Max-Planck-Institute for Meteorology: Coupled climate models systematically underestimate the radiation response to surface warming

Do climate models realistically represent the coupling between Earth’s surface warming and the top-of-the atmosphere radiation? Dirk Olonscheck, research scientist at Max Planck Institute for Meteorology, and Maria Rugenstein, Assistant Professor at Colorado State University, show that models systematically underestimate the observed coupling strength. Thereby, the models accumulate too much energy in the atmosphere, an effect that contributes to a possibly too high climate sensitivity of some models.

A realistic representation of the top-of-the-atmosphere (TOA) radiation response to surface warming is key for trust in climate model projections. However, whether climate models reproduce the observed coupling between TOA radiation and global and local surface warming was unresolved so far, partly because of the shortness of the observational record.

Olonscheck and Rugenstein tackle the problem of the 22-year short observational record of TOA radiation by accounting for the internal variability of the climate system, which is strong on such short timescales. They compare the observed trend to initial-condition large ensembles of eleven climate models. These large ensembles allow one to account for both the internal variability within the period 2001-2022 and for structural differences between the models. The authors find that coupled climate models systematically underestimate observed TOA radiation trends even if they reproduce observed surface temperature trends (Figure 1).

Graphic: TOA radiation — Figure 1: Observed (black dot) and simulated (colored dots) 2001–2022 trends in global mean TOA radiation (left) and global mean surface temperature (right). Each filled dot represents one realization of a large ensemble; black circles represent the ensemble mean. The vertical dashed line and gray shading shows the observed trend ±2 standard errors of the 22-year linear regression. The observations are derived from the satellite-based CERES instruments. Credit: Olonscheck

The authors test for different hypotheses why models underestimate the observed TOA radiation trend and show that the underestimation is caused by a too weak local coupling between surface warming and TOA radiation, referred to as response bias (Figure 2). The too weak coupling stems from both larger regions and greater magnitudes of underestimating than overestimating the observed TOA radiation trends (more blue than red regions). This response bias stems from both the models' inability to reproduce the observed large-scale surface warming pattern and from errors in the atmospheric physics affecting short- and longwave radiation. The relative role of the two reasons differs strongly across models.

Graphic: Pattern of response bias of net TOA radiation trends — Figure 2: Pattern of the response bias of net TOA radiation trends to observed surface warming. The percentages indicate the global area for which the models overestimate (red) or underestimate (blue) the observed TOA radiation response to surface warming, and the number in black shows the magnitude of the global mean response bias in Wm−2 decade−1. Credit: Olonscheck

Finally, Olonscheck and Rugenstein find that the systematic response bias in surface-TOA coupling strength over the last 22 years correlates with the long-term climate response, called effective climate sensitivity. Models with a stronger response bias, that is, a weaker TOA radiation response to surface warming, accumulate too much energy in the atmosphere. This overestimated accumulation results in a higher effective climate sensitivity of these models (Figure 3). Hence, the findings are a new line of evidence that less-sensitive models more realistically reproduce climate change over the last 22 years than more-sensitive models. The findings imply that a better understanding of both the role of surface temperature patterns and model errors in representing atmospheric physics is needed to be able to better constrain the Earth’s climate sensitivity.

Graphic on reaction bias TOA — Figure 3: Relationship between the 2001–2022 response bias of global mean TOA radiation to surface warming and effective climate sensitivity (EffCS) for eleven large ensembles. Credit: Olonscheck

Original publication

Olonscheck, D., & Rugenstein, M. (2024). Coupled climate models systematically underestimate radiation response to surface warming. Geophysical Research Letters, 51, e2023GL106909. https://doi.org/10.1029/2023GL106909

Contact

Dr. Dirk Olonscheck
Max Planck Institute for Meteorology
dirk.olonscheck@mpimet.mpg.de

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