Over the weekend, climate scientists from around the world convened to tackle one of Earth's most pressing problems: what can humanity do to fight climate change? It’s been a massive process, with the UN’s International Panel on Climate Change (IPCC) bringing together dozens of the world’s top climatologists to study the issue for nearly three years. The final result is a 33-page summary of nearly every option available to humankind, from energy production to urban policy, the most definitive and influential document on how governments should respond to the changing world. But among the broad range of options, there's one word that's found nowhere in the document: geoengineering.

"Geoengineering" is nowhere to be found

It's still a controversial idea, essentially proposing high-altitude aerosol drops to reflect back sunlight and counter the warming effects of a carbon-heavy atmosphere. Done right, it could be a crucial way of slowing down climate change, but skeptics worry the effects could spiral out of control and see the process as fighting pollution with more pollution. Even research related to geoengineering has become the topic for passionate debate, pitting curious scientists against a chorus of intense skepticism. (At one recent conference, Al Gore called the idea "utterly insane.") Many had hoped the IPCC's latest report might open the door to more research into geoengineering, allowing scientists to get a better sense of its risks and benefits — but in the group's recommendations to world leaders, there wasn't a single mention of the process, even as it heaped praise on alternative processes like carbon dioxide removal, which would use chemicals or bacteria to pull CO2 directly from the atmosphere. Even as the IPCC launches a full-court press against the effects of climate change, geoengineering is one idea that's still seen as too dangerous to consider.

Russia pushed hard for geoengineering

As expected, the politics of the issue were hard-fought. Russia was pushing hard to include geoengineering as an option, but other countries hated the idea, seeing it as a false solution advanced to protect the country's immense oil wealth by delaying conservation. Individual nations are given a powerful veto over individual portions of the executive summary, so the only solution was to leave geoengineering out of the document entirely. The process will still likely be mentioned in the underlying scientific research, released later this week, but anyone hoping that geoengineering would get a show of political support from the IPCC came away disappointed.

Geoengineering is seen as "a scientific oddity rather than a policy option"

That leaves would-be climate engineers in a difficult place. A UN moratorium still prevents any testing in the open atmosphere, and it will be difficult for the science to progress without stronger political support. Scientists are pushing for tests rather than full-scale climate engineering, but as climate scientist Ken Caldeira puts it, "the distinction between research and deployment is often a fuzzy one in the public mind." Even preliminary experiments, releasing small amounts of sulfate in the atmosphere and study the effects, face a de facto ban from the international community, forcing Caldeira and his colleagues to rely on computer simulations to develop their ideas. The latest IPCC report indicates that geoengineering research may have to rely on those computer models for the foreseeable future. "This might be where it's at for the next few years," says Caldeira, with climate change organizations "treating geoengineering as a scientific oddity rather than a policy option."

"We'd much rather see it addressed."

It's not the first time controversy has kept the idea off the table. In November, the IPCC working group's co-chair Ottmar Edenhofer was quoted as saying, "geoengineering is one option, and it should be included in a portfolio of other options" in a Nature article. After a few weeks, he walked back the claim, saying he was not speaking as an IPCC co-chair and would need a more thorough scientific review before any recommendations could be made. "A number of questions have to be answered before knowingly altering the Earth’s radiation balance can be included as a reliable component in a portfolio of mitigation options," Edenhofer wrote. Supposedly, they were questions that would be answered in this week's report.

Still, even critics of geoengineering don't see silence as a solution. "We'd much rather see it addressed," says ETC Group's Pat Mooney, a prominent activist against geoengineering. "The realities are still there. You still have a carbon energy industry with $20 trillion of assets in the ground." Even if the IPCC was able to fend off Russia's advances this time, Mooney is concerned the group has just kicked the can down the road, setting the stage for proposals to surface more forcefully in the future. "It still needs to be debated in the open."