We can't just burn carbon now and suck it out of the atmosphere later

This article was originally published by the Boston Globe and is available online here.

As complex as the climate system is, its response to human carbon dioxide emissions is remarkably simple. Like the water level in a bucket slowly rising as water flows in, carbon dioxide levels have been rising — filling the atmosphere. And higher concentrations produce a stronger greenhouse effect, which traps more heat and accelerates the warming of the earth’s land and oceans. 

The simple arithmetic of cumulative emissions produces “carbon budgets” — estimates of the amount of carbon we can emit if we want to limit warming to a specific amount. Right now, the carbon budget for a 50:50 chance of limiting warming to no more than 1.5 degrees Celsius above preindustrial levels — a target set by a group of 200 nations with the Paris Agreement of 2015 — is 130 billion tons. Annual emissions are some 40 billion tons, which means the budget will be exhausted in three years. We will soon overshoot the 1.5 degree target. 

The good news is that if we stopped all carbon dioxide emissions tomorrow, the natural carbon cycling processes of the earth would drain the extra carbon we have added to the atmosphere and temperatures would stabilise and then fall. The bad news is that it would take about 100,000 years to get back to preindustrial levels. If we want global temperatures to decrease anytime soon, we need another approach: carbon dioxide removal, often abbreviated to CDR. 

If we think of the atmosphere as a bucket that we have filled with water, then CDR would be like putting a hose into the bucket and siphoning out the water. If the siphon removed the same amount of water as was flowing in, that would mean we’ve reached net zero emissions. Reducing global temperatures would require going further to net negative, in which more carbon dioxide flows out than in.

There have been numerous proposals for how this might be achieved. They can be loosely grouped into two categories: nature-based and engineered. Nature-based means trying to speed up natural carbon cycling processes. The most common approach is planting trees. As trees grow, they draw down carbon from the atmosphere and store it in biomass and soils. Simply increasing the total number of trees on the planet could have a noticeable impact on atmospheric concentrations of carbon dioxide. Although the effect is hard to establish exactly, each year around two billion tons of carbon dioxide are already removed by existent tree planting schemes, around 5 percent of annual emissions.  

The longer we continue to pour carbon dioxide into the atmosphere, the greater the amount of carbon we will need to remove if we want to get back to 1.5 degrees. We are already at the point where tree planting will not be enough. Alternative nature-based proposals include creating vast seaweed plantations with regular sinking of the seaweed, so that its carbon drops to the bottom of the ocean, or scaling up the production of biochar — a form of charcoal often used to improve soil for farming. If done well, these and other schemes could remove a few more billion tons a year. If done badly, they could devastate biodiversity, displace millions of people from their land, and threaten food and water security.

These concerns have driven interest and funding toward the second category of CDR: engineered. There are pilot programs that in effect seek to accelerate the natural take-up of carbon dioxide by the world's oceans. Most activity is around direct air capture (DAC). These systems, which resemble large air conditioning units, pass air over a chemical filter, which strips out carbon dioxide molecules. The captured gas is then compressed and pumped into storage sites deep in the earth’s crust. This approach does not need large amounts of land, water, or biomass to operate. But it does need a lot of energy: Using DAC to remove 100 percent of current emissions would exceed present-day global electricity generation. This largely explains why the amount of carbon removed by DAC is tiny, on the order of a thousand tons a year. 

DAC will improve, but the laws of thermodynamics unavoidably limit efficiency gains. The energy stored in fossil fuels is the result of carbon having been concentrated by photosynthesis many millions of years ago. Liberating this energy dilutes the carbon to such a degree that we measure its presence in the atmosphere in parts per-million. Concentrating all that gas back again will always be energy-intensive.

But couldn’t we just throw vast amounts of renewable energy at DAC? The problem with this approach is that we desperately need to use renewables now to decarbonize legacy fossil fuel systems. Add in exponentially increasing electricity demands from data centers being driven by the AI boom and you can see how big the renewable supply gap could become. 

Whichever way you sum it up, the benefits of not emitting the carbon dioxide in the first place vastly outweigh those of a burn-now-CDR-later approach. This is not to say that DAC or any other type of CDR is pointless. If we rapidly decarbonize industrial processes and food systems and thereby halt further ecosystem destruction, a portfolio of CDR approaches could be part of a solution. But we are far away from that scenario. Emissions in 2024 were at an all-time high. At this point, promises to recover after overshoot are just that — promises. 

Overshooting 1.5 degrees could be the moment when we fully realize that past and current approaches have not worked and that we need new thinking about the climate and ecological crisis. Unfortunately, there is scant evidence that any such awakening is on the horizon. In fact, policy makers may double down on the existing course by going beyond CDR into the even more fantastical realm of solar geoengineering approaches. One of these is sulfate aerosol injection (SAI), which would involve spraying millions of tons of sulfurous compounds in the high atmosphere each year in order to reduce the amount of energy from the sun that reaches the planet’s surface. As unhinged as that sounds, SAI is garnering increasing interest and financing. 

The clearest path to a safe and stable climate begins with the rapid phase-out of fossil fuels. We must stop carbon dioxide flooding into the atmosphere. Increasing our reliance on overshoot Hail Marys like DAC and SAI increases the dangers we face because they lock us into technological solutions. If these fail to deliver — and well-informed people have very little confidence that they will succeed — we face catastrophe.