Carbon Budget

A carbon budget is the total amount of carbon dioxide (CO2) that all humans together can still release into the air if we want to keep global warming below a chosen limit, such as 1.5°C or 2°C. Think of it like a fixed family budget for a month: once the money is spent, it is gone.

Every tonne of CO2 that any country releases is spent from the same shared budget. The idea of carbon debt follows from this: some countries have already spent far more than their fair share of the budget, so they are said to "owe" the others.

Why does warming depend on the total, not the yearly amount?

CO2 is a very long-lived gas. Once it goes into the air, a large part of it stays there for hundreds to thousands of years. So the CO2 released by a British coal factory in 1880 is still warming the planet today. This means global temperature depends mainly on the total CO2 added up over time (called cumulative emissions), not on how much any country released in one year. This is why history matters so much in climate talks.

What did the scientists find?

The Intergovernmental Panel on Climate Change (IPCC) found, with high confidence, a near-straight-line relationship between cumulative CO2 emissions and warming. In its Sixth Assessment Report (AR6, 2021), it said that every 1,000 gigatonnes of CO2 released causes about 0.45°C of warming (likely range 0.27°C to 0.63°C). This number is called the TCRE (Transient Climate Response to Cumulative CO2 Emissions).

Because the link is so direct, scientists can work backwards: if we want to stop at 1.5°C, they can tell us roughly how much more CO2 the world can release. That remaining amount is the remaining carbon budget.

Where did the idea come from?

Scientists had studied the link between cumulative emissions and warming for years. The IPCC's Fifth Assessment Report (AR5, 2013–14) made the carbon budget a central idea in climate policy. The IPCC's Special Report on 1.5°C (2018) and AR6 (2021) updated the numbers. Since then, the Global Carbon Project, a group of scientists who publish a yearly "Global Carbon Budget" report, keeps track of how fast the remaining budget is being used up.

How big is the budget, and how much is left?

• From 1850 to 2019, humans released about 2,390 Gt of CO2 in total (IPCC AR6). This is roughly four-fifths of the total budget for a 50% chance of staying within 1.5°C.
• At the start of 2020, the IPCC estimated the remaining budget at about 500 Gt CO2 for a 50% chance of staying within 1.5°C, and about 1,150 Gt CO2 for a 67% chance of staying within 2°C.
• The world now releases about 42 Gt of CO2 every year (about 42.4 Gt in 2024, as per the Global Carbon Budget 2025).
• As of the Global Carbon Budget 2025 report, only about 170 Gt CO2 was left from the start of 2026 for a 50% chance of 1.5°C. At current rates, that is used up in about 4 years. For 2°C, about 1,055 Gt CO2 (around 25 years) was left.

So the 1.5°C budget is almost finished. This is why the question "who used the budget so far?" has become so sharp.

Who used the budget? The idea of carbon debt

Different studies use different years and methods, but they all show the same broad picture. One widely cited analysis of CO2 from 1850 to 2021 found that the United States alone released about 20% of all historical CO2, followed by China (about 11%), Russia (about 7%), Brazil (about 5%) and Indonesia (about 4%). India's share of historical emissions is only about 3–4%, even though India has around 17% of the world's population.

The IPCC (AR6) also noted that South Asia as a whole contributed only about 4% of historical cumulative CO2 emissions while having about 24% of the world's population.

From this, many developing countries make a simple argument. Rich countries became rich by burning coal, oil and gas for over 150 years. In doing so, they used up most of the shared carbon budget. Poorer countries, which still need energy to remove poverty, are now told to stop using the same fuels. They say this is unfair. The unpaid share of "carbon space" that rich countries took is the carbon debt (also called climate debt).

Where did "climate debt" enter the talks?

The idea was pushed strongly by Bolivia in 2009, ahead of the Copenhagen climate summit (COP15). Bolivia, along with Cuba, Dominica, Honduras, Nicaragua and Venezuela, argued that developed countries owe two kinds of debt:

• Emissions debt: for using more than their fair share of the atmosphere.
• Adaptation debt: for the harm that climate change now causes poorer countries, which must spend money to protect themselves.

They asked for repayment through money and the transfer of clean technology. The word "debt" was never accepted into any formal UN climate agreement. But the same idea lives on through the principle of Common but Differentiated Responsibilities and through demands for climate finance and for a "fair share" of the carbon budget.

Total emissions vs per person emissions

Another way to look at fairness is per capita (per person) emissions. India's per capita CO2 emissions are about 2.2 tonnes a year [figure varies by source and year], less than half the world average of about 4.7 tonnes. The per capita figure for the US is about 14 tonnes. India often asks that the debate move from "which country emits most this year" to "how much each person has emitted, today and over history".

India's position and Indian examples

• India argues that developed countries have used more than their fair share of the carbon budget and must reach net zero (no net addition of CO2) well before 2050, so that developing countries get "carbon space" to grow.
• At the same time, India has taken its own targets. In its updated NDC (August 2022), India committed to reduce the emissions intensity of its GDP by 45% by 2030 from the 2005 level, and to get about 50% of its installed electricity capacity from non-fossil sources by 2030. India's long-term goal is net zero by 2070, announced at COP26 in Glasgow (2021) as part of the Panchamrit (five promises).
• In April 2026, India submitted its new NDC for 2031–2035 (NDC 3.0). It aims to cut the emissions intensity of GDP by 47% by 2035 from 2005 levels, reach 60% non-fossil installed power capacity by 2035, and create an additional carbon sink of 3.5–4 billion tonnes of CO2 equivalent through forest and tree cover.
• India still depends heavily on coal for electricity. Its official position is that fossil fuels will remain part of its energy mix during a gradual, fair transition.

Commonly confused concepts

• Carbon budget vs carbon credit: A carbon budget is the whole world's remaining limit of CO2. A carbon credit is a certificate that one tonne of CO2 was cut or removed, which can be bought and sold in a carbon market.
• Cumulative emissions vs annual emissions: Annual emissions are what a country releases in one year (China is the largest annual emitter today). Cumulative emissions are the total over many years (the US is the largest cumulative emitter).
• Carbon debt (climate debt) vs "carbon debt" in forestry: In climate diplomacy, carbon debt means rich countries' excess past emissions. In forest science, the same words describe the extra CO2 released when a forest is cut for bioenergy, which takes years of regrowth to "repay". UPSC uses the first meaning in IR questions.
• Emissions intensity vs absolute emissions: Emissions intensity is CO2 released per unit of GDP (how "dirty" each rupee of output is). India's NDC targets intensity, so total emissions can still rise as the economy grows. Absolute emissions are the total tonnes released.
• Net zero vs zero emissions: Net zero means any CO2 still released is balanced by CO2 removed (by forests or machines). Zero emissions means nothing is released at all.

Issues, criticism and the way forward

• Is it fair? Developing countries say that asking everyone to cut at the same pace ignores history. Developed countries reply that today's big annual emitters, including China and India, must also cut, because the physics of warming does not care who emits.
• Which years count? Should history start in 1750, 1850 or 1990 (when the science of climate change became widely known)? Each choice changes the shares, so the numbers are often debated.
• Should carbon debt allow more fossil fuel use? Some experts warn that the poorest people in developing countries suffer most from heat, floods and droughts. So using carbon debt to justify unlimited coal use could hurt them too. They argue the "repayment" should come as money and technology, not as permission to pollute more.
• Money still falls short. At COP29 (Baku, 2024), developed countries agreed to mobilise $300 billion a year by 2035 under the New Collective Quantified Goal (NCQG). Developing countries said this is far below the trillions needed.
• Way forward: Experts suggest early net zero by rich countries, much larger climate finance and adaptation funding, cheaper access to clean technology, and fair "just transition" plans for coal-dependent regions and workers in countries like India.

Concepts to Know

• Gigatonne (Gt): One billion tonnes. 1 Gt of CO2 is about the weight of 1 billion small cars put together.
• IPCC (Intergovernmental Panel on Climate Change): A UN scientific body set up in 1988 by the World Meteorological Organization (WMO) and UNEP. It does not do its own experiments; it reviews all published climate science and writes big "Assessment Reports" that governments use.
• NDC (Nationally Determined Contribution): Each country's own climate action plan under the Paris Agreement, updated every five years.
• 1.5°C and 2°C limits: The Paris Agreement (2015) aims to keep warming "well below 2°C" above pre-industrial levels (1850–1900), and to try to limit it to 1.5°C.
• Carbon sink: Anything that absorbs more CO2 than it releases, such as forests, soil and oceans.
• Just transition: Moving from fossil fuels to clean energy in a way that protects workers, communities and poor people who depend on the old fuels for jobs and cheap energy.