Ideas to block or reflect sunlight:
• Buildings with white roofs and light-colored pavement reflect more sunlight back into space. Pros: Little chance of unintended effects. Con: Wouldn’t create a significant climate change by itself.
• Send sulfur dioxide particles into upper atmosphere. Pro: Likely to work, since this happens in nature when volcanoes erupt. Cons: Could change rainfall patterns, making it wetter in some places, drier in others. Might thin the ozone layer or have other unintended effects. Would have to be done repeatedly.
• Space-based mirrors. Orbiting mirrors would reflect sunlight away from Earth. Pro: Could be concentrated over key areas needing cooling, such as Earth’s poles. Con: Expensive.
• Cloud whitening. Ships would spray seawater into the lower atmosphere, leaving shiny salt crystals that brighten clouds and reflect sunlight. Pro: Uses only seawater so may be legal under current international law. Con: Would have to be done perpetually.
• Ocean whitening (microbubbles). Ships would leave wakes full of tiny air bubbles, which are whiter and more reflective than the rest of the ocean surface. Pros and cons: See cloud whitening.
• Fields of light-colored plants, perhaps genetically altered to be reflective. Pro: Would reflect sunlight back into space. Con: Might cause unintended consequences.
Ideas to remove CO2 from the atmosphere (all take a long time to create a substantial effect):
• Plant (genetically modified?) trees to absorb CO2. Pro: It works and has little risk. Cons: Genetic modification could pose risks. Not likely to have much effect by itself.
• Create artificial trees to absorb CO2. Pro: Would remove CO2. Con: Costly, unproven technology.
• Ocean fertilization with iron or other nutrients. Pros: Algae blooms would suck in CO2 and then sink to the ocean depths. Cons: One study suggested the blooms might be toxic. Might have other effects.