When an object has the gravity to pull itself into a sphere, astronomers say that it's in hydrostatic equilibrium. And that's why the Earth is round. Of course, the Earth isn't perfectly round. Because it's turning on its axis approximately once every 24 hours, the Earth's equator bulges outwards.
More frequent droughts and shifting precipitation patterns lower water levels in rivers, lakes and streams, leaving less water to dilute pollutants. Higher temperatures cause more frequent algal blooms and reduce dissolved oxygen levels, both of which can cause fish kills and do significant harm to ecosystems.