Man has influenced the environment in three very different ways: first a dramatic reshaping of the landscape to create efficient agriculture and urban life; secondly major interference in the biogeochemical cycles of carbon, nitrogen, phosphorus, and metals, changing the physics and chemistry of the environment; thirdly thousands of chemicals, foreign to the planet and its life forms, have been used extensively in the environment, some of them deliberately to poison life.
The consequences of the these three kinds of impact have been discussed in the earlier sections of the book. Here we will study how these types of pollution and environmental impact spread into the environment on a local, regional, and global scale, how they interact with each other either by counteracting each other or by reinforcing each other, and finally how long they last, the longevity of environmental impact.
The pollution chain is the route a pollutant takes from its production to the environment. Some chemicals are easily taken up by life forms and may accumulate in organs and tissues, and stay in food chains as they migrate from prey to predator, even from mother to child.
Chemicals have special effects on ecosystems. An ecosystem might be completely disrupted if one key species is badly damaged, and prey-predator relationships are changed. Typically, ecosystems hit by pollution loose diversity and biomass. At the same time environments that are less diverse, both as landscapes, and as ecosystems, are more vulnerable to environmental impacts.
Compared to the 1950s and 1960s, when the threat from chemical pollution was first grasped seriously, much has happened. Many chemicals have been banned and new chemicals have been designed so they do not accumulate in ecosystems. But old chemicals still leak from society into the environment, and new threats are continuously discovered. Lately, pollutants that influence the sexual differentiation in animals, the so-called endocrine disruptors, are creating a new situation, a chemical panorama that seems more threatening and finally may reach man.
Environmental impacts interact in several ways, either to reinforce one another or sometimes dampen each other. Landscape changes make the environment more or less susceptible. A modernised monotonous production landscape enhances eutrophication since the factors that reduce nitrogen and phosphorus flows are absent. At the same time an ecosystem that has relatively few species is less able to withstand the impacts of pollution and changes in general, e.g. the Baltic Sea ecosystems, it is less robust. An environment that has changed but is able go back to its original status after an impact has ceased, is called resilient.
Some of the impacts that man has had on the environment will last a very long time. Changes in infrastructure, roads, buildings, etc., will last perhaps to the next ice age. Also landscape changes, e.g. deforestation and drainage, may be very long lasting. Forests will take hundreds of years to be more natural and a "virgin" forest will probably take a thousand years to establish itself. A chemical impact will only last as long as the chemical survives. However, changes in biogeochemical cycles will take hundreds or thousands of years for global impacts to adjust, if mechanisms are available.
Some changes, the extinction of species, are irreversible. Even if we will in the long run be able to manage the environment, it is already clear that our children will live in an environment that is less rich and less diverse than ours.