through offsets. However, a 2021 complaint to the US Department of Transportation alleged that Delta's offset claims were misleading because many purchased credits were from projects with questionable additionality. Delta announced a shift toward higher-quality credits after 2023 and has publicly moved away from REDD+ credits that failed the Guardian investigation.

Lufthansa's Compensaid program is notable for offering passengers a choice between conventional tree-planting offsets (priced at roughly $7–$20 per tonne equivalent) and direct SAF contributions priced at the actual cost premium — approximately $0.10–$0.50 per passenger on short-haul, $0.50–$3.00 per passenger on long-haul. Funding SAF rather than offsets is arguably more effective because it addresses the source of emissions rather than compensating elsewhere.

EasyJet announced in 2019 that it would offset all of its flights' CO₂ emissions at a cost of approximately $32 million per year — an impressively transparent commitment for a budget carrier. The airline used a mix of forestry and renewable energy projects. In 2022, easyJet shifted its strategy toward SAF investment rather than offsets, citing the credibility concerns around voluntary carbon markets.

Qantas' carbon offset program "Fly Carbon Neutral" offers Gold Standard-certified offsets across project categories including wind energy in India, landfill gas capture, and household efficient cookstoves in Africa. The airline reports approximately 15% of passengers choose to offset, and the program has funded the retirement of over 7 million tonnes of CO₂e credits since its launch.

Criticism and Limitations

The voluntary carbon market faces three fundamental criticisms that apply in varying degrees to all offset types: additionality failure, permanence failure, and the moral hazard of offsetting versus reducing.

Additionality failure — funding projects that would have happened anyway — is particularly prevalent in older renewable energy credits and REDD+ forest protection projects. A 2022 Nature Climate Change study found that only one-third of voluntary carbon market credits represent genuine emissions reductions, suggesting the market as a whole overcredits by a factor of three.

Permanence failure — carbon stored in nature being re-released — became dramatically visible during the 2021 Pacific Northwest heat wave, which killed an estimated 1 billion tonnes of forest carbon in weeks; the 2023 Canadian wildfire season, which released approximately 2.3 billion tonnes of CO₂ from forests; and ongoing Amazon deforestation that is releasing carbon previously credited under REDD+ programs. These events highlight that nature-based storage is not permanent on the centuries-to-millennia timescales relevant to CO₂.

The moral hazard criticism is perhaps the most fundamental: buying offsets can provide psychological permission to continue emitting at high levels, when the most effective climate action would be emissions reduction at source. Airlines promoting offset programs that are cheap relative to their actual climate cost may be misleading passengers about the effectiveness of those programs.

Better Alternatives to Offsets

Several alternatives to conventional carbon offsets more directly address aviation's climate impact. Contributing directly to SAF development — through airline programs that transparently price the cost premium of actual SAF purchases, or through investing in SAF producers — funds emissions reduction at source. Programs like Lufthansa's Compensaid (SAF option), SAS's SAF contribution, and Finnair's SAF purchase program allow passengers to pay the true incremental cost of blending SAF into actual flights.

High-permanence removal credits — including biochar, enhanced weathering, ocean alkalinity enhancement, and DAC — provide durable CO₂ removal even at today's high prices, for those willing to pay. The cost is not trivial: genuinely offsetting a transatlantic economy return flight via DAC costs $450–$1,100 versus $15–$30 via conventional offsets. But the climate benefit is real and permanent.

Reducing the number and length of flights remains the most effective personal strategy. Replacing a short-haul flight with rail — for example, Brussels (BRU) to Amsterdam (AMS), where Eurostar and Thalys trains make the city center-to-city center journey competitive in time with flying — eliminates the emissions entirely rather than compensating for them. Concentrating necessary long-haul travel into fewer trips, and flying economy rather than business, multiplies the emissions reduction of these structural choices.