Concerns about global climate change and its impacts on the environment and the economy are encouraging policy-makers and stakeholders to explore a range of options to reduce emissions of carbon dioxide (CO 2 ) and other greenhouse gases (GHGs). Congress is considering legislation that would, among other things, provide incentives for parties to reduce or mitigate GHG emissions or to sequester (store) additional CO 2 . The possible use of forestry and agricultural practices and lands to mitigate or sequester CO 2 is part of the debate. However, substantial uncertainty exists about current ability to accurately quantify, monitor, and verify the amount of carbon sequestered by various agricultural and forestry practices. By comparison, measuring the carbon from a discrete point source, such as a power plant, is relatively easy and precise. Incorporating the agriculture and forestry sectors in an emissions reduction program will likely require a firm basis for measuring carbon inventories and change for forestry and agricultural practices and lands. Farm and forest activities can be both a source and a sink of GHGs, releasing GHGs through plant and animal respiration and decomposition and removing CO 2 through photosynthesis, storing it in vegetation and soils (a process known as sequestration). A range of land management, agricultural conservation, and other farmland practices can reduce or abate emissions and/or sequester carbon. These include tree planting, soil conservation, manure and grazing management, and land retirement, conversion, and restoration. Many of these activities, however, may be impracticable for an emission trading program because they might not meet credible standards for quantifying, monitoring, and verifying emission reduction or carbon storage. Reliable tools and techniques are needed for carbon inventories and carbon change on forests and agricultural lands. The ability to measure carbon levels allows countries that have committed to reducing GHG emissions to measure their current annual emissions and carbon storage (and changes in carbon stocks). Current estimates show that forests account for a significant share of estimated existing carbon stocks globally; agricultural lands account for a small share of stored carbon. Also, the ability to measure carbon levels provides the means to estimate the mitigation potential of forest or agriculture activities that sequester additional carbon in soils or vegetation (i.e., result in a net reduction compared to estimated baseline conditions or current sequestration). This may allow a farm or forestry activity to be recognized as a way to mitigate or offset emissions—through voluntary action, an emissions trading market, or a regulatory program. For an emissions trading program to be credible, a participating entity is usually required to meet a series of established protocols that specify what, when, where, and how to measure changes in carbon. Protocols provide technical guidelines or standardized rules for quantifying, monitoring, and verifying the mitigation of an activity. They specify requirements on project eligibility, scale and baseline measurements, measurement frequency, and verification. The difficulty is developing credible protocols that are quantitatively defensible and readily applicable across areas with differing land uses, weather, and other site-specific conditions. Protocols also address, to varying degrees, concerns about the validity of activities, such as additionality, leakage, and permanence. Protocols may be either voluntary or set by regulation. In one voluntary market, the Chicago Climate Exchange (CCX) has protocols for a range of soil and land management projects, including agricultural methane, soil carbon, rangeland soil carbon management, and tree planting projects. The Regional Greenhouse Gas Initiative (RGGI)—the first regional mandatory, market-based effort to reduce GHG emissions—is developing technical standards for a narrower set of offset projects from the agricultural and forestry sectors, providing for afforestation and methane reduction from livestock operations. Individual requirements of current protocols and standards can vary widely by program. Numerous methods exist to measure forest and agricultural carbon. The appropriate measure to use in specific circumstances depends on several variables, including the purpose for measuring the carbon, the scale and basis to be measured, the frequency of the measurement, and how the measurement is to be verified. Two geographic scales are commonly used for measuring GHG emissions—the national/regional level to report GHG emissions and participate in broad efforts to reduce emissions; and the local/site-specific level for projects to offset emissions. Regardless of scale, the emission reduction or carbon sequestration is compared to a baseline —the historic GHG emissions or carbon stocks at a specified point in time. The scale and baseline timing are typically specified in the protocol of the reporting, marketing, or regulating organization. Sometimes, for projects with multiple land uses, the land is stratified into the various land uses (e.g., cropland, pasture, sapling forest, mature forest), with a different baseline established for each use. Protocols typically identify when GHG emissions must be measured. An initial measurement is needed to establish the baseline. This must be done prior to the onset of a project, to allow for measuring the change that results from the action. Occasionally, a historic baseline is specified; for example, the Kyoto Protocol identified 1990 emissions as the baseline for measuring emission reductions. Other options include a current level, or other level whereby a project is compared to "business as usual." The protocols also identify the frequency and timing of measurements. For example, CCX contracts for agricultural projects require annual measurements to assure that the emission reduction or carbon sequestration is actually occurring. Frequency of measurement also depends on the rate of change in carbon storage. Some carbon pools, such as forest soils, change relatively slowly (unless the forest is disturbed), and measurement once a decade may be sufficient. For other carbon pools, such as pastures or managed lands, differences within and across years can be substantial, and may require more frequent measurement. Timing can be critical, and alternative measurements may vary widely. The amount of carbon stored in vegetation, in particular, varies over the course of a year, with carbon sequestered during the spring, carbon stored in foliage at its maximum in late summer, and carbon released during the winter as the deciduous leaves decompose. Thus, consistent timing for annual measures is an important element for agricultural and forestry carbon projects. Verifying the emission reduction or carbon sequestration is critical in efforts to mitigate climate change. It is particularly important for agriculture and forestry projects, as these activities are harder to measure reliably than other types of GHG offsets. One question is who will be responsible for verifying changes in carbon, which raises questions about the role of a regulatory agency for accrediting claimed changes in carbon levels from an activity. Existing programs typically recommend or require that the carbon offset be verified by an independent entity. Independent verification may be an auditing function, to assure the reality and accuracy of the carbon offset for markets (buyers and sellers), regulations (emitters and regulators), and reports (emitters and reporting organizations). One source has prescribed several qualities for independent verification: an "independent, qualified, third-party verifier" using "approved methodologies and regulations" and "whose compensation is not in any way dependent on the outcomes of their decisions" and who follows set procedures to avoid conflicts of interest. As voluntary and regulated markets for GHG emissions offsets develop, qualified, independent organizations to verify carbon offsets will be needed. Entities qualified to verify agriculture and forest carbon offsets must be proven to be knowledgeable about carbon measurement. One source notes: "To provide good quality and trustworthy oversight, a sufficiently rigorous accreditation process will be necessary to ensure that the verifiers have the needed expertise." This process could parallel the development of independent auditors for certifying sustainable forestry programs. Basic approaches for measuring agricultural and forest carbon inventories and change include on-site measurement, indirect measurement from off-site tools, and estimation using process models or inferences. A hybrid approach involving a combination of approaches (e.g., combining modeling with on-site sampling and independent verification) might improve the accuracy enough to be useful while still containing costs. Because of the inherent challenges associated with balancing the cost of measuring carbon and the accuracy of these measurements, any practicable system for measuring forest and agricultural carbon might require a mix of these different methods and approaches, rather than a single approach. Direct measurement of the carbon content of agricultural and forestry soils and vegetation through field sampling and site-specific laboratory estimates is perhaps the most accurate way to measure carbon levels and changes. However, this is time-consuming, costly, and often requires continuous sampling and replication via a census of soil and vegetation carbon for all agriculture and forestry projects, and may be infeasible. Also, it cannot cover large areas. Samples can be taken and the results extrapolated, based on soil survey, land cover, climate, and other spatial data. Sampling patterns (e.g., a grid, random, or stratified random), intensity (e.g., the area to be sampled), and frequency are likely to be specified in the protocols, and many sources discuss sampling methods for agriculture and forestry projects. The more intensive and frequent the sampling, the greater the cost, but the higher the likely accuracy of the data. Most experts suggest some sampling to ensure the accuracy of models or off-site measures, especially performed consistently over time. As with verification, the entity that measures the on-site carbon can affect perceptions of the accuracy of the measurement. Landowners or other offset sellers can perform the measurement—both at the outset of the project (for the baseline) and periodically during the life of the project. This could reduce costs, because they are commonly on the site, but raises questions of credibility, since they have an interest in the reported carbon levels. Ensuring that verification is conducted by independent verifiers might be sufficient to assuage market concerns over credibility, but could involve high project verification costs. Tools exist to calculate carbon content without actually being on the site. Remote sensing—using photographic and other images from aircraft or satellites—can be used to measure carbon-related factors. For example, infrared imagery can detect live biomass, with variations in the image reflecting variations in the type and level of biomass.