One tonne of elemental carbon sequestered corresponds to 3.67 tonnes of CO2 removed, based on the molecular weight ratio (44/12 = 3.67).

• 67 tonnes: Incorrect calculation.
• 67 tonnes: Ratio of oxygen mass to carbon (32/12 = 2.67), not total CO2.
• 67 tonnes: Derived from the stoichiometric ratio of CO2 to C: 44 / 12 = 3.667 ~ 3.67 tonnes.
• None of the above: Incorrect.

In terms of the total natural greenhouse effect maintaining Earth's atmospheric temperature, water vapour and clouds contribute 60-70%, far exceeding carbon dioxide (~20-25%).

• Water vapour and clouds: The dominant natural greenhouse gas absorber, accounting for approximately 60% to 70% of Earth's atmospheric warming.
• Carbon dioxide: Contributes about 20-25% to the total natural greenhouse effect, though it is the primary driver of anthropogenic enhancement.
• Methane: Potent greenhouse gas but contributes 4-9% to total warming.
• Nitrous oxide: Contributes approximately 2-4% to the natural greenhouse effect.

FACE stands for Free Air Carbon dioxide Enrichment, a field research methodology used to investigate plant and ecosystem responses to elevated atmospheric CO2 without using chambers.

• Free Air Carbon Enrichment Technology: Omits 'dioxide'.
• Free Air Carbon monoxide Enrichment Technology: Incorrectly specifies carbon monoxide.
• Free Air Carbon dioxide Enrichment Technology: Correct scientific designation (FACE technology).
• Free Air Carbon dioxide Enhancement Technology: Uses 'Enhancement' instead of 'Enrichment'.

Among anthropogenic greenhouse gases responsible for global warming, carbon dioxide (CO2) contributes the largest share (approximately 65-76%).

• CFC: Chlorofluorocarbons have high global warming potential but account for only ~10-12% of total warming.
• N2O: Nitrous oxide accounts for approximately 6% of anthropogenic radiative forcing.
• Methane: Second largest anthropogenic contributor, accounting for ~16-18% of global warming.
• CO2: Contributes over 65% to anthropogenic global warming due to massive emissions from fossil fuel burning and land-use change.