Ok, I teach this to my biology students: Algae are members of the protoctists (essentially organisms that don't quite fit the definition of animal, plant, or fungus). They grow rapidly in fertiliser causing light to be blocked from water plants. They die leaving behind a smorgasbord of food for bacteria and fungi and other decay-eating microorganisms. As they live, like us they need oxygen to perform respiration.....taking oxygen from the water, leading to larger organisms such as fish to die, leading to more bacteria etc etc etc finally causing anoxic water which smells and nothing larger than a cell is alive in it.

Or you watch older movies set in the future with dates that have already passed!

Earths magnetic field is linked to the cores rotations. I would assume the slowing would cause a decrease in the field that will give less protection to the sun's radiation. I'd also assume the change will be negligible for us normies (non-scientists) to worry about.