The Earth is hurtling toward an average temperature that will rise by 1.5 °C compared to the pre-industrial period One climate model suggests that the probability of reaching this threshold in 2023 is now 55 in %. 1.5°C was the preferred maximum warming limit set by the United Nations in the landmark 2015 Paris Agreement on climate change. Climate scientists use a variety of models to make predictions. Exceeding the Paris limit requires a long-term warming trend of 1.5°C or more, but some research groups that track average annual temperatures in isolation are already predicting 1.5°C warming this year. In May, the report The World Meteorological Organization said there is a 66% chance that the average annual temperature will exceed 1.5°C of warming between 2023 and 2027. (Katharine Sanderson)