In theory, reducing the concentration of heat-trapping carbon dioxide in the atmosphere might be one way to keep the world under the two-degree Celsius target for warming established by the 2015 Paris climate agreement. But in practice, removing carbon dioxide is far from simple. There are major questions about scale, cost, speed and energy requirements. In most cases, the carbon dioxide that was removed would have to be buried underground indefinitely — and carbon storage technologies have only been deployed on a small scale so far.

Some removal technologies are more fanciful than others, but as for which are most viable, it could be argued that none are, at least not yet. In a report last month, the European Academies Science Advisory Council offered a pessimistic outlook for carbon removal, saying that it offered only “limited realistic potential” to have a climate impact. The authors argued that the world should not count on removal technologies to make up for a failure to sharply reduce or eliminate emissions in the first place.

There are five major approaches to carbon dioxide removal:

Plant more forests. Trees remove carbon dioxide naturally, incorporating it into their tissues as they grow. Worldwide, forests store about one billion to two billion tons of carbon annually, offsetting a chunk of the roughly 10 billion tons emitted by human activity. Reforestation and afforestation, properly managed, could remove a lot more and keep it out of the atmosphere. But planting forests is slow work — as Icelanders know well — and requires a lot of land. The world is currently much better at cutting down forests than planting new ones.

Crush a lot of rock. This technique is called enhanced weathering, and is based on the fact that some types of rock weather by naturally combining with carbon dioxide in the air or water. One suggested approach would use the mineral olivine, which is plentiful, crushing it into fine sand and spreading it on land, perhaps along coastlines. But mining, crushing and transporting the billions of tons needed would be expensive and energy intensive. And the carbon removal would still be exceedingly slow.