Couplet ERC project

The Couplet project was funded during 2018 to 2024 by an advanced grant (786427) from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme to myself as principal investigator.

Jonah Bloch-Johnson, Matt Couldrey, Giorgio Graffino, David Hassell, Pietro Salvi and Quran Wu were employed to work on Couplet for various periods.

We collaborated with Paulo Ceppi, Andrew Williams and Cael, and others who attended Global Cupcake in 2023.

Summary

Research in recent years has revealed that the climate sensitivity (the degree of global warming which results for a given amount of extra heating of the climate system) is not constant, but changes over time as the climate evolves. Moreover, its value is different for each of the various agents that have forced climate change to occur during the past 150 years, which include effects of air pollution and volcanic eruptions as well as CO2 and other greenhouse gases. The hypothesis of Couplet is that the variations of the climate sensitivity are related to variations in the geographical patterns of temperature change, which are determined by the responses of both atmosphere and ocean to the forcing agents.

The objective of the project is to develop new frameworks for describing and predicting the variations of the coupled atmosphere–ocean climate system, taking into account the influences on and the effects of the geographical patterns. Improved scientific understanding will enable more precise projections for given emissions scenarios of greenhouse gases and other pollutants. National and international plans for adaptation to and mitigation of the consequent climate change depend on such information.

From climate model experiments, we have made various discoveries that were unexpected and advance the state of knowledge substantially:

  • That the climate sensitivity varies over time during the last 150 years because of the varying importance of different drivers of change, especially greenhouse gases and volcanic eruptions, but the historical variation simulated by climate models (black line and grey envelope in the figure) is quite different from what observations indicate (blue line and envelope, magenta cross).

  • That there are physical relationships among the climate sensitivity, the change in the vertical profile of atmospheric temperature, and the geographical pattern of surface temperature change.

  • That there are physical relationships among the uptake of heat by the ocean at high latitude (especially in the Southern Ocean), the vertical profile of seawater density (especially in mid-latitudes) and the strength of the Atlantic meridional overturning circulation (which is especially influential in the North Atlantic).
We have formulated new quantitative formulae for climate sensitivity and ocean heat uptake efficiency based on these discoveries.

In addition, we have made a new reconstruction of historical ocean heat uptake on the basis of observed surface temperature change, using an existing method which we substantially improved.

The figure shows the historical time-variation of climate sensitivity in climate models with simulated sea surface temperature (SST) (black line, with grey envelope showing the uncertainty) and observed SST (blue line and envelope), compared with an estimate using the observed Earth energy balance (magenta), and the model-mean value for doubled atmospheric CO2 (red line).

Publications

  1. Andrews, T., Bodas-Salcedo, A., Gregory, J. M., Dong, Y., Armour, K. C., Paynter, D., Lin, Pu, Modak, A., Mauritsen, T., Cole, J. N. S., Medeiros, B., Benedict, J. J., Douville, H. and Roehrig, R., 2022. On the effect of historical SST patterns on radiative feedback. J. Geophys. Res., 127, e2022JD036675 10.1029/2022jd036675 
  2. Bloch-Johnson, J., Rugenstein, M. A. A., Alessi, M. J., Proistosescu, C., Zhao, M., Zhang, B., Williams, A. I. L., Gregory, J. M., Cole, J., Dong, Y., Duffy, M. L., Kang, S. M. and Zhou, C., 2024. The Green's Function Model Intercomparison Project (GFMIP) Protocol. J. Adv. Model Earth Syst., 16, e2023MS003700 10.1029/2023ms003700 
  3. Bloch-Johnson, J., Rugenstein, M. and Abbot, D. S., 2020. Spatial radiative feedbacks from internal variability using multiple regression. J. Climate, 33, 4121-4140 10.1175/jcli-d-19-0396.1 
  4. Bloch-Johnson, J., Rugenstein, M., Stolpe, M. B., Rohrschneider, T., Zheng, Y. and Gregory, J., 2021. Climate sensitivity increases under higher CO₂ levels due to feedback temperature dependence. Geophys. Res. Lett., 48, e2020GL089074 10.1029/2020gl089074 
  5. Cael, B. B., Bloch-Johnson, J., Ceppi, P., Fredriksen, H-B., Goodwin, P., Gregory, J. M., Smith, C. J. and Williams, R. G., 2023. Energy budget diagnosis of changing climate feedback. Sci. Adv., 9, eadf9302 10.1126/sciadv.adf9302 
  6. Ceppi, P. and Gregory, J. M., 2019. A refined model for the Earth's global energy balance. Clim. Dyn., 53, 4781-4797 10.1007/s00382-019-04825-x 
  7. Gregory, J. M., Andrews, T., Ceppi, P., Mauritsen, T. and Webb, M. J., 2020. How accurately can the climate sensitivity to CO₂ be estimated from historical climate change? Clim. Dyn., 54, 129-157 10.1007/s00382-019-04991-y 
  8. Gregory, J. M., Bloch-Johnson, J., Couldrey, M. P., Exarchou, E., Griffies, S. M., Kuhlbrodt, T., Newsom, E., Saenko, O. A., Suzuki, T., Wu, Q., Urakawa, S. and Zanna, L., 2024. A new conceptual model of global ocean heat uptake. Clim. Dyn., 62, 1669-1713 10.1007/s00382-023-06989-z 
  9. Hassell, D. and Bartholomew, S. L., 2020. cfdm: A Python reference implementation of the CF data model. J. Open Source Soft., 5, 2717 10.21105/joss.02717 
  10. Lin, Y-J., Hwang, Y-T., Ceppi, P. and Gregory, J., 2019. Uncertainty in the evolution of climate feedback traced to the strength of the Atlantic Meridional Overturning Circulation. Geophys. Res. Lett., 46, 12331-12339 10.1029/2019gl083084 
  11. Newsom, E., Zanna, L. and Gregory, J., 2023. Background pycnocline depth constrains future ocean heat uptake efficiency. Geophys. Res. Lett., 50, e2023GL105673 10.1029/2023gl105673 
  12. Rugenstein, M., Bloch-Johnson, J., Gregory, J., Andrews, T., Mauritsen, T., Li, C., Frölicher, T. L., Paynter, D., Danabasoglu, G., Yang, S., Dufresne, J-L., Cao, L., Schmidt, G. A., Abe-Ouchi, A., Geoffroy, O. and Knutti, R., 2019. Equilibrium climate sensitivity estimated by equilibrating climate models. Geophys. Res. Lett., 47, e2019GL083898 10.1029/2019gl083898 
  13. Salvi, P., Ceppi, P. and Gregory, J. M., 2021. Interpreting the Dependence of Cloud-Radiative Adjustment on Forcing Agent. Geophys. Res. Lett., 48, e2021GL093616 10.1029/2021gl093616 
  14. Salvi, P., Ceppi, P. and Gregory, J. M., 2022. Interpreting Differences in Radiative Feedbacks From Aerosols Versus Greenhouse Gases. Geophys. Res. Lett., 49, e2022GL097766 10.1029/2022gl097766 
  15. Salvi, P., Gregory, J. M. and Ceppi, P., 2023. Time-evolving radiative feedbacks in the historical period. J. Geophys. Res., 128, e2023JD038984 10.1029/2023jd038984 
  16. Williams, A. I. L., Jeevanjee, N. and Bloch-Johnson, J., 2023. Circus tents, convective thresholds, and the non-linear climate response to tropical SSTs. Geophys. Res. Lett, 50, e2022GL101499 10.1029/2022gl101499 
  17. Wu, Q. and Gregory, J. M., 2022. Estimating ocean heat uptake using boundary Green's functions: a perfect-model test of the method. J. Adv. Model Earth Syst., 14, e2022MS002999 10.1029/2022ms002999 

Jonathan Gregory