The carbon budget explained: how much CO2 can we emit and still save the climate?

If we want to limit the rise of average global temperatures to 1.5 degrees, we can emit only a limited amount of CO2. This is the carbon budget.

We know that greenhouse gas emissions caused by human activities (from energy to transportation, industry to farming) are speeding up global warming at a rate that threatens our planet’s natural balance as we have known it to be. But when, exactly, will we cross the line? The European Union and Joe Biden’s US have promised to reach carbon neutrality by 2050, and China by 2060. But, while we wait for this historic transition to take place, how much CO2 can we still emit into the atmosphere without unalterably compromising our future climate? To answer all these questions, experts have devised an ad hoc indicator, known as the carbon budget.

What does carbon budget mean?

The carbon budget is the amount of CO2 that humanity can emit while still having a chance to contain global warming within 1.5 degrees centigrade compared with preindustrial levels, as advocated by the Paris Agreement. Fridays for Future uses a simple metaphor to explain the concept: if our atmosphere is a bathtub and we want to avoid it filling up completely and flooding the house, we need to turn off the tap.

In reality, our planet can count on certain natural carbon sinks forests and oceans – but even these can no longer keep up with the rate at which we’re pouring greenhouse gases into the atmosphere. Specifically, according to data from the Global Carbon Project, in the decade between 2010 and 2019, terrestrial ecosystems absorbed 12.5 gigatonnes of CO2 per year, and oceans absorbed 9.2 gigatonnes. In 2020, these two immense sinks compensated for 54 per cent of CO2 emissions, but it must be said that it was a unique year. According to the most reputable estimates, containment measures for the coronavirus pandemic caused global emissions to fall by 7 per cent.

forests
By protecting forests we can reduce atmospheric CO2 while preserving biodiversity © Marc Pell/Unsplash

How much more CO2 can we emit?

Scientists have made different calculations to work out how much of the carbon budget humanity still has at its disposal. A think tank called MCC, using data from the Intergovernmental Panel on Climate Change (IPCC), stated that at the end of 2017 we had a budget of no more than 420 gigatonnes of CO2 if we want to stay within 1.5 degrees of warming, that is 42 gigatonnes per year (or 1.332 tonnes per second).

The problem, however, is that in 2019, before the paralysis caused by coronavirus, global emissions reached 52.4 gigatonnes. This was revealed by the United Nations’ Emissions Gap Report. In fact, if we also account for emissions related to changes in land use (deforestation above all), the figure rises to 59.1. At this rate, we will have burned through our remaining carbon budget by the end of 2027 for the 1.5-degree scenario, and by 2045 for the 2-degree scenario. The MCC published a countdown to these dates: the Carbon Clock.

The well-known SR15 Report drafted by the IPCC is more optimistic, claiming that, starting in 2020, we can still emit 495 gigatonnes of CO2. Doing so, however, only gives us a 50 per cent chance of hitting the 1.5-degree goal. On 23rd September 2019, at the Climate Action Summit in New York, Greta Thunberg said it best: “Maybe 50 per cent is acceptable to you. But those numbers don’t include tipping points, most feedback loops, additional warming hidden by toxic air pollution or the aspects of justice and equity. They also rely on my and my children’s generation sucking hundreds of billions of tonnes of your CO2 out of the air with technologies that barely exist. So a 50 per cent risk is simply not acceptable to us – we who have to live with the consequences”.

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