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At the outset of the century the ideas with mass followings remained the great religions. The doctrine of the Bible includes various injunctions about nature.
The god of the ancient Hebrews enjoins:
" Be fruitful and multiply and fill the earth and subdue it and have dominion over the fish of the sea and over the birds and over every living thing that moves upon the earth" . Genesis 1:26-29
In Genesis 9 :1-23 God tells Noah and his sons:
"Be fruitful and multiply and fill the earth. The fear of you and the dread of you shall be upon every beast of the earth and upon every bird of the air and upon everything that creeps upon the ground and all the fish of the sea; into your hand they are delivered. Every moving thing shall be food for you and as I gave you the green plants, I give you everything."
Who could ask for clearer licence to appropriate all the net primary productivity of the planet? These and other biblical passages inspired an argument to the effect that Christianity or the Judeo-Christian tradition uniquely encouraged environmental despoliation.
Every durable body of scripture is ambiguous, self-contradictory and amenable to different interpretations to suit different circumstances, but these and other constraints affected environmental outcomes only modestly.
A variation on the Judeo-Christian theme is the notion that Western humanism, rationalism or the scientific revolution uniquely licensed environmental mayhem by depriving nature of its sacred character. It is true that western science helped recast environments everywhere indirectly by fomenting technological change.
Environmental change on earth is as old as the planet itself. Homo Sapiens has altered earthly environments for about four million years but there has never been anything like the twentieth century.
For most of human history people could not pollute the air except by kicking up a little dust. Then half a million years ago fire was harnessed. Landscapes were torched, releasing carbon dioxide and other gases into the air. But this made little atmospheric impact. Natural processes including the steady work of trillions of microbes and the occasional major volcanic eruption governed the atmosphere.
Since 1900 one thing above all else accounts for air pollution - fossil fuel combustion. In 1900 coal combustion caused most of the air pollution. Most of the things that changed environments came from economic activities.
The human race has undertaken a gigantic uncontrolled experiment on the earth and in environmental history the twentieth century stands alone because of the acceleration of so many processes that bring ecological change. Most of these processes are not new, timber has been cut, ore has been mined, wastes have been generated, crops have been grown and animals hunted for a long time. But since 1945 these things have been done more than ever before.
The ecological peculiarity of the twentieth century is a matter of scale. Air pollution previously only had a local impact but it has now grown on so comprehensive and large a scale that it affects the fundamentals of global atmospheric chemistry.
The environmental history of the twentieth century is different not only because ecological changes were greater and faster but also with increased intensities. For example incremental increases in fishing brought total collapse in some oceanic fisheries. Accumulation of many increased intensities may throw some grand switches producing some very basic changes on the earth.
The twentieth century would also appear equally unusual if one chartered the long term use of fresh water use, timber use or mineral use for industrial output All of these boomed after 1900 as did the generation of solid waste and of air and water pollution.
The use of artificial fertilisers allowing perhaps an extra two billion people to eat , systematically widened the gaps between rich and poor farmers around the world. Fertilisers mostly miss targets and become water pollutants . Estimates vary but something between half of the fertilisers applied end up in the waters of agricultural communities and their downstream neighbours.
The impact of chemical fertilisers did not stop with chemistry. They strongly influenced the choice of crops after 1950. Those that respond well to fertilisers, maize for example, spread far and wide replacing those that did not. Hence the trend towards more and more people eating fewer and fewer varieties of food. Two-thirds of grains now come from three plants: rice, wheat and maize.
Chemical fertiliser use also made food production dependent upon the fossil fuels needed for fertiliser production. It has recast the global phosphorus and nitrogen cycles and strongly favours all species that thrive on heavy diets of these nutrients. Whilst the chemistry industries of the twentieth century replenish the soil nutrients, they have helped to contaminate the soil.
The effects of these planetary changes to humanity remain as yet unclear.