How do weather and climate extremes respond to anthropogenic forcing?

Our understanding of the climate system suggests that, in response to human forcing through greenhouse gas & aerosol emissions and land use, weather and climate extremes are potentially amplified both by thermodynamic and dynamic changes. In other words, an extreme event such as a heatwave can become more intense through thermodynamic modulations, e.g., background warming and drier soils providing fuel for stronger surface sensible heating, but potentially also through atmospheric circulation changes.

Especially at regional scales, however, such anthropogenic modulations can be masked or enhanced by internal climate variability. Moreover, sea surface temperature patterns simulated by climate models systematically deviate from observations, which points to a flawed response to external forcing (e.g., Wills et al., 2022). Considering how strongly the surface ocean interacts with the atmosphere, this suggests that large-scale winds may also fail to adequately respond to anthropogenic forcing. This raises critical questions: Where do we expect notable dynamic contributions in the first place? Are observations consistent with these expected changes? And how do dynamics compare to thermodynamics driving weather and climate extremes?

To tackle these questions, we employ simulations using CESM2, a global Earth System Model, with which we disentangle the responses to anthropogenic forcings in thermodynamic state and atmospheric circulation. In doing so, we obtain the physical model truth with regards to how extreme weather and climate events are altered under additional global warming through purely thermodynamic and dynamic pathways.

 

References

Wills, R. C. J., Dong, Y., Proistosecu, C., Armour, K. C., & Battisti, D. S. (2022). Systematic climate model biases in the large-scale patterns of recent sea-surface temperature and sea-level pressure change. Geophysical Research Letters, 49, e2022GL100011. https://doi.org/10.1029/2022GL100011