Direct Air Capture of CO2 Is Needed After the World Reaches Net Zero Emissions

Alan Emery
9 min readFeb 17, 2024

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Introduction

The concept of capturing carbon dioxide (CO2) from the air or from the source of emissions has been around for a long time, but recently a number of fossil fuel companies have proposed using the concept direct air capture (DAC) after the CO2 is in the atmosphere to offset their use of fossil fuels. To bring climate change under control it will be necessary not only to stop carbon emissions, but it will also be necessary to bring the atmospheric level of CO2 back to between about 320ppm and 350ppm. This is necessary so that the temperature of the Earth can climb back down from the level it reaches when we have completed the transition away from fossil fuels to a carbon-free energy source, such as nuclear, deep geothermal, or in some instances, renewables plus some means of long-term storage of energy for release when either wind or sun is not consistently present.

The transition from fossil fuels to other forms of carbon-free energy is moving very slowly on a world scale; certainly not quickly enough to control the worst of the damages from the anthropogenic climate change impacts. The proposed use of direct capture and storage to “give the world more time to solve the problem of climate change caused by humans,” is not really a smart idea at this time if technology is used because the rate of emissions is far, far higher than the present technology could even nearly achieve. Better to carryout research and development to be ready once Net Zero Emissions have been reached.

Given that we actually know how to solve climate change from a technological perspective right now, undertaking the task of reaching net zero emissions as fast as possible while maintaining world energy demand is far more logical.

So… why doesn’t the world just get on with doing the job?

For most people, and certainly for politicians reacting to public pressure, the idea of spending a lot more money right now to help someone in the distant future is not very popular. For sure, some of that altruism is happening, but it is very tiny compared to the full need. The idea of a tax to provide the incentive to move to “greener” ways of doing things is great if the choices are truly available for people to make a decision. Unfortunately, there are no real choices for most people.

This leads to the presumption that nothing will be done about climate change unless it is right now cheaper than the existing systems. The logic for capturing and sequestering CO2 today is gaining support, but only if it can be used as a carbon offset to allow more fossil fuels to be used. CO2 can also be used to pump oil up from depths, thus allowing more use of fossil fuels.

Ocean fertilization can be used to supply the limiting nutrients for ocean plants, speeding up their growth and intake of carbon. That creates a difference in the partial pressure of CO2 in the ocean compared to the air so the ocean takes up CO2 from the air to replace the extra carbon the plants in the ocean use up once their limiting nutrients are supplied. This process is under experiment now and review by the international authorities to determine the scale of experiments that can be used to understand if this process will work without and unanticipated negative consequences. We already know it cannot be used in near-shore environments but it is likely it would work well in the nutrient-poor waters of the open ocean.

I have no doubt that if ocean fertilization works, and if the fossil fuel companies were funding it, they would want to be able to measure the amount of CO2 removed from the air so as to claim it as a carbon offset, thus allowing more use of fossil fuels. If instead, ocean fertilization were funded by governments, and if governments disallowed the reduction of CO2 to change the goal of reaching Net Zero Emissions as soon as possible, that would make the transition much faster and the resulting damages much less.

At the same time, there is a HUGE advantage to pulling CO2 out of the air after we reach net zero emissions. Therefore, it is imperative to not only know how to remove CO2 from the air, but it is also imperative to have the infrastructure ready to go.

The Probable Results of Starting to Transition away from Fossil Fuels by 2030 and Completing the transition (Net Zero Emissions) by 2110.

For example, if the world gets serious by 2030 and manages to reach net zero within 80 years after that, the following chart illustrates that even the most optimistic estimate for natural CO2 reduction is that the world will start at ~800ppm CO2 and take at least 300 years to get to 400ppm CO2, and then it tails off from there over the next 10,000 years back down to 280ppm. In the least optimistic case (the carbon sinks are very slow to take up the CO2), the return to a modest 335ppm would take literally thousands of years.

Atmospheric levels of CO2 decline at potentially different rates depending on the efficiency of the carbon sinks at the time.

Any of these scenarios result in temperatures that are basically way above what human civilization could adapt to. Even the most optimistic temperatures assuming a rapid cool-down would begin at well over 4C and fall to at best, 2.5C and then rise again to 4C. in the worst instance of a slow cool-down, the temperatures would continue to rise to about 9C global average over the next 900 years. These changes could easily wipe out our civilizations and possibly even humans as a species.

The changes in global average temperature reflect the rate at which CO2 is removed from the atmosphere — in this case by natural sinks. In all cases, the levels are in the danger zone, and in the extreme case, they are lethal in many parts of the world.

Given these potential levels of temperature, the ocean will react as well to these natural changes in temperature, that in turn are the result of the natural decline in CO2. In all cases, whether the temperatures cool down quickly (the best situation) or slowly (the worst situation), the likely sea level rise by 2070–2080 is on the order of 1 meter. The sea level rise doesn’t stop there because the temperatures will all be above the point where there is no sea level rise. By 2200, the most optimistic sea level rise would be on the order of 4 meters and it will hover there for another 100 years. If the cool-down rate is slow, the likely sea level rise (as a result of continuing high global temperatures) could reach over 6m by 2300.

By 2070, the seal level will likely be higher than pre-industrial time by between 0.4m and 2m. Assuming a 1m rise by then the sea level changes will again depend on the temperature changes which in turn depend on the rate at which CO2 is removed from the atmosphere by natural means.

Beyond 2300 and into the next millennium, the most optimistic would be about 10 m, and the possibility of a worst case of about 30 m sea level rise. The increase in sea level under natural cool-down would not slow until about the year 7000.

Likely sea level changes will only slow down as the temperature drops below 1.0C below the preindustrial global average temperature.

Advantages of Direct Air Capture and Sequestration of CO2 on Climate Impacts

By comparison, if the use of direct air capture (DAC) were implemented immediately at net zero (2110), the situation would be quite different and far better. For example, the CO2 levels in the air would come down quickly. In my analysis, I am assuming a rate of reduction of about 20 billion tons of CO2 per year (20Gt/yr). To accomplish that rate of removal would require something like 10,000 DAC plants each capable of drawing 2,000,000 tons of CO2 from the air each year. That capacity per plant is reasonable, the number of plants assumes about 100 countries participate with an average of 100 DAC plants each. The chart below shows some options.

The Emissions of CO2 and the subsequent Direct Air Capture Rates are Illustrated as a series of options, highlighting 40 and 80 year periods to reach Net Zero Emissions before beginning Direct Air Capture at 20Gt/yr until the atmospheric level of CO2 is at about 335ppm.

Given a reduction to 335ppm CO2 by 2300, the temperature changes would still be nasty, but it would be survivable. If the DAC plants were activated at net zero by 2110, the atmosphere by the year 2220 (110 years) would have about 335ppm CO2, which is quite reasonable for long-term adaptation.

The time required to bring the atmospheric level of CO2 back down to 335ppm using a removal rate of 20Gt/yr depends on how long it takes for the transition away from fossil fuels, which in turn creates the probable level of CO2 in the atmosphere at Net Zero Emissions.

The temperature regime following the introduction of DAC is dramatically different from natural reduction rates. Instead of rising rapidly, the temperature declines rapidly. Under a fast cool-down (the most optimistic,) the global average temperature would be back into the safe zone at 2C by 2400 and then it would continue to fall until it reached a stable temperature roughly equivalent to today’s global average temperature. In the worst case, a slow cool down, it would take 900 years to get back to the safe zone, but the maximum temperature would be less than 4C at 2110. Nasty, but quite survivable if we are prepared.

By using Direct Air Capture after the world reaches Net Zero Emissions at a rate of 20Gt/yr, the average global temperature falls back to the safe zone after a period of between 400 and 900 years (as opposed to never), and does not rise as high as 4.0C. Not great, but not nearly as bad as letting nature remove the CO2.

Finally, if we consider the sea level changes under DAC, these are also far less disastrous than allowing nature to remove the CO2 with consequently dangerous temperature and sea level rises. Instead, in the best case, the sea level rise is about 5m over a period of about 350 years, and then it declines to our familiar level by the year 2900. In the worst case, the sea level rises as much as 12m by the year 3000 before declining back to our familiar level by the year 4300. It is important to be aware that both of these maximum sea levels are extremely damaging. At 4 m sea level rise, essentially all coastal cities will be inundated and in many cases be destroyed by storms once they are underwater. But the sea level does not continue to rise beyond that forcing extensive loss inland and also forcing literally billions of people to leave their homes and businesses. Thus, using DAC after reaching net zero emissions is far less damaging than letting natural sinks take up the CO2.

By removing CO2 from the air faster than nature can accomplish the task, the potential sea level rise is significantly reduced. The possible sea level rises of between 5m by 2300, and in the worst case 12m by 2700 are both extremely damaging, but not nearly as bad as natural removal of CO2 would cause. Adaptation planning needs to start now, not once the incursions become globally destructive.

The next obvious question is “Can the world afford the cost of DAC?” The following chart illustrates the annual cost of removing 20Gt of CO2 per year until the excess CO2 is removed, assuming the cost of removal is about $100/ton, the delivery of energy is retained at the demand level, sold for $0.06/Kwh, with a profit to industry of 10%.

In this illustration, the world reaches Net Zero Emissions by 2110 and then begins to remove CO2 from the atmosphere at a rate of 20Gt/yr. The peak deficit to pay for the infrastructure is at about the year 2170, while still earning revenue from the production of electricity so there is no failure to deliver power to the world. That peak deficit would be on the order of $34 trillion.

It is very expensive, but not impossible. At the worst year, the global debt would be about $35 trillion. That sounds huge — and it is. But to put it into perspective, the US alone currently (2024) has a debt on the order of $34 trillion.

Needless to say, all of this depends on the world getting its act together seriously by 2030 to reduce its CO2 levels down to net zero emissions by 2110 and then using DAC to bring atmospheric CO2 levels down to about 335ppm within a period of 80 years (2190).

To summarize, it is probably a poor idea to use CO2 removal during the transition away from fossil fuels if the fossil fuel industry is in control. If they are funding the removal, they will use it as a carbon offset to keep using fossil fuels. What is required is for any artificial removal of CO2 during the transition to be used in the accounting for the final level of CO2 in the atmosphere, not as a carbon offset to enable more use of fossil fuels. The saving in damages to the world biological and human systems is HUGE. The cost is also huge, but well within the limit of what the world economy could undertake.

What do you think will actually happen? Scientific and engineering results predict that the world can reach net zero emissions within this century if our leaders get serious about climate change. Science also predicts very dangerous climate impacts if our leaders do not get serious in time. Social history predicts that our world of technological marvels and relatively prosperous human civilization will end under the conditions predicted by science if no action is taken.

Given that we can achieve a net zero emissions before the end of the century and given that we can afford to remove CO2 from the air considerably faster than nature, if we prepare in advance; what are we waiting for? How do we change the minds of our leaders to care for us, rather than for their profits?

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Alan Emery

Scientist (PhD marine sciences). Looking for solutions. Focus: ecology, evolution, global warming, energy transition, biodiversity, Indigenous Knowledge.