Climate Brief: Greenhouse Gas Removal Part 1: Natural Processes

Introduction

Removing and sequestering greenhouse gases to reduce their concentrations in the atmosphere has been studied over the last two decades as a way to mitigate global warming. Greenhouse gases are more than just carbon dioxide (CO₂); methane (CH₄), nitrous oxide (N₂O), nitric oxide (NO), nitrogen dioxide (NO₂), other nitrogen oxides (NOx), and the chlorofluorocarbons (CFCs) are also significant greenhouse gases. However, for the sequestration discussion, we will concentrate on CO₂, as it accounts for 76% of all greenhouse gas emissions. The fraction of different gases accounting for total emissions is shown in the pie chart below.

Anthropogenic sources of greenhouse gases

All told, human activities add 50 gigatons (Gt) of CO₂ to the atmosphere every year. The global sources by economic activity are shown in the next pie chart. Note that direct energy production (electricity) and agricultural production account for 49% of the total emissions … almost half. 

Each of these sectors potentially can mitigate some of its greenhouse gas emissions through currently available technology. Familiar examples include electricity produced by solar and/or wind energy, transportation run with electric motors, and dietary changes (for example: vegetarian or vegan diets; and on a more modest level, reducing or eliminating beef consumption).

Naturally Occurring Sinks for Greenhouse Gases

The greenhouse gas additions noted above contribute to the land-atmosphere-ocean system in proportions shown in this graphic. We’ll discuss the land and ocean CO₂ sinks in the next sections.

Land-based Sinks of CO₂

Plant photosynthesis is the largest natural sink for CO₂ on land. They use CO₂, soil water, and soil nutrients in photosynthesis, and transpire O₂ and water vapor as byproducts. 

Addition to and creation of soil is another natural CO₂ sink. Biomass (decaying dead leaves and other plant material) and animal excreta are added to soil, becoming part of the soil matrix. Manure production, while better for the environment than man-made nitrogen fertilizers, emits methane (CH₄) as it decays, thus adding greenhouse gas to the atmosphere. Methods are now being developed to reduce these emission through better agricultural practices.

Ocean-based sinks of CO₂

The graphic above shows CO₂ uptake at the ocean surface. The amount of uptake depends on the how saturated the water is with respect to CO₂, relative to its atmospheric concentrations. Because its concentration has been rising, globally averaged CO₂ in water has been increasing.

Without going too much into the chemistry, it turns out that CO₂ is very “attracted” to water and combines with it to make carbonic acid, which dissociates into carbonate and bicarbonate ions, and lowers the ocean’s pH, making it more acidic.

Ocean acidification interferes with the shell formation by corals and shellfish in surface waters. At the base of the ocean food chain, plankton in the deeper ocean can also be adversely affected, with profound implications higher up in the food chain, and ultimately human beings. This is a major limitation for use of the ocean in CO₂ sequestration.

What Are Some Realistic CO₂ Sequestration Strategies?

The goal of all reduction strategies is to ultimately generate a net loss of CO₂ from the atmosphere from human activities. In Part 1, we’re covering only CO₂ removal through sequestration through natural and what I’ll call “passive technological” means. This does not absolve human civilization from having to transition from carbon energy use to renewable energy technologies, and ultimately living in harmony with the planet we call home.

methods of sequestration

Here, CO₂ gas is taken out of the atmosphere and sequestered for long periods of time through natural and augmented natural means. These methods include: 

Carbon farming

The idea here is to use agricultural methods to increase the rates that CO2 is naturally stored in the soil as organic matter, and in the crops themselves. About 10% of farmland is cultivated using these methods, usually with financial incentives from government to make carbon farming profitable. (Ed.: I don’t believe Big Ag is doing any of this, as it eats into their profits.)

Afforestation

Trees are planted where there were none before, to increase carbon storage through photosynthesis. Governments and NGOs are funding these efforts. A side benefit of afforestation is increased soil quality and carbon levels, and soil moisture retention.

These efforts are limited to the area favorable for afforestation activities, though a 2019 study concluded we could add 9 million km² of new forests to the current global coverage. Ultimately, this could store up to 25% of the current atmospheric “carbon pool” through CO₂ decrease and introduction of more O₂.

Examples of programs for afforestation can be found in Canada, China (!), Spain, Great Britain, India, Israel and North Africa. 

Reforestration

Besides creating new forests, projects are ongoing to restore existing and former forests naturally and intentionally. As things stand, forest area decrease has slowed by about 40% between the 1990s and 2010s, to 4.7 million hectares (2.47 acres). This is, unfortunately, before major impacts on forest area from Brazil’s abandonment of Amazon rainforest preservation under Jair Bolsonero (President since 1 January 2019).

A goal was set by the United Nations to increase global forested area 3% between 2017 and 2030. It now seems unlikely that this goal will be met.

“Blue Carbon” Sequestration

The ocean and coastal ecosystems are a big sink for CO₂ through wetland plant growth and decay with subsequent storage in wetland soils. The same general process takes place in the deep ocean with photosynthesis by plankton, its consumption by marine animals, and whatever is left sinking to the ocean floor after death. 

Wetlands

Wetland are an important carbon sink in the ocean/coastal ecosystem. A particular type of wetland that is highly efficient at storing carbon is the mangrove forest, which can store up to 10 times the amount of carbon per unit area than tropical rainforests. As mangroves forests die, they go through decay and, given the proper conditions, becomes peat through the action of fungus and bacteria. Termites also process the mangrove leaves, wood and root systems to build their nests and help in peat formation. Peat provides effective storage for carbon since it doesn’t emit carbon products unless it is burned (e.g. peat fires in Siberia beneath winter snowcover).

The Deep Ocean

In the deep ocean, augmenting natural uptake of CO2 by plankton is also being considered. Last year research done in support of the National Academy of Sciences, found that plankton absorb about twice as much greenhouse gas emissions than previously thought. Without this action and other ocean sinks, the current CO₂ levels would be at 550-600 ppm, rather than the current level of 420 ppm.

Because oceans cover 70% of the planet’s surface, it is thought that restoring the ocean ecosystem to optimize its sequestration potential may be the most effective blue carbon approach. It’s been thought that ocean fertilization to increase plankton production might increase deep ocean uptake and sequestration, but research over the last few years has suggested only small amounts of carbon would be permanently sequestered.

Discussion

There is something very attractive about using existing natural cycles to remove greenhouse gases. Unintended consequences could be less likely, and generally their costs are less than more sophisticated technologies we might develop.

Unfortunately, natural carbon dioxide removal, even augmented, is not likely to be enough to prevent sufficient warming to cause substantial, permanent damage. The damage doesn’t respect country boundaries, which will likely increase already existing international conflicts, and create new ones.

We’ve already seen extreme weather events that would have been impossible without anthropogenic global warming. Without serious, sustained action, we’ll see increasing crop failures, famines, and water shortages that could result in millions of deaths.

So if natural CO2 capture and sequestration isn’t enough, what additional options are available to get us out of this unfolding crisis? Stay tuned for part 2, all about the technological, human endeavors to remove greenhouse gases from the atmosphere.