Imagine if all human life ceased on Planet Earth. This doesn’t have to be a terrible thing, say we decided to leave en masse to give the Earth a break for ten years. What would we see from our orbiting space stations looking down on the recovering Earth? Perhaps we could leave some Land Remediation Services to sort things out in the worst places. What is land remediation? It is where the soil, water and ground is reclaimed so it can be used again. How long would that take naturally, if at all?
First of all we’d see the lights go out. Presuming that we’d remember to turn off and deactivate the Nuclear Power stations (otherwise they will go bang like Chernobyl and that will make the problem even worse) all the electricity will slowly stop. Fuel based centres will run out and solar panels will get so dirty the rays will not penetrate it. Dam based hydroelectric could still keep running. Places like the Hoover Dam were designed to go on for decades.
The seas would start to cover flood defences and the vegetation would start to grow unchecked. It doesn’t take long for the plants to find holes in concrete and cement. After about twenty five years all the cities would be covered.