It’s come to this. With Earth at its hottest point in recorded history, and humans doing far from enough to stop its overheating, a small but growing number of astronomers and physicists are proposing a potential fix that could have leaped from the pages of science fiction: The equivalent of a giant beach umbrella, floating in outer space.

The idea is to create a huge sunshade and send it to a far away point between the Earth and the sun to block a small but crucial amount of solar radiation, enough to counter global warming. Scientists have calculated that if just shy of 2 percent of the sun’s radiation is blocked, that would be enough to cool the planet by 1.5 degrees Celsius, or 2.7 Fahrenheit, and keep Earth within manageable climate boundaries.

The idea has been at the outer fringes of conversations about climate solutions for years. But as the climate crisis worsens, interest in sun shields has been gaining momentum, with more researchers offering up variations. There’s even a foundation dedicated to promoting solar shields.

A recent study led by the University of Utah explored scattering dust deep into space, while a team at the Massachusetts Institute of Technology is looking into creating a shield made of “space bubbles.” Last summer, Istvan Szapudi, an astronomer at the Institute for Astronomy at the University of Hawaii, published a paper that suggested tethering a big solar shield to a repurposed asteroid.

Now scientists led by Yoram Rozen, a physics professor and the director of the Asher Space Research Institute at Technion-Israel Institute of Technology, say they are ready to build a prototype shade to show that the idea will work.

To block the necessary amount of solar radiation, the shade would have to be about a million square miles, roughly the size of Argentina, Dr. Rozen said. A shade that big would weigh at least 2.5 million tons — too heavy to launch into space, he said. So, the project would have to involve a series of smaller shades. They would not completely block the sun’s light but rather cast slightly diffused shade onto Earth, he said.

Dr. Rozen said his team was ready to design a prototype shade of 100 square feet and is seeking between $10 million and $20 million to fund the demonstration.

“We can show the world, ‘Look, there is a working solution, take it, increase it to the necessary size,’” he said.

Proponents say that a sunshade would not eliminate the need to stop burning coal, oil and gas, the main drivers of climate change. Even if greenhouse gas emissions from fossil fuels were to immediately drop to zero, there’s already excessive heat-trapping carbon dioxide in the atmosphere.

The Earth’s average temperature is on the brink of rising 1.5 Celsius over the preindustrial average. That’s the point beyond which the chances of extreme storms, drought, heat waves and wildfires would increase significantly and humans and other species would struggle more to survive, scientists say. The planet has already warmed 1.2 degrees Celsius.

A sunshade would help stabilize the climate, supporters of the idea say, while other climate mitigation strategies were being pursued.

“I’m not saying this will be the solution, but I think everybody has to work toward every possible solution,” said Dr. Szapudi, the astronomer who proposed tethering a sunshade to an asteroid.

It was 1989 when James Early of the Lawrence Livermore National Laboratory suggested a “space-based solar shield” positioned near a fixed point between the Earth and the sun called Lagrange Point One, or L1, some 932,000 miles away, four times the average distance between the Earth and the moon. There, the gravitational pulls from the Earth and sun cancel each other out.

In 2006, Roger Angel, an astronomer at the University of Arizona, presented his proposal for a deflective sun shield at the National Academy of Sciences and later won a grant from the NASA Institute for Advanced Concepts to continue his research. He suggested releasing trillions of very lightweight spacecraft at L1, using transparent film and steering technology that would prevent the devices from drifting off orbit.

“It’s just like you just turned a knob down on the sun,” Dr. Angel said, “and you don’t mess with the atmosphere.”

The sunshade idea has its critics, among them Susanne Baur, a doctoral candidate who focuses on solar radiation modification modeling at the European Center for Research and Advanced Training in Scientific Computation in France. A sunshade would be astronomically expensive and could not be implemented in time, given the speed of global warming, she said. In addition, a solar storm or collision with stray space rocks could damage the shield, resulting in sudden, rapid warming with disastrous consequences, Ms. Baur said.

Time and money would be better spent on working to reduce greenhouse gas emissions and removing carbon dioxide from the atmosphere, she said, with a small portion of research devoted to “more viable and cost-effective” solar geoengineering ideas.

But sunshade proponents say that at this stage, reducing greenhouse gas emissions will not go far enough to allay climate chaos, that carbon dioxide removal has proved extremely difficult to realize and that every potential solution ought to be explored.

A fully operational sunshade would have to be resilient and reversible, Dr. Szapudi said. In his proposed design, he said 99 percent of its weight would come from the asteroid, helping offset the cost. It would still likely carry a price tag of trillions of dollars, an amount that is far less than what is spent on military weapons, he said.

“Saving the Earth and giving up 10 percent of your weapons to destroy things is actually a pretty good deal in my book,” Dr. Szapudi said.

He held up Tesla as an example of an idea that once seemed wildly ambitious but within 20 years of its founding became the world’s top manufacturer of electric vehicles.

Morgan Goodwin, executive director of the Planetary Sunshade Foundation, a nonprofit organization, said one reason sunshades haven’t gained as much traction is that climate researchers have been focused, quite naturally, on what’s happening within the Earth’s atmosphere and not on space.

But the falling costs of space launches and investments in a space industrial economy have widened possibilities, Mr. Goodwin said. The foundation suggests using raw materials from space and launching solar shade ships into L1 from the moon, which would cost far less than setting off from Earth.

“We think as the idea of sunshades become more understood by climate folks, it’s going to be a pretty obvious part of the discussion,” said Mr. Goodwin, who is also the senior director at the Angeles chapter of the Sierra Club.

The Technion model involves affixing lightweight solar sails to a small satellite sent to L1. Their prototype would move back and forth between L1 and another equilibrium point, with the sail tilting between pointing to the sun and being perpendicular to it, moving like a slat on a venetian blind. This would help keep the satellite stable and eliminate the need for a propulsion system, Dr. Rozen said.

Dr. Rozen said the team was still in the predesign phase but could launch a prototype within three years after securing funds. He estimated that a full-size version would cost trillions (a tab “for the world to pick up, not a single country,” he said) but reduce the Earth’s temperature by 1.5 Celsius within two years.

“We at the Technion are not going to save the planet,” Dr. Rozen said. “But we’re going to show that it can be done.”