Carbon Budget

Climate scientists have shown that there is a direct relationship between the total amount of carbon dioxide (CO2) humans have emitted since the industrial revolution — when we started to burn large amounts of fossil fuels — and higher global temperatures. This rise in temperature disrupts natural weather patterns and causes natural disasters − storms, heat waves, floods and droughts − to be more intense, leading to increased loss of life. Higher temperatures also cause ocean acidification, which threatens marine biodiversity, and rising sea levels, which puts humans and ecosystems along the coastline at risk. 

These consequences of climate change can be mitigated by reducing the  – and eventually stopping – the rise in global temperatures, which is dependent on the total CO2 we emit, not the rate at which we emit it. This is very different from many other pollutants and has a profound implication: if we want to limit global temperature rise to 1.5°C above pre-industrial levels — or any other reasonable level — we will eventually need to stop emitting CO2 from fossil fuel combustion and industrial processes. The amount of CO2 that we can emit is our carbon budget.

Why is the carbon budget important? 

The carbon budget is a helpful tool to measure the progress countries and the world are making towards mitigating climate change. By representing the cumulative carbon emissions from human activity, it provides a metric that has a direct relationship to the climate impact of the progress made. In that way, it offers a transparent and accountable method of tracking.  

Like a financial budget, the carbon budget sets a limit on how much can be spent to reach a goal. The global carbon budget is a single envelope of emissions that gets thinner each year we continue to release CO2.

How is the carbon budget calculated?

Calculating a carbon budget is a complex process that involves modeling the relationship between carbon emissions and global average temperature increase. These models are based on the relationships between the atmosphere, oceans, land and living things, and how they interact and affect the climate. In addition, calculating the carbon budget also needs to account for the effects of other, non-CO2 greenhouse gases, such as methane, and the uncertainty in observational data and historical emissions.

Because there is uncertainty in both the data and the complex behaviour of Earth’s climate, statements of the budget include probabilities that tell us the specific chance of staying within a temperature limit for the stated budget.

As long as we continue to emit CO2, the remaining carbon budget will shrink each year. According to estimates, the remaining carbon budget at the start of 2025 is 235 gigatonnes of CO2 for a 50 per cent chance to keep global warming within 1.5°C of pre-industrial levels. Based on 2024 emission levels, that budget would be depleted in six years.

In addition, as knowledge of climate science improves, we can expect the carbon budget to change. For example, a 2023 study that uses more recent models to account for the climate impact of non-CO2 greenhouse gases suggests that the remaining 1.5°C carbon budget may be significantly smaller than estimated in the IPCC report Climate Change 2021: The Physical Science Basis. This only further underscores the urgency of climate action.