Waternet, the public water cycle utility of Amsterdam and surroundings, has the ambition to operate climate neutrally in 2020. This requires a reduction of greenhouse gas (GHG) emissions of 48 kton CO2-eq. An inventory was made of measures to realize the target either in 2020 or in 2050. For all measures the effects on GHG emissions and on costs were determined. To comply with two core values of Waternet – economic effectivity and sustainability – the measures were prioritized based on CO2 effectivity, defined as costs per ton GHG emission reduction. To realize the target in 2020, 34 measures have to be implemented. The total investments are € 60 million, while the measures result in a decrease in yearly costs of € 5 million from 2020 onwards. In the case where the target has to be realized in 2050, 10 measures have to be implemented with a total investment of € 100 million and a decrease in yearly costs of € 16 million from 2050 onwards. As the cumulative cost savings in 2050 are € 50 million higher for the case where the target is already reached in 2020, and the uncertainty is lower, the realization of the target in 2020 is preferred.