Five systems, and the fact that makes them one problem: they are coupled. Pressure on any one propagates through the others.
Climate
The atmosphere is a shared medium with no property lines, which is why it has been the cheapest place to put things. Warming is not primarily experienced as heat; it is experienced as the destabilisation of everything that was calibrated to a stable climate — harvest calendars, water infrastructure, insurance, settlement patterns and the boundaries of where diseases live.
Water
Fresh water is the constraint that binds most quickly, because it cannot be substituted. Groundwater in several of the world’s major agricultural basins is being withdrawn faster than it recharges — a form of borrowing from a future that has no say in the loan. Warming intensifies both flood and drought in the same regions, which is a harder engineering problem than either alone.
Soil
Topsoil is a living system, not a substrate: a handful contains more organisms than there are people on Earth. It forms at the rate of roughly a centimetre per century and is being lost, across much of the world’s cropland, considerably faster. This is the least visible of the five and arguably the most consequential, because everything eats from it.