Wikipedia Deep Dive

Carbon budget

10 min read

Imagine you're given a credit card with a fixed limit—no option to increase it, no rollover to next month. Once you hit that limit, the card stops working. That's essentially what a carbon budget is for our planet, except the stakes are considerably higher than a declined transaction at the grocery store.

A carbon budget is the maximum amount of carbon dioxide we can collectively emit into the atmosphere before we trigger dangerous levels of warming. It's not a vague aspiration or a symbolic target. It's a hard number, grounded in physics, that tells us how much CO2 humanity can release before we cross temperature thresholds like 1.5 or 2 degrees Celsius above pre-industrial levels.

The concept emerged from a remarkable scientific discovery: the relationship between cumulative carbon emissions and global temperature rise is almost perfectly linear. It doesn't matter much when we emit that carbon or where it comes from. What matters is the total amount. Emit a trillion tons of CO2, and you get roughly the same temperature increase whether you spread it over fifty years or a hundred.

Two Ways to Think About the Budget

Scientists talk about carbon budgets in two ways. The total carbon budget counts everything from the start of the industrial revolution in 1750 to the present and into the future. It's the comprehensive accounting of our carbon spending across centuries.

The remaining carbon budget, on the other hand, is what we have left to spend starting from today. This is the number that tends to make headlines, because it tells us how much time we have before we blow past our climate goals at current emission rates.

As of January 2023, researchers calculated that we have about 250 gigatons of CO2 left in our budget if we want a fifty-fifty chance of staying below 1.5 degrees of warming. At 2022 emission levels—roughly 40 gigatons per year—that's about six years of business as usual before we've spent the entire budget.

Let's be clear about what this means. It doesn't mean we have six years before we need to act. It means that if emissions stayed exactly where they are now instead of continuing to rise, we'd exhaust our carbon budget by around 2029. The clock is ticking, and the alarm is set for a time that's uncomfortably close.

The Math Behind the Budget

Calculating a carbon budget requires synthesizing multiple lines of evidence. Scientists need to estimate how much warming humans have already caused, how much additional warming each ton of CO2 will produce (a measure called Transient Climate Response to Cumulative Emissions, or TCRE), and how much warming might continue even after we stop emitting entirely—something known as the Zero Emissions Commitment.

They also have to account for Earth system feedbacks that might amplify warming beyond what CO2 alone would cause. Think of permafrost thawing and releasing methane, or forests dying and releasing stored carbon, or ice sheets melting and exposing dark ocean water that absorbs more heat. These feedback loops can eat into the carbon budget faster than emissions alone would suggest.

The estimates vary depending on several key choices. What temperature target are we aiming for? What probability of success are we willing to accept? And critically, what happens with non-CO2 greenhouse gases like methane, nitrous oxide, and hydrofluorocarbons?

That last question turns out to matter enormously. Depending on how successful we are at reducing methane and other short-lived climate pollutants alongside CO2, the remaining carbon budget could be 220 gigatons higher or lower. If we aggressively cut methane emissions from oil and gas operations, agriculture, and waste, we buy ourselves more room in the CO2 budget. If we don't, that budget shrinks correspondingly.

From Global to National

A global carbon budget is useful for understanding the big picture, but climate policy happens at the national level. Countries need to know their individual emissions budgets to set meaningful reduction targets. This is where things get politically fraught.

How do you divide up a finite global carbon budget among nearly two hundred countries with vastly different histories, populations, wealth levels, and development needs? This isn't a purely technical question. It requires value judgments about fairness, responsibility, and equity.

One influential principle comes from the United Nations Framework Convention on Climate Change, or UNFCCC: "common but differentiated responsibilities and respective capabilities." The phrase is deliberately vague, but the core idea is that while climate change is everyone's problem, countries bear different levels of historical responsibility and have different capacities to act.

The United States and Europe industrialized early, burning coal and oil for more than a century to build wealthy economies. From 1750 to 2019, humanity emitted roughly 2,390 gigatons of CO2. A disproportionate share of that came from countries that are now rich. Under this principle, those countries should have smaller remaining emissions budgets because they've already used up so much of the total budget.

This argument has been central to climate negotiations since the 1990s. Developing countries point out that they didn't cause the problem but are often most vulnerable to its consequences. They argue for larger emissions budgets to pursue economic development and reduce poverty, while wealthy nations should cut emissions more aggressively and provide financial and technical support.

Three Principles for Sharing the Budget

Beyond historical responsibility, other equity principles have been proposed for allocating carbon budgets.

The egalitarian principle says every person on Earth has an equal right to emit carbon. Under this approach, you'd allocate budgets proportionally to population. China and India would get much larger budgets than, say, Switzerland or Denmark, simply because they have far more people. This principle appeals to countries with large or rapidly growing populations but raises an uncomfortable question: do individuals really have a "right to pollute"?

The sovereignty principle takes a different view, asserting that nations themselves have equal rights regardless of population size. One method that follows this logic is called grandfathering, which allocates budgets based on emissions at some baseline year. If your country emitted a lot in, say, 2010, you'd get a proportionally larger budget going forward.

This approach was used in early climate agreements like the Kyoto Protocol and the European Union's Emissions Trading Scheme. Unsurprisingly, it tends to favor developed countries that had high historical emissions. Recent scholarship has criticized grandfathering as fundamentally unfair, pointing out that it creates "cascading biases" against poorer states and essentially treats past pollution as conferring future rights to pollute more.

There's no perfect formula for dividing the carbon budget. Any allocation method embeds assumptions about fairness that different countries will contest based on their interests. What's clear is that the global budget is finite, the distribution matters enormously for climate justice, and the political negotiations over who gets to emit what are inseparable from the science.

Tracking What's Left

Several organizations produce annual updates to the remaining carbon budget, attempting to answer the question: how much time do we have left?

The Global Carbon Project is perhaps the best known. In April 2022, they published data showing that after COVID-19 lockdowns briefly reduced emissions in 2020, global CO2 emissions rebounded sharply by 4.8% in 2021, returning to 2019 levels. China and India surpassed their 2019 emission levels, while the United States and European Union remained below them.

As of January 1, 2022, the Global Carbon Project estimated that 420 gigatons of CO2 remained in the budget for a fifty-fifty chance of staying below 1.5 degrees of warming. That's about eleven years of 2021 emission levels.

But here's the critical point: that doesn't mean we have eleven years before we need to act. It means that if emissions stayed constant at 2021 levels instead of rising, we'd exhaust the budget in eleven years. Given that emissions have been trending upward, the actual timeline is shorter.

Other trackers show different numbers based on different methodologies and probability thresholds. The Mercator Research Institute on Global Commons and Climate Change, or MCC, displayed "7 years 1 month left" as of May 2022. If you wanted an 83% chance of staying below 1.5 degrees—a more conservative target—the budget dropped to about 6.6 years.

By October 2023, researchers updated the budget to account for new data and a better understanding of how reduced air pollution from particulates affects warming. Ironically, as we clean up smog and particulate pollution, we remove a slight cooling effect they had been providing, which means the remaining carbon budget is even smaller. Their updated estimate: 250 gigatons of CO2 from January 2023, or about six years at current emission rates, to have a fifty-fifty shot at 1.5 degrees. To hit that target, humanity would need to reach net-zero CO2 emissions by 2034.

For 2 degrees of warming—a less ambitious but still challenging goal—the budget is considerably larger: 1,220 gigatons, or roughly thirty years at current rates.

What Carbon Budgets Mean for Policy

Carbon budgets reframe climate policy in stark terms. Unlike annual emissions targets that can be delayed or revised, a cumulative budget forces a reckoning with total lifetime emissions. It makes clear that every year of delayed action shrinks the remaining budget and makes the required future cuts steeper.

This is different from the more common approach of setting annual emissions targets for a specific year, like reducing emissions 50% below 2005 levels by 2030. Those targets can coexist with a carbon budget, but they don't necessarily guarantee we'll stay within it. A country could hit its 2030 target and still overshoot its carbon budget if it doesn't sustain reductions afterward.

Carbon budgets also highlight an uncomfortable truth: there's no room left for slow, incremental change. If we want a good chance of limiting warming to 1.5 degrees, we need to cut global emissions in half by 2030 and reach net-zero by mid-century. Those aren't aspirational goals. They're what the physics demands.

The concept has proven powerful in climate negotiations because it's grounded in observable reality. You can argue about the fairness of different allocation schemes, but you can't argue with the fundamental constraint that cumulative emissions drive temperature rise.

The Credit Card with No Increase

Returning to that credit card analogy: we're holding a card with a spending limit that can't be raised, and we're charging it faster than ever. The remaining balance tells us we're much closer to the limit than most people realize.

The carbon budget concept doesn't tell us exactly how to allocate the remaining emissions among countries, what technologies to deploy, or how to balance climate action with economic development. Those are political and ethical questions that require negotiation and compromise.

What it does tell us is that the total amount matters more than the timing, that we've already spent most of our budget, and that the remaining balance is shrinking fast. Whether we stay within that budget—and how we choose to share it—will determine what kind of climate future we're building for the generations that follow.