2 Degrees Celsius

An international climate goal and scientific-policy benchmark to limit the rise in global average temperature to within 2℃ above pre-industrial levels.

2 Degrees Celsius

Overview

2 Degrees Celsius (2℃, Two Degrees Celsius) refers to the international climate policy goal of keeping the rise in global average temperature within 2℃ above pre-industrial levels. The Paris Agreement, adopted in 2015, strengthened this goal further by explicitly stating efforts to limit the increase to 1.5℃, and 2℃ is being reinterpreted not as a “safe upper limit” but as a dangerous threshold beyond which hard-to-reverse climate feedbacks may be set in motion.

Key Details

Definition and Origins

The figure “2℃” first appeared in the scientific literature in the 1970s. In 1975, the economist William Nordhaus proposed 2℃ as a reference point in his cost–benefit analysis of climate change, and in 1996, when the EU Council of Environment Ministers adopted it as an official policy target, it became established as an international benchmark. Thereafter, by way of the 2009 Copenhagen Accord and the 2010 Cancún Agreements, the wording “holding the increase in the global average temperature to well below 2℃ above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5℃” was codified in Article 2 of the 2015 Paris Agreement.

Scientific Basis

The number 2℃ is not a mere political consensus but is grounded in the physical characteristics of the Earth system. Representative examples include:

Sea level rise: With a 2℃ rise, sea level is estimated to rise by about 0.4–0.6 m by 2100, roughly 10 cm higher than under the 1.5℃ scenario.
Coral reef loss: Even at 1.5℃, 70–90% of tropical coral reefs disappear, but at 2℃, effectively more than 99% are lost.
Arctic sea ice: With a 2℃ rise, the probability that summer Arctic sea ice disappears completely once every few decades increases.
Tipping points: Hard-to-reverse thresholds—such as the collapse of the Greenland ice sheet, methane release from permafrost, and the savannization of the Amazon rainforest—are densely concentrated in the 1.5–2℃ range.

Relationship with the 1.5℃ Target

The IPCC (Intergovernmental Panel on Climate Change) Special Report on Global Warming of 1.5℃ of 2018 quantitatively demonstrated that the difference between 1.5℃ and 2℃ is “not a matter of 0.5℃ but a matter of survival.” In almost every indicator—population exposed to heatwaves, population facing water scarcity, rate of loss of species habitat—the 2℃ scenario is significantly worse than 1.5℃. With this report as a turning point, the center of gravity of the international community shifted from “within 2℃” to “defending 1.5℃.”

Position in International Agreements

The Paris Agreement’s temperature goal (Article 2), adaptation goal (Article 7), mitigation goal (Article 4), and implementation review (Article 14, Global Stocktake) framework all take the 2℃/1.5℃ temperature benchmarks as their axis. Each country’s submitted Nationally Determined Contribution (NDC) is likewise evaluated on the basis of its contribution to these temperature goals.

Criticism and Debate

Criticism of political convenience: It is pointed out that 2℃ is a number chosen to suit political negotiability rather than scientific calculation.
Argument of unequal responsibility: The structural inequality whereby developing countries with smaller historical emissions suffer greater climate damage is a key issue.
Caution against technological optimism: Scenarios that rely excessively on carbon dioxide removal (CDR) technologies are criticized as having low feasibility.
Regional variation: A global average of 2℃ can mean a local increase of 3–4℃ or more depending on the region, and the limitations of discussions centered on averages are pointed out.

Recent Trends

The Situation in 2024–2025

Realization of a temporary overshoot of 1.5℃: 2023 and 2024 were recorded as the first years in observational history in which the annual average temperature exceeded 1.5℃ relative to pre-industrial levels. However, this is not based on the long-term average, and the 1.5℃ target of the Paris Agreement presupposes a 20–30 year average.
Emissions plateau and the gap: The UNEP Emissions Gap Report warned that even if all current NDCs are implemented, a rise of 2.6–3.1℃ by 2100 is expected. This means that even the 2℃ goal is difficult to achieve under current policies.
Follow-up to the Global Stocktake (GST): Reflecting the results of the first GST held in Dubai in 2023, countries must submit new NDCs containing 2035 targets by 2025. Consistency with the 1.5℃ pathway becomes a key evaluation criterion.
Acceleration of the energy transition: A sharp drop in the levelized cost of renewable energy, the spread of electric vehicles, and rising electricity demand from AI data centers are combining to drive a restructuring of power systems.
Rise of the adaptation discourse: As awareness grows that mitigation alone cannot avoid 2℃, the operation of the Loss and Damage fund and the expansion of adaptation finance have emerged as major agenda items.

Implications

2 Degrees Celsius is changing in character from a “goal to be kept” to a “line that must not be crossed.” Science clearly shows the gap between 1.5℃ and 2℃, but policy has moved more slowly than the pace needed to narrow that gap. Ultimately, the core of the 2 Degrees Celsius debate converges not on the number itself but on the questions of distribution and speed: how quickly, by whom, and at what cost will emissions be reduced?