If it comes to geoengineering to save the biosphere, then think spherical and Brazil-sized. Regard the Space Bubbles Project from MIT

We keep a small part of our radar on geoengineering projects - we much prefer a post-consumerist, zero-carbon, techno-enabled cosmolocalism, so that our burden on the planet is lifted by our own hands, with human self-determination to the fore. But we couldn’t resist the interplanetary audacity of MIT’s Space Bubbles Project.

From their press release (media pack here):

‘Space Bubbles,’ inspired by an idea originally proposed by astronomer Robert Angel, is based on the deployment of a raft in space consisting of small, inflatable bubbles with the aim of shielding the Earth from a small portion of solar radiation.

This project is part of a solar-geoengineering approach—a set of technologies aiming to reflect a fraction of sunlight coming to the Earth—to contest climate change. Unlike other Earth-based geoengineering efforts, such as dissolving gases in the stratosphere for increasing its albedo effect, this method would not interfere directly with our biosphere and therefore would pose less risks to alter our already fragile ecosystems.

The raft itself (researchers hypothesize a craft roughly the size of Brazil) composed of frozen bubbles would be suspended in space near to the L1 Lagrangian Point, a location between the Earth and the sun where the gravitational influence of both the sun and the Earth cancel out.

This proposal addresses many questions: How to engineer the best material for the bubbles to withstand outer space conditions? How to fabricate and deploy these bubbles in space? How to make the shield fully reversible? What are the potential long-term effects on Earth’s ecosystem?

Visualisation of the Solar Bubbles as they shield Earth

“Space-based solutions would be safer – for instance, if we deflect 1.8% of incident solar radiation before it hits our planet, we could fully reverse today's global warming.”
Early, James T. "Space-based solar shield to offset greenhouse effect." Journal of the British Interplanetary Society 42 (1989): 567-569

This PDF download is a more technical presentation of the Space Bubbles Project.