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Introduction Human Population Growth Agriculture Land Use Change Water Use Synthetic Fertilizers and Pesticides Livestock Fossil Fuel Combustion and Climate Change The Greenhouse Effect Consequences of Climate Change Urbanization and Infrastructure Invasive Species Mechanisms of Invasive Species Impact Positive Human Influences
For most of human history, our species was one of millions, modestly affecting the ecosystems we lived in. Then something changed. About 10,000 years ago, some human populations began farming. About 250 years ago, some began burning fossil fuels on an industrial scale. These two transitions transformed humanity's relationship with the natural world more completely than any previous development in our 300,000-year history.
Today, humans influence every ecosystem on Earth. We have altered more than 75 percent of the land surface. We have changed the chemistry of the atmosphere. We have moved species between continents, rerouted rivers, and removed entire mountains. No corner of the planet is untouched by human activity.
Understanding the full scope of human influences on the environment is not cause for despair. It is the foundation for making better decisions.
All human environmental impacts ultimately stem from one underlying driver: the size and consumption patterns of the human population.
The global human population reached:
The rate of population growth has slowed, but the absolute numbers added each year remain enormous. The global population is projected to reach approximately 9.7 billion by 2050 and possibly peak around 10 to 11 billion later in the century.
Agriculture is the human activity with the greatest total environmental footprint. It occupies approximately 50 percent of all habitable land and uses approximately 70 percent of all freshwater withdrawn from rivers, lakes, and groundwater.
The conversion of natural habitats to agriculture has been the primary driver of terrestrial biodiversity loss throughout human history. Forests, grasslands, wetlands, and other natural ecosystems have been converted to cropland and pasture on every inhabited continent.
Approximately 80 percent of current agricultural land expansion is occurring in tropical regions, the areas of highest biodiversity.
Irrigation allows agriculture in areas that would otherwise be too dry for crops. Approximately 40 percent of global food production comes from irrigated land.
Consequences of agricultural water use:
The development of synthetic nitrogen fertilizers in the early 20th century (the Haber-Bosch process) transformed agriculture and enabled food production to keep pace with rapidly growing populations. This is one of the most consequential technological developments in human history.
However, only approximately 50 percent of applied nitrogen fertilizer is taken up by crops. The remainder enters waterways, causing eutrophication, contributes to atmospheric nitrous oxide (a potent greenhouse gas), and disrupts natural nitrogen cycles.
Pesticide use has increased yields but has also reduced insect diversity dramatically. Insect populations in some regions have declined by over 75 percent in recent decades. This collapse in insect populations threatens pollination services for both wild plants and crops, and removes the foundation of many terrestrial food chains.
Livestock farming occupies approximately 70 percent of agricultural land, either as pasture or as cropland growing animal feed.
Environmental impacts of livestock farming:
The burning of coal, oil, and natural gas releases CO₂ that was stored in geological deposits over millions of years. Since the industrial revolution, atmospheric CO₂ concentration has risen from approximately 280 parts per million to over 420 parts per million, a level not seen in at least 3 million years.
The greenhouse effect is the natural process by which certain gases in the atmosphere trap heat that would otherwise escape to space, warming Earth's surface to temperatures that support life.
Greenhouse gases include water vapor, CO₂, methane, nitrous oxide, and ozone. Without the natural greenhouse effect, Earth's average surface temperature would be approximately -18°C instead of the current average of approximately +15°C.
The enhanced greenhouse effect arises when human activities increase the concentrations of greenhouse gases beyond natural levels, trapping more heat and warming the climate.
Rising temperatures:
Changing precipitation patterns:
Sea level rise:
Ocean acidification:
Biological impacts:
Urban areas now house approximately 56 percent of the global population, a proportion projected to rise to approximately 68 percent by 2050.
Urban expansion converts natural habitat and agricultural land directly. Road networks fragment habitats over far larger areas than the roads themselves. Dams block fish migration and flood terrestrial habitats. Power lines cause bird collisions and electrocution.
Infrastructure also enables the exploitation of remote areas previously inaccessible to commercial extraction. Road building in the Amazon has been the single greatest driver of deforestation, opening previously intact forest to logging, mining, and agricultural conversion.
Human activities have dramatically accelerated the movement of species between previously isolated regions, through deliberate introduction, accidental transport in ballast water and cargo, and escape from agriculture or the pet trade.
Invasive species are now recognized as the second most important driver of biodiversity loss globally after habitat destruction.
Predation on naive prey: Island species that evolved without mammalian predators have no behavioral defenses against introduced rats, cats, and snakes. Brown tree snakes introduced to Guam eliminated virtually all native forest bird species.
Competition: Invasive species can outcompete native species for resources. Japanese knotweed in Europe and kudzu in the United States grow faster than native plants and shade them out.
Disease introduction: Introduced species can carry pathogens to which native species have no immunity. Chytrid fungus, introduced to amphibian populations worldwide, has driven approximately 90 amphibian species to extinction and caused population declines in hundreds more. Dutch elm disease, introduced to Europe and North America, destroyed billions of elm trees.
Hybridization: Introduced species may interbreed with closely related native species, eroding the genetic distinctiveness of native populations.
Not all human influences on the environment are negative. Conservation science, environmental legislation, and changing social values have produced measurable positive outcomes.
These successes demonstrate that human influences on the environment can be positive when informed by scientific understanding and supported by effective policy. The same capacity for impact that has caused environmental damage can be directed toward environmental restoration.