The atmospheric background concentration of CO2 has been increasing since continuous monitoring began in Mauna Loa in 1958, recently passing the troubling milestone of 400 ppm monthly mean concentration. This CO2 increase has been clearly linked to changing global radiative forcing and its major source was identified as emissions from burning of fossil fuels (see IPCC working group I). The global atmosphere accumulated 5.4 ± 0.2 PgC in 2013. The global fossil fuel CO2 emissions were estimated to be 9.9 ± 0.5 PgC and 0.9 ± 0.5 PgC from land-use change. These emissions are partly counteracted by oceanic uptake and a terrestrial biospheric sink. It is notable that the biospheric sink shows large inter-annual variability and the future of the global CO2 sink is hard to predict (see Global Carbon Project). Another uncertainty is introduced as the CO2 emissions from fossil burning do not encompass comprehensive atmospheric observations, but rely solely on fuel use statistics and have become the largest uncertainty in the net budget of atmospheric CO2. Therefore, it is crucial for our understanding of the global carbon cycle to properly quantify fossil fuel CO2 emissions and accurately track the atmospheric CO2 concentration increase. So why would we care about cities when addressing the global net CO2 emissions? Emissions from urban areas are actually the biggest contribution of the global budget for CO2 emissions from fossil fuel burning (FFCO2). They are currently estimated to be responsible for about (53%-87%) of the total global CO2 emissions related to energy use and predicted to further increase (see IPCC working group III). This is why our chase for Greenhouse Gases, which started in the clean air of the arctic has moved to better understanding the Greenhouse Gases emissions at our doorstep.