Human well-being ultimately depends on the health of the ecosystems which envelope and sustain us. We exploit ecosystems for the food, water, and timber we need for everyday living. We depend on ecosystem processes to regulate natural cycles and keep diseases at bay. We rely on them for recreation, instruction and mental and spiritual enrichment. We know that without healthy ecosystems we could not survive, and yet we are transforming and degrading them at an ever-increasing rate. Our demands on nature are growing as our populations expand and our societies develop. We continually and carelessly modify the Earth’s natural cycles through over- exploitation of freshwater, exhaustion of soils, depletion of forests and wildlife, and excessive use of pesticides and fertilizers. We pollute our air, water and soil. We propel greenhouse gases into our upper atmosphere, changing our climate and putting even more pressure on our ecosystems. We drive animals and plants out of their natural habitats and push them toward extinction. How can we halt and reverse this degradation of the Earth’s ecosystems even as we make increasing demands on their services? To answer this challenge, and to better understand the consequences of our actions, in 2001 United Nations Secretary General Kofi Annan launched a comprehensive scientific study, the Millennium Ecosystem Assessment , focusing on ecosystem changes over the course of past decades and projecting those changes into the future.