Some proposals are uncontroversial, such as using charcoal to lock carbon dioxide into soil or scattering carbon-absorbing gemstones. Richard Branson, the billionaire chairman of Virgin Group Ltd, has offered a $25 million prize for the best solution in the field known as geoengineering.

Other ideas to cool the planet have scientists worried about unintended consequences. There are proposals, untested at scale and with uncertain costs, to block the sun’s rays with airborne particles or seed the oceans with carbon-absorbing iron. That they’re even being considered reveals both frustration over government inaction and skepticism that policy alone will solve the problem.

“For the last 20 to 30 years, governments, at the back of their minds, have assumed that mitigation is the main way forward,” said Mark Maslin, a fellow at the UK’s Royal Geographical Society. Researchers now realize that the planet needs “other urgent ways of dealing with CO2.”

Interest in geoengineering comes after two decades of United Nations talks that have yet to produce a global climate-change agreement. Envoys from about 200 nations will meet December in Paris, where they’re expected to finalize a pact to curb carbon emissions.

Two degrees

There is a sense of urgency. Researchers are seeking to limit warming to 2 degrees Celsius from pre-industrial times.

“To achieve that we will have to actually do some sort of geoengineering,” Maslin said.

Global surface temperatures have already risen about 0.85 degrees Celsius since 1880, according to a 2014 UN report. The researchers found that while the unintended consequences of manipulating the climate may be significant, “some basic inquiry does seem appropriate.”

A National Academy of Sciences panel echoed those concerns. In a February report, it found little evidence that researchers will be able to deploy geoengineering anytime soon. It also concluded that the US should study the technologies as a “last-ditch” tool.

Branson’s prize

Tinkering with the planet’s climate may carry more risk than efforts to reduce carbon emissions, said David Titley, a professor in Pennsylvania State University’s department of meteorology.

“Climate intervention involves techniques that are of high and unknown risk,” he said. “The risks for mitigation and adaptation are understood and manageable.”

Branson’s Virgin Earth Challenge began in 2007 and announced 11 finalists in 2011. The winner must be able to remove 1 billion tonnes of carbon from the atmosphere annually for 10 years and be economically viable, among other criteria. Branson hasn’t said when the prize will be awarded, if ever.

The goal is to “find true breakthroughs and hopefully create new ways of attacking the climate change problem,” Branson said in an interview.

“CO2 reduction at any sufficient scale is unlikely to happen soon, considering the fact that the priorities of China and India are on developing their economies, and both dispose of huge coal reserves,” said Olaf Schuiling, scientific adviser at Smart Stones, one of the Earth Challenge finalists.

Smart stones

Smart Stones, based in the Netherlands, is working with olivine, a yellow-green mineral found abundantly in the earth’s crust. Once a favourite of Egyptian jewellery makers, olivine absorbs CO2 as it weathers. The idea is to mine olivine, crush it and scatter it over land.

A tonne of olivine can capture about a tonne of CO2. Cost estimates range from £3 to £41 ($4.60 to $63) a tonne. “I could make sure that every year as much CO2 is absorbed by this method as is emitted by humans,” he said.

Another finalist is Zurich-based Climeworks AG, which is developing mobile systems to capture CO2 in filters. The gas is injected into greenhouses to promote plant growth or used in carbonated drinks.

The Biochar Co., also a finalist, takes waste wood from lumber mills and bakes it at high temperatures to produce biochar, a black compost-like material that can be added to soil to boost its quality and productivity.

Biochar also locks CO2 into soil for hundreds or even thousands of years. For every pound of biochar added to soil, about 1.5 pounds of CO2 is sequestered, the company said.

Mount Pinatubo

Some geoengineering ideas have scientists worried. Solar radiation management, which seeks to reduce warming by blocking the sun’s rays, may trigger changes in regional rainfall and temperature patterns that aren’t well understood.

The effect would be similar to the aftermath of the 1991 eruption of Mount Pinatubo in the Philippines, one of the largest volcanic blasts of the 20th century. The volcano spat as much as 20 million tons of sulfur into the upper atmosphere, shielding the earth from the sun’s rays and causing global temperatures to drop by nearly half a degree Celsius in a single year.

The technology “is on the edge of science fiction and would be highly problematic,” Maslin said. “‘It might keep the average temperature slightly lower than it should be, but it mucks up climate anyway.’’