How might you remove greenhouse gases from the atmosphere?
Lots of different substances, including water, can be classified as greenhouse gases. However, it is carbon dioxide in particular that has been the biggest contributor to global warming and ocean acidification so far.
Carbon dioxide going into and out of the atmosphere forms part of the Earth’s global Carbon Cycle.
The Carbon Cycle
You might have heard of The Carbon Cycle (above diagram): a continuous, planet-wide exchange of Mother Nature’s preferred building block: carbon.
Carbon is being swapped between plants, animals, soils, oceans and the atmosphere constantly. Plants are made up of between 50 and 60 percent carbon, depending on the species, and they get all this carbon by absorbing CO2 molecules from the air via the process of photosynthesis. And because carbon accounts for just 27.3 percent of the weight of a carbon dioxide molecule, every ton of carbon stored in plants is responsible for removing over three-and-half tons of CO2 from the atmosphere. When they die, plants return much of that carbon back to the air. Similarly the oceans both absorb and re-emit carbon.
Carbon emissions: upsetting the balance
The problem right now is that humankind’s extra output of carbon dioxide from burning long buried fossil fuels and released from the soil due to changes in the way we use land (“land use change”) is overwhelming the finely-tuned balance of the natural carbon cycle. This means CO2 levels in the atmosphere are going up.
We’re making progress, but it’s also proving harder than many people had hoped to cut the levels of emissions from the world’s economic activities.
Removing GHGs from the Atmosphere
As well as cutting our emissions from fossil fuels and land use change in the first place, are there environmentally friendly ways we could take CO2 and other greenhouse gases out of the atmosphere, to try and bring the carbon cycle back to equilibrium and help society arrest global warming?
The answer appears to be yes.
In fact, the approaches taken by our finalists cover five broad methods:
‘Biochar’ is eco-branding for humble charcoal. The ‘bio’ refers to ‘biomass’, and by converting biomass (such as a dead tree) into charcoal, a large fraction of the carbon that tree has absorbed over its life can be locked away in a stable form, rather than rotting and returning to the atmosphere. Charcoal is locked-up carbon that, a bit like coal, won’t be released unless it’s burned.
The good news is that there are lots of commercial uses for charcoal that don’t involve setting fire to it to grill your burger. In certain circumstances, for example, it can provide a lot of benefits when mixed into soils, making certain types of farmland more productive and protecting them from erosion and loss of nutrients. The process also produces renewable fuels. But there are still a lot of questions about how different biochars behave in different soil systems and how all these complex effects add up to shape the total “carbon balance” of biochar. Nonetheless, all of our finalists are working hard on commercially sensible, sustainable ways to help their clients enjoy biochar’s many useful properties:
Bio-energy with carbon capture and storage (BECCS)
As we’ve known since we invented fire, burning biomass is a way of releasing energy. This ‘bio-energy’ is used today in everything from small wood fires to biomass-fuelled power stations, but it ultimately all comes from photosynthesis: plants absorbing sunlight to convert CO2 in the atmosphere into all the carbon-based molecules that make up biomass. However, the carbon those plants took from air as they grew finds its way back into the atmosphere when they are burnt. By linking bio-mass energy production to a ‘carbon capture and storage’ process that traps and buries this CO2 underground, the net result can be clean energy production that can remove more carbon dioxide from the air than it emits.
As an approach it faces some key challenges. One field or area of forest can only produce so much biomass each year, so sourcing enough to make a difference to the amount of carbon dioxide in the air sustainably, without damaging natural forests or food production, will not be easy. There are also political and financial challenges in combining two very different technologies and communities (Bioenergy on the one hand and Carbon Capture and Storage on the other), especially while both sectors are still just getting started and face their own challenges. However there are organisations committed to driving the ‘Bio-CCS’ or ‘BECCS’ agenda forward, like our finalist in this category…
Direct Air Capture (DAC)
Technologies for chemically capturing CO2 from air have been around for many years. Submarines and Space Stations have air scrubbers to keep the atmosphere breathable for their crew. The problem, however, is that they use quite a lot of energy, and would likely be responsible for emitting more carbon than they removed (if powered by fossil fuels) making them infeasible and costly to implement on an industrial scale.
Finding materials that naturally bind with CO2, even at the low concentrations we find in the atmosphere, isn’t hard. Carbon is Mother Nature’s preferred building block partly because of its tendency to form bonds with other materials. But the stronger the bond a material forms with CO2, the more energy it takes to break that bond and recover the CO2. This energy cost has been the biggest barrier to capturing CO2 economically. Another challenge is that carbon dioxide forms less than 0.1% of the air around us, so to remove each kilogramme of CO2 the scrubber needs to filter over 1,000 times that much air. Moving that much air takes energy and also means any air capture plant needs to be big if it’s going to catch a lot of carbon!
That’s where our Direct Air Capture finalists come in. All of them use CO2 adsorption-desorption systems, but all are developing inventive designs and technologies to overcome these challenges and capture carbon from the air in a way that is both clean and cost-effective.
Enhanced Silicate Weathering (land)
Carbon’s tendency to bond with other materials includes interactions with rocks. In fact, this is part of the Earth’s natural weathering processes, the main way that Nature rebalances carbon dioxide levels over time spans of millions of years. By efficiently grinding up the most carbon-hungry rocks and spreading them over soils, it’s possible to greatly accelerate this natural process to permanently lock up carbon directly from the atmosphere.
While the basic principle is proven, and lab trials are promising, there is still a lot we don’t understand about what affects the rates of reaction in the field. Adding the minerals to real-world natural systems involves all kinds of complex chemical and biological interactions and feedbacks that can either accelerate or hinder the reaction, and could also bring environmental risks. Caution is also required in choosing the right minerals; impurities in these carbon hungry rocks could potentially lead to contamination of the environments they’re added to. Lastly, it can be difficult to accurately keep track of just how much carbon you have captured, as the reaction can take many years. Fortunately there is work being done to find ways of overcoming these concerns and making this a viable approach. One of the leaders in this field is our finalist…
Land Management / Ecosystem Sequestration
From this… to this…
One of the questions we first get asked when telling people about the Virgin Earth Challenge is: ‘taking greenhouse gases out of the air? What, you mean, like, trees?’
Well, yes, trees, grasses, soils… land and ecosystems, rocks and the world’s oceans all have direct interactions with the atmosphere as part of the global carbon cycle – a constant give and take that has been overwhelmed and unbalanced by our prodigious use of fossil fuels and our clearance of natural ecosystems. Could we use our knowledge of these systems to encourage them take up and store more carbon?
Here’s one example of doing just that.
Large herds of grazing animals evolved to roam around natural grasslands, eating the grasses and being chased by large predators; at the same time, the grasses themselves evolved in this same relationship to the movement of the grazers. When this time-honoured cycle continues, grasslands flourish and store carbon in grasses and soils. However, modern farm-based grazing practices have disrupted this cycle and many of the processes that keep grasslands healthy bringing desertification and loss of carbon as a result.
The good news is that farms can be moved over to a planet-friendly system of grazing that mimics the same evolutionary cycle and puts the carbon back. There’s a commercial upswing too, stimulating the growth of grass and reducing the need for fertilisers and livestock feeds. The main challenges are in putting exact numbers on just how much carbon is taken up, and for how long, and working out how to account for future risks if those ecosystems became degraded and the carbon was re-released. A pioneering leader in the field of grassland restoration is our finalist…
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