In pictures: Global sunshade

Getting a 20-million tonne sunshade into space, however, is not easy... or cheap.

As one of the world's foremost minds on glass optics, Roger Angel decided the best thing to do, instead of building one big shade, would be to construct many smaller optics that together make up the completed structure.

But a space shuttle can only carry payloads of up 25 tonnes to low-Earth orbit. To get the glass discs into space therefore requires a different approach.