The worldwide emission of CO2 is still rising and therefore the agreed temperature limits are slowly disappearing. Can we then remove part of that emission from the air? In theory it is. But the practice is full of objections.
CO2 is a persistent greenhouse gas that works largely cumulatively: the longer we emit, the higher the concentration increases. Even thousands of years after us, the CO2 concentration will be even higher. This is partly because CO2 in the air does not break down.
Nature can absorb CO2; think of plants via photosynthesis and shellfish and coral in the formation of lime compounds. The problem is that there is no capacity for that; nature emits approximately as much CO2 as it absorbs. Because man adds extra carbon from fossil earth layers, the cycle is no longer closed. And so the concentration increases every year.
Unless there is a magic dial. The magic words are 'negative emissions': taking in CO2 instead of emitting it. It sounds futuristic, but is already incorporated in the official climate scenarios of the International Energy Agency (IEA) and UN climate panel IPCC. If we still want to stay below 1.5 degrees of warming, we can't even live without it.
Problem: negative emissions do not exist in practice - not on a scale that comes close to our emissions. NU.nl lists five plans that could potentially change that - including the reason why it is not getting off the ground
"Planting a trillion trees"
Swiss researchers took the headlines last year: plant a trillion trees, as 'best and cheapest' action against climate change. US President Donald Trump also supports the plan, he said in Davos.
Due to calculation errors, the effect is often exaggerated. If emissions continue to rise, it will be 3 to 5 degrees warmer. The Swiss forest plan can take a maximum of 0.3 degrees away from this.
Another objection is space: forests are being cut down in many places in the world, including the United States. Stopping doing this is proving difficult enough - while the demand for agricultural land and biomass is increasing. Moreover, a mature forest no longer absorbs net CO2, but it can lose it. One forest fire and it's in the air again.
Compensate with olivine emissions?
Nature has one more trick. Some rocks bind CO2 while they weather, such as olivine. That process is naturally very slow. But we can speed it up: by digging up these rocks, crushing them and spreading that grit over warm beaches, where the circumstances are the best.
To solve the climate problem along this route, olivine mining of a similar size as fossil fuel extraction is needed. This is not possible without high (energy) costs and environmental damage.
'BECCS' - biomass with CO2 storage
This option has one major advantage: if you filter the CO2 from the flue gasses at a biomass plant and store it underground ('CCS'), you can theoretically generate energy, while removing net CO2 from the air: a win-win situation.
Two major problems: the technology for CCS is so expensive that it can only work with a very high carbon price, which will be stopped for the time being. In addition, there are concerns about environmental damage caused by the large-scale use of biomass.
Design for a 'carbon scrubber' from the Canadian company Carbon Engineering. (Photo: Carbon Engineering)
Fertilizing oceans and huge filter machines
The world's most powerful carbon vacuum cleaner is much smaller than a tree: plankton. Plankton often has one mineral deficiency on the open oceans: iron. If we spread that around, more plankton can grow, which then absorbs more CO2.
Research is disappointing: extra iron causes a short-lived plankton bloom, but also an increase in species that eat plankton residues on the ocean floor. In addition, CO2 is released again. The search at sea continues - including (sinking of) seaweed is a candidate for CO2 storage.
Finally, ' carbon scrubbers ': machines that filter the air and bind CO2. Main problem: very expensive. About 250,000 should be built there, and it costs hundreds of euros per tonne of CO2. Preventing these emissions by replacing fossil energy with renewable energy is many times cheaper.