1. Solving for Legacy CO2: Returning to a Safe Climate

A Diversified, Portfolio Approach to Carbon Sequestration

We have the tools and knowledge to return Earth’s climate to safe levels of greenhouse gases, with dedicated effort over decades. Undoubtedly, this would be a massive undertaking. Yet doing so would cost us far less, in purely economic terms, than a climate that continues to overheat.

Many climate scientists consider a 350 parts per million (ppm) carbon dioxide (CO2) to be a safe threshold for our climate. Earth’s climate is now above 410 ppm on average, with seasonal variations. On our current course, we’ll eclipse 450 ppm by 2034 - a level considered “high risk” by leading international organizations.

We should not resign ourselves - and future generations - to living with the chaos and extremes of a 400+ ppm climate. To protect our own lives and those of our kids, we ought to stabilize our climate at a safe level, because we can.

For many people of the world, this is a matter of survival. But it is also about our quality of life - the freedom of billions of people to live well rather than merely survive - that will be severely impacted by a 400+ ppm climate.

We must aggressively deploy clean, zero emissions technology to meet humanity’s growing demand for power and fuel, yet even this is only the first step - crucial, yet insufficient by itself - towards returning to a safe global climate. 

Truly solving for global warming requires that we remove long-lived (i.e. “legacy”) CO2 from the atmosphere, and store it in the Earth for thousands of years (effectively permanent). This will require harnessing our industrial, technical and logistical leverage to capture and store CO2 on a massive scale, akin to America’s wartime mobilization in World War II, except in multiple countries.

CO2 resides in the atmosphere from 300 to 1000 years according to NASA, trapping heat and warming the planet. Most anthropogenic (human caused) CO2 in the atmosphere is from the 1760s onward, after the fossil fuel-intensive Industrial Revolution began in Europe and America.

Nature both sequesters CO2 from the atmosphere, and emits it back up at nearly even levels. At an accelerating rate in recent centuries, anthropogenic CO2 is being added beyond what nature can offset, leading to higher concentrations of greenhouse gases. NASA explains how we know this:

Scientists know the increases in carbon dioxide are caused primarily by human activities because carbon produced by burning fossil fuels has a different ratio of heavy-to-light carbon atoms, so it leaves a distinct “fingerprint” that instruments can measure.

Essentially, CO2 from the past is causing global warming today. However, we can’t simply pass off the blame to distant generations, since half of our current atmospheric CO2 levels are the result of emissions only dating back to 1980. Global warming has been caused by many of us living today, and so it is our responsibility to fix the problem.

Most legacy CO2 has been emitted by the wealthy nations of Europe, the U.S., Canada, Australia, Japan and China.

Historical CO2 Emissions by Data. Credit: Our World in Data

In terms of climate justice, therefore, these are the nations that should remove legacy CO2. Practically speaking, these are also the only nations currently equipped for such a task in terms of wealth, infrastructure and industrial capacity.

We’ll need technology and industry to take on legacy emissions, which may prompt the question: if industrialization got us into this problem, how can industry also be the solution?

To answer this, we need to examine the different ways that CO2 can be sequestered, which fall into two broad categories: biological sequestration and geological sequestration. 

Biological sequestration occurs via: (A) Photosynthesis, when plants or phytoplankton pull CO2 from the atmosphere to grow; or (B) Soil microbial activity binds dead organic matter from plants or animals to minerals in the ground.

Geological sequestration occurs when non-living elements and processes, such silicate rocks and minerals, capture CO2 and bind it slowly over thousands of years.

At least, that’s how it works in nature. The natural rate of geological sequestration occurs too slowly to make a dent in legacy CO2 on a relevant timeline for us.

Instead, we’ll need to artificially imitate or accelerate geological processes to lock away carbon for thousands of years, which requires technology and industry. Moving forward, these artificial methods are what I’ll call geological sequestration.

Geological sequestration is necessary because there are practical constraints on biological sequestration methods, limiting how much legacy CO2 they can remove. For example, land use conflicts eventually emerge planting trees and agriculture or urbanization, especially as billions more people populate the earth.

Even in an optimal scenario, plants and soil microbes can only offset decades of humanity’s annual emissions before they max out, leaving lots of legacy CO2 to continue warming the planet.

Additionally, plants die within decades and slowly release their CO2 back into the air. Soil storage is more long lived, but could release its carbon if regenerative approaches are abandoned or extreme weather strikes. Thus, biological methods are limited, insecure and temporary.

Geological methods, on the other hand, can securely store CO2 for thousands of years underground in practically unlimited quantities. For example, Texas’ Permian Basin alone could store 100 years worth of global emissions, and there are other storage basins across the U.S., not to mention the rest of the world. .

Because geological sequestration methods are artificially accelerated and CO2 storage space is massive, they can be scaled up far beyond natural levels and can therefore remove legacy emissions.

Once captured, CO2 can be stored deep underground for thousands of years, where it has little chance of escaping. This means that the climate removal gains are effectively permanent.

Since geological processes must be artificially induced, we are doing the work, rather than nature. That means investments of time, money, energy and infrastructure are required, which is the main drawback.

For example, enhanced weathering requires gathering mineral rocks, crushing it and transporting it to a beach or farm to be deposited. The natural process of sequestration is greatly sped up, but machines and transportation are required.

Critics also point out that the investment in mechanical carbon removal diverts funding from the clean energy transition. It is true that investing in clean energy to displace fossil fuels is more efficient than removing CO2 after it’s been emitted.

However, this point only holds until we reach a low or zero carbon emissions economy, after which the function of removing legacy CO2 is far more valuable. Our goal shouldn’t be to only reach zero carbon emissions annually, it should be restoring a safe climate, which will require drawing down gigatons of CO2. A zero emissions economy ought to be the priority, but in reality we’ll need to start building out carbon removal projects far ahead of time.

Artificial geological sequestration is crucial to a 350 ppm climate, but it is also expensive and inefficient. This is where biological methods can help. Under the main goal of reaching 350 ppm, we can use both geological and biological methods for optimum mix of cost, energy efficiency and carbon removal rate.

Because nature does most of the work, biological sequestration is very energy efficient and affordable. People will need to seed trees and mangroves, or manage soils using regenerative practices, but no mines, rail lines or new power plants are needed. Furthermore, plants and soil microbes start pulling down CO2 quickly, putting a dent in greenhouse gas levels while geological projects are developed.

Biological methods also provide non-sequestration benefits:

• Healthy soils hold more water, making them resilient to extreme heat, rain or drought

• Trees clean the air, mitigate flooding and create beneficial habitats for wildlife

• Mangroves nurture young fish and protect coastal settlements from high winds and waves

What is proposed in this series is a “drawdown portfolio” approach, which utilizes cheaper, faster biological methods to reduce the heavy lifting required by expensive, slow to deploy geological projects. Trees, soil restoration and aquatic vegetation can be harnessed as much as possible for CO2 drawdown, while geological methods are ramped up to pull down the remaining CO2 needed for our goal.

If global tree planting efforts miss their targets or enhanced weathering doesn’t pencil out, then additional direct air capture plants could be built to make up the difference.

The actual share of carbon expected from each method isn’t explored here. Real world application will help to clarify the feasibility of each approach. Instead, my focus in this series is to explore six of the most promising options for humanity to include in its drawdown portfolio.

Here are links to each article exploring these six drawdown methods:

Planting Trees (Biological)

Farming Carbon (Biological)

Blue Carbon (Biological)

Bioenergy & CO2 Capture (Geological)

Enhanced Weathering (Geological)

Direct Air Capture (Geological)