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Throughout its 4-billion-year life, Earth has undergone many changes in the distribution and abundance of life forms, including human inhabitants, and in the living and nonliving features of the ecosystems with which humans interact. Early in the twenty-first century, the United Nations Millennium Assessment Report, the collective work of over 1300 scientists worldwide, painted a disturbing picture of life-supporting ecosystems that are gravely stressed by human activities to an extent that is unsustainable even in the medium-term. This conclusion has been reinforced by recent publications of the Intergovernmental Panel on Climate Change,1 and the Millennium Ecosystem Assessment.2

Atmospheric composition and climate have changed many times. Sometimes air and ocean currents that determine climate and weather have been altered by tectonic plate movements. The impact of large meteors or massive volcanic activity that block sunlight by filling the air with dust and gases such as sulfur dioxide have occasionally produced sudden climate changes leading to great extinctions.3 Variation in solar radiation, oscillation of Earth's axis, or passing clouds of interstellar dust may induce ice ages and periods of interglacial warming.4 Minor seasonal fluctuations are associated with many intervening variables that make weather forecasting one of the most inexact of all sciences.

A consensus has developed among scientists in the relevant disciplines that human activity is adversely affecting Earth's climate;5 and there is compelling evidence that human activity is changing the biosphere in other ways besides climate.6 The changes represent a new scale of human impact on the world unlike anything in recorded history. Collectively, the changes endanger both human health and future prospects for many other living creatures. Global warming and stratospheric ozone depletion have attracted the most attention, but the changes go beyond these two processes.

The term global change covers several interconnected phenomena:7 global warming (“climate change”); stratospheric ozone attenuation; resource depletion; species extinction and reduced biodiversity; serious and widespread environmental pollution; desertification; and macro and micro ecosystem changes, including some that have led to emergence or reemergence of dangerous pathogens.8 These phenomena are mostly associated with industrial processes or result from the increased pressure of people on fragile ecosystems.9 All are interconnected and some are synergistic—some processes reinforce others. What makes these human-induced changes different from those through history is the rate of current change and its reach.10

Ecological integrity, the ability of ecosystems to withstand perturbations, is dependent on three factors: population, affluence, and technology.11,12,13 All three factors are interdependent and can operate synergistically, accelerating declines in systems upon which life, including human life, depends for sustenance. As declines accelerate, thresholds are exceeded as the buffering capacity of these systems is challenged, resulting in system flips or collapses. When large-scale ecosystems collapse, all life that these systems have nurtured over centuries and millennia can then either ...

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