Carbon budgets

What do we mean by carbon budgets?

•    We know that every tonne of CO2 emitted leads to more global warming – the more we emit the more we warm the planet. 
•    If we want to stop warming, we have to stop emitting – the sooner we stop, the lower the warming.
•    Advances in the science have helped us work out how much CO2 can still be emitted before we reach 1.5°C and 2°C of warming above pre-industrial (1850-1900) levels. This can be used to guide global decisions on how to tackle climate change.

The role of the carbon cycle

The amount of greenhouse gases in the atmosphere – especially CO2 – is increasing due to human activity. But they are also affected by natural processes such as plant growth or dissolving in ocean water. Processes which affect the amount of CO2 in the atmosphere are referred to as sinks if they remove it and sources if they emit more of it. Over history, carbon sinks have removed approximately half of the CO2 emitted by human activity – thus reducing how much climate change we experience. But we know this won’t necessarily continue forever. Understanding the natural sources and sinks of carbon therefore is essential to help us plan how to achieve desired climate goals.

Schematic of global climate cycleFigure 1. schematic of global climate cycle


This is why the IPCC WG1 report always includes a chapter dedicated to the carbon cycle. Met Office scientists and research have been central to this chapter over multiple Assessment Reports. This article describes this essential part of the IPCC AR6 around the behaviour of the carbon cycle and how it determines the remaining carbon budgets


How will carbon sinks behave in future?

The more CO2 we put into the atmosphere the more is removed by natural sinks. Over the last few decades land and ocean sinks have removed a fairly constant 56% of CO2 emissions from human activities. But we know that this will change if we increase or decrease our emissions. It is also affected by the climate – as the world warms, natural sinks are expected to weaken.

The IPCC AR6 carbon cycle chapter assesses how the carbon cycle has behaved in the past to absorb this constant fraction of our emissions, and also assess how it may change in the future under a range of different emissions scenarios.

Because the sinks may begin to saturate, and because higher levels of warming have an increasing effect on them, the sinks become less effective as we emit more CO2. In other words, more of our emissions are left in the atmosphere. This means that if we compare high emissions scenarios to low emissions ones, not only is more CO2 emitted, but a greater fraction of it is left in the atmosphere.
CO2 emissions - IPCC figure SPM7

Figure 2. IPCC figure SPM.7. Cumulative anthropogenic CO2 emissions taken up by land and ocean sinks by 2100 under the five illustrative scenarios.


IPCC figure SPM.7 (above) shows how this works – the bars show the total (“cumulative”) CO2 emitted from 1850-2100 under the range of future scenarios. The higher the bars, the greater the emissions. For high emissions scenarios, the land and ocean sinks (the coloured portion of each bar) are larger in magnitude, but a smaller proportion of the total. This means that the amount remaining in the atmosphere (grey portion of the bars) increases more quickly. The pie, or donut-charts below denote the same data but as a proportion of the total. These clearly show that under the lower emissions scenarios the land and ocean take up a greater fraction of our emissions, while as emissions get higher, this sink-fraction declines leaving more CO2 in the atmosphere.

Now that we can quantify the amount of our emitted CO2 which remains in the atmosphere, we can determine how much we need to limit our emissions to stay within the warming levels of the Paris Agreement. The amount of CO2 we can emit without breaching these levels is known as a “carbon budget”. We have emitted a lot of this budget already – the amount still left to be emitted is known as the “remaining carbon budget”.


Carbon budgets - how do we work them out?

The IPCC’s Special Report on 1.5°C  presented the five different components that we need to know in order to work out the remaining carbon budget associated with a warming level.

We need to know:
•    how much the world has been warmed by human activity so far;
•    how strongly future warming is related to emissions;
•    whether the world will carry on warming if we stop emitting;
•    what other greenhouse gases we emit; and
•    any further feedbacks or surprises which might affect the calculation – this last one is because we know that our knowledge is incomplete and things like thawing permafrost might further change the climate by releasing CO2 from frozen ground.

What is the remaining carbon budget for 1.5°C and 2°C climate goals?

We have good estimates of all of the above, but there is still an uncertainty range – i.e. we don’t know all of these to 100% accuracy – this means that we have to calculate a range of possible carbon budgets for given levels of confidence of success. The carbon budget for a 50% chance of meeting 2°C  for example is 1350 GtCO2 – but if we want to make sure we have a 67% or 83% chance of meeting the goal, we must restrict emissions to 1150 or 900 GtCO2.

The IPCC report puts all these aspects in a big table - below - to allow us to read off a carbon budget that meets our requirements.
Historical CO2 emissions - IPCC table

Table SPM.2. Estimates of historical CO2 emissions and remaining carbon budgets. Estimated remaining carbon budgets are calculated from the beginning of 2020 and extend until global net zero CO2 emissions are reached. They refer to CO2 emissions, while accounting for the global warming effect of non-CO2 emissions.  Global warming in this table refers to human-induced global surface temperature increase, which excludes the impact of natural variability on global temperatures in individual years.


Taking the 67% chance column – in IPCC language this gives us a “likely” chance of meeting our goals, then the remaining carbon budget for 1.5°C and 2°C  is 400 and 1150 GtCO2 respectively from the start of 2020. At current rates of about 40 GtCO2 per year, this would be exhausted in 10 and 29 years. 

What does net zero mean?

The link between CO2 emissions and warming means that if we want to stop warming we have to stop emitting. Simply reducing our emissions isn’t enough – if we keep emitting a small amount then the world will keep warming. So, we must cut our CO2 emissions to zero.

Some aspects are very hard to stop, but we might be able to invent ways to remove the CO2 from the atmosphere. This is called “negative emissions technology”. The important thing for climate is that the total emissions – or “net” emissions are zero. Hence it is referred to as “net zero” – meaning we might still emit some CO2 somewhere if we can remove it somehow elsewhere. Techniques to do this exist but have never been utilised on a large enough scale, so it is much safer to simply reduce our emissions as much as possible and as fast as possible.

If we phase out our emissions steadily to zero then we have to reach zero emissions by around 2040 or 2077 for a “likely” chance to stay within 1.5°C and 2°C respectively. The IPCC WG3 report due out in March 2022 will assess many scenarios targeted at 1.5°C and 2°C to determine how feasible these budgets are and the potential requirement for negative emissions. 


What about other greenhouse gases?

CO2 is not the only greenhouse gas (GHG) – although it is the main cause of human climate change. Other greenhouse gases – such as methane (CH4) and nitrous oxide (N2O) – also contribute to climate change. If we reduce emissions of these then this also helps reduce the total warming. The more we can reduce these other emissions, then the more CO2 can be emitted – or conversely the more we emit of other greenhouse gases, the more our CO2 emissions have to be cut. 

This is why the remaining carbon budget calculation has to allow for other gases (“non-CO2 GHGs”).

Reductions in non-CO2 gases, especially CH4 may have other benefits too such as improved air quality.

The IPCC report assessed different ways to measure the equivalence of other GHGs to CO2. This is required to allow CO2 removal to compensate for continued emission of other GHGs which may be hard to eliminate. Using the GWP100 metric, net-zero GHG emissions typically follows net-zero CO2 by a few years in most scenarios and may lead to a slow cooling of global climate.

But whatever GHG we consider, rapid action now pays dividends in both the near and long-term. Achieving our global climate goals remains challenging but possible. The IPCC report improves our knowledge of how we can achieve this.

The Met Office is committed to aligning its own activities to the clear scientific message that carbon dioxide emissions, along with other greenhouse gases, need to be reduced. Earlier this year we announced our target to become a net zero greenhouse gas emitter by 2030. You can find out more about our journey to net zero here.