My (obviously unmodelled, hypothetical) low-earth-orbiting solution would be to use 'intelligent' rings of what I will call "dancing shadow leaves" (akin to the

My (obviously unmodelled, hypothetical) low-earth-orbiting solution would be to use 'intelligent' rings of what I will call "dancing shadow leaves" (akin to the 'shadow square' system that provided day/night cycles for Ringworld). Vast numbers of relatively small 'leaves' made of graphene-like material would be connected into a large number of rings held together by conductive nanotube-like 'wire'. The leaves would use electrostatic charge to change their angle and to jump to slightly higher or lower orbit (to avoid orbiting debris). Being able to change their angle appropriately for their changing position above the earth they would shade or not shade different areas on the ground. A supercomputer capable of modeling the affects of such shading, would use the system to have some control of over the weather in some areas. Just the ability to cool areas of the oceans that spawn tropical storms might allow the system to pay for itself. And ideally the leaves would also be photovoltaic and beam power to the surface. As long as I am dreaming, the leaves would be transparent to the wavelengths most supportive of photosynthesis so that they provide shade without lowering CO2 uptake. And before anyone chants "Ringworld is unstable" of course my rings's orbits would be constantly adjusted by using solar radiation pressure, electrodynamic tether thrusting and/or ion propulsion. To be able to shade most of the planet would require ~100X the surface area of the L1 proposal. But using recent nanomaterials and the much smaller transportation distances for LEO might make up the difference in cost. In addition a much smaller coverage system might still be enough to keep critical areas of glacier and tundra below freezing for more of the year and keep the most critical tropical storm spawning areas of ocean cooler.