We present first consistent comparisons of CO2 and CH4 emission estimates based on inventories and observations for the top world emitters. We cover fossil CO2, net land CO2, and anthropogenic CH4 emissions. The aim is to identify, quantify and explain divergences between global inventories, atmospheric inversions, process models and national inventories submitted to the UNFCCC. This report is the first of three annual updates. The analysis will be updated every year throughout the CoCO2 project, incorporating the latest data and model results, with more detailed uncertainty assessment included in future reports. Where possible, we include relevant non-CoCO2 estimates from the VERIFY project and satellite inversion results from other studies.