Northern and tropical landscapes constitute complex mosaics of diverse ecosystems ranging from forests to wetlands, natural steppes and agricultural crops. Climate, socio-economic transformations and changes in land use are altering the balance of greenhouse gas exchange between such ecosystems and the atmosphere. This session will focus on new findings from flux measurement programs, the application of inverse modelling techniques, development of new measurement technologies, and discussion of issues related to scaling, quantifying and understanding variability in trace gas emissions and the growth rate of atmospheric greenhouse gases. Contributions are welcomed which investigate poorly characterised but potentially significant sources of atmospheric trace gases, links between spatial and temporal variability in fluxes and external factors, consistency or heterogeneity in the stable isotope composition of fluxes from individual sources, and year-to-year variability in sources and sinks at a process level through numerical modelling.