Humans have in habited the Earth for only a fraction of its history, yet they have changed the face of the planet. Their impact is obvious to the eye, even from space: city lights and gas flares by night, sprawling urban areas and the unnaturally straight lines of intensive agriculture by day. With other sensors, vast tract of pollutions are obvious on land, in water and in the atmosphere. Mapped over just a few decades, the destruction of forests, the spread of deserts, the reduction in stratospheric ozone and the increase in green house gases are dramatic. All are the result of human activity. In the past, the Earth has displayed a remarkable resilience, globally if not locally: today, an exploding population that demands ever increasing affluence may be pushing the limits of the Earth’s resources and its ability to process waste materials.
At almost every stage the complex natural cycles between land, sea and atmosphere can be influenced, augmented or upset by human activity. Most emission follows the hydrological cycle. They can wash into ground water and rivers, dissolve in water vapour in clouds and fall again to Earth in rain.
Mining and Industry can release toxic metals and other wastes into ground water, and they release metals into the atmosphere as fine particles. They produce acidic gases, such as Sulphur dioxide and nitrogen oxides, and green house gases, notably carbon dioxide. Human activities, using fossil fuels, notably power generation and transport, are the biggest net emitters of greenhouse gases since they extract the carbon from non renewable sources.