Bring rain and reduce air pollution.

Clean up cities and bring rain with solar energy: With global warming the land is heating up faster than the ocean in terms of temperature (although about 90% of the solar energy absorbed by Earth is going into the oceans). When this happens the relative humidity of the air decreases more when blowing from sea to land. One can have floating spray pumps in the ocean to increase relative humidity (the pumps could be operated by wave motion). Another method to get more rain is to have this sort of system near the sea: Where there is land not being used (say on rocky hills, etc) erect pipes a few metres in diameter and a few hundred metres high and reflect sunlight onto these pipes using mirrors. The air in the pipes will be heated and rise and convectional rain is far more likely than usual to occur. It should be very cheap system to implement without the need for machinery to operate it. The bottom part could consist of a greenhouse. Often one will get 6 kWh or so of sunshine per day on a one square metre horizontal surface. Theoretically at 101.325 kPa this could heat 3600 cubic metres of air by 5 deg C (just the solar energy on one square metre could do this) starting off with 24 deg C air. With a 5 deg C temperature difference one can expect about 600 cubic metres per second out of the pipe (tower) if it is 400 m high or so (to research this Google stack effect draft, etc). To supply the energy for heating the 600 cubic metres coming out one needs roughly 5x1.2x600 kJ of energy per second (volumetric heat capacity of air is about 1.2 kJ per cubic metre per deg C and temp rise is 5 deg C and volume is 600 cubic metres). If sun power is 0.5 kW/ sq metre then we need 5x1.2x600/0.5 square metres of radiated surface. ie we need 7200 square metres of radiated surface. Therefore we need an area of about 84 m by 84 metres to supply energy to heat 600 cubic metres of air by 5 deg C every second. Of course there will be heat losses, so a bigger area is needed. This device can also be used to move polluted air out of cities by convection.
Links: 1) Cool roofs could be reducing rainfall https://www.scientificamerican.com/article/cool-roofs-may-have-side-effects-on-regional-rainfall/
2) Colour of land could affect rainfall: https://eos.org/articles/more-intense-rains-in-u-s-midwest-tied-to-farm-mechanization?utm_source=Eos%20Primary%20List&utm_medium=email&utm_content=more-intense-rains-in-u-s-midwest-tied-to-farm-mechanization&utm_campaign=5022c1d8f8-Weekly_All_Content_Digest&utm_term=0_f923f18da4-5022c1d8f8-522497757
3) Regarding cool roofs, etc, http://iopscience.iop.org/article/10.1088/1748-9326/11/6/064004/meta says, 
"The lowered wind speeds and vertical mixing during daytime led to stagnation of air near the surface, potentially causing air quality issues." 

If space is a problem then one could build a greenhouse with black floors to absorb solar energy and holes in the floors to let hot air rise through. See following diagram: