Cities account for most anthropogenic greenhouse-gas emissions, CO2 being most important. We evaluate the net urban contribution to CO2emissions by performing a meta-analysis of all available 14 annual CO2budget studies. The studies are based on direct flux measurements using the eddy-covariance technique which excludes all strong point sources. We show that the fraction of natural area is the strongest predictor of urban CO2 budgets, and this fraction can be used as a robust proxy for net urban CO2emissions. Up-scaling, based on that proxy and satellite mapping of the fraction of natural area, identifies urban hotspots of CO2emissions; and extraction of 56 individual cities corroborates their inventory-based estimates. Furthermore, cities are estimated as carbon-neutral when the natural fraction is about 80%. This fresh view on the importance of cities in climate change treats cities as urban ecosystems: incorporating natural areas like vegetation.