If we consume more of a natural resource in a year than the Earth can replace in a year, we will eventually run out of that natural resource. For each natural resource, the process of resource depletion will be running at an individual rate. You can go to http://www.overshootday.org and http://www.footprintnetwork.org to read more about how scientists calculate these rates and combine them to produce the concept of ‘Overshoot Day’.
It’s an idea that has its difficulties, but it’s a compelling way of representing the ecological problem that humanity is creating for itself.
Fish out of water 