Science can outline the dangerous consequences of various levels of warming for humans and other life on Earth, but these are possible destinations and not roadmaps.

The goals of the 2015 Paris Agreement are based on the implications of what 1.5°C and 2°C of warming would mean for humans and other species. They set the relatively ‘safe’ boundaries within which some irreversible but not catastrophic damage will occur. The IPCC reports and other research show that limiting the global average temperature rise this century to well below 2°C will mean we can avoid the most dangerous consequences of climate change. If we do, there is also hope of adapting and building a resilient and more sustainable future.

Yet there is no thermostat for the whole Earth into which we can easily dial these numbers. The temperature goals alone are not enough to guide climate change policy because the level of warming we achieve will depend on a complex range of decisions made over time by governments and businesses around the world. This national, regional and local decision-making will be effective in setting policy targets and creating incentives for change through governing greenhouse gas (GHG) emissions rather than temperature.

To translate temperature into emissions and make the Paris goals actionable, scientists use so-called carbon budgets. In some ways, they work just like a financial budget: there is a cap on total spend – the amount of greenhouse gases we can emit, and this ensures we stay within the ‘safe zone’ and avoid going into debt – to future generations in this case. But carbon budgets are also quite different. A look at your financial accounts and income streams is usually enough to judge personal spending limits, whereas in a carbon budget scientists must also calculate the total amount of greenhouse gas emissions compatible with various levels of warming.

With these calculations included, carbon budgets can show us several things: how we are actually doing (the historical budget), how much more time we can keep ‘spending’ at current levels (the remaining budget) and what a fair and equitable allocation would look like when sharing the total budget between countries.

How do carbon budgets work?

In your own budget, the final figure – how much money you have available without needing to worry about overspending – is only possible with details of your income and costs. Similarly, a carbon budget starts with scientists identifying the sources of carbon coming into the atmosphere and the sinks capturing carbon from it (such as forests or the ocean). Advances in climate and earth sciences mean it is possible to build a balance of the carbon cycle in nature and then add an additional source: emissions from human activity.

In a personal financial budget it can be important not to drop below zero to avoid being unable to pay for something or resorting to costly loans. In the climate system, the amount of carbon in the atmosphere – measured as a concentration in ‘parts per million’ (ppm), creates the ‘greenhouse effect’ and gives the level of global average temperature. It is this temperature level we are concerned about, since exceeding the Paris Agreement’s 2°C limit would bring consequences that countries have agreed are unacceptable.

To balance your own budget, you can cut some costs, try to earn more, or get a loan and dip into your future income to repay the loan with interest. To balance a carbon budget, we can also ‘cut costs’ by figuring out how to adapt to the consequences of global warming, but the options are quite limited – we can’t negotiate with the Greenland ice cap for a reduction in climate change-driven melting, for instance. ‘Earning more’ for a carbon budget means deep emissions reductions through things like renewable energy and energy efficiency and other measures. And ‘getting a loan’ means transferring responsibility to future generations that would have to act more aggressively – not just to reduce emissions but to bring them to negative levels by successfully capturing more greenhouse gases than are emitted.

Lastly, a financial budget may have income and spending in various currencies – as it is not possible to add and subtract these different currency units directly, we convert them to a common currency for comparison. Likewise, budgets are calculated for each of the greenhouse gases in the atmosphere and then made comparable by ‘converting’ them into units of carbon dioxide equivalent (CO2e) – carbon dioxide is used since it is the dominant greenhouse gas emitted from human activity.

Matjaz Krivic / Climate Visuals Countdown

So what’s our carbon budget?

The most well-known exercise in tracking our carbon budget comes from the Global Carbon Project – an international research project within the Future Earth research initiative on global sustainability, and a research partner of the World Climate Research Programme. In 2022, more than 100 researchers got together to build the 17th edition of the budget for CO2.

Firstly, the Global Carbon Project’s budget describes trends in global emissions of CO2 from energy and land use. For instance, in 2022 fossil carbon emissions continued to rise and reached 36.6 billion tonnes of CO2 – 1.0% above the year before. This rise was slightly more than the previous pre-pandemic peak in 2019 and is far from what is needed to reach the Paris Agreement goals. Instead, a rapid decrease at the scale of about 1.4 GtCO2 each year is required to reach net zero CO2 emissions by 2050.

The global carbon budget also tells us how much we are able to emit and still stay on track for 1.5°C, 1.7°C or 2°C of warming – that is, 380, 730 and 1,230 billion tonnes of CO2, respectively. To make these figures less abstract, scientists usually present them as years of ‘doing the same thing we were doing last year’. Based on 2022 emissions levels, we have nine more years until we breach the 1.5°C limit, and just 18 and 30 more years before we exceed the higher boundaries.

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