While scientists and environmentalists have discussed ecosystem services implicitly for decades, the Millennium Ecosystem Assessment (MA) in the early 2000s popularized this concept. There, ecosystem services are grouped into four broad categories: "provisioning", such as the production of food and water; "regulating", such as the control of climate and disease; "supporting", such as nutrient cycles and oxygen production; and "cultural", such as spiritual and recreational benefits. To help inform decision-makers, many ecosystem services are being valuated in order to draw equivalent comparisons to human engineered infrastructure and services.