L1 is roughly 1 million miles away from the Earth in the direction of the Sun, roughly 1/100 of the distance (0.01AU). The Sun's diameter is roughly 100 times the diameter of Earth. This means that at L1, a parasol that is big enough to cast shade on Earth has to be TWICE the size of Earth, roughly 16,000 miles in diameter.
So you're not only talking about one HUGE FREAKIN' parasol, you're talking about one that is TWICE the size of Earth.
Ain't happenin'.
HA! I didn’t know that! Thanks for educating me!
Your math is close for a total earth disk sized shadow. The proposal is to block 1 or 2 percent of the total insolation that would be a disk 150 miles in diameter for a 1% interception at L1 with carbon nanotubes and spinning the disk it could be literally atoms thick to block the IR light wavelengths. Solar photon thrust would be a very real issue you have just created a solar sail on a scale that has been proposed to send probes to proxima Cent. The closer star to terra. You would need to tack the sail against the solar wind to put the thrust vector against the wind vector otherwise it will.rapidly accelerate to sol C3 escape velocity. The theory is valid and a sail of a few hundred miles in diameter is within human technical reach. Should it be done is the better question at least if they messed up the calculations and it cooled too much just tilt it with the solar wind vector and away it fly out of the solar system forever.