Carbon Measurement Approaches and Accounting Frameworks

D.2.1 Inventory Measurements or “Bottom-Up” Methods

Measurements of carbon contained in biomass, soils, and water, as well as ecosystem measurements of carbon dioxide (CO₂) and methane (CH₄) exchanges between land and water ecosystems and the atmosphere, constitute carbon inventories and are sometimes referred to as bottom-up approaches. Generally, carbon stocks in land ecosystems are measured with remote sensing and field sampling, which may be repeated over time to estimate changes in stocks. In addition, the exchange of CO₂ and CH₄ between land and water ecosystems and the atmosphere may be observed directly by using gas concentration measurements, directly measur- ing fluxes or estimating fluxes from assessments of energy consumption and sales (in the case of fossil fuel flux). Measurements in specific environments, such as urban areas, often combine demographic and activity data (e.g., population and building floor areas) with “emissions factors” that estimate the amount of CO₂ released per unit of activity. Emis- sions of CO₂ and CH₄ released from large sources (e.g., power plants) may be observed directly.

D.2.2 Atmospheric Measurements or “Top-Down” Methods

Observations of atmospheric concentrations of CO₂ and CH₄ are obtained using air sampling instruments on the ground, towers, buildings, balloons, and aircraft or remote sensors on satel- lites. Top-down approaches infer fluxes from the terrestrial land surface and ocean by coupling these atmospheric gas measurements with carbon isotope methods, tracer techniques, and simulations of how these gases move in the atmosphere. The network of greenhouse gas (GHG) measurements, types of measurement techniques, and diversity of gases measured has grown exponentially since SOCCR1, providing improved estimates of CO₂ and CH₄ and increased temporal resolution at regional to local scales across North America.

D.2.3 Ecosystem Models

Terrestrial and marine ecosystem models are used to estimate quantities or fluxes of carbon that may be difficult or impossible to measure directly over large areas. The models typically are evaluated and calibrated using measurements at a limited num- ber of sites representing different ecosystems. The models are then used to apply these measurements to larger areas or regions based on knowledge of ecosystem characteristics such as species composi- tion, soils, weather, physiography, or management history. Ecosystem models also are used with top- down atmospheric measurements to attribute GHG observations to specific terrestrial or ocean domains of interest.