Assimilation - See data assimilation, below.

AVHRR - Advanced Very High Resolution Radiometer. An instrument carried aboard NOAA polar-orbiting satellites.

CO2 Weather - A loose term referring to the movement of air masses with high- or low-concentrations of CO2 across a continental-scale region. These atmospheric CO2 patterns move with the winds and follow "real" weather dynamics before disappearing through atmospheric mixing. See the CO2 Weather page.

CASA - Carnegie-Ames-Stanford Approach. A terrestrial ecosystem model driven by satellite measures of vegetation greenness (NDVI). See Potter et al. [2003].

CDIAC - The Carbon Dioxide Information and Analysis Center, a CO2 analysis program of the U.S. Department of Energy located at the Oak Ridge National Laboratory (ORNL).

CIRA - Cooperative Institute for Research in the Atmosphere. A joint NOAA-Colorado State University research institute.

CIRES - Cooperative Institute for Research in Environmental Sciences. A joint NOAA-University of Colorado, Boulder research institute.

Column Average - The average mole fraction of carbon dioxide in a vertical column of air starting at some point on the earth's surface and extending to the top of the atmosphere. This is a relevant quantity for comparison with satellite observations.

Data Assimilation - The process of constraining model predictions to be consistent with observations. Models express our understanding of basic physics and chemistry, but this grounding is often not sufficient to produce a realistic simulation. Observations inform us about the state of the carbon system at one place and at one time, but do not by themselves tell us about other places and times. Data assimilation brings these together to yield a simulation that is consistent both with theory and with observations.

ECMWF - The European Centre for Medium-Range Weather Forecasts. CarbonTracker uses the ECMWF operational "forecast" model for the meteorological drivers of the TM5 transport model.

EDGAR - Emission Database for Global Atmospheric Research. A fossil fuel emissions database. See

Emissions Inventories - In CarbonTracker, fossil fuel inputs of carbon dioxide to the atmosphere are estimated from industry, EPA, and other governmental estimates of the consumption of coal and petroleum. These are called "emissions inventories" for fossil fuels.

EnKF - Ensemble Kalman filter. An inversion scheme based on the classic Kalman filter algorithm. The EnKF uses the power of parallel computers by numerically representing the covariance matrix of parameter uncertainty with a large ensemble of slightly different realizations.

ESRL - Earth System Research Laboratory. A NOAA laboratory in Boulder, Colorado.

EVI - Enhanced Vegetation Index. A satellite measure of vegetative greenness. Follow-on to the NDVI, but using the MODIS instrument instead of AVHRR.

Flux - The flow of mass across a surface. In CarbonTracker, we estimate the transfer of carbon mass between the land and the atmosphere, and the ocean and the atmosphere.

GFED2 - Global Fire Emissions Database, version 2. Refs: Randerson et al. [2005] and Randerson et al. [2006].

GMD - Global Monitoring Division. Part of the NOAA ESRL complex in Boulder, Colorado.

GOSAT - Japanese Greenhouse Gases Observing Satellite, launched in 2009.

GPP - Gross primary production. Amount of carbon fixed by plants during photosynthesis.

MAGL - Meters above ground level

MASL - Meters above sea level

MBL - Marine boundary layer. Technically, the lowest level of the atmosphere over the ocean. Practically, this is an idealized construct of air representing the well-mixed lower atmosphere away from continental sources and sinks of CO2.

MODIS - Moderate-Resolution Imaging Spectroradiometer.

Mole Fraction - The mole fraction is defined as the number of molecules of CO2 in any given air parcel divided by the total number of all molecules (except water) in that parcel. For CO2 it is usually expressed as parts per million, abbreviated as ppm. We use the mole fraction of CO2 in (dry) air because it is a conserved quantity: it does not depend on the pressure, temperature, water vapor or condensed water content, which are all highly variable. The CO2 mole fraction is much less variable, and depends only on the history of its sources and sinks, almost all of which are caused by surface processes such as photosynthesis and respiration, and the burning of coal, oil, or natural gas. It is the magnitudes of these source processes that we want to estimate.

MSC - Meteorological Service of Canada

NDVI - Normalized Difference Vegetation Index. A measure of vegetation greenness derived from AVHRR observations.

NEE - Net ecosystem exchange. The final balance of carbon surface exchange by an ecosystem, after accounting for photosynthesis, autotrophic and heterotrophic respiration. NEE = NPP - Rh. Other definitions of NEE may include or exclude later fluxes (e.g., crop and wood harvest), loss of carbon to water runoff, and fire emissions.

NOAA - National Oceanic and Atmospheric Administration

NPP - Net primary production. Gross primary production (fixation of carbon via photosynthesis) minus autotrophic respiration (release of CO2 by the same plant).

OCO-2 - NASA CO2 observing satellite, launched in July 2014.

pCO2 - The partial pressure of carbon dioxide. Contribution of CO2 to the total atmospheric pressure. Often the amount of CO2 dissolved in seawater is expressed as pCO2, since differences between atmospheric and oceanic pCO2 imply a disequilibrium that will drive a net exchange of CO2.

PgC/yr - petagram of carbon per year. A unit of emission equal to 1x1015 grams of carbon released per year, or one billion metric tons of carbon per year (GtC/yr). Technically, this is not a flux, since flux has dimensions of mass per unit area per unit time. The carbon dioxide emission is larger than the carbon emission by the ratio of molecular mass of CO2 to the atomic mass of carbon: 44/12.

ppm - The mole fraction is defined as the number of molecules of CO2 in any given air parcel divided by the total number of all molecules (except water) in that parcel. For CO2 it is usually expressed as parts per million, abbreviated as ppm. A more formal way of expressing ppm is micromoles per mole (μmol mol-1).

Q10 - Rate constant for temperature dependence of heterotrophic respiration. Microbial respiration--the breakdown of organic carbon into CO2--is often modeled as an increasing function of temperature.

Residual - Difference of simulated and measured quantities. We compute these as model-minus-observed, which casts the observation as a "control" variable. This sign convention means that positive residuals indicate the model is too high, and negative ones means the model is too low.

Rh - Heterotrophic respiration. Conversion of organic carbon compounds into CO2 by microbial processes. "Heterotrophic" refers to the fact that the agent doing this conversion is distinct from the plant which created the organic carbon compound via photosynthesis.

Selection scheme - Method by which data providers choose which parts of a measurement time series are appropriate for analysis by models. Usually this involves screening out observations with obvious contamination from local sources, often using wind direction or other criteria. These schemes are usually implemented by our data providers, who are most familiar with the conditions at their observing sites.

Sink - Any process by which carbon dioxide is removed from the atmosphere. Sinks may be natural (e.g., photosynthesis and reforestation) or due to human activities (e.g., management of forests and other carbon sequestration strategies).

Source - Any process by which carbon dioxide is added to the atmosphere. Sources may be natural (e.g., soil and plant respiration) or due to human activities (e.g., combustion of fossil fuels and deforestation).

TCCON - Total Carbon Column Observing Network. See

Time Series - Observations of a quantity taken regularly over time. The most famous time series in atmospheric carbon dioxide analysis is the Keeling curve, representing CO2 concentrations at Mauna Loa, Hawaii from 1958 onwards.

TM5 - Transport Model, version 5. An off-line model of global atmospheric tracer transport. Uses meteorological driver fields from other sources to model the advection and diffusion of atmospheric gases. Supports two-way nested grids to provide higher-resolution simulations over certain areas of the globe. See

TransCom - A special project of the International Geosphere-Biosphere Programme (IGBP), Global Analysis, Interpretation, and Modeling (GAIM) Project, the objective of which was to quantify and diagnose the uncertainty in inversion calculations of the global carbon budget that result from errors in simulated atmospheric transport, the choice of measured atmospheric carbon dioxide data used, and the inversion methodology employed. See

Uncertainty Estimates - CarbonTracker's fluxes are statistical quantities, which have associated "error bars". These error bars are expressed as estimates of a confidence interval. Generally, we report the standard deviation of the flux estimate as the uncertainty; this means that we believe that there is a 68% probability that the unknown, true flux lies within one standard deviation of our best guess flux.

WMO - World Meteorological Organization