"Some flows, like sunlight or wind, cannot be affected by human action. No matter how many solar panels I put on my roof to absorb sunlight, they will have no effect on what the sun does tomorrow. But other flows--"critical-zone resources," in the jargon--can be exploited to exhaustion. Consider an archetypical critical-zone flow: the run of salmon swimming upstream to spawn. Drop a net across the watercourse and the fish will swim right into it. As long as the number of fish taken from the river every year doesn't exceed the number of survivors from that year's crop of newborns, fishing can continue indefinitely--the supply won't go down, no matter how many years people put in nets. But leave the net in too long one year and it will take every single salmon and there will be no more fishing after that. Catching the last fish is just as easy as catching the first--laying the net across the stream doesn't get more costly as the supply diminishes. With critical-zone flows, things typically go fine until they suddenly don't."