The Mid-Latitude Carbon Sink, 1990-Present

Several of the earliest inverse analyses of global CO₂ concentration data suggested a sink in the northern hemisphere was required to correctly match the interhemispheric concentration gradient. This result was challenging to ecologists of the time as there was then no coherent ecological explanation for large fluxes in the north without corresponding larger sinks in the tropics, and generated an enormous amount of research. These results were synergistic with satellite vegetation indices showing northern "greening" although greening need not increase carbon storage. Several decades later, Stephens showed that inversions that best matched independent data suggested a smaller, though still significant northern sink, and also implied a significant tropical sink, a result much more consistent with ecological theory. Since then, inversions over the time period spanned by greenhouse gas satellites have shown an increased northern sink, possibly due to changing climate and land use, and show that the global distribution of sources and sinks may be highly dynamic in time and space. Most efforts to explain sinks evident in atmospheric concentrations have emphasized variation in plant growth, rather than fire or decomposition, but again, new space based measurements suggest a more complex picture, with changes to plant growth, mortality and respiration all playing roles.The global terrestrial carbon cycle may be dominated by dynamic fluxes, with source and sink regions changing with time, rather than exhibiting a stable carbon climatology, and variation in time and space may be driven by any and all of the major gross fluxes (plant growth, decomposition and fire) rather than being dominated by the best-understood process, photosynthesis. The early finding of a mid-latitude sink has stood the test of time, but current understanding is far more nuanced and complex than even recent work suggested. Space-based observations fill a major observational scale gap and open the door to far deeper understanding through constraints on grass as well as net fluxes, thus complementing the surface observing network with its far high accuracy and precision. Considerable work remains to optimally integrate multiple observing modes and provide an efficient means of monitoring climate-driven change to the carbon cycle.