How can environmental problems be local, regional, or global? What are some examples?

Consider a lake. Suppose we would like to estimate the phosphorus level that would be expected in a completely well mixed lake under steady state conditions given some combination of phosphorus inputs and sinks. If there is too much phosphorus, this will cause euthrophication. Rate of addition of P = Rate of removal of P is the equation that could be used. However, this is a lake (the boundary of this condition is the lake boundary). This is a local environmental problem. All lakes have different values in terms of their sedimentation rates, incoming P concentration, surface area, stream inflow and outflow rates, etc.

A regional environmental problem can be depicted as international river. Such as Asi (Orontes) river. It starts its journey from Lebanon, passes western Syria and enters Turkey. Regularly rivers increase their flow rates during their journeys. But due to heavy irrigation in Syria, water quantity drops when this river reaches Turkey. Water quality also decreases in its way.

Finally, a global environmental problem is greenhouse effect. Whoever introduces carbondioxide and other gases into the atmosphere, the source cause atmospheric increase of such gases everyhere. Right now carbondioxide concentration is over 400 ppm and still increasing. This is a global problem.