Imagine flooding a desert half the size of the Sahara. Using 238 trillion gallons of desalinated ocean water to do the job. Creating millions of 1-acre-square micro-reservoirs to grow enough algae to gobble up all of Earth’s climate-changing carbon dioxide. For an encore: How about spreading the water and fertilizer (the dead algae) to grow a vast new forest of oxygen-producing trees?

A Silicon Valley venture capital firm, Y Combinator, unveiled the radical desert flooding plan as one of four “moonshot” scenarios that it hopes innovators will explore as potential remedies to catastrophic global warming.

But would it work? And should it even be tried?

With unlimited capital and political will — both far from given — experts said the scheme would stand a chance of reducing dangerous greenhouse gas levels. But while they generally believe the climate crisis has become severe enough to push even extreme options onto the table, the experts cautioned against interventions that might create as many problems as they solve.

“We do not want to have this be purely profit driven,” said Greg Rau, a University of California, Santa Cruz climate scientist and part of the team that helped Y Combinator craft the request for proposals. “We are trying to benefit the planet, not just make money. So we need this kind of research and development first, but then oversight and governance over how any of this is deployed.”

The Y Combinator proposal grows out of what is now the consensus of climate scientists — that humanity needs to move beyond slowing the production of carbon dioxide and begin removing excess levels of the gas already straining Earth’s atmosphere.

The startup accelerator that helped finance Airbnb, Dropbox and Reddit asked innovators last month to come forward with specific proposals on desert flooding and three other extreme plans for reducing greenhouse gas concentrations. The existential threat posed by climate change requires research into solutions that the investment firm itself conceded could be “risky, unproven, even unlikely to work.”

Y Combinator said it had a rush of interest in its challenge. It declined to say how many took up the desert flooding option. But Sam Altman, Y Combinator's president, predicted that in 2019 his firm will fund three companies to pursue the “Plan B” climate solutions.

A host of scientists who have studied Earth’s ecosystems, climate change and bio-engineering said further exploration might be warranted. But they were quick to cite many reasons that desert flooding is not likely to succeed.

Massive size

Y Combinator called filling 1.7 million acres of arid land with 2-meter-deep pools of water “the largest infrastructure project ever undertaken.” Just to pump ocean water inland and desalinate it would require an electrical grid far greater than the one Earth now devotes to all other uses.

“It’s a desert for a reason,” said Lynn Fenstermaker, a research professor at Nevada’s Desert Research Institute. “Flooding the desert and then keeping the water there, in an already water-poor area with all the evaporation, is hard to imagine.”

Y Combinator doesn’t deny the magnitude of the challenge. “Economies of scale as well as breakthroughs in material science and construction technology will all be necessary for success,” its proposal says.

Unprecedented cost

Y Combinator pegs the price tag at $50 trillion. That’s roughly half the entire globe’s economic productivity for a year. Altman said in an interview that the cost for any solution will need to drop into the billions to become more realistic. “You can do a lot of things that require spending more money than you will ever be able to get,” Altman said, “and it just doesn’t come.” Brought to a more realistic price, he believes that governments will pay.

Destruction of unique ecosystems

Many species would be wiped out by massive man-made flooding of deserts. “People think there is nothing valuable in the deserts, but that is far from the truth,” said Henry Sun, a microbiologist and research professor at the Desert Research Center. “These diverse species deserve, and need, the desert to survive.” Most of the world’s countries would set a high bar, Sun said, before destroying habitat.

Potential for making things worse

Interfering with nature can have unexpected consequences. Katherine Mackey, a University of California, Irvine climate scientist, noted how Australia has long tried, and failed, to combat overpopulation of native species by introducing non-native creatures. Famous case in point: toads were introduced in 1935 to tame sugar cane-eating beetles. But the toads couldn’t climb sugar cane. So the beetles thrived, alongside their new neighbors — an out-of-control toad population.

“Saying that we intervened and created a problem with global warming, so let’s further intervene, that’s not the thing to do,” Mackey said. “That’s not how you fix the problem, by replacing it with another problem.”

Distracting from more workable solutions

Climate scientists believe that most, if not all, the needed solutions for limiting new greenhouse gas emissions and reducing current CO2 concentrations already exist. Environmentalist Paul Hawken has catalogued solutions in his Project Drawdown. Taken in total, Hawken has said, they would reduce emissions and sequester enough carbon to more than meet the goals laid out by world leaders in the 2015 Paris climate accord.

Among the carbon-sinking projects that would require little or no new technology: creating new forests in degraded pasture, farm and other land; criss-crossing fields with trees to create so-called silvopastures, which absorb far more carbon than open fields; and preserving and restoring peatlands, the boggy wetlands that store carbon at twice the rate of the world’s oceans.

“I think it’s far easier to, for instance, make our houses better insulated and solar powered, than to flood deserts. Also, from a timescale perspective, how quickly will this technology be available at scale?” said Mathis Wackernagel, president of the Global Footprint Network, an Oakland, California-based sustainability think tank. “If we want to solve climate, why not bet on the easy stuff?”

The case for a moonshot

Altman’s answer is that he and Y Combinator already support, and fund, pragmatic green energy companies. But big reductions in greenhouses gases are still needed and don’t appear to be coming quickly enough, the firm said.

Y Combinator suggests that flooding the deserts may be less risky than another solution on its list — fertilizing the oceans with massive amounts of iron or other nutrients to spur the growth of CO2-gobbling phytoplankton. Compared to ocean phytoplankton seeding, “doing so in desert reservoirs reduces systemic risk and exposure of the marine ecosystem to our widespread meddling,” Y Combinator’s request for proposals says.

“The window for easy solutions is already closed. Doing nothing is guaranteed suicide,” the firm said. Altman, 33, added: “All of this stuff is scary. However, runaway global warming where we all die is also quite scary. ...None of this is where we would like to be. But here we are.”

Activists and scientists interviewed by NBC News said exploration of far-out solutions is warranted — as long as researchers, governments and funders don’t lose focus on other fixes.

“The concern I have is that Silicon Valley seems very excited about these moonshots, when maybe what we need is a bunch of Boeing 737s,” said Armond Cohen, executive director of the Clean Air Task Force, a clean energy think tank and lobbying organization. “And to do it on a 10- to 15-year development cycle, not one that takes 30 years.”