Paris Agreement Temperature Goal and the 1.5°C Threshold

The Paris Agreement is the world's main climate treaty, adopted in 2015. Its central aim is a temperature goal: keep the rise in the Earth's average temperature well below 2°C above pre-industrial levels, and pursue efforts to limit it to 1.5°C. These two numbers are the measuring stick for every climate decision in the world. When news says "the 1.5°C goal is slipping away", it is talking about this goal.

Why does it exist?

Burning coal, oil and gas releases CO2, which traps heat. More heat means stronger heatwaves, floods, droughts, melting ice and rising seas. Every extra bit of warming makes these worse. Countries needed one clear, shared number to plan around. Without it, every country would decide for itself what "safe" means, and nobody could check progress.

What exactly does the treaty say?

The goal is in Article 2.1(a) of the Paris Agreement. It says the aim is "holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C above pre-industrial levels". Article 2.2 adds that the treaty will be carried out according to equity and common but differentiated responsibilities and respective capabilities, in the light of different national circumstances.

What does "pre-industrial" mean?

It means the time before factories and machines began burning large amounts of coal. Scientists use the average temperature of 1850 to 1900 as this starting line, because that is when reliable thermometer records begin worldwide.

Where did the numbers come from?

The goal grew step by step.

• In the 1970s, economist William Nordhaus used about 2°C as a rough limit in his studies (1975 and 1977). He reasoned that more warming would push the climate outside the range seen in several hundred thousand years.
• The European Union adopted 2°C as a policy goal in 1996.
• The Copenhagen Accord (2009) recognised the 2°C limit, and the Cancun Agreements (2010) formally adopted it under the UN. Cancun also set up a review (2013 to 2015) to consider whether the limit should be tightened to 1.5°C.
• Small island countries, grouped as AOSIS (Alliance of Small Island States), and the poorest countries campaigned with the slogan "1.5 to stay alive". For low-lying islands, 2°C could mean their land going under the sea.
• The UN review found that 2°C should be seen as a "defence line", not a safe "guardrail".
• At COP21 in Paris (December 2015), countries agreed on "well below 2°C" plus "pursuing efforts" for 1.5°C. They also invited the IPCC to write a special report on 1.5°C.
• The IPCC's Special Report on Global Warming of 1.5°C (SR1.5) was approved in Incheon, South Korea, in October 2018.
• At COP26 in Glasgow (2021), countries "resolved to pursue efforts" to limit warming to 1.5°C, making 1.5°C the main working target in practice.

Why does half a degree matter so much?

SR1.5 compared a 1.5°C world with a 2°C world.

• Coral reefs: about 70 to 90% would decline at 1.5°C; more than 99% would be lost at 2°C.
• Sea level: by 2100, sea level would be about 10 cm lower at 1.5°C than at 2°C. This means about 10 million fewer people exposed to risks from rising seas.
• Arctic sea ice: an ice-free Arctic summer would happen about once a century at 1.5°C, but about once a decade at 2°C.
• Species: at 1.5°C, about 6% of insects, 8% of plants and 4% of vertebrates would lose more than half their range; at 2°C these become 18%, 16% and 8%.

How is the goal supposed to be met?

The treaty does not set targets for each country. Instead it works like this:

1. Each country submits an NDC (its own plan) every five years.
2. Each new NDC must be stronger than the last one. This is called the ratchet (a tool that only turns one way).
3. Every five years, a Global Stocktake checks whether all the NDCs together are enough.
4. Article 4.1 adds a long-term aim: emissions should peak as soon as possible and reach a balance between emissions and removals (net zero) in the second half of this century.

The Paris Agreement entered into force on 4 November 2016, after at least 55 countries responsible for at least 55% of world emissions had joined (Article 21). India ratified it on 2 October 2016, Gandhi Jayanti. The United States left the agreement twice; its second exit took effect on 27 January 2026.

How is it measured?

A single hot year above 1.5°C does not mean the goal has been broken. The goal refers to long-term warming, averaged over many years. Scientists, including the IPCC, usually use a 20-year average. So when the World Meteorological Organization (WMO) confirmed that 2024 was about 1.55°C (±0.13°C) above pre-industrial levels, it said this was the first calendar year above 1.5°C, but not yet a breach of the Paris goal. Long-term, human-caused warming is close to 1.4°C [as of 2025].

The carbon budget link

Warming depends on the total CO2 humans have ever released. So scientists calculate a "carbon budget": how much CO2 the world can still release for a given limit. A 2025 scientific update estimated the remaining budget for a 50% chance of 1.5°C at about 130 billion tonnes of CO2 from the start of 2025. At today's emission rates, this would be used up in a little over three years.

India's position and Indian examples

India supports the Paris goal but stresses fairness. It argues that developed countries used most of the world's carbon budget and must reach net zero much earlier, so that developing countries have room to grow. India's targets include a 45% emissions-intensity cut by 2030 and 47% by 2035 (from 2005 levels), and net zero by 2070. India is also highly exposed to warming: heatwaves, glacier melt in the Himalayas, erratic monsoons and sea-level rise along a 7,500 km coastline.

Commonly confused concepts

• One year above 1.5°C vs breaching the Paris goal: One hot year (like 2024) is weather plus climate. The Paris goal is about a long-term average of about 20 years.
• "Well below 2°C" vs "1.5°C": "Well below 2°C" is the firm commitment. "1.5°C" is what countries agreed to "pursue efforts" towards. In practice, since Glasgow 2021, 1.5°C has become the main target.
• Temperature goal (Article 2) vs net zero goal (Article 4.1): The first is a limit on warming; the second is about balancing emissions and removals in the second half of the century.
• Pre-industrial baseline vs recent baseline: News reports often say "0.8°C above the 1991 to 2020 average". That is a different baseline. The Paris goal always uses 1850 to 1900.
• Paris Agreement vs Kyoto Protocol: Kyoto (1997) set binding targets only for developed countries. Paris (2015) asks all countries to make their own plans.

Issues, criticism and the way forward

• Pledges fall short: Even if all NDCs are met, warming is projected at about 2.3 to 2.5°C (UNEP, 2025).
• No enforcement: The treaty has no penalty if a country misses its NDC. It relies on transparency and peer pressure.
• Equity dispute: Developing countries say the remaining budget must be shared fairly; developed countries push all large emitters to cut fast.
• Overshoot is now likely: Scientists now speak of returning to 1.5°C after overshooting it, which needs carbon removal and has risks of its own.
• The way forward: stronger NDCs, tripling renewable energy capacity and doubling energy efficiency by 2030 (agreed at COP28, Dubai, 2023), cutting methane, and more climate finance.

Concepts to Know

• COP (Conference of the Parties): The yearly meeting of all countries that have joined the UN climate convention (UNFCCC). COP21 was held in Paris in 2015.
• UNFCCC (United Nations Framework Convention on Climate Change): The parent climate treaty, adopted in 1992 at the Rio Earth Summit. The Kyoto Protocol and the Paris Agreement work under it.
• IPCC (Intergovernmental Panel on Climate Change): A UN body set up in 1988 that reviews climate science and reports to governments. It does not do its own research; it assesses published studies.
• Ratification: The formal step by which a country agrees to be legally bound by a treaty it has signed.
• Carbon budget: The total amount of CO2 the world can still release while staying within a chosen warming limit.