Human activities have profoundly altered Earth's ecosystems. Deforestation, the large-scale clearing of forests for agriculture, logging, or urban development, leads to loss of biodiversity, increased carbon emissions, soil erosion, disrupted water cycles, and loss of habitat for countless species. Approximately 10 million hectares of forest are lost each year, primarily in tropical regions of South America, Africa, and Southeast Asia. Deforestation also contributes to climate change by releasing stored carbon dioxide when trees are burned or decomposed, and by removing carbon sinks that would otherwise absorb atmospheric CO₂.
Pollution and climate change further stress ecosystems. Ocean acidification is the decrease in ocean pH caused by the absorption of excess atmospheric CO₂, which forms carbonic acid in seawater. Since pre-industrial times, ocean surface pH has decreased by about 0.1 units, from roughly 8.2 to 8.1, representing about a 26% increase in hydrogen ion concentration. This reduces the availability of carbonate ions, making it harder for corals, mollusks, and plankton to build calcium carbonate shells and skeletons. Coral bleaching occurs when stressed corals expel their symbiotic zooxanthellae algae, turning white and becoming vulnerable to disease and death. The greenhouse effect, the process by which greenhouse gases such as CO₂, methane, and nitrous oxide trap heat radiated from Earth's surface, underlies these climate changes. Weather describes short-term atmospheric conditions lasting hours to days, while climate is the long-term average of weather patterns in a region over 30 or more years.
Other human impacts include bioaccumulation, the gradual accumulation of substances like pesticides or heavy metals in an organism's tissues over its lifetime, and biomagnification, the increasing concentration of a toxin in organisms at successively higher trophic levels of a food chain. Eutrophication is the excessive enrichment of water bodies with nutrients such as nitrogen and phosphorus, leading to algal blooms, oxygen depletion, and aquatic dead zones where dissolved oxygen is too low to support most marine life. Biological oxygen demand, or BOD, measures the amount of dissolved oxygen needed by aerobic microorganisms to decompose organic matter in water, with high BOD indicating high organic pollution and low water quality. Carbon sequestration, the long-term capture and storage of atmospheric CO₂ in plants, soils, geologic formations, or the ocean, offers one strategy to mitigate climate change.
Sustainability is meeting present needs without compromising the ability of future generations to meet their own needs, balancing environmental, social, and economic factors. Sustainable development integrates economic growth with environmental protection. An ecological footprint measures the amount of biologically productive land and water area required to produce the resources a population consumes and absorb its waste. Ecological overshoot occurs when humanity's demand on nature exceeds the biosphere's regenerative capacity. The precautionary principle states that if an action or policy has a suspected risk of causing harm, protective measures should be taken even without full scientific certainty. Sustainable practices such as agroforestry, which integrates trees and shrubs with crops and livestock to enhance biodiversity, improve soil health, and increase carbon sequestration, demonstrate how human systems can align with ecological principles.