You might have heard scientists and climate advocates use the term “carbon budget” when talking about climate change.

“The world is getting close to exceeding its carbon budget,” for example, or, “Countries have less than 10 years to fight climate change to stay within the carbon budget.” 

The United Nations’ blockbuster report on climate change from 2018, by the Intergovernmental Panel on Climate Change (IPCC), dedicates significant time to unpacking "the carbon budget." Meanwhile, youth climate activist Greta Thunberg regularly invokes the “carbon budget” when urging countries to stop emitting greenhouse gases into the atmosphere. 

While the concept of a carbon budget might sound as dry and straightforward as a financial planning statement that lays out how much a person can spend each month, it’s actually complicated and freighted with the complexities of geopolitics. 

The carbon budget essentially outlines a point of no return — a threshold beyond which climate change will get much worse, when the fruits of all efforts to save the planet will be squandered.

Understanding what it means is critical to confronting the climate crisis. 

What Is the Carbon Budget?

The carbon budget refers to how much carbon dioxide countries can release into the atmosphere before the world is guaranteed to warm at least 1.5 degrees Celsius compared to pre-industrial levels — the maximum increase set by the goals of the Paris climate agreement.

So, if countries release another 420 gigatons of carbon dioxide into the atmosphere, there’s a 67% chance we’ll keep temperatures below a 1.5-degree increase; and if we release 580 gigatons more carbon dioxide, there’s a 50% chance, according to the latest IPCC report.

In other words, even if we release fewer than 420 gigatons, there's a 33% chance of exceeding the 1.5 degrees Celsius limit. Even the most sophisticated scientific analyses of climate change have a broad range for error because there are many factors that are hard to pin down precisely, such as how much methane will be released in the Arctic as ice melts or how much more heat oceans can absorb.

Those measurements of gigatons — 420 and 580 in the examples above — make up what’s known as the carbon budget. 

The carbon budget rises to 1,670 gigatons of carbon dioxide if the world wants to have a 50% chance of keeping temperatures from rising more than 2 degrees Celsius — a considerably riskier proposition. 

In 2018, to give you an idea of scale, countries released an estimated 40.8 gigatons of carbon dioxide into the atmosphere from driving cars, flying planes, generating electricity from coal and natural gas, raising livestock, and all the other activities that release emissions. 

What Do We Need to Do to Stay Within Budget? 

The IPCC says that global carbon dioxide output has to quickly fall to around 20 to 30 gigatons a year, and then sharply drop toward zero gigatons a year to stay within the budget. 

But the world is headed in the opposite direction. 

Emissions are currently expected to reach 42.4 gigatons annually in 2020, rise to an estimated 49.4 gigatons per year in 2030, and keep on increasing. That means that the 1.5 degree carbon budget would most likely be all used up before 2030, on our current trajectory, setting the world up for catastrophic environmental consequences.

The only way to stay within the carbon budget is to reach “peak emissions” within the next few years and then reduce them as fast as possible to zero. 

Greenhouse gas emissions like carbon dioxide don’t have an immediate effect on the climate. They sit in the atmosphere for decades and even centuries, so the gases entering the atmosphere today will have a warming effect later on. 

That’s one of the most vexing challenges of climate change — the delay of consequences — and arguably why countries have been so slow to act.

What’s It Going to Take to Phase Out Fossil Fuels? 

Now that climate change is undeniably impacting countries —  from droughts in East Africa to unprecedented forest fires in the Arctic to rising sea levels throughout the Pacific Islands — a wake-up call is occurring.

But phasing out fossil fuels is an immense global undertaking, requiring enormous investments in renewable energy and an overhaul of infrastructure around the world. 

Some countries will have a much harder time ending their reliance on dirty energy sources than others. 

Over the past decade, China has skyrocketed to become far and away the biggest emitter of carbon dioxide in the world, accounting for more than a third of global emissions, according to the IPCC. India's emissions have also been rapidly rising. 

The United States and the European Union, meanwhile, each account for around an eighth of global emissions. Russia and Japan are the next biggest emitters. 

The fate of the 1.5 degrees Celsius carbon budget — those 420 gigatons — essentially falls to these countries. Curbing carbon dioxide levels depends on how rapidly they transition to renewable sources of energy and move beyond the fossil fuel industry. 

There have been major structural setbacks in recent years, however. 

The United States, for example, is producing more oil right now than at any point in its history. China and India still rely heavily on coal. Long-term fossil fuel projects are getting approved all around the world, even though the majority of remaining oil reserves have to stay within the ground. The global food system is causing mass deforestation and shifting toward more meat production. 

Then there are the feedback loops caused by climate change. 

As permafrost in the polar regions melt, methane — a far more potent greenhouse gas than carbon — is being released. Droughts and rising temperatures are making wildfires more likely, incinerating forests that store carbon. And as the world warms, humans are relying more on air conditioners that release other harmful emissions.

Amid these challenges, however, signs of progress are also emerging. 

Renewable energy sources generate electricity at a lower price than fossil fuels. Many countries are rapidly scaling their wind and solar power sectors. Multinational companies have vowed to reduce emissions throughout their supply chains and champion the Paris climate agreement. Investments have been made in projects to remove carbon from the atmosphere. And mass movements of climate activists are forcing world leaders to act. Climate emergencies have subsequently been declared by hundreds of governments. 

The activist Greta Thunberg, in particular, has galvanized people around the world, rapidly becoming one of the foremost climate activists.

She recently spoke to the French parliament and chastised those assembled for failing to adequately address climate change. She highlighted the inexplicable invisibility of the carbon budget in discussions of climate change. 

“At current emissions levels, that remaining budget is gone within roughly eight-and-a-half years,” she said. “These numbers are as real as it gets, though a great number of scientists say they are too generous, these are the ones that have been accepted by all nations through the IPCC, and not once, not one single time, have I heard a journalist, politician, or business leader even mention these numbers.”

“It is almost like you don’t know they exist,” she said. “As if you haven’t even read the latest IPCC report on which the future of our civilization depends.”