In the past they did not have to think too much about the impact that their decisions had on the natural world.
But that is now changing.
More and more people believe that we should live within the rules set by nature.
In other world, they think that in a world of fixed and limited resources, what is used up today will no longer be available for our children.
We need to look at each human activity and try to change it or create alternatives if it is not sustainable.
The rules for this are set by nature, not by man