Clouds still form over the open ocean and drift toward the coast, but they empty into the dry air above the sealed coastal waters and never reach the land. The machinery of rain keeps running, yet the last step fails, so the water never arrives where it is needed. A system can look intact and still deliver nothing.
Waste that people treated as gone comes back as the force that destroys them. The industrial waste poured into the oceans returns as the very thing that withholds the rain. Inside the story this reads less as an accident than as the sea's simple retaliation.
Rain over land starts as water rising off the ocean, so anything that blocks that rise cuts the supply at its root. Here, waste dumped into the sea for decades forms a thin skin on the water that stops almost all evaporation. The skin lets air through, so the sea itself stays alive while the land beyond it dies of thirst.
A worldwide drought arrives as the peak of shorter droughts that had been coming more often for a decade. Only as dry zones spread across the map and food grew hard to supply did anyone attempt to control the weather. By then the cause lay somewhere nobody had looked.
As a river dries into lagoons and then a maze of creeks, it stops linking the people who lived along it. Each household becomes its own island, cut off in space and also from shared time and routine. Once the old connections are gone, survivors' minds and society start to crack apart.