In this paper, we propose using marginal emission factors instead of average emission factors for determining the impact of adding variable renewable electricity to the generation mix. Average emission factors assume constant emissions over time, which does not reflect reality. Therefore, they cannot be used for e.g. accurately determining the mitigated CO2 emissions by renewables, or for scheduling shiftable loads in order to have the lowest CO2 emissions. To solve this, we provide a method to construct the marginal emission profiles via the merit order and demonstrate the method by composing these for the case of the Netherlands. Using this method, we re-evaluate the CO2 impact in 2014 of photovoltaic-generated electricity to be 0.42 Mt - compared to 0.36 Mt using the average emission factor - and for wind-generated electricity to be 3.6 Mt instead of 2.9 Mt CO2 (an increase of 14.3% and 24.2%, respectively). Furthermore, we show the impact of CO2 price on the merit order and show that even high CO2 prices of 50 to 75 €/tCO2 are not sufficient to phase-out new coal-fired power plants.