Satellite observations of XCO2 show greater biases apparently over oceans than over the land surface. However, no effective ways to evaluate space-time XCO2 variations over wide geographical areas exist. Observations on commercial ship tracks and aircraft routes, together with atmospehric model calculations, provide a new reference XCO2 dataset for the otherwise inaccesible areas of the world. High quality satellite observations are a requirement for better understanding of the carbon cycle in response to climate change.