Humans warmed Earth. Can't we just cool it? Here are some ideas.

Could we MacGyver our way out of climate change?

Ask scientists and you’ll get a range of answers: “Probably not.” “Maybe?” and “Are you insane?”

And yet ideas for geoengineering – literally engineering ways to either lower carbon dioxide levels or temporarily cool the planet while they come down – are increasingly being floated as a climate stopgap, even though they seem more like movie plot lines than serious suggestions.

Dehydrate the stratosphere. Create bubble curtains to keep warm water from melting glaciers. Fill the skies with dust or gas to reflect the sun’s rays. Add iron to the oceans so they can absorb more carbon dioxide. Build enormous industrial parks that do nothing but suck carbon dioxide out of the air and bury it deep underground.

The ideas sound outlandish, and even if they do work, critics worry they could backfire spectacularly. But amid the climate crisis, these moonshot ideas have gained attention from experts and investors.

There's a big catch: None of these ideas would allow humanity to keep burning fossil fuels without consequence. They're intended as a stopgap while people reduce fossil fuel use.

Here's a look at a few ideas for how humans could help cool the planet:

Carbon capture: Efforts underway to suck carbon dioxide out of the air

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 technology already exists, and at least one version is fast becoming noncontroversial. Even the United Nations' Intergovernmental Panel on Climate Change sees it as central to mitigating climate change.

On the controversial side are projects that capture CO2 from oil and natural gas wells and from industrial processes, then pump it back underground to make it easier to bring up more oil and gas – which doesn’t do enough to actually stop fossil fuel use, critics say. Because there are subsidies for this technology, it effectively pays polluters to pollute, they say.

The less controversial approach involves constructing huge fanlike machines that suck in air, pull out carbon dioxide via chemical processes and then store it underground.

Last year the Global Thermostat plant near Denver opened. It can capture about 1,000 tons of CO2 a year. The plant is a pilot project demonstrating that the technology can work on a large scale, but experts say far more and bigger plants would be necessary to make a real difference. Humans produce about 35 billion tons of carbon dioxide every year. About 500 billion tons of carbon dioxide would need to be removed to return to preindustrial levels of carbon dioxide in the atmosphere, according to researchers at the Massachusetts Institute of Technology.

Pilot projects are also underway in Switzerland, Norway, Canada and Iceland.

Solar geoengineering: Use sulfur to create reflective clouds

Solar geoengineering involves launching sulfur dioxide or other materials into the stratosphere so they reflect some of the sun's energy back to space, artificially cooling the Earth.

It's exactly what some volcanic eruptions do: spew huge amounts of dust and sulfur dioxide high enough that the material goes into the stratosphere where rain and other weather systems can't quickly wash it out of the air.

In 1991 the eruption of Mount Pinatubo in the Philippines 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 almost 1 degree Fahrenheit in 1992 and 1993 before finally washing out.

No one has yet worked out how to get the aerosols up in the atmosphere to reflect sun rays at the kind of scale that would be necessary.

Though the proposal wouldn’t reduce the amount of greenhouse gases in the atmosphere, it could buy time, said Wake Smith, a lecturer at the Yale School of the Environment and author of "Pandora's Toolbox: The Hopes and Hazards of Climate Intervention."

“It's a very immature technology, a very controversial technology, and so we shouldn't put too much faith in it yet," Smith said. "But if we heated the Earth too much, there is at least one tool that looks like it would help.”

Already two startups are working in the area, though neither has gotten very far.

In 2022, a company called Make Sunsets began doing small launches, deploying biodegradable balloons full of helium and sulfur dioxide that burst in the stratosphere. A U.S.-Israeli startup called Stardust Solutions founded in 2023 has so far gotten at least $15 million in funding to work on the technology required. 

Marine cloud brightening: Create reflective clouds over the ocean

Clouds are white and reflect sunlight, which lowers the amount of heat added to the atmosphere.

“If we can make clouds last a bit longer, particularly over the oceans because no one really wants more clouds over land, and maybe make them whiter than gray, that would reflect more solar energy,” said Hugh Hunt, a professor of engineering at the Centre for Climate Repair at the University of Cambridge in England.

This is already being tested in Australia to cool the waters around the Great Barrier Reef.

“The good thing about all this is that you can spray this stuff from boats, where it doesn't feel quite so scary as going 20 kilometers up with planes spraying sulfuric acid. You’re just spraying saltwater from the boat,” said Shaun Fitzgerald, director of research at Cambridge’s Center for Climate Repair.

The downside is that the method works only in small areas, like cooling a reef, rather than being scalable to large portions of the planet.

The upside is that it's short-term. “You stop spraying your saltwater and the effect on the clouds will stop within a day,” he said.

In April, a test of this technology was begun in San Francisco Bay by scientists at the University of Washington – and quickly shut down by local officials who worried there were possible health and environmental risks.

A bubble curtain for glaciers: Slow melting by insulating the ice

The world’s glaciers are melting, a problem because of their potential to significantly raise sea levels.

A study released in May found that the Thwaites Glacier on the vast West Antarctica Ice Sheet may be eroding more quickly than had been realized. It's known as the Doomsday Glacier because it contains enough water to raise sea levels by as much as 50 feet. (Even newer research says the nightmare scenario is unlikely, for now.)

Glaciers melt because warmer, deep, dense saline water from the ocean is being pushed under them, known as seawater intrusion.

One idea is to stop the warmer ocean water from reaching the base of the glaciers using a curtain of air bubbled up from the bottom, which would act as insulation and cause the warmer water to curl around and return to sea rather than under the glacier.

“We’re testing methods to pump air along a pipe with holes in it to create a bubble curtain to disrupt the flow of water,” Fitzgerald said.

Ocean fertilization: Trap CO2 in dead algae

The idea here is to fertilize the oceans with iron to stimulate algae and plankton blooms, which would pull CO2 out of the water and into their bodies.

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.

When these blooms die off, they would fall to the ocean floor and store the CO2 there.

But it’s not clear that the CO2 would stay put, or how well the technology would work. A study in 2023 suggested ocean iron fertilization might rob nutrients from surrounding areas and reduce overall numbers of fish and other marine life by as much as 5%.

Contributing: Ramon Padilla