One of the important outcomes of exponential growth is that the growth that happens in one doubling period is equal to all the growth that has ever happened before.  However, not everything that is growing exponentially has always done so, or always will do so; so this rule, and others, won't apply in all practical cases. 

For example, human population has grown exponentially since a few-thousand years ago; but prior to that, it rose and fell for hundreds-of-thousands of years, depending on circumstances.  This means that the increase in population that occurs in one doubling period now, won't be more than the number of people that have ever existed before that doubling period, because of the very long period of human existence without growth before exponential population growth began.

This is also true for any resources that we have used throughout a large part of our existence; these are mostly renewable resources such as water, timber, and animal products.  However, most of the non-renewable resources that are causing us supply or pollution issues now have only been used by us in the last few-hundred or at most thousands of years; and for many of these our use has been growing exponentially since we first began to use them. 

For these resources the outcomes of exponential growth will apply, and the use that happens in one doubling period will be more-or-less equal to all the use of them that has ever happened before that doubling period.

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resource recycling and economic growth

exponential growth

economic growth

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