We explore how and when temperature could exceed 50°C in Paris in CMIP6 climate model simulations of the 21st century.
Pascal Yiou, LSCE, ESTIMR
Motivations
Reaching a surface temperature of 50°C in a heavily populated region, like Paris, would have devastating effects. Although such a high value seems far from the present-day record of 42.6°C, its occurrence cannot be dismissed by the end of the 21st century, due to the continuous increase of global mean temperature. In this paper, we address two questions that were asked by the City of Paris to a group of scientists: When does this event start to be likely? What are the prevailing meteorological conditions? We base our study on the CMIP6 simulation ensemble. Many of the CMIP6 yield biases in temperature. Rather than using methods of bias correction, which are not necessarily adapted to high extremes, we propose a pragmatic approach of model selection in order to seek such high temperature events that are deemed realistic. We analyze the meteorological conditions leading to first occurrences of such hot events and their common atmospheric patterns. This paper (Yiou et al., 2024) describes a simple data mining approach (on a large ensemble of climate model simulations) which could be adapted to other regions of the world, in order to help decision makers anticipating and adapting to such devastating meteorological events.
Result Synthesis
We have identified events corresponding to moderate Global Mean Surface Temperature (GMST) increase (between 2.1 and 2.8°C). Those events come with spates of tropical nights (minimum temperatures over 20°C), which would exacerbate the impacts on health of exceeding 50°C during the day.
Some of the Paris events correspond to larger scale events, where temperature exceeds 50°C (e.g., in the south west of France) and large parts of the country have TX above 40°C. Therefore, a crisis in Paris would also be reflected in other regions of France, where temperatures could even be higher, especially in urban areas. We have not quantified the probability of exceeding this high threshold, because it is rarely observed more than once in each simulation. This probability is deemed to be negligible for a global surface temperature increase lower than 2°C within the next two decades. It increases to ~1% chance if the global surface temperature increases by 2.7°C (Figure 1).
If global surface temperature (GMST) increases by more than 4°C since 1950-2000, the probability of such hot events increases dramatically in the CMIP6 database.
Figure 1: Distributions of GMST increase (GWI) values when TX>48°C in Paris, from reference baselines of GMST in 1950-2000. The vertical bars indicate all occurrences of TX>48°C. The colors indicate the SSP scenario of the TX>48°C event occurrences. The black line is an empirical probability density function of the year of occurrence of TX>48°C(Yiou et al., 2024).
We have shown that the prevailing large-scale conditions during those hot events correspond to El Niño states and intense AMO. Such large-scale features tend to increase surface temperature over the northern midlatitudes.
References
Yiou, P., Vautard, R., Robin, Y., de Noblet-Ducoudré, N., D’Andrea, F., and Noyelle, R.: How could 50 °C be reached in Paris: Analyzing the CMIP6 ensemble to design storylines for adaptation, Climate Services, 36, 100518, https://doi.org/10.1016/j.cliser.2024.100518, 2024.