I was asked this question the other day in my environmental science class. Below is the question and my response
What evidence suggests that the human population has reached its capacity on the planet? How can the health of the environment be measured?
When talking about the world as a whole, this is a huge topic. I would rather take things down to the lower levels. Say a family of six lives on 3 acres of land and they are able to provide 80% of the items needed to live. There is no waste and everything produced is able to be reused or composted, 80% is enough to survive but the standard of living is nothing like what we are used to in the higher developed nations such as the United States. Say two families of 5 work together on 6 acres and are able to diversify what they farm and trades that they make between them, now both sets of families are able to live at a higher level and sustain for themselves through trade. Add a third family and the need to have more children goes down but the ability to create a more robust trade system grows. Now you can have one family that focuses on food, one on cattle, and maybe one that works on other items such as technology. When gathering as a community the human population can accomplish great things.
Once people are gathered in very large groups is when the balance can be thrown off. Meaning in a large metropolitan area, say New York City, people no longer focus on things needed by the community like food production. The areas that are not as highly populated are then taken and farmed to produce the food that is needed and shipped to different area. As technology advances so do things like yield but there will be a point when the vast diversity of crops grown dwindles down to only a few strands of each crop. When the population of any given area has given up its ability to produce what it needs on its own I believe that they have then passed over the capacity. As a whole, the planet is on the verge of reaching this level unless people start looking for a way to become more self sustaining where they choose to live.
The health of the environment cannot be measured per say but by any one test or type of test. There are many different valuables that need to be found and or compensated before a large picture can be drawn for the entire planet.