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 global carbon cycle, as described in the Intergovernmental Panel on Climate Change’s 4th Assessment Report by Working Group One (IPCC AR4 WG1). The arrows represent exchanges of carbon between different parts of the Earth, such as from fossil fuels into the atmosphere. The red arrows show the changes that are a consequence of human activity. More information can be found on the NOAA’s Geophysical Fluid Dynamics Laboratory’s website [click the image].

  1. 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 constantly being swapped between plants, animals, soils, oceans and the atmosphere. 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.

    Because carbon accounts for just one-third 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 billions of tonnes of carbon every year.

  2. Carbon Emissions: Upsetting the Balance

    The problem right now is that humankind is emissions a lot of extra greenhouse gases. The biggest ones: carbon dioxide (CO2 ) and methane (CH4) are carbon-based.

    So, the extra greenhouse gas emissions from human activities are overwhelming the finely-tuned balance of the natural carbon cycle. This means CO2 levels in the atmosphere are going up.

  3. Removing GHGs from the Atmosphere

    As well as cutting our emissions in the first place, are there environmentally friendly ways we could take CO2 and other greenhouse gases back out of the atmosphere? Could these help bring the carbon cycle back into balance, and help society solve, or even reverse, global warming?

    The answer appears to be yes.

    In fact, the approaches taken by our finalists cover five broad methods:

  1. Biochar

    ‘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 biochar that don’t involve setting fire to it. In certain circumstances, for example, biochar can provide a lot of benefits when mixed into soils. This can make certain types of farmland more productive, protect them from erosion and help them keep valuable nutrients.

    The process of making biochar also produces heat and/or liquid fuels.

    But there are still a lot of questions about how different biochars behave in different soil systems. It all feeds in to how all these complex effects add up to shape the total “carbon balance” of biochar.

    Nonetheless, our biochar finalists are working hard on commercially sensible, sustainable ways to help their clients enjoy biochar’s many useful properties:

  2. Bio-energy with Carbon Capture and Storage (Bio-CCS)

    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, ultimately, all biomass energy comes from photosynthesis: plants using sunlight to convert CO2 in the atmosphere into all the carbon-based building blocks of life.

    However, the carbon those plants took from air as they grew finds its way back into the atmosphere when they are burnt. This is ‘low carbon’, or, at best ‘carbon neutral; in climate speak.

    By linking biomass energy production to a ‘carbon capture and storage’ process that traps and buries this CO2 underground, the net result can be cleaner energy production that can remove more carbon dioxide from the air than it emits.

    As an approach, Bio-CCS faces some key challenges. A field, or an 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, without damaging natural forests or impacting 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…

  3. Direct Air Capture (DAC)

    Technologies for chemically capturing CO2 from air have been around for many years. Submarines and Space Stations have air COscrubbers to keep the atmosphere breathable for their crew.

    The problem, however, is that these small systems use quite a lot of energy.

    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.

    That’s where our Direct Air Capture finalists come in. All of them use CO2 adsorption-desorption systems – where the CO2 bonds with a material, solid or liquid, and then gets re-released in a concentrated form. They are all 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.

  4. Enhanced Silicate Weathering

    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 re-balances carbon dioxide levels over geologic time spans.

    By efficiently grinding up the most carbon-hungry rocks and spreading them over soils, it’s possible to greatly accelerate this natural process, and permanently lock up carbon directly from the atmosphere.

    The basic principles are well-documented, and early lab and field trials are promising. But there is still a lot to be better understood 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. These might accelerate or hinder the reaction, and could also bring unintended 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…

  5. Land Management / Ecosystem Sequestration

    From this…                                                         to this…

    Paddock - before and after

    Images courtesy of Savory Institute’s partners. Tony for scale.

    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. It’s a constant give and take – that has been overwhelmed recently by human activities up until this point.

    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 many farms can be moved over to a planet-friendly system of grazing. One that mimics the same evolutionary cycle and puts the carbon back into the soil.

    There can be 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…


Check out our links page to find references for each approach.

You may find our Frequently Asked Questions section useful too.