Editor’s note: From “climate adaptation” to “ecosystem services,” environmental jargon is everywhere these days. Conservation International’s blog looks to make sense of it in an occasional explainer series we’re calling “What on Earth?”

In this installment, we explore the “carbon budget” — what it is, why it’s important, and how a new study about it is making waves.

What is the ‘carbon budget’?

It’s generally defined as the maximum amount of carbon that humans could emit into the atmosphere and still keep global average temperatures below an increase of 2 degrees Celsius.

What does that mean, in layman’s terms?

In other words: If we keep burning fossil fuels and cutting down forests, at some point we will have put enough carbon dioxide into the atmosphere to assure that global warming will reach a catastrophic point of no return.

Renowned climate policy expert — and Conservation International Distinguished Fellow — Christiana Figueres likens the carbon budget to a bathtub that’s already 60 percent full. If you keep adding water, at some point in the near future the tub will overflow.

Got it. What’s so important about 2 degrees?

A global average temperature of more than 2 degrees Celsius above pre-industrial temperature averages is widely considered to be, well, bad. After that point, scientists say, the effects of climate change will become extreme, destructive and likely irreversible. Many policy experts, in fact, are calling to restrict average temperature rise to a more conservative 1.5 degrees Celsius. 

So if the world agrees to reduce its emissions by X percent, will that keep us on budget?

Partly. Not only do we have to eliminate nearly all carbon emissions, we have to figure out how to actually remove carbon from the atmosphere on a large scale.

And emissions are still rising; in fact, two recent studies found that we’ll almost certainly top 2 degrees by 2100, barring drastic and immediate changes. And much of our carbon budget, sadly, has already been spent.

How much?

As in the bathtub metaphor, we’re about 60 percent spent: The Global Carbon Project and others estimate that about 2 trillion metric tons of carbon dioxide have been emitted into the atmosphere since the onset of the Industrial Era (around 1870), with about 1.2 trillion metric tons left to go before we hit the 2-degree threshold.

Figuring out where to make cuts in the budget — and who should make them — is not easy and sometimes runs into controversy.

What controversy?

Let’s bring in an expert to explain.

“The reason that the carbon budget is controversial for developing countries is because most of the already-spent budget is due to past emissions by countries that used up their budgets to become fully developed — think the United States and Europe,” says Shyla Raghav, a climate change expert at Conservation International. “Now, the implication is that developing countries will have to slash their carbon budgets, which raises arguments about justice and equity.”

In other words: Why should still-developing countries cut their emissions, when already-developed countries became developed in no small part because of the emissions they produced since the Industrial Revolution? Discussions about this continue, with debates about if and how the remaining budget should be allocated across time and countries, Raghav says. This is — partly — why a new study about the size of the carbon budget is significant.

OK, I’ll ask: What study?

New research out this week suggests that the world’s carbon budget is much higher than what we previously thought. The study’s authors write that we could — theoretically — keep global warming below 1.5 degrees C even if emissions continue at their current rates for another 20 years.

So does this change the entire calculus for fighting global warming?

Not entirely. Let’s bring in an expert to explain.

“This is the most radical departure from general understanding of our carbon budget in a little while,” and as such has drawn skepticism and scrutiny from experts, says Will Turner, senior vice president of global strategies at Conservation International. “But let’s assume it’s right and see where that takes us. If it’s right, great — we need all the help we can get.”

But even if it is right, Turner says, its findings offer no panacea: As the authors note, their results mean that keeping warming to 1.5 degrees C is no longer “impossible” but can now be considered “not yet [an] impossibility” and will likely continue to require steep and difficult emissions cuts by 2030.

What does ‘steep and difficult emissions cuts’ look like in light of this study?

“It’s still much more than we are doing now, much more than governments and others are even pledging to do now, plus decades of mitigation eliminating emissions and removal of carbon dioxide that’s already in the atmosphere,” Turner says.

Oh. How are we going to do that?

In addition to a shift to renewable energy, we have to look to forests, Turner says.

“Stopping deforestation is still one of the biggest and most essential ways to eliminate emissions, and reforestation is still the only technology we have for removing carbon dioxide from the atmosphere at scale,” he says. Protecting tropical forests, Turner notes, can provide at least 30 percent of all global mitigation action needed to limit global warming to 1.5 degrees Celsius.

Whatever climate scenario awaits, we’re going to need forests and ecosystems as part of the solution.

Bruno Vander Velde is CI’s editorial director. 

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Further reading

• What on Earth is a ‘carbon offset’?
• Calculate your carbon footprint and reduce your impact
• What on Earth is ‘natural capital’?
• What on Earth is ‘sustainable’ coffee?