The Millennium Ecosystem Assessment (MA) was a four-year, international effort to document the contribution of ecosystems to human well-being, assay the current state of ecosystem health, and offer a prognosis for how the capacity of ecosystems to support human needs may change under different management scenarios. The intent was to provide decision-makers scientifically credible information to help them manage ecosystems more sustainably while meeting human development goals.

The MA was a remarkably broad-based effort. Completed in 2005, it involved over 1300 scientists from 95 countries. It found that humans have altered the structure and functioning of the world's ecosystems more substantially in the second half of the twentieth century than at any time in human history. As a result, 15 of the 24 ecosystem services the MA assessed are now being degraded or used unsustainably (MA 2005a:viii, 1, 6).

This unsustainable use stems from the fact that humans often favor some kinds of ecosystem production—such as the provisioning services of food and fiber production—at the expense of other services that ecosystems can render, such as biodiversity, water purification, or natural pest control. The MA showed that such trade-offs among different ecosystem services are the norm. Particularly over the past hundred years, human management of provisioning services (food, timber, water, and other commodities) has degraded the ability of ecosystems to provide regulating services, such as flood control or pollination. Cultural services such as recreation and the aesthetic and spiritual appreciation of nature have also suffered.

At the same time, the findings of the MA have shed new light on the importance of ecosystems to the poor and how ecosystem degradation impairs the livelihoods of the poor. Poor people, particularly those in rural areas in developing countries, are more directly dependent on ecosystem services and more vulnerable when those services are degraded or lost (MA 2005a:2-14).

The MA findings document many examples of the human toll on ecosystems. Approximately 35 percent of mangroves have disappeared in the last two decades. Twenty percent of the world's coral reefs have been lost and an additional 20 percent are degraded. Water withdrawals from rivers and lakes have doubled since 1960. Nitrogen flows to the environment have also doubled, while phosphorous flows have tripled between 1960 and 1990. Landings from inland and marine fisheries have declined due to overexploitation. Fuelwood used for energy is scarce in many parts of the world. Some 10-20 percent of drylands are degraded (MA 2005a:2, 26, 31, 34).