In order to avoid catastrophic consequences of climate change, there is…

Concept: International climate policy states its goal as an upper limit on the rise in global average surface temperature, measured against the pre-industrial baseline — the 1850–1900 average — and not against any recent year. The goal is carried by two nested figures rather than one: an outer ceiling that warming must not exceed, together with a stricter figure that parties undertake to pursue, because projected damage rises steeply as warming approaches and passes the outer ceiling.

Application

1. The below-2 °C ceiling was already the operative international formulation when this paper was set. The Copenhagen Accord (2009) recorded it, and the Conference of the Parties adopted it in the Cancun Agreements (2010), which held that deep cuts in global emissions are required to hold the increase in global average temperature below 2 °C above pre-industrial levels.

2. The same Cancun decision mandated a periodic review, carried out between 2013 and 2015, of whether that goal should be strengthened to 1.5 °C. At the date of this examination, therefore, 2 °C was the agreed ceiling and 1.5 °C was the stricter figure formally under consideration; 1.5 °C was not yet itself an adopted limit.

3. The Paris Agreement (2015), Article 2.1(a), later settled both figures in their present wording: hold the increase in global average temperature to well below 2 °C above pre-industrial levels, and pursue efforts to limit the increase to 1.5 °C.

4. The two figures the international process actually works with are therefore 2 °C, the ceiling already adopted when this paper was set, and 1.5 °C, the strengthening threshold then under formal review and adopted at Paris the following year. Among the ranges offered, the only one bounded by those two figures is 1.5 °C to 2 °C.

Cross-check

• Global average surface temperature had already risen about 0.85 °C above the 1850–1900 baseline by 2012 (IPCC AR5) and about 1.1 °C by 2020. A limit whose upper end is 0.5 °C or 1.0 °C would therefore already have been breached, so it cannot serve as a forward-looking ceiling.

• IPCC assessments place the severe outcomes the agreement exists to avoid — large-scale crop and water stress and near-total loss of warm-water coral reefs — at and above 2 °C. A limit running from 2.0 °C up to 3.5 °C would therefore admit exactly what the agreement is designed to prevent.

• A note on the wording: 1.5 °C and 2 °C work as two nested upper limits, not as a range inside which warming is meant to sit. 2 °C is the outer ceiling and 1.5 °C the stricter figure — placed under formal review by the Cancun decision and adopted in Paris as the target parties undertake to pursue. Neither is a lower bound below which warming should not fall.

• Result: the range that captures the limit the world's countries have converged on for the rise in average surface temperature above pre-industrial times — the adopted below-2 °C ceiling together with the 1.5 °C figure the process moved to adopt — is 1.5 °C to 2 °C.