People have come up with a few ideas like that. However, instead of building a solid object, which would be huge and thus hugely expensive, these ideas involve changing the atmosphere to make it reflect more sunlight. One idea, marine cloud brightening, involves putting stuff in the air over the ocean that makes more clouds form, and makes clouds whiter, thus reflecting more sunlight. Another idea involves spraying certain sulfur-based chemicals in the upper atmosphere, which reflect sunlight.
Why haven’t we done these? While we think they would work, they might also have side effects we didn’t predict. And because these projects would have a huge main effect (changing the climate of the entire world), any side effect could be just as huge. So, it’s kinda scary to embark on a project like this.
Additionally, right now we are trying to fight climate change by cutting back emissions of greenhouse gases. If we start cooling the planet through engineering projects, maybe we won’t try as hard to cut emissions. This is dangerous, because cutting emissions is a way better way to solve the problem. Climate change is happening because we screwed with the environment. The best way to fix it is to stop screwing with the environment, not to screw with the environment in multiple ways and hope they cancel each other out.
This is why: https://www.youtube.com/watch?v=0qksm5cRtcU
In all seriousness, it would be insanely costly to build and maintain. There’s also the problem of who would be footing the bill for it (do all countries contribute? What if some countries decide not to pay?), and who would be paying to constantly maintain it.
There would also likely be unforseen consequences of doing this, though probably not as dramatic as the example in the video.
There are plans, it’s called geoengineering which involves sending planes high in the atmosphere and spraying aerosols that stay suspended in the air to reflect back some of the light. The problem is it’s very expensive and you’d have to get the whole world to agree on it and there may be unintentional consequences.
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