Humans have been extremely successful as a species. 200,000 years ago, only a few thousand humans lived on Earth. In 1999, the Earth's human population exceeded 6 million. Our impact on the planet is gigantic. Humans have transformed half of the land on Earth for use as pasture (26%), farming (11%), forestry (11%), and housing, industry, services, and transport (2 to 3% total).

Humans use more than half of the easily accessible freshwater on Earth. We have altered the flow of about two-thirds of all rivers, creating artificial lakes and altering natural environments.



Construction of a new ski run in White River National Forest, Colorado. Courtesy Steve Holmer, photographer.

The oceans have also been affected by our presence. Although humans use only 8% of the primary productivity of the oceans, two-thirds of marine fisheries have been fished to their limits or beyond. In the 20th century, half of all coastal mangrove forests were destroyed by human-caused pollution, as were 10% of coral reefs.

By applying fertilizer for agricultural purposes, humans have altered Earth's nitrogen cycle. The amount of nitrogen entering the cycle more than doubled in the 20th century. Humans currently contribute 50% more to the nitrogen cycle than do all natural sources combined. The presence of excess nitrogen has caused excessive growth, leading to impoverished forest soils. In the oceans, high nitrogen concentrations cause toxic algal blooms and the formation of oxygen-free "dead" zones.

Through burning fossil fuels, humans have increased the carbon dioxide content of the Earth's atmosphere by 30% over pre-industrial levels, and have increased the methane content of the atmosphere by 145% over natural levels. Fossil fuel use is closely associated with increasing global temperatures.

Humans have had an enormous effect on the Earth's plant and animal life. Since 1600, 484 animal species and 654 plant species have become extinct. The dodo bird (shown below in an 1890 painting by German artist F. John), a native of the island of Mauritius, was unknown to humans until 1598, when Portuguese sailors landed.

The sailors took advantage of the flightless bird's lack of fear of humans to overhunt them, driving the dodoes to extinction in 1681. Humans have caused other species to disappear from Earth, less directly but just as permanently, by depriving them of necessary habitat.

The impact of human activities on Planet Earth depends upon our sheer numbers, the quantities of resources we consume, and the effects of our waste products. In many ways, human impact on Earth exceeds that of all other living species combined. It is clear that the future of our planet is in our inexperienced hands.