Suck it up

Reducing carbon dioxide in the atmosphere

There are frequent news stories about ways of limiting global warming by sucking carbon out of the atmosphere. Some of the stories make me think “Madness” but others make me conclude “Brilliant”. As I started researching this newsletter I realized that when I read these stories, I’ve generally gotten the wrong end of the stick. This is so even though I’ve done research on the role of the land in the global carbon cycle for 15 years. I’ve used models, satellite data, and ground based observations to probe our understanding of the flux of carbon between the atmosphere and the land surface. And so when I’ve read about carbon capture and storage (CCS) or carbon dioxide removal (CDR), I assumed I knew what those terms meant, but I didn’t. Since reducing carbon in the atmosphere through CCS or CDR may go a significant way towards reducing global warming, it is worth understanding what the terms actually mean - if only because the techniques lend some hope to the usually dire climate news.

CCS, carbon capture and storage, is “the process of capturing carbon dioxide before it enters the atmosphere, transporting it, and storing it for centuries or millennia.” Typically, CCS involves separating carbon from other emissions in the flues of cement plants and energy plants. Then the separated carbon is stored in geological formations or by turning it into stable minerals.

On the other hand, CDR, or carbon dioxide removal, is already well known to us for the mechanics of CDR already work on a massive scale. Trees and other plants, along with the soil, absorb and store about 1/4 of the carbon we emit. Furthermore, the oceans absorb and store a similar fraction For the purposes of defintions, however, CDR includes only human driven absorption of carbon, and thus the historical absorption of 1/2 of our emissions by the land and the oceans are not included in CDR. For example, CDR could include the amplification of the natural land and ocean sinks via management practices such as reforestation. It’s the kind of painstaking detail that I am certainly prone to ignoring and has led to my own confusion when reading about carbon reduction in the atmosphere.

So having cleared up the definitions of CCS and CDR what is their potential for helping us avoid dangerous climate change? In order to avoid global warming above 1.5 degrees Celsius, the Intergovernmental Panel on Climate Change (IPCC) has established three milestones: 1. net emissions need to start dropping today, 2.By 2050, net emissions need to be zero and 3. Net emissions need to be negative thereafter. Net emissions are what we emit minus what is absorbed. CCS aims to reduce emissions while CDR aims to enhance absorption - both reduce the amount of carbon in the atmosphere. If CCS, along with other emission reductions, can reduce our total emissions down to the rate of CDR by 2050, we will have reached zero net emissions.

Let’s start with CCS. Carbon can be captured in smoke stacks by various processes. One of the most developed techniques is running the emissions through an aqueous solution that contains amines (derivatives of ammonia) which separate carbon dioxide and sulfur dioxide from the main gas flow. This is variously referred to as gas sweetening or amine scrubbing. Amiie scrubbing has the potential to capture up to 90% of CO2 in the flues of fossil fuel energy plants and industrial plants, but unfortunately, that’s not the whole story. CCS requires an additional 25% of energy to run the plants cutting deeply into the emissions savings. And in the case of coal power plants, CCS doesn’t help to clean the energy used to mine the coal and transport it to the plant. Some studies have concluded that there will actually be more emissions per delivered power in coal plants using CCS than those without CCS. Furthermore the excess fuel that will be needed to power the mining, transportation and scrubbing of the emissions will increase all the ecological, land use, and air and water pollution associated with mining. So even before we get to the storage issues of CCS, it doesn’t appear to be helpful for reducing emissions from fossil fuel energy plants.

To my surprise, storing the captured carbon is less of a problem. Estimates suggest that North America alone has enough underground caverns to store over 900 years worth of CO2 emissions at current emission rates. There is some concern about leakage of the CO2 back into the atmosphere but this has been estimated at a loss rate of only 1% every 100 years. Alternatively we could store the scrubbed out CO2 in mineral form. When CO2 reacts with metal oxides, like olivine, it produces stable carbonates such as calcite. Unfortunately, the energy needed to run a power plant with CCS and mineral storage is 60-180% more than a plant without CCS. As of 2020, carbon capture and storage was capturing and storing only 1/1000th of our global emissions. What’s more, according to Wikipedia, many projects have failed to deliver on promises of reduced emissions. So it appears the prospects for CCS at fossil fuel plants to limit global warming are quite poor.

CCS may have a role to play in conjunction the a CDR scheme and that is bio energy carbon capture and storage. The IPCC (AR5) estimated that growing crops, which absorb carbon, and then burning those crops to run power plants, and collecting those carbon emissions, could sequester up to 22 gigaton of carbon per year. That is MASSIVE, half of what we emit every year. However, there are some major questions about the feasibility and sustainability of large-scale bio energy and carbon capture and storage deployment. And that’s even before we address the political and economic questions. For example, we’d need 58 million hectares to grow crops for every one GtCO2 sequestered. A GtCO2 is a giga-ton of carbon dioxide, or a billion tons. For every GtCO2 reduction of atmosphere carbon via bioenergy and capture, would require 1% of currently farmed to be converted to energy crops. To get up to 22 GtCO2 we’d need 22% of currently farmed land. This would likely lead to more deforestation, biodiversity loss, food insecurity and more greenhouse gas emissions. Estimates for sustainable deployment of growing crops for energy and capturing their emissions estimates that bioenergy and carbon capture could only remove about 1 GtCO2 / year.

There is a whole suite of other CDR schemes, but most, like the CCS and the bio energy scheme have serious technical, political and/or financial barriers. For instance direct air capture is another favorite. It uses giant fans to blow air across grids of calcium can capture carbon. But it is currently extremely expensive and has been estimated to need 1/4 of global energy supplies to remove less than 1/8 of the carbon we emit. The impracticalities of direct air capture seem inescapable given that we can’t yet efficiently scrub out the carbon when in its denser form in smoke stacks. What hope is there in putting the carbon genie back into the bottle once it’s dispersed in the atmosphere?

Several other CDR schemes involve mucking about with the ocean. For instance, we could use the mineral storage capacity of olivine, as used in the smokestack scrubbing scheme discussed above. But in this instance, instead of using tanks full of amines to clean smokestack air, we could sprinkle olivine rich sand into the ocean at places of high water flow, like coastlines. The tumbling of the sand would abrade the olivine and expose fresh surfaces, which then would uptake oceanic carbon. This mineralized carbon would then sink to the ocean floor, much like corals and shellfish do to form limestone. Removing the carbon from the liquid ocean in this way would both lower oceanic acidification and also cause the ocean to absorb more atmospheric carbon. This is referred to as ‘enhanced weathering’ and it could sequester far more carbon than we’ve emitted thus far. Exciting as it sounds, we don’t yet know how feasible it is. Questions like how quickly would the olivine break down into smaller chunks and how quickly would it absorb carbon remain unanswered. We also don’t begin to understand the full consequences. But it does have potential and is worth continued research.

While none of the above schemes offer a sure fired, ecologically sound, doable solution today, there is one method of CDR which does work. Growing plants. :^) Forestry management and increased forest cover could absorb up to 10 GtCO2 / year, with ample side benefits such as increased soil fertility, increased biodiversity, more water filtration and enhanced income from agroforestry. Real world considerations mean that forestry management would probably only be able to sequester around 3 GtCO2 / year of absorption, but that’s almost 7% of our global emissions. Other land management practices could sequester even more carbon. Estimates of the maximum feasible reductions in atmospheric carbon dioxide are given here in the parantheses in GTCO2 / yr for various schemes: decreased deforestation (6), decreased forest degradation (2), decreased conversion of peatlands (1), agroforestry (6), soil carbon sequestration in croplands (7), carbon sequestration in pastures (3), and various other agricultural management schemes (3). Added together, it is technically feasible that these additional practices could absorb an additional 28 GtCO2 / year. But again, the boring old real world of political and economic feasibility have to be considered. Several different analytical approaches yield an estimate for an achievable carbon uptake by all land management practices of about 14 GtCO2 / year. That’s a about 1/3 of the decrease in atmospheric carbon inputs that we need by 2050.

And what is very helpful about these land uptake practices is that they are ready to go and they have positive knock on effects. Given the political will, they are quick to implement and have rapid uptake as they rely on enhancing natural sequestration methods. This is important for long term goals but also important for our short term goal of reducing net emissions straight away. In other words, the natural world can give us the breathing room we need by ramping up carbon absorption so we can develop the infrastructure to run on renewables. That’s good news. We could actually save two birds with one shovel full of soil: nurturing the wilderness and working towards meeting Paris Accord targets. If we undertake all of enhanced land absorbtion schemes listed above we’d only have to get our emissions down to 14 GtCO2 / year by 2050, not 0 GtCO2 / year, to meet net zero emissions.

Lest I leave you without a way to be proactive, remember that we can reduce emissions by altering consumer behavior too. Drastically reducing food waste and shifting towards a plant based diet have the potential to reduce emissions by 4.5 GtCO2 / yr and 8 GtCO2 / yr, respectively. Reaching net zero emissions is looking more and more feasible if we’d all eat our veggies and our leaders would wise up and get gardening.