“There is something increasingly desperate about placing more faith in technological cleverness when it is the unrelenting desire to command the natural world that has brought us to this point.”—Clive Hamilton
Stratospheric Aerosol Injection
The Plan: Release sunlight-blocking sulfate aerosols into the stratosphere with the aim of replicating the temporary drop of 0.6 degrees Celsius in global surface temperatures caused by the 1991 eruption of Mount Pinatubo.
The Cost: At about $18 billion per year for each degree Celsius of warming avoided, many geoengineers consider this the most affordable and realistic prospect, although it would result in changed weather patterns, such as reduced rainfall.
The Plan: Float sixteen trillion small mirrored disks in orbit a million miles above the earth’s surface, creating a reflective but permeable sheath around the planet.
The Cost: To surround the globe, engineers would need to fire missiles with stacks of 800,000 disks every five minutes for ten years, at an estimated cost of around a trillion dollars.
Bioenergy with Carbon Capture and Storage
The Plan: Cultivate trees in order to draw CO2 out of the atmosphere, then harvest and burn them for fuel at specially designed power plants that capture released carbon and store it underground.
The Cost:Each gigaton of stored CO2 would require up to 700 million hectares of land, reducing the amount available for agriculture and preservation.
Marine Cloud Brightening
The Plan: Create a reflective cloud cover by spraying seawater from land or via thousands of robotic boats at the rate of ten cubic meters per second. The liquid evaporates quickly, leaving behind salt particles.
The Cost:At an estimated $100 million to $200 million per year, the price would be less than that of the annual UN Climate Change Conference. However, saltier clouds would reduce global mean precipitation, threatening resource-rich rainforests.
Earth Radiation Management
The Plan: Use cloud-seeding drones to thin out high-altitude cirrus clouds, which trap thermal radiation released by the earth’s surface. With fewer of these clouds in the way, more of the earth’s heat would dissipate into space.
The Cost: Although the 140 tons of bismuth triiodide required to seed the troposphere would cost only $19 million per year, the process could worsen floods and droughts for up to 65 percent of the global population.
The Plan: Allow degraded forests to revert to their natural states. The World Resources Institute estimates that reforesting 2.2 billion acres, or 6 percent of the earth’s surface area, would capture two-thirds of man-made carbon emissions.
The Cost: The method relies on poor nations drastically reducing their available agricultural land, which would increase food prices and hunger. The incentives to forestry managers could cost over $280 per ton of CO2, or almost $1.7 trillion in total.
Direct Air Capture
The Plan: Remove carbon from the air by means of machines fitted with solid sorbent filters, which trap CO2 molecules by chemically binding with them.
The Cost: At anywhere from $100 to $230 per metric ton of CO2 removed, the process currently depends on the use of fossil fuels. A 2019 study predicted that a full-scale rollout of DAC machines would consume a quarter of global energy in 2021.
The Plan: Dump iron or urea into seawater to stimulate the growth of phytoplankton, which absorbs atmospheric CO2 through photosynthesis and organically sequesters the carbon by sinking to the ocean floor when it dies.
The Cost: Ocean macronutrient fertilization could cost as little as $20 per ton of CO2 sequestered, but uncontrolled algae growth would disrupt the marine food chain and deplete deep-ocean oxygen.