Rotterdam 's population of approx . Nothing paints a more sustainable urban picture than green rooftops . 90 % of the city is below sea level and is therefore facing a high risk of flooding . Rotterdam aims to become the most sustainable port city in the world .
Rotterdam 's population of approx . 90 % of the city is below sea level and is therefore facing a high risk of flooding . Rotterdam aims to become the most sustainable port city in the world .