Early Warning of Crossing the 1.5°C Global Temperature Change Threshold

With the human induced increase in global temperatures continuing, the question if and how we might exceed the 1.5°C warming level enshrined in the long-term temperature goal of the Paris Agreement has received increased public and scientific interest. Identifying the level of human induced warming at any given year is subject to a range of uncertainty including from short-term natural variability. A single year, or even several consecutive years, above 1.5°C thus does not imply that the human induced warming level is reached but does provide an early warning of the risk of crossing that threshold.

Here we find that under an emission pathway following current policies, a single year above 1.5°C might imply that a crossing of the global warming threshold could materialise within 11 years thereafter (66% or likely range). For a three (5) year consecutive average, this time window decreases to 5 (2) years. If 1.5°C is reached in 2024, according to our analysis it would mark an unusual event (about 1-in-25 years) under a current policy scenario that reaches 1.5°C around 2040 (central estimate). We find that stringent emission reductions in the near-term can increase the chances of never crossing 1.5°C. Under a scenario of stringent emission decline, an exceedance of 1.5°C in one or several years may be observed without the long-term warming level ever being breached.

The occurrence of a single year at or above 1.5°C should therefore be taken as a final warning for the need for very stringent near term emission reductions to keep the Paris Agreement long-term limit within reach.