Capturing CO2 emissions using direct-air-capture (DAC) technology requires almost as much energy as that contained in the fossil fuels that produced the carbon dioxide in the first place, according to new analysis.

In 2020, the world used 462 exajoules (EJ) of energy from fossil fuels, which resulted in 32 billion tonnes of CO2 emissions. Capturing that carbon dioxide through DAC — which sucks the greenhouse gas out of the air — would require 448EJ, according to calculations by Australian maths-as-a-service company Keynumbers.

That 448EJ is the equivalent of 124,444TWh — more than five times the annual global electricity consumption in 2020 (23,177TWh, according to Enerdata). And that doesn’t even include the energy that would be required to then transport and store the captured CO2.

“The world would need just as much energy to clean up the energy it made a mess with in the first place,” said Keynumbers founder John Poljak. “Not quite what the circular economy had in mind.”

Generally speaking, DAC technology works by using giant fans to draw in air, with the CO2 (roughly 0.04% of the content of air) bonding to chemicals known as sorbents. When the sorbent is saturated, it is heated to 80-100C to release the captured carbon dioxide.

The world’s largest DAC facility, Climeworks’ $10m-15m Orca plant, was opened in Iceland last week, and is due to capture 4,000 tonnes of carbon dioxide from the air every year — the equivalent to the emissions from about 870 cars. The captured CO2 is then mixed with water and injected into basalt rock 1km underground, where it slowly turns into a solid carbonate mineral over a two-year power.

So, theoretically, eight million such plants would be needed to capture the world’s annual carbon emissions, at a cost of $80trn-120trn.

Of course, Orca is just a pilot project and costs would inevitably fall with economies of scale and technology development.

Indeed, Climeworks’ rival, US-based Carbon Engineering, believes it will require 8.8GJ per tonne of captured CO2, rather than the 14GJ-per-tonne figure used in Keynumbers’ calculations. With this lower number, the world would need a mere 284EJ (78,888 TWh) per year to capture global annual carbon emissions.

It has also been stated that nature (trees, plants, soils and oceans) absorbs about half of the planet’s annual carbon emissions. So, at best, using current technology, the world would need 142EJ (39,444 TWh) each year — almost double the annual global electricity production — to suck all our carbon emissions from the air.

No-one is suggesting that the world should capture all its greenhouse gas emissions using DAC, but the amount of energy required does raise questions about whether the technology makes any sense. After all, the world needs to rapidly decarbonise its existing energy usage — not add to the load required.

And with major oil & gas producers such as Chevron and ExxonMobil ploughing millions of dollars into DAC — rather than investing in clean energy — it is difficult to argue that current direct-air-capture technology is little more than greenwashing, a poor excuse to keep emissions high on the promise that they can be plucked from the air at a later date.

While economies of scale and efficiency improvements would undoubtedly help, it seems more likely that a massive technology breakthrough would be needed to reduce the energy required by DAC to an acceptable level.