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A giant parasol that hovers high in orbit to block the sun. Refreezing the melting poles by making submarine-sized ice cubes. Pumping extra carbon dioxide deep underground for indefinite storage. Spraying sulfur dioxide into the atmosphere to form clouds to artificially cool the Earth.

As global warming becomes more dire – and nations dither about decreasing emissions – could these controversial technological fixes known as geoengineering buy us time to move away from burning fossil fuels?

Scientists say that transition needs to be well underway in the next 20 years. While geoengineering isn't a replacement for moving to a carbon-neutral economy, it has increasingly been seen as something humans need to explore as a stopgap.

We don't have much time, said Hugh Hunt, an engineering professor at Cambridge University in England, where he directs the Center for Climate Repair.

More: Were the predictions we made about climate change 20 years ago accurate? Here's a look

He compares geoengineering to the often harsh and sometimes dangerous chemotherapy used to treat cancer.

"I’m no more pro-geoengineering than someone working on chemotherapy is pro-chemotherapy. But if someone has cancer, you try chemotherapy,” he said.

As wacky as some of these ideas might sound, they may turn out to be just as serious as the problem humankind now faces. Here are some planet-saving ideas being discussed by global scientists, along with their pros and cons:

Sucking carbon dioxide out of the atmosphere Bill Campling/USA Today Network; Getty Images

The idea: Sucking carbon dioxide out of the atmosphere

Carbon dioxide (CO2) is the main culprit in climate change. In the atmosphere, the gas acts as a blanket, keeping the heat in. The more of it there is, the hotter things get. Multiple groups around the world are working on ways to pull the gas out of the atmosphere and store it deep underground.

The pros: An elegant, fast solution

Putting CO2 back underground can be as simple as planting (or replanting) forests and using cover crops to capture and store the gas. A more technical approach involves constructing huge fanlike machines that suck in air, pull out carbon dioxide via chemical processes and then store it underground. Pilot projects to do this are underway in Switzerland, Norway, Canada and Iceland.

"All it's doing is taking the carbon dioxide out of the air and putting it back underground, where humans took it from in the first place," said David Hawkins, director of climate programs at the Natural Resources Defense Council, an environmental advocacy group based in New York City.

The cons: Scaling it up

The technology is still in its infancy and would need to be deployed on a global scale, which would be neither cheap nor easy. It would require years and lots of experimentation to create robust systems that are cost-effective.

Spraying sulfur dioxide into the atmosphere could form clouds that would artificially cool the planet. Bill Campling/USA Today Network; Getty Images

The idea: Creating clouds with sulfur

Spraying sulfur dioxide into the atmosphere could form clouds that would artificially cool the planet by reflecting solar radiation away from the planet.

The pros: A cooler Earth

This plan takes its cue from a natural experiment that actually has made the only recent dent in global warming's rise in the past few decades – the 1991 eruption of Mount Pinatubo in the Philippines, which blasted more than 15 million tons of sulfur dioxide 21 miles high. The haze scattered and reflected sunlight away from the Earth and cooled global atmospheric temperatures 0.7 to 0.9 degrees Fahrenheit in 1992 and 1993 before finally washing out.

The cons: Stopping could cause problems

Getting that much sulfur dioxide into the atmosphere wouldn’t be easy, and airplanes would have to continuously fly into the upper atmosphere to maintain the clouds because they would last only about a year if spraying stopped. Some scientists contend that a long-term, planetary-scale program of this type could be dangerous because suddenly stopping that spraying could devastate the planet if it led to rapid rewarming, according to a study on the potential biological effects of climate intervention released in 2018.

"Fertilizing" the oceans with iron to stimulate algae and plankton blooms could result in pulling CO2 out of the water. Bill Campling/USA Today Network; Getty Images

The idea: Dump iron shavings into the ocean

About 30% of the world’s oceans have low iron levels, which keeps the growth of microscopic ocean plants and animals such as algae and plankton down. The idea here is to fertilize the oceans with iron to stimulate algae and plankton blooms. These would pull CO2 out of the water and into their bodies. When they died, they would fall to the ocean floor, storing the CO2 there.

The pros: The ocean is a carbon sink

Oceans take up two-thirds of the surface of the Earth, so storing large quantities of carbon dioxide deep underwater could be a good bet. And more algae and plankton could be good for the world’s fisheries, potentially increasing populations of fish and marine mammals

The cons: It might not stay put

There’s no solid data that this would actually work. Some researchers say the algae and plankton are quickly eaten by microbes and other creatures close to the water’s surface and never make it to the ocean floor. There’s also concern that radically changing the ecology of large swaths of the ocean could have unintended consequences for the health of ecosystems that much life on Earth depends on.

A team in China has suggested deploying a swath of 5,000-square-foot shields made out of a thin reflective film in orbit to decrease the amount of solar energy that reaches Earth's surface. Bill Campling/USA Today Network; Getty Images

The idea: A giant sunshade orbiting the Earth

A team in China has suggested deploying a vast swath of 5,000-square-foot shields made out of a thin reflective film in orbit above the Earth to decrease the amount of solar energy that reaches the surface. They imagine building a series of foldable shields that could be deployed by rockets and unfolded in space.

The pros: Less sunlight = less warming

Variations on the solar shade idea have been floated by several groups, who suggest blocking sunlight before it reaches the Earth. The Chinese researchers estimate their plan could achieve as much as a 1.2% reduction in solar input.

The cons: A million rockets

The Chinese team calculated that covering the necessary area with shields would take 1 million launches to deploy the shields into space using the Chinese Long March 5 rockets. The cost and energy required to place the shields would be massive.

The polar ice caps are melting at an alarming rate, and various plans to reverse the trend have been suggested. Bill Campling/USA Today Network; Getty Images

The idea: Refreeze the polar ice caps

The polar ice caps are melting at an alarming rate, and various plans to reverse the trend have been suggested. One involves filling submarines with water, filtering out the salt to raise the water's freezing point by 3 degrees and then allowing it to freeze. The resulting rectangular ice blocks, 16 feet thick and 82 feet wide, would be pushed out into the water. These might then freeze together during the winter to create more ice cover. Another suggestion involves using wind power to spray seawater onto the polar ice caps where it would freeze more quickly, replenishing Arctic ice.

The pros: Ice is reflective

Thicker ice is more likely to survive the summer than thin ice. The more ice the better, because ice reflects a high proportion of the sun’s rays while open ocean water absorbs them. Once the ice is gone, the entire area heats up faster.

The cons: That's a lot of wind pumps

A scientist in England calculated it would take 10 million submarines to replenish the polar ice that has melted over the past 40 years. For the thicker ice, the scientists project that to replenish just 10% of the area of the Arctic Ocean would require 10 million wind-powered pumps.

Covering glaciers with blankets is a tactic one Swiss town has actually been using for years. Bill Campling/USA Today Network; Getty Images

The idea: Wrap glaciers in thermal blankets

This is actually something that the residents of the Swiss town of Obergoms have been doing for the past eight years. In the spring, they drag large thermal blankets around parts of the Rhône Glacier to protect it from the sun’s rays. The glacier contains an ice grotto that has been carved out every year since 1870 and is a major tourist attraction, and residents are trying to keep it from melting away. The glacier has lost an average of 33 feet in thickness in the past 10 years.

The pros: The glacier is melting more slowly

Glaciologists say the effort reduces melting by 50% to 70% each summer.

The cons: Costly and ultimately futile

The process of covering the ice takes hours and costs thousands of dollars in materials and manpower each spring. And despite the work, the glacier is still receding. The Swiss say it has retreated 4,600 feet since 1856.

Contributing: Doyle Rice in Washington, D.C.