How it works: The idea here is to use satellite-guided sailing ships to crisscross the oceans, constantly spewing a fine stream of sea mist into the clouds.
Turns out particles, in this case the salt in the sea mist, will cause clouds to become denser, reflecting more sunlight back into space and keeping the planet cooler.
The novel ship design actually dates back nearly a century, when the German inventor Anton Flettner built one that crossed the Atlantic. The mist towers are hollow and rotate in the wind, acting as sails. It can cross the Atlantic faster than a conventional sail boat and do so without a crew. The power for the mist pumps is generated by the turbine under the hull.
Each ship would cost $2 or $3 million, making the entire program cost just a few billion dollars.
About 1500 ships would be needed to maintain current temperatures, according to John Latham, a scientist at the National Center for Atmospheric Research.
Beyond the machine: Like most scientists studying geoengineering, Latham said it is not a substitute for reducing greenhouse gas emissions as it does not address other problems associated with too much carbon dioxide.
He also said the idea, like most other geoengineering ideas, needs much more study before it's deployed on a wide scale, as any other potential side effects are still unknown.
Space-based Frisbee deflectors
This is perhaps the most far-out idea here.
How it works: Deflect the sun's rays with a cluster of translucent disks, each two feet in diameter, placed in orbit between the earth and the sun.
The number of disks needed for the job is staggering: Some 16 trillion, according to Roger Angle, a professor of astronomy and optical sciences at the University of Arizona.
Why it might not work: The limiting factor here is how to launch these things into space, as it would be far too costly to send 16 trillion disks up by rocket.
So Angle is proposing a yet-to-be-built magnetic gun with a mile-long barrel to shoot the disks into space. Even then, he said it would still cost trillions of dollars.
"This is obviously pretty futuristic," he said. "But if we get to the panic point, you need to know a little bit about what your options are."
A permanent haze
You can go a step further than just whitening clouds by actually putting more clouds in the atmosphere.
How it works: Volcanic eruptions have shown that putting sulfur in the atmosphere cools the planet. Rain drops react with the sulfur particles high in the stratosphere, forming clouds that keep out sunlight.
The clouds are thin, so the sky would be less like London and more like a hazy day at the beach.
The sulfur is cheap, and comes from an activity we're well versed in: burning oil and coal. It could be delivered into the stratosphere by high flying jets, artillery, or hot air balloons, pictured here.
Another idea is to deliver the sulfur to the stratosphere via an extremely long hose kept aloft by a series of balloons. This method is said to be cheaper than others.
Why it might not work: A major downside is what effect this could have on rain patterns, said Alan Robock, an environmental sciences professor at Rutgers University. It may disrupt the monsoons in Asia, rains that sustain food crops for billions of people.
And like most geoengineerng schemes, it would need to be done continuously, for as long as carbon levels stayed high, or else the earth would rapidly warm up again.
Carbon-sucking machines
We've put it in the atmosphere for so long, why not take it out?
How it works: This device relies on panels made of a thin plastic membrane that traps carbon dioxide, pictured here atop a supporting structure.
The panels can be cleaned by placing them in a humid environment. The carbon can then be buried deep underground.
Each machine can capture the same amount of carbon as 1,000 trees. About 1 million of them would need to be deployed to keep carbon in the atmosphere at an acceptable level, according to Columbia Professor Klaus Lackner, who designed the system.
The main problem: The system uses electricity, so it would need to be supplied from renewable sources to produce substantial results. And some have questioned whether carbon stored underground will actually stay there for the long-term or eventually seep back up to the earth's surface.
Ocean seeding
The ocean currently acts as a huge sponge for carbon dioxide, absorbing some 60 billion tons a year.
But all that carbon is making the oceans more acidic and reducing the amount carbon-sucking plankton.
How it works: This idea involves seeding the oceans with iron or other elements to stimulate plankton growth. Already, dust storms off the continents that blow iron into water are known to cause plankton blooms, seen here.
The iron is available as a byproduct of steel making, and could be dispersed for little more than shipping costs, said Dan Whaley, chief executive of Climos, a company working on the idea.
Why it might not work: Concerns remain over how additional plankton will impact the oceans. A main concern is what happens when the plankton dies? Some say it will safely sink down to the ocean floor. Others fear it will decompose near the surface, releasing its carbon into the atmosphere.
Beyond the idea: Climos is one of the few actual companies operating in the geoengineering space - most of the work is done by university professors or other researchers.
Climos' funding comes in the form of grants, but it's conceivable that some firms could make a handsome profit if any of these ideas pan out. Geoengineering schemes may qualify as carbon offset projects, where companies that emit carbon pay others to take it out of the atmosphere. Carbon offset projects may become mandatory under greenhouse gas laws.
But investors beware: Most of these ideas are far from reality.
White deserts
White roofs may cool cities, but white deserts may cool the planet.
How it works: This audacious plan would cover the world's great deserts with white plastic sheeting, like a tarp but thicker, in an attempt to reflect more of the sun's rays back into space.
This wouldn't take place on sand dunes or in deserts like those in the American Southwest that have lots of plants, but rather the vast barren stretches of the Sahara, the Gobi, or the Arabian Peninsula. Some have also talked about doing this over the ocean, although specifics on that idea are in short supply.
The big problem: Cost. In order to reflect enough light to balance increasing carbon emissions, an area the size of Missouri would need to be covered each year. That would cost an estimated $500 billion annually, said Alvia Gaskill, president of the consulting firm Environmental Reference Materials who has done some calculations on the idea.
There's also the issue of space. It could only be carried out for 40-60 years before all the deserts were covered.
Biochar
Currently farmers, foresters, and others that dispose of plants and trees usually leave them in the field to rot, or they burn them. Both those actions release carbon into the atmosphere.
How it works: This plan calls for farmers and the like to feed their waste into a machine that turns it into charcoal, seen here. The charcoal - or biochar - is then buried in the soil.
That would keep up to 40% of the carbon in the plant out of the atmosphere, and make the soil richer at the same time, said Jim Fournier, president of Biochar Engineering Corp.
Why it might not work: Questions remain over whether biochar could absorb enough carbon to make a difference in global warming.
Hurricane-killing wave pump
How it works: The contraption is basically a ring several hundred feet in diameter, with layers of old tires making up the outer wall.
In the middle, a funnel runs down several hundred feet into the ocean. The force of waves breaking over the outer tire ring forces warm water down the funnel. It mixes with the cooler water below before rising to the surface again, colder than before. This would create a constant exchange of warm and cold water.
Why it might succeed: Hurricanes rely on warm water for their strength, this process denies them that and may weaken the storms, said Ken Caldeira, a scientist at the Carnegie Institution who, along with Bill Gates, is one of the scientists who came up with the idea. It may also absorb more carbon dioxide from the atmosphere, since cold water dissolves more CO2 than warm water.
Why it might not succeed: Some question how much carbon it can absorb, and also whether it could be deployed on a scale that would weaken a big storm. Others say it will simply make hurricanes go elsewhere.
How it works: The idea here is to use satellite-guided sailing ships to crisscross the oceans, constantly spewing a fine stream of sea mist into the clouds.
This is perhaps the most far-out idea here.
You can go a step further than just whitening clouds by actually putting more clouds in the atmosphere.
We've put it in the atmosphere for so long, why not take it out?
The ocean currently acts as a huge sponge for carbon dioxide, absorbing some 60 billion tons a year.
White roofs may cool cities, but white deserts may cool the planet.
Currently farmers, foresters, and others that dispose of plants and trees usually leave them in the field to rot, or they burn them. Both those actions release carbon into the atmosphere.
How it works: The contraption is basically a ring several hundred feet in diameter, with layers of old tires making up the outer wall.