Twenty-five years and counting: stable isotopes of greenhouse gases measured by the NOAA-CU INSTAAR cooperative program
Sylvia Michel1, James White1, Bruce Vaughn1, Owen Sherwood1, Andrea Sack1, Isaac Vimont1, Amy Steiker1, John Miller1,2, Caroline Alden1, Edward Dlugokencky2, Kenneth Masarie2
1 INSTAAR, University of Colorado, Boulder, Colorado
2 NOAA Earth System Research Laboratory, Global Monitoring Division, Boulder, Colorado
In 2015 we marked the 25th anniversary of stable isotope measurements in the NOAA Global Greenhouse Gases Reference Network. Our records of δ13C and δ18O of CO2 (since 1990), δ13C of CH4 (since 1998), and our up-and -coming measurement systems for δD of CH4, isotopomers of N2O, and δ13C of CO are important tools for understanding sources and sinks of greenhouse gases in this era of global change. However, in order to be useful, isotopic measurements of atmospheric trace gases require a high level of precision and accuracy, and we will discuss some of the challenges of our instrumentation, calibration, and ties to primary standards. We will also explain some of our quality assurance procedures, and how we ensure that we are providing the highest quality data to users (as the data is publicly available). Finally we will discuss some of our scientific findings, highlighting recent changes in δ13C of CH4. Whereas previous increases in concentration had only subtle changes in the carbon isotope ratio, the recent rise in CH4 is concurrent with a significant decrease in δ13C of CH4. Our data and simple models suggest an increase in tropical, biogenic sources of methane with only a small contribution from extraction of natural gas and oil or from thawing Arctic permafrost. For d13C of CO2, our long record of atmospheric measurements continues to show the remarkable anti-correlation with CO2 concentrations at both seasonal and inter-annual timescales, demonstrating the strong role of the terrestrial biosphere in modulating atmospheric CO2. These analyses underscore the value in measuring stable isotopes of greenhouse gases across a wide global network.