Many species have become extinct because of hunting and overharvesting, the conversion of wetlands and forests to croplands and urban areas, pollution, the introduction of invasive species,
and other forms of human-caused destruction of their natural environments. Indeed, current rates of human-induced extinctions are estimated to be about 1,000 times greater than past natural (background) rates of extinction, leading some scientists to call modern times the sixth mass extinction.
This high extinction rate is largely due to the exponential growth in human numbers: growing from about 1 billion in 1850, the world’s population reached 2 billion in 1930 and more than 7.7 billion in 2019 and is expected to reach about 10 billion by 2050.
As a result of increasing human populations, habitat loss is the greatest factor in current levels of extinction. For example, less than one-sixth of the land area of Europe has remained unmodified by human activity,
and more than half of all wildlife habitat has been eliminated in more than four-fifths of countries in the paleotropics (the Old World tropics that span Africa, Asia, and Indonesia).
In addition, increased levels of greenhouse gases have begun to alter the world’s climate, with slowly increasing surface temperatures expected by the middle of the 21st century to force many species to migrate toward the poles and up mountain slopes in order to remain in habitats with the same climate conditions.
Most ecologists, conservation biologists, and climate scientists worry that global warming will contribute greatly to species extinctions.
For example, one study released in 2015 that examined 130 extinction models from previous studies predicted that 5.2 percent of species would be lost as a result of global warming alone with a rise in average temperatures of 2 °C (3.6 °F) above temperature benchmarks taken before the start of the Industrial Revolution. The study also predicted that about 16 percent of Earth’s species would be lost if surface warming increased to about 4.3 °C (7.7 °F).