To achieve a net-zero emissions target, where greenhouse gas emissions and removals are balanced, governments and businesses must do two things: reduce emissions as much as possible and remove all remaining pollutants from the atmosphere.

To estimate how quickly emissions need to be reduced, analysts often turn to the so-called carbon budget, a well-established framework for calculating the amount of greenhouse gases that can be emitted to meet or stay below a certain level of global warming.

(World leaders have committed to limiting the increase in global average temperature to well below 2 degrees Celsius above pre-industrial levels, and ideally to 1.5 degrees Celsius.)

However, quantifying the amount of carbon dioxide removal available to achieve these temperature targets is an analytical area that is less frequently addressed.

To address this problem, researchers at the University of Oxford have now developed a “CO2 removal budget.”

Think of carbon removal as the “net” in net zero.

CO2 can be removed in many ways, including through nature-based approaches such as restoring forests or peatlands.

In addition, there are technologies such as direct air capture and storage, so-called biochar and bioenergy, and carbon capture and storage (BECCS).

But much of this technology has not yet reached its full potential – and time is running out.

The planet is warming so quickly that without rapid and drastic action, the critical 1.5 degree threshold will almost certainly be exceeded.

It has therefore become essential to scale up technologies and projects that can remove billions of tons of climate-damaging gases from the atmosphere and thus reduce global temperatures.

Oxford academics Ben Caldecott and Injy Johnstone, who authored the paper, argue that while growth of this industry is essential if we are to have any chance of achieving a net zero target, carbon removal will always be “a fundamentally finite resource” that must be allocated responsibly.

“Carbon removal is not free and has significant economic constraints,” Caldecott said in an interview. “So if a company that could easily reduce emissions instead chooses to use part of its available budget for carbon removal, what does that mean for other actors? This finite resource must be distributed fairly, and there will undoubtedly be trade-offs.”

In other words, those industries that find it easier to reduce their emissions should maximize those efforts, so that more of the world’s limited capacity to remove carbon from the atmosphere is left to those that naturally have limited opportunities to do so.

Companies in sectors where emissions reductions are “hard” to achieve, such as steel producers and airlines, may have a greater need for such measures under certain circumstances, the authors say.

The capacity for CO2 removal and storage is generally limited by physical factors as well as a number of economic, institutional and technological barriers.

Caldecott and Johnstone said the question remains important: “Who has the right to access CO2 removal methods that are already feasible and who has the responsibility to develop further potential.”

So how much carbon do we need to remove from the atmosphere and at what cost to meet our climate goals?

And what is possible given the current state of CO2 removal technology?

Assuming a minimum price of $100 per tonne of CO2 removal and taking into account what is actually feasible from a technical and economic perspective (not to mention political variables), the authors estimate that in a scenario in which global warming is limited to about 1.5 degrees Celsius, there will be a global CO2 removal deficit of 49 gigatons of CO2 between 2025 and 2100.

A deficit means that the demand for carbon removal will exceed the projected use of such technologies.

If humanity were to accept a warming of two degrees Celsius – which would have catastrophic consequences from a climate change perspective – there would be potential for a small surplus of 12 gigatons of CO2 in terms of the carbon removal budget.

Nature does not have an unlimited capacity to store carbon.

And all natural carbon sinks could eventually release some of the stored CO2 back into the atmosphere.

Novel carbon removal techniques, such as direct air capture (DCA), in which machines suck CO2 out of the atmosphere and bury it deep underground, are less likely to reverse the trend.

However, even the most complex technologies can cost up to $1,610 per ton.

And it can take a long time to build the necessary infrastructure.

“CO2 removal is like the new gold,” Johnstone said. “It is extremely valuable, scarce and should be reserved for only a select number of use cases.”

Robert Höglund, an expert in CO2 removal, takes a different view.

He says that the main constraint on the number of deportations is “not the finite resources, but the willingness to pay.”

However, he agrees with Caldecott and Johnstone that while removals need to be done on a larger scale, the top priority of companies and governments seeking to eliminate carbon emissions should be to reduce emissions.

“We should plan as if CO2 removal is scarce, but build so that it is in abundance,” he said.

“There are large uncertainties about our ability to remove CO2 and the focus should be on preserving the initial carbon budget,” Johnstone said. “Any discussion about carbon removal must start with the fact that it is much easier to reduce emissions than to pull them back out of the atmosphere.”