Human activity is anything that humans do. It is often classified into activities that directly generate income, referred to as economic activities, and those that are related to personal satisfaction, or non-economic activities.
The natural environment usually changes only very slowly over time as a result of evolution, natural climatic change, and geological and other changes. However, it has been changing rapidly and at an ever-increasing speed during the past several hundred years as a result of the accelerating growth in human population and in human activity. Some of the types of human activity that have been occurring on a massive scale and having the greatest effects on the natural environment include the burning of fossil fuels, the use of plastics and other substances derived from fossil fuels, deforestation, hunting, and the introduction of alien species.