Monocle in the sky
One way to reduce temperatures on earth would be to reduce the amount of sunshine reaching us. Hence, a concave lens, 1,000 km in diameter and only a few mm thick, floating in space between the earth and the sun at a distance of about 1.5 million km from the earth. The lens would disperse the rays from the sun reducing sunlight on earth by 0.5 to 1 percent. It’s a bit heavy, though, at 1.57 trillion kg and at $24 quadrillion, a tad expensive.