Across its four-billion-year history, the course of life’s evolution has been intimately tied to the Earth’s physical environment. Global cooling led to the relatively recent spread of grasslands, which then triggered an evolutionary shift in many herbivorous mammals from browsing to grazing. Conversely, the evolution of life has altered the physical environment. Photosynthetic bacteria released free oxygen into the early oceans and atmosphere, making Earth habitable for later types of organisms. Humans, with their increasing population and expanding technology, have altered the landscape, the flow of rivers, the distribution of flora and fauna, and atmospheric chemistry in ways that affect the climate. Earth system processes also influence where and how humans live. For example:

With human populations increasing the world over, the emission of greenhouse gases has also increased dramatically. These gases alter the chemical composition of the atmosphere and directly influence the planet’s climate. It is generally agreed that the rapid and immense pouring of carbon dioxide into the atmosphere will lead to global warming, which will have incredible impacts throughout the world.

Around three million years ago, a land bridge formed between North and South America. For the first time in more than 150 million years, the two continents were linked, and the mammals inhabiting both lands migrated across the bridge. Horses, mastodons, cats, and dogs moved south, while opossums, porcupines, ground sloths, and armadillos moved north (to name a few). Today, half the mammal species in South America are descended from North American migrants.

Throughout the Southwest, water diversion for agriculture and other uses has changed and continues to change remarkably. The Colorado River effectively no longer reaches the Gulf of California due to a series of dams and diversions along its course. Much of its flow is diverted outside of its basin, for use in California and Nevada. Water is also taken to quench the thirst of nearby cities like Denver and Phoenix. The Colorado River serves the needs of 30 million people in seven US states and Mexico, and 70% of its flow is diverted to irrigate 2.2 million hectares (5.5 million acres) of land. It also supports production of 4,200 megawatts of electric generating capacity. These changes coupled with long-standing drought in the region—likely enhanced by human-induced climate change—have brought a wide range of changes to the basin. When we ask, “Why does this place look the way it does?” the role of humans must be central to our answer.