Q1: I'm a member of the press and would like more information, to whom do I turn?
Q2: Can I use whatever I find on these pages?
Q3: How do I cite or acknowledge CarbonTracker work?
Q4: What data are available beyond those currently offered through FTP, and how do I get them?
Q5: Why is CarbonTracker almost a year behind the current date?
Q6: How reliable are your products?
Q7: Where can I find more technical details on CarbonTracker?
Q8: I'm having trouble using the CarbonTracker results, where can I get help?
Q9: Are CarbonTracker results available for earlier releases?
Q10: What are the units of CO2 emissions that CarbonTracker uses?
Q11: What are the units of atmospheric CO2 abundance that CarbonTracker uses?

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Q1: I'm a member of the press and would like more information, to whom do I turn?
A1: Please contact Pieter Tans (NOAA ESRL Carbon Cycle Group Chief) to discuss CarbonTracker, get the latest carbon cycle insights, or obtain additional material.

Q2: Can I use whatever I find on these pages?
A2: Yes, all our results are free to be used by the public, scientists, and others. This includes all figures, numbers, data files, and even the Fortran source code. We encourage you to contact us with questions to ensure proper representation of the results, and we welcome any feedback and possibility for cooperation. Please acknowledge the CarbonTracker efforts when you use them in your scientific endeavors.

Q3: How do I cite or acknowledge CarbonTracker work?
A3: We ask that scientific work that relies heavily on CarbonTracker products is discussed with us before publication, to ensure proper representation of our work and co-authorship if appropriate. Detailed instructions for acknowledgment are available at our citation page.

Q4: What data are available beyond that currently offered through FTP, and how do I get them?
A4: In addition to the available weekly global fluxes and North American mole fraction data, we can generate global 1x1 degree biological fluxes at 3-hourly resolution, as well as optimized mole fraction data for any other part of the world. We have time series of estimated fluxes for 50+ AmeriFlux eddy-covariance sites, optimized CO2 time series for 250+ locations worldwide, and routines to sample column CO2 abundances at other times of day to coincide with satellite overpasses of for instance AIRS, GOSAT, or OCO-2. We also offer the full covariances of our estimated parameters to any interested parties. Please contact us to discuss the sharing of these results.

Q5: Why is CarbonTracker almost a year behind the current date?
A5: CarbonTracker operates with at least a one-year delay. Our next release will update flux estimates through the end of 2016. There are several reasons for this delay, but the most important one is that there is a lag in our receiving air sample flasks from around the world, analyzing their contents, and performing the requisite quality control. This process involves many persons and many hours of meticulous work. As a result, the CO2 mole fraction data for 2017 won't be available for our CarbonTracker modeling efforts until summer 2018. Preparing the CarbonTracker product itself requires another two months or so of modeling effort.

We do provide more up-to-date results via the CarbonTracker "Near-Real Time" (CT-NRT) product, at http://www.esrl.noaa.gov/ccgg/carbontracker/CT-NRT/. CT-NRT is an experimental research project using provisional CO2 observations, different flux models, and a revised assimilation scheme, with real-time atmospheric transport. Users are encouraged to contact us before using CT-NRT data.

While CarbonTracker results are lagged by a year, the raw flask and in-situ observations themselves are posted as soon as they are available, even as the quality-control efforts are underway. For the most up-to-date observational data, you can visit our interactive data visualization page at: http://www.esrl.noaa.gov/dv/iadv/.

Q6: How reliable are your products?
A6: The reliability of our products depends on the time and location for which they are assessed. North American results will generally be more reliable than those in other parts of the world due to the focus of CarbonTracker on this region. Also, results that are aggregated in space (e.g., whole continents) or time (e.g., monthly averages) will be more robust than local or instantaneous estimates. For the fluxes, formal uncertainty estimates on the biological fluxes (full covariance matrices are available upon request) give some indication of the random errors we expect. However, systematic errors (e.g., biases) might dominate these at specific times and locations. Calculated mole fraction data will generally be reliable to within the specified errors at each site, while mole fractions at other locations will be better constrained in the proximity of assimilated sites. Assessing our products against independent data and quantifying their reliability is an important and ongoing task for the CarbonTracker team. We welcome any help and assistance, or feedback you might have on this issue. The CarbonTracker team will generally quote conservative formal uncertainty estimates on all website products.

Q7: Where can I find more technical details on CarbonTracker?
A7: The most complete information about CarbonTracker is available on our documentation page. For technical details beyond the documentation page, we suggest you read the literature, visit our collaborators page, see our release notes, or contact us.

Q8: I'm having trouble using the CarbonTracker results, where can I get help?
A8: Send us an email describing in detail what you are trying to do, and what problem you run into. We will make every attempt to help you along.

Q9: Are CarbonTracker results available for earlier releases?
A9: Yes. Previous versions of CarbonTracker have been archived and are available on our versions page.

Q10: What are the units of CO2 emissions that CarbonTracker uses?
A10: CO2 emissions are reported in mass of carbon released into the atmosphere per unit time, over some assumed area. Please note that this is not mass of CO2, which is approximately 44/12 times larger due to the inclusion of two oxygen atoms in the molecule. We generally report emissions in petagrams of carbon per year, or PgC/yr. A petagram of carbon is 10¹⁵ gC, or 1 billion metric tons C. A billion metric tons is equivalent to a gigaton of C, or GtC. To convert to mass of CO2, multiply by 44/12. Thus an emission of 1 PgC/yr is equivalent to 3.67 GtCO2/yr.

Q11: What are the units of atmospheric CO2 abundance that CarbonTracker uses?
A11: Atmospheric CO2 abundances are reported as a dry-air mole fraction, in parts per million ("ppm"). This is the ratio of the number of CO2 molecules in the sample to the total number of molecules, excluding water molecules. Parts per million are equivalent to micromol/mol (or μmol mol-1), so we sometimes report abundances using that form. This ratio means that for a sample of dry air having a CO2 abundance of 400 ppm, out of every million molecules, 400 are CO2. Historically, abundances were sometimes erroneously reported as parts per million by volume ("ppmv"), which is only equivalent to ppm if you assume that CO2 and dry air (consisting mostly of N2 and O2 molecules) act as ideal gases. This is not generally true, and as a result the CO2 community now reports dry-air mole fractions as ratios of the number of molecules, not of their volumes.