Extreme Temperatures Beyond Net Zero

Climate change is the critical challenge facing humanity. The impacts of global warming are already being experienced around the world, with the rapidly increasing risk of severe heatwaves a particularly dangerous threat to societies and ecosystems. The exceptional summer of 2022 – the hottest on record in Europe, with the UK breaking the 40C threshold for the first time – is an ominous portent of the future.

Research on extreme temperatures has predominantly focused on the effects of the ongoing ramp-up in CO2 concentrations (Fischer & Knutti 2015). However, with CO2 emissions expected to slow and potentially decrease over coming decades as the world transitions towards ‘Net Zero’, there is also an urgent need to understand the response of extreme temperatures to CO2 ramp-down and global cooling. Yet fundamental understanding of extreme temperatures in a cooling climate is limited. For example, as CO2 concentrations in the atmosphere decrease, for how long will dangerous heat events persist? Will hot days cool faster or slower than the average day? And will extreme temperatures – like other features of the climate system, for example sea ice – exhibit hysteresis in response to CO2 ramp-up and subsequent ramp-down, with different behaviour for the same CO2 concentration? These open questions have important implications for future risks related to extreme temperatures, including heat stress & wildfires.

The goal of this IAPETUS2 project is to transform understanding of extreme temperatures leading up to, and beyond, Net Zero. The project will build on a recent study – led by Dr Michael Byrne – of extreme temperatures in a warming climate (Byrne 2021), and will combine new climate simulations with theory to tackle three key objectives:

1. Decompose the physical processes driving changes in extreme temperatures during the CO2 ramp-up phase, and how these processes vary across latitude (e.g. between the tropics and extratropics).
2. Investigate, for the CO2 ramp-down phase, whether cooling is amplified for hot days relative to the average day – as predicted by theory (Byrne 2021) – and how land humidity influences the pace of cooling.
3. Identify whether extreme temperatures exhibit hysteresis in response to CO2 ramp-up and ramp-down, and examine the physical origins of any hysteresis identified.

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Image Captions

Projected warming of the hottest 5% of days in the tropics relative to the average warming (data from the GFDL-CM4 climate model).


To address the project objectives, a suite of state-of-the-art climate simulations will be performed on the ARCHER2 high performance computing facility. Not only will these innovative simulations advance fundamental understanding of extreme temperatures, they will also develop valuable skills for the PhD student in coding, scientific modelling and high-performance computing.

The student will perform an ensemble of fully-coupled simulations using the Community Earth System Model (CESM2; Danabasoglu et al. 2020), and potentially also UKESM, to investigate the response of extreme temperatures to CO2 ramp-up and ramp-down. An ensemble of five simulations with different initial conditions will be used to robustly assess the warming and cooling of extreme temperatures in response to changes in CO2 concentrations. All simulations will have a horizontal resolution of approximately 1 degree latitude x 1 degree longitude with 40 vertical levels. For each simulation, the CO2 concentration will be ramped up by 1% per year from the pre-industrial value of 280 ppm until the concentration reaches 1120 ppm after 140 years. The CO2 concentration will then be ramped down by 1% per year, reaching its pre-industrial value of 280ppm after a total simulation time of 280 years.

The ensemble of CO2 ramp-up/ramp-down simulations will be analysed to decompose the physical processes driving changes in extreme temperatures during CO2 ramp-up and how these processes vary across latitude (e.g. between the tropics and extratropics). This analysis will have important implications for identifying the physical origins of – and potential pathways to reducing – the large uncertainties in extreme-temperature projections. For the CO2 ramp-down phase, the student will investigate whether cooling is amplified for hot days relative to the average day and how land humidity couples to changes in extreme temperatures. To identify any hysteresis in response to changes in CO2 concentration, the student will compare extreme temperatures (specifically high percentiles of daily-mean temperature) at equal CO2 concentrations in the ramp-up phase versus in the ramp-down phase. Differing extreme-temperature statistics for the same CO2 concentration will be evidence of hysteresis. The physical origins of any hysteresis in extreme temperatures will be examined, with one hypothesis being that asymmetry in land humidity responses to CO2 ramp-up and ramp-down could drive hysteresis. The simulations and analyses in this project will produce a robust assessment of extreme temperatures in response to potential future changes in CO2 concentrations. Moreover, this research will advance fundamental understanding of how the climate system works, both in the future and in the past, and will have implications for predicting and managing heat risks in the potential scenario of decreasing CO2 concentrations over the coming decades and century.

Project Timeline

Year 1

Year 1 will involve a literature review to allow the student to develop understanding of heatwave dynamics and physical aspects of climate change. The student will also begin using the CESM climate model and setting up the planned CO2 ramp-up/ramp-down simulations.

Year 2

Year 2 will focus on running the suite of CESM simulations to investigate extreme temperatures in response to CO2 ramp-up and ramp-down. Initial analyses of the simulations will quantity the warming of extreme temperatures relative to the average temperatures, and investigate the processes driving the trends in extreme heat. The student will potentially present the key results at the American Geophysical Union’s annual meeting in San Francisco

Year 3

Year 3 will focus on the CO2 ramp-down phase, and in particular the rate of cooling of extreme temperatures and whether there is evidence of hysteresis. This will involve substantial Big Data analyses and the student will also draft a research article on this portion of the project. The student will attend and present the research at an international conference on extreme weather events.

Year 3.5

Year 3.5 will focus on writing the PhD thesis and drafting a research article on the analyses conducted during Year 3.

& Skills

The student will be trained in several aspects of physical climate science including atmospheric dynamics, heatwaves, climate modelling and climate change. The student will also be trained in highly sought-after technical skills in computational modelling, high-performance computing, and Big Data analyses.

References & further reading

[1] Byrne (2021): Amplified warming of extreme temperatures over tropical land, Nature Geoscience, vol. 14, pp. 837–841.[2] Fischer & Knutti (2015): Anthropogenic contribution to global occurrence of heavy-precipitation and high-temperature extremes, Nature Climate Change, vol. 5, pp. 560–564.[5] Danabasoglu et al (2020): The Community Earth System Model version 2 (CESM2). Journal of Advances in Modeling Earth Systems, vol. 12, e2019MS001916

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