Geoengineering to Combat Climate Change: Carbon Dioxide Removal

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Tree plantations such as this could cover previously deforested land. Image by Leon Brooks

In February, the National Academy of Science released two reports on environmental engineering to counteract climate change. The first report covered Carbon Dioxide Removal (CDR), and the second covered albedo modification. Which of these techniques is the most effective means of helping the environment?

CDR vs. Albedo Modification

CDR is the intentional removal of CO2 from the atmosphere, either through enhancing natural processes or using man-made technology. Albedo modification artificially increases the amount of sunlight reflected back out to space in order to lower earth’s temperature by limiting the amount of solar energy absorbed.

While CDR addresses the root of the problem –too much CO2 in the atmosphere – albedo modification only seeks to mask it. The National Academy of Science chose to release its findings for the two types of climate engineering separately because the potential risks, benefits and recommendations are vastly different.

The report concluded albedo modification should only be used as a last-ditch effort, but that CDR is relatively safe and should be looked into further. Since the use of multiple CDR approaches is likely, let’s explore the science behind the different types of carbon dioxide removal.

Land Management: Afforestation and Reforestation

Land management practices that remove carbon dioxide include afforestation, reforestation and low-till agriculture. Afforestation is the growth of forest on land that has been deforested for at least 50 years; reforestation is the growth of forest on land that was recently deforested.

Afforestation and reforestation both harness trees’ natural ability to capture CO2 through photosynthesis and store it in the wood and soil. The uptake of CO2 is greatest in new growth forest, and tapers off as the forest matures and the growth rate of plants and trees levels off.

Trees reach their maximum rate of CO2 uptake after about 30 to 40 years of growth. The rate at which forests can remove carbon from the atmosphere also depends on the temperature, precipitation, site geology and history, atmospheric CO2 concentrations, and the species composition of the trees. Warmer and wetter climates create faster growth that stores carbon more efficiently; this is why tropical forests store more carbon than temperate forests, which store more carbon than boreal forests.

Afforestation and reforestation already remove substantial amounts of carbon from the atmosphere – on the order of 1 gigaton per year. This is mainly due to forest reclaiming abandoned agricultural land in the northern hemisphere. It is also due in part to faster plant growth caused by increasing CO2 concentrations in the atmosphere.

Unfortunately, the potential to expand on this natural carbon sink is limited. Old growth forests release as much CO2 through respiration and decay as they remove from the atmosphere through photosynthesis; this makes old growth forest a net wash in terms of the carbon budget. However, old growth forest provides many other benefits, such as the preservation of plant and animal species that require a specific habitat to thrive, and old growth forests also store a massive amount of carbon.

One of the most important strategies for preventing worsening climate change is to halt tropical deforestation, which could add up to 1,800 gigatons of CO2 to the atmosphere. Another limiting factor is the potential for more widespread forest disease or faster decomposition of carbon in soil due to a warmer, wetter climate.

In the worst case scenario, forests could become a carbon source rather than a sink for atmospheric greenhouse gases.

Land Management through Agriculture

Currently, most farmers plow their fields before planting crops, releasing carbon into the atmosphere by speeding up decomposition in the soil. One way to keep more carbon in the soil is to plant more varied crops, combining grasses and legumes; another is to eliminate or reduce tillage. Cover crops are non-market crops that are planted for the purpose of increasing organic matter in the soil and improving soil quality. Soil with better structure can store more carbon, in addition to being more resistant to pests and erosion.

Accelerated Weathering

Natural chemical weathering of limestone, such as this in Kongsberg, Norway, could be tweaked to remove carbon from the atmosphere faster. Image by Mahlum.

Weathering is another part of the carbon cycle that naturally removes CO2 from the air and stores it on land or in the ocean. Through chemical reactions known as carbonate and silicate weathering, CO2 in the atmosphere eventually either dissolves in ocean water and settles to the sea floor as carbonate sediment, or becomes stored on land as solid calcium carbonate.

Accelerated weathering would speed up this process by bringing concentrated CO2 in contact with the appropriate type of rocks on land, or tweaking ocean chemistry by releasing ground-up minerals into the ocean.

Dissolving calcium carbonate lowers the acidity of the ocean, allowing it to absorb more CO2. The more CO2 the ocean absorbs, the more carbonic acid is formed, and the greater the need for more minerals.

The drawback to this approach is the energy use and environmental damage caused by extra carbonate or silicate mining to get the minerals, and the energy used to transport the minerals.

Ocean Iron Fertilization

A more controversial way to tweak the natural carbon cycle is through ocean iron fertilization. The biological pump in the ocean is a process where phytoplankton and other tiny ocean plants remove CO2 from the atmosphere through photosynthesis and convert it to organic matter, which ends up stored as inorganic carbon in the deep ocean. Iron fertilization enhances the natural biological pump by adding iron to parts of the ocean to stimulate the growth of phytoplankton. Unfortunately, this would have serious unintended consequences, such as producing toxic algae blooms and interfering with the ocean food web.

Bioenergy with Carbon Capture and Sequestration (BECCS)

Like reforestation, BECCS uses photosynthesis to remove CO2 from the atmosphere through the growth of biomass. Unlike reforestation, the biomass is then further processed to produce bioenergy or biofuel, and then the carbon may be captured, concentrated and sequestered to prevent it from ending up in the atmosphere again.

The benefit of this approach is the relatively eco-friendly energy produced. The downside is that the processing (oxidation and gasification), transport, and sequestration all use energy to create this bioenergy, which may or may not be so eco-friendly. If, for example, the energy used to capture the CO2 or pump it underground comes from a coal-fired power plant, this becomes a much less practical option – and if the biomass is converted to a liquid fuel, the carbon will not be sequestered at all.

The best case scenario is if the fuel is either converted directly to heat or burnt in a smoke stack where emissions can be captured.

Another limitation to this approach is the lack of availability of land for growing the biomass crops. Furthermore, a fraction of the nitrogen fertilizer used for biomass cultivation gets converted to nitrous oxide, a powerful greenhouse gas.

Direct Air Capture and Sequestration (DACS)

DACS uses technology to directly capture CO2 from the atmosphere and dispose of it through sequestration. The chemical separation or scrubbing technology used is like that of scrubbers in traditional carbon capture and sequestration (CCS), except that it removes the CO2 from ambient air rather than a smoke stack.

On the one hand, this means that it can actually lower existing CO2 concentrations in the air rather instead of preventing further emissions (like with CCS). But on the other hand, it takes far more energy to remove a pollutant that is less concentrated, as it is in the atmosphere. It takes at least twice as much energy to capture CO2 from the atmosphere than from a smoke stack. If the energy used by the scrubbers were natural gas or coal, the amount of CO2 emitted would actually be greater than the amount captured.

Geological Sequestration of Carbon Dioxide

Unlike land management practices where the storage of carbon is built into the natural cycle, BECCS and DACCS require sequestration to keep the captured carbon trapped. Geological sequestration traps CO2 by pumping it underground into depleted oil and gas reservoirs or saline aquifers at high pressure.

Although geological sequestration has the capacity to store large amounts of CO2, there are risks involved. Like fracking, geological sequestration injects large volumes of fluid into brittle rock, which may over-pressurize the reservoir, causing earthquakes and CO2 leakage. Even after a successful injection, sites require long-term monitoring to ensure their stability.

This technology is not completely untested, as it has already operated at a handful of coal-fired power plants successfully for the past several years, but on a very small scale.

Ocean Sequestration of Carbon Dioxide

Ocean sequestration, where CO2 is injected into the mid-depth or deep ocean, is another potential means of reducing atmospheric CO2 that the researchers investigated in this report. However, the idea lacks support because of concerns over ocean acidification and subsequent damage to sea life. When the ocean absorbs CO2 (either naturally or through man-made intervention), carbonic acid forms in the process.

Reducing Atmospheric CO2: CDR Methods

Arguably, reducing the amount of CO2 in the atmosphere by CDR methods rather than cutting emissions is a bit like turning to liposuction rather than a diet to lose weight. Preventing emissions in the first place would obviously be preferable because of the drawbacks, limitations and costliness of CDR. However, political reality has to be acknowledged, and supplementing emissions reduction with safe forms of CDR may be necessary to avoid reaching the point where the scarier version of climate engineering, albedo modification, becomes our only option.

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