The world is at a critical juncture when it comes to the fight against climate change. With global temperatures already rising and the devastating effects of climate change becoming more and more apparent, reducing greenhouse gas emissions has become a top priority.

One key tool in this fight is the concept of the global carbon budget, a framework that sets a limit on the amount of carbon that can be emitted while still keeping global warming below a certain threshold. But what exactly is the global carbon budget, and how does it work?

The budget for remaining carbon emissions

In their latest assessment report, the Intergovernmental Panel on Climate Change reports on the near-linear relationship between CO2 in the atmosphere and global warming. For every 1,000 gigatons (Gt) of CO2 emitted, the planet warms by about +0.5˚C.

Between 1850 and 2019, the IPCC estimates that humans have emitted about 2,390 Gt of CO2 in the atmosphere.  To not exceed 1.5˚C of warming, the overall cumulative CO2 “budget” is about 2,900 Gt of CO2 – so the IPCC says that as we transition our economies to clean energy, we can only emit about another 500 Gt of CO2.

Of course, CO2 is not the only greenhouse gas, but for simplicity, the budget concept focuses on CO2 as the largest contributor to warming.

It is also important to recognize that because climate systems are complex when climate scientists speak of future forecasts, they have to speak in terms of probabilities. There is no formula that guarantees that staying below 2,900 GtCO2 will limit warming to +1.5˚C. In fact, the IPCC believes that this is the level of CO2 emission that gives us a 50/50 chance of staying below +1.5˚C of warming.

If we wanted to a 67% chance, we’d need to limit remaining additional emissions to less than 400 GT of CO2, to have an 83% chance we’d need to limit to 300 Gt.

This notion of a probability range is an additional reason to act urgently that many people may not fully appreciate.  Of course, the probabilities may end up working in our favor – there’s a chance we exceed “the budget” and still stay below 1.5˚C – but given that this is literally a bet on the planet – I’d feel more comfortable stacking the odds in our favor by cutting emissions as much as possible as quickly as possible.

Is there really a budget?

In short, no, not really. As part of the 2015 Paris Agreement, 196 parties entered into a binding agreement to limit global warming to well below 2˚ C above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5˚C.

To achieve this goal, countries that signed the Paris Agreement committed to reducing their greenhouse gas emissions, including CO2 emissions, in order to reach net zero – a balance between emissions and removals – by 2050. The specific reduction targets set by each country are called Nationally Determined Contributions (NDCs), and they are updated every five years to ensure that countries are on track to meet the collective global goal.

At the end of the day, we don’t yet know if all the combined NDCs will keep us under 2,900 GtCo2.

Ok, but, how is it going?

So far, global progress on reducing emissions has not been great – but there are some bright spots.

The most recent assessment of global carbon emissions comes from the Global Carbon Project (GCP) – a collaborative scientific project aimed at providing reliable, peer-reviewed, information on the global carbon budget.

Last November, the GCP announced that total global CO2 emissions would reach 40.6 GtCO2 –  a record level with no sign of decrease.  This was driven primarily by fossil fuel emissions of CO2 which grew by 1.0% compared to 2021 to reach 37.5 GtCO2.  Emissions from land-use change (such as deforestation) are projected to be 3.9 GtCO2 in 2022.

At the current rate, the GCP warned that the remaining carbon budget would be exhausted within about 9 years, leading to a 50% chance of greater than 1.5˚C warming.  The GCP highlighted that to reach zero CO2 emissions by 2050, a decrease of about 1.4 GtCO2 each year would now be required. For perspective, that is comparable to the fall in 2020 emissions observed during the COVID-19 lockdowns.

The bit of bright news that the GCP reported was that the long-term growth rate of fossil CO2 emissions had slowed.  The average rise peaked at +3% per year during the 2000s, while growth in the last decade has been about +0.5% per year.  Growing more slowly is nowhere near enough, but it is going in the right direction. 

There were other bright spots in the regional data. China’s CO2 emissions were forecasted to fall, the US continued to be on a longer-term downward trend and the EU collectively also continued to cut emissions.

While there are some positive signs, leaders will need to take meaningful action if we are to have a chance to limit warming close to 1.5˚C.   Our window of opportunity to secure a livable future is closing – let’s work to compel our governments to act immediately and effectively before it does.