"To block the necessary amount of solar radiation, the shade would have to be about a million square miles, roughly the size of Argentina...."

"A shade that big would weigh at least 2.5 million tons — too heavy to launch into space, he said. So, the project would have to involve a series of smaller shades. They would not completely block the sun’s light but rather cast slightly diffused shade onto Earth.... Dr. Rozen said his team was ready to design a prototype shade of 100 square feet and is seeking between $10 million and $20 million to fund the demonstration. 'We can show the world, ‘Look, there is a working solution, take it, increase it to the necessary size,' he said.... 'I’m not saying this will be the solution, but I think everybody has to work toward every possible solution,' said Dr. Szapudi, the astronomer who proposed tethering a sunshade to an asteroid."

From "Could a Giant Parasol in Outer Space Help Solve the Climate Crisis? Interest in sun shields, once a fringe idea, has grown. Now, a team of scientists says it could launch a prototype within a few years" (NYT).