Clouds, being white, are much more reflective than the murky oceans underneath. Increasing the amount of cloud in the atmosphere, or making clouds 'more white', therefore, will increase the albedo (reflectivity) of Earth. Imagine Earth as a dull mirror; cloud modification polishes up part of the mirror, allowing more energy to be reflected away from the surface.
The most likely candidate of the proposed schemes is to have a fleet of unmanned ships patrolling the oceans and spraying seawater mist into the air to thicken and whiten clouds. Modelling studies have indicated that a doubling of atmospheric CO2 can be compensated by doubling the droplet concentration in stratocumulus clouds across about 3% of Earth's surface.
2. Ocean Fertilization
The food chain of the ocean is rooted by phytoplankton, tiny photosynthesising organisms that combine carbon (absorbed into the ocean from the atmosphere) with nutrients to grow. They are similar to trillions of tiny plants floating in the waters, supporting life on every other trophic level and taking up carbon dioxide in the process.
When phytoplankton die, they sink to the bottom of the ocean, safely locking away the carbon they absorbed from the atmosphere above. Therefore, fertilizing the nutrient-sparse areas of ocean with iron - being cheap and abundant compared to, say, nitrogen - will cause blooms of phytoplankton that will increase the carbon sink effect of phytoplankton. To date, at least 13 international trials have taken place, and ocean fertilization looks set to become one geoengineering project that may be a reality in the short term. It benefits from other methods because it is relatively inexpensive (compared to other geoengineering projects at least), and allows for private profiteering if governments and industry invest to offset carbon credit costs.
3. Bio-Energy Carbon Capture
Bio-energy with Carbon Capture and Storage (BECCS) is the only technology on this list that is currently fully operational. It effectively kills two birds with one stone - providing energy and sequestering atmospheric CO2 to create negative carbon emissions. It is also relatively simple, bio-fuels are grown, cultivated and burnt for energy, with the emissions captured and stored deep within geological formations underground for storage.
BECCS has the advantage of being scalable and relatively cheap, with less unpredictable climatic impacts when compared to other geoengineering methods. Forecasts predict BECCS could one day create negative emissions of between 5 and 20 billion tonnes of CO2 per year, with current pilot projects removing around half a million tonnes per year.
4. Stratospheric Aerosols
In 1815, Mount Tambora, a volcano in Indonesia, exploded in what was the biggest volcanic event for over 1,300 years. The year that followed became known as 'The Year Without Summer', where average global temperatures dropped by 0.4 - 0.7°C, causing food shortages and socio-economic effects felt across the globe.
The reason? The super-colossal volcanic event ejected a huge plume of aerosols and dust, which, carried by jet stream winds, circumnavigated the Earth and blocked a proportion of sunlight from reaching the surface. Stratospheric aerosol injection aims to basically copy this natural event (albeit on a smaller scale than Mount Tambora's eruption), by dumping aerosols from planes, artillery or balloons into the high atmosphere every few years. Not only is this technologically viable now, it may also be relatively cheap and manageable. After all, if there were unforeseen negative impacts it would only take a few years for the climate system to recover after aerosol injection was stopped.
5. Space Sunshade
The first proposal is to launch 16 trillion small disks, each about half a metre across and 5 micrometres thick, into a Lagrangian point, where together they would block about 2% of sunlight. Launching of the mirrors would require new technologies, such as huge cannons with 0.6 mile wide barrels to consistently fire the mirrors into space. Of course, such a futuristic project would take decades to implement and trillions of dollars in investment.
The second proposal would be to put one huge concave Fresnel lens into a Langrange point to spread solar radiation around Earth. The lens would have to be truly monumental, perhaps 1,000km across, although only a few millimetres thick. Whilst this proposal is cheaper and probably more technically viable than the above proposal, it's certainly no steal at an optimistic $20 billion price tag.
Whilst an exciting proposal, it is really quite difficult to avoid words such as 'ridiculous' when considering its merits. It is therefore difficult to see space mirrors being the most economical and effective of geoengineering solutions.
Is geo-engineering the solution?
There are a number of not-inconsequential issues that climate geoengineering must face. Who, for example, would foot the bill for what are extremely expensive schemes? What about their effectiveness? Most of these proposals, after all, have never been tested in the 'real world', existing only in laboratories, computer models and on the back of scientists calculations. What about the inevitable and significant side effects to our weather? And what if funding dried up, or the system failed? Would it cause potentially disastrous sudden climate change? And isn't this just a stop gap solution, at best buying us time and at worse reducing real impetus for fossil fuel management?
Unfortunately, however, the cost of doing nothing may ultimately outweigh the cost of doing something. With the exception of greenhouse gas management, these proposals tackle the symptoms, not the cause. So is geoengineering a final solution? Probably not. A necessary experiment? Probably so.
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