1. Background
   1. Most studies focus on the droughts.
   2. The background aridity in a warming world is less unknown.
   3. Note that the droughts rely on the background aridity.
2. Method
   1. The ratio of precipitation to potential evapotranspiration (P/PET).
   2. Higher P/PET means a drier world, and vice versa.
   3. P/PET should be 1 globally and on climate time scales.
   4. P shows no difference from ocean to land.
3. Result
   1. P/PET is decreasing and projected to be decrease in the future over land.
   2. Thus, P/PET over ocean should be in the increasing trend.
   3. Thus, PET increases faster than P over land.
   4. Trend: PETland > Pland = Pocean > PETocean
4. Mechanism
   1. Land is warmer than Ocean.
5. What are droughts? What are heatwaves? Should we define them relatively or absolutely?
6. So, tropical cyclones will decay faster on land in the future?