Variations in climate sensitivity and its role in climate projections

Introduction and aims

This PhD will revisit our understanding of how well we can make  projections of the Earth’s surface temperature. The project will examine how the interplay between the different drivers of climate change might cause the Earth’s response to vary in time. The subsequent results should lead to more constrained projections of future climate change and a better understanding of its uncertainty.

Understanding how and why the Earth’s energy budget changes is central to understanding climate change and climate sensitivity (Forster, 2016). Exciting work at the Met Office has been using atmospheric model intercomparison (AMIP) simulations to explore how equilibrium climate sensitivity varies with time over the 20th century (Andrews et al. 2018). This work found that feedbacks depend crucially on how the pattern of surface temperature change drives different feedbacks in the system that have be seen in recent satellite records of energy budget changes (Loeb et al. 2020). Both supervisors are leading work for the Radiative Forcing Model Intercomparison Project (RFMIP) as part of the Climate Model Intercomparsion Project phase 6 (CMIP6) whose data, when analysed will allow for an investigation of feedback changes through time within a coupled modelled framework for the first time.

The PhD, through a combination of CMIP6 analysis and dedicated climate model runs with the Met Office climate model (HadGEM3) will build a comprehensive understanding of time varying changes in climate sensitivity, what causes this variation and how predictable such variation might be.  Together with improved estimates of radiative forcing and its uncertainty from RFMIP this will lead to a better understanding of the uncertainty in 21st century climate change projections, and the relationship between observed and modelled estimates of climate sensitivity. This links directly to the goals of the EU Horizon 2020 CONSTRAIN project led by Leeds in which the Met Office is a key partner, as well as the Met Office Hadley Centre Climate Programme.

CMIP6 image
CMIP6 climate model comparison exercise

 

Methodology and student development

The PhD is designed with the students development in mind, beginning with simple analysis of model output to complex analysis techniques on large multi-model datasets.  The PhD is designed to take advantage of a plethora of new data from idealised climate change and historical simulations as part of CMIP6.  In particular, data produced from RFMIP, the Cloud Feedback Model Intercomparison Project (CFMIP) and the Decadal Climate Prediction Project (DCPP) will be used to examine reasons for variation in sensitivity between models, including any dependency on different ocean base-states. The forcing data from RFMIP will allow time dependent sensitivities to be computed from the historical and future simulations for the first time, aligning with simulations from the Scenario Model Intercomparison Project (ScenarioMIP) and Detection and Attribution Model Intercomparison Project (DAMIP).

The work will allow the student to test how climate sensitivity parameters may vary in the future and whether there is any dependence on individual drivers of historical climate change (e.g. aerosols vs. greenhouse gases).  Where appropriate these integrations will be supplemented by dedicated radiative kernel analyses and modified HadGEM3 simulations to better understand radiative adjustment processes, especially those related to land surface forcing and Earth System processes. This should lead a better a understanding of land-surface changes and Earth System processes and their role in historical climate change.

The aim of these planned model analyses is to understand why equilibrium climate sensitivity and the transient climate response vary with time, and investigate major sources of uncertainty.  Using these model insights and by employing ARGO float measurements of ocean heat content and CERES Earth’s energy budget estimates, we plan to generate a much better understanding of how the Earth’s sensitivity might vary going forward and give an indication of how predictable such trends in sensitivity might be.

Schematic of ARGO float operation. https://www.euro-argo.eu/Outreach/Education/On-the-way-to-an-oceanic-treasure-hunt/How-Argo-Floats-work 

The project would:

  1. Analyse energy budget, surface temperature data, ocean heat content trends, and climate feedbacks within CMIP6 experiments;
  2. Perform a few dedicated short HadGEM3 integrations as needed to better understand the physical processes involved and test various hypothesis, especially those related to land-surface forcing;
  3. Analyse the Earth’s energy budget through ARGO float and CERES observations for detailed comparisons with model energy budget data.

The student will be expected to undertake regular visits to the CASE supervisor and the Understanding Climate Change group to ensure effective collaboration and interactions. The CASE supervisor will assist the student in learning how to perform HadGEM3 experiments on high performance computing facilities.

Output and potential for high impact

The student would be expected to submit a PhD by publication, publishing approximately  three high quality papers from their PhD in leading journals. The student will be part of the Priestley International Centre for Climate which promotes working across disciplines on climate solutions. This centre offers both a student society and the opportunity to apply to its scholars program.  The results of the project, in particular the revised constraints on future climate projections, will be valuable to the UK Climate Projections (UKCP) team, and the 2022 UK Climate Change Risk Assessment Exercise. The student would benefit from being part of the wider EU CONSTRAIN program, a vibrant international research program, led by Piers Forster.

 

References:

General article on latest understanding of climate sensivivity by Piers Forster,: CarbonBrief 2020

Forster PM, 2016: Inference of Climate Sensitivity from Analysis of Earth’s Energy Budget, Annual Review of Earth and Planetary Sciences, 44, pp.85-106. https://www.annualreviews.org/doi/abs/10.1146/annurev-earth-060614-105156

Andrews T, Gregory JM, Paynter D, Silvers LG, Zhou C, Mauritsen T, Webb MJ, Armour KC, Forster PM, Titchner H. 2018. Accounting for Changing Temperature Patterns Increases Historical Estimates of Climate Sensitivity. Geophysical Research Letters. 45(16), pp. 8490-8499. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL078887

Loeb NG, Wang H, Allan RP, Andrews T, Armour K, Cole JNS, Dufresne J, Forster P, Gettelman A, Guo H, Mauritsen T, Ming Y, Paynter D, Proistosescu C, Stuecker MF, Willén U, Wyser K. 2020. New Generation of Climate Models Track Recent Unprecedented Changes in Earth’s Radiation Budget Observed by CERES. Geophysical Research Letters. 47(5). https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2019GL086705

Sherwood, S. C., Webb, M. J., Annan, J. D., Armour, K. C., Forster, P. M., Hargreaves, J. C., et al. (2020). An assessment of Earth’s climate sensitivity using multiple lines of evidence. Reviews of Geophysics, 58, e2019RG000678. https://doi.org/10.1029/2019RG000678