The problem is the excess of CO2 hovering in the lower atmosphere, from burning fossil fuel, that traps heat closer to the ground level. Projections for global warming must also include heat from the Earth's core.
Exposed source of water will evaporate quickly. Cover them.
There can be water pools above the clouds of CO2. Rain will be excessive at this level. Grab the highlands. Trap and redistribute water on the highlands to the ground levels.
Fresh water source will be scarce when the ground heats up. This will occur way before mass extinction.
Given high CO2 levels, this can occur in the next two decades...
But first with excesses from the melting polar caps, it will rain and flood. Have a nice decade or two.
