Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs. That single definition, from the Brundtland Commission, is the lens for this whole topic: every environmental problem is a case of present use borrowing against the future, and every solution is a way of repaying that loan.
Worked example: Burning coal produces sulphur dioxide, a primary pollutant emitted directly into the air. In the atmosphere it can form sulphuric acid and fall as acid rain, damaging forests, soils and buildings far from the source. The chain — emission, transformation, distant harm — is why air pollution is treated as a regional problem, not only a local one, and why the answer to "which gas causes acid rain" is about sulphur and nitrogen oxides rather than carbon dioxide.