World

CARBON DIOXIDE REMOVAL

Roughly half of global carbon emissions are removed naturally from the atmosphere each year. Human-­devised CO₂ removal strategies, including restoring forests or sprinkling iron dust on the oceans to spur rapid plankton growth, typically boost natural processes. Another approach: Two North American start-ups and a Swiss firm have built facilities for harvesting small amounts of carbon from the air.

CO₂ could be trapped by thousands of giant artificial trees equipped with filters.

STRATOSPHERIC AEROSOLS

When Mount Pinatubo erupted in the Philippines in 1991, it spewed sulfur dioxide into the stratosphere, which reflected so much sunlight back into space that for a time the Earth was cooled by about 0.5°C (0.9°F). A riff on that event—injecting sulfate particles into the upper atmosphere—has generated more buzz than any other geoengineering idea. But managing solar radiation only temporarily masks the effects of global warming.

Harvard physicist David Keith theorizes that by 2070, 50 customized jet planes distributing a million tons of sulfur a year could halve the rate at which Earth is warming.

SPACE SUNSHADE

Launching sunshades into space is the most way-out—literally—geoengineering proposal. Trillions of extremely thin, lightweight disks could be shot into orbit a million miles above Earth to reflect sunlight. But deploying this global sunshade would be tremendously costly and time-consuming: Twenty electromagnetic launchers would have to send a stack of 800,000 disks into space every five minutes for ten years.

The “shade” would be 16 trillion small disks, each equipped with tiny adjustable solar sails to prevent it from drifting away.

MARINE CLOUD SEEDING

Marine clouds, which cover about one-fifth of the Earth’s surface, already help cool the planet; making them bigger would mean more cooling. Physicist John Latham proposes injecting water droplets into the clouds to increase the amount of sun they reflect; engineer Stephen Salter would have unmanned, satellite-controlled, wind-­powered ships travel the oceans, spraying seawater high into the air. Both ideas have yet to be tested.

Seeding low-level marine clouds with an aerosol made using seawater would improve their ability to reflect sunlight away from Earth.