Plan B for saving the planet? (Getty Images)

You can strip CO2 from the air with chemical filters or by boosting reactions occurring as rocks weather. Colin Axon of Brunel University in Uxbridge, UK, and Alex Lubansky at the University of Oxford estimated what it would take to remove the 30 gigatonnes of CO2 we emit every year.

That would mean processing 75,000 Gt of dry air. Scaling up proposals to filter air would use 180 Gt of clean water per year, depriving 53 million people of water, on top of the 66 per cent of the world’s population who will face water shortages by 2025.

To make matters worse, mopping up carbon dioxide with chemical filters would use an enormous amount of energy and be prohibitively expensive. A study last year in Nature pegged the cost of carbon dioxide removal at about $600 per ton, which is about seven times more pricey than even the high-end estimates of carbon taxes deemed necessary to curtail the world’s emissions.

Another promising way to strip carbon dioxide out of the air would involve rock weathering. This already happens naturally, over millions of years: The carbon dioxide that’s dissolved in rainwater interacts with silicate rock to produce bicarbonate ions, which are carried to the ocean and sequestered out of reach. Whenever you read about past geological periods, many millions of years ago, in which carbon dioxide levels naturally fell and the Earth cooled, rock weathering was probably involved.

So could we speed this process up today? Realistically, probably not. Axon and Lubansky estimate that we would need about 100 gigatons of olivine, a common silicate rock, which is 12,500 times more than is currently being produced worldwide. Oh, we’d also need to spread it about half an inch thick across 3.6 billion square kilometers of dry land, which is about 1,000 times more land than the Earth has available.

So much for that idea. What about trees? Trees, after all, absorb and store carbon-dioxide naturally. (Indeed, deforestation is responsible for around one-fifth of mankind’s greenhouse-gas emissions.) But simply halting deforestation is a huge enough task. It would be extremely difficult to go even further and plant enough new trees to vacuum up all of the excess carbon-dioxide in the air. One study in Nature Geosciences last year estimated that converting all of the Earth’s cropland to forests would lower global temperatures by just 0.45°C. (Granted, several of these strategies combined might start to add up to serious reductions.)

Other, more radical geoengineering schemes have involved ideas like fertilizing the ocean with iron so that plankton starts blooming and absorb the CO2 out of the air. But that runs the risk of harming fisheries or creating oxygen-depleting “red tides” in the seas. Meanwhile, there’s the mad-scientist proposal to inject the air with fine particles in order to reflect a portion of sunlight back into space. But that, too, could wreak havoc — by disrupting rainfall patterns, for instance. See this earlier post for a primer on the hazards of geoengineering.

All of the sudden, weaning the world off fossil fuels and curbing greenhouse gas emissions starts to look easy by comparison.

Update: As commenters pointed out, trees are one blindingly obvious carbon-sucking “technology,” so I added it to the discussion above.