Who could afford to geoengineer the planet?

Proponents and opponents of solar geoengineering often point to the same price estimates: Cooling the planet could cost about $10 billion a year.

That sum is within reach of some private firms, small states, and even a few wealthy individuals. For champions of the untried technology, that’s an exciting prospect—a chance to buy time as humanity decarbonizes. For skeptics, the figure raises fears that someone might act alone, with unknown side effects for the whole planet.

But new detailed engineering cost models by Wake Smith, a research fellow at the Harvard Kennedy School, show that sustaining a climate-altering program for the decades needed to stabilize or reduce global temperatures could instead cost more than a trillion dollars. At that expense, only the largest states or coalitions could fund the effort, sharply limiting who could act alone.

What is solar geoengineering?

Solar geoengineering, also called solar radiation management (SRM), refers to methods that would reflect sunlight to cool the Earth. One of the most widely discussed approaches, stratospheric aerosol injection, would use airplanes to release sulfur particles or another substance high in the atmosphere, mimicking the effect of some large volcanic eruptions.

The resulting particles would gradually disperse and fall out of the stratosphere, so the flights would have to continue to maintain the cooling. If greenhouse gas concentrations in the atmosphere remained high, stopping suddenly could cause temperatures to jump, a risk known as termination shock.

Cutting greenhouse gas emissions remains essential to slowing climate change. Some critics worry that even researching geoengineering could weaken that effort by encouraging the belief that countries can keep burning fossil fuels and cool the planet later.

Still, some researchers think SRM may be needed to manage what is now being termed “climate overshoot” – the period during which the world continues its current warming trajectory and goes beyond the Paris Agreement’s aspiration to limit warming to no more than 1.5°C above pre-industrial levels. But the risks—including damage to the ozone layer, altered rainfall and uncertain impacts on agriculture—remain poorly understood.

Reaching the stratosphere

Smith identifies three constraints to SRM deployment: the equipment, the money, and the geopolitics.

Start with airplanes. The most efficient way to create a globally effective layer of planetary sunscreen would be to inject it above the tropics at an altitude of roughly 20 kilometers, or 12 miles above sea level. Delivering large quantities of material at that height would require a new kind of aircraft.

“It’s not that we can’t get there. It’s just that no customer needs a very high-altitude bomber, which is essentially what this is. It flies up and dumps stuff out the back and lands without its payload,” says Smith, who is also an adjunct faculty member at the Yale School of the Environment.

From the moment anyone decides to design a new plane, “it’s roughly a decade until you have the first aircraft, and thereafter you’ve got to build the fleet, however many of them you need,” Smith adds.

He estimates that at most a dozen countries could build such an airframe, and far fewer could make the engines. Governments closely monitor that technology, he said.

What would it cost?

Full deployment would require backers capable of paying the bills for generations. Smith estimates that a global program could cost a trillion dollars or more over 50 years. That includes developing a fleet of novel aircraft, building ground infrastructure to support the program, maintenance, crews, fuel, and continuous flights to disperse up to 10 megatons of sulfur particles annually.

“It’s hard to imagine anybody commencing deployment unless they’re confident and capable of funding it for decades at least,” he says. “Once we’re in trillion-dollar scales, it’s only the largest states or coalitions among them that I can imagine trying to move forward on this.”

The third constraint is political. Even with funding and aircraft ready, a project’s backers would have to reckon with the United States or China telling them they could not proceed unilaterally. Smith considers that the biggest obstacle of all.

Pole position

Smith doesn’t expect geopolitics to block field trials, however. And there is a way to begin them sooner.

A proposal called High-Latitude Low-Altitude, or HiLLA, would concentrate injections in subpolar regions, above 54 degrees north and south latitude. At those latitudes, the stratosphere is lower, allowing injections at about 15 kilometers, or nine miles. Refitted commercial aircraft, such as Boeing 777s, could do the work, avoiding the wait for a purpose-built plane.

Smith puts the cost of a five-year trial at $100 million to $1 billion—a feasible amount for philanthropists, nonprofit groups, small states, and venture capital investors to raise.  

But the approach is inefficient, requiring roughly three times as much sulfates as a program that spreads injections more evenly around the planet. Researchers are investigating other compounds, but using more sulfates would also add to health concerns—exposure can cause lung damage and raise questions about the risk-risk trade-offs between lower heat stress mortality and higher air pollution mortality. Concentrating injections near the poles could also disrupt weather and rainfall elsewhere.

Beyond a field trial, the costs again become prohibitive for most. Using HiLLA to halt further global warming for 50 years would cost over $2 trillion, Smith estimates, adding that it is “not a plausible long-term strategy.”

The upside of cost and delay

So, while tests “could be funded by a wide variety of actors,” Smith says, trillion-dollar deployments would require “large states or coalitions among them.”

For Daniel Schrag, a professor of environmental science and engineering at Harvard, the cost and delay are “very reassuring.” He says the delay would give scientists time to study the risks and governments and communities time to debate deployment.

And at this cost, even a superpower would seek help paying, Schrag argues: Sharing the bill “in some ways guarantees some multilateral control.”