Gabriel Chiodo

Stratospheric composition and climate

Via its modulation of the amount of UV absorption in the stratosphere, ozone provides a crucial chemistry-climate feedback. This feedback can either operate through ozone-induced perturbations in radiative forcing, or dynamical effects on the tropospheric circulation. However, the magnitude of this chemistry-climate feedback, along with that of other stratospheric composition changes (e.g. water vapor and methane) remains largely unknown.

I have developed a new line of research aimed at quantifying stratospheric composition feedbacks. My research has shown that stratospheric ozone chemistry reduces the climate sensitivity to solar (Chiodo and Polvani, 2016) and the large-scale circulation response to anthropogenic greenhouse-gases (Chiodo and Polvani, 2017). Models are consistent in terms of the ozone response to abrupt 4xCO₂forcing (Chiodo et al., 2018), although the structure and magnitude of the ozone changes near the tropopause can have widely different impacts on tropospheric climate (Chiodo and Polvani, 2019). I also co-led recent studies  revealing that feedbacks from stratospheric water vapor substantially contribute to climate sensitivity, but the uncertainty across models is large (Banerjee, et al., 2019), and was involved in another study, revealing that CMIP6 models may over-estimate the feedback (Nowack et al., 2023).

Aside from studying feedbacks on long-term climate projections, my research has also explored the feedback exerted by ozone on inter-annual time-scales, with focus on the Arctic stratosphere. I co-led recent studies showing that ozone modulates the surface effects of Sudden Stratospheric Warmings (Oehrlein et al., 2020). Most remarkably, my research group at ETH has shown, for the first time, that depletion events in the Arctic (such as e.g., 2020) can lead to wide-spread surface climate anomalies across the Northern Hemisphere (Friedel et al., 2022a), motivating further work on assessing the role of ozone as a source of predictability at S2S time-scales. Changes in Arctic ozone can also modulate the timing and surface impacts of the Vortex Breakdown (Friedel et al., 2022b). However, we also found that the variance of Arctic ozone is expected to decline in the future, due to the projected decline in Ozone Depleting Substances (Friedel et al., 2023b). Despite the decline in the variance, we also show in another recent study, that long-term changes in Arctic ozone during springtime may have sizable impacts on stratospheric and surface climate, and need to take into account in future projections (Chiodo et al., 2023).

Solar Radiation Modification

Solar Radiation Modification (SRM) could be a potential Climate Intervention measure to supplement other tools (e.g. CDR) to help reduce climate risks, although the focus should be on reducing anthropogenic greenhouse gas (GHG) emissions. SRM techniques such as stratospheric aerosol injections (SAI) have been shown to be effective in the short-term. However, there is still considerable uncertainty concerning potential adverse side effects, including impacts on the ozone layer. I was involved in recent studies showing the sizable uncertainty across models in terms of the radiative forcing efficiency of SAI in models that interactively simulate the enhancement of the aerosol layer following artificial SO₂ injections (Weistenstein et al., 2022); differences in transport and microphysics (e.g. Vattioni et al., 2024) are the biggest source of uncertainty across models. Also, the role of stratospheric heating in the broader climatic impacts of SRM was highlighted in another study I co-authored: Wunderlin et al., (2024).

In particular, my former group at ETH, in close collaboration with the Climate group at PMOD/WRC led by Dr. Sukhodolov, has undertaken the very first comprehensive assessment of climatic and chemical impacts of a new type of SRM technique: injection of solid particles (e.g., Calcite and Alumina). This effort was led by Dr. Vattioni, former PhD student in my group, by combination of 3-D aerosol-chemistry climate modeling, and laboratory efforts at the Paul Scherrer Institute  and IAC-ETH. A first paper on this topic (Vattioni et al., 2023) has shown, for the first time, that the effects of Alumina particles on the ozone layer are extremely uncertain, ranging between almost negligible and up to global reduction in ozone by 9%, which would be more than twice as much as the ozone loss caused by chlorofluorocarbons in the late 1990s. This paper will motivate, in the years to come, a concerted effort in measuring uptake coefficients in laboratory studies. Also, the development of a new microphysics scheme for solid particles achieved by Dr. Vattioni will be the basis for a number of future modeling studies on SRM. Taken together, this research line has shown the potential of solid particles for reduced side effects in the climate system over e.g. SO₂ injections, but it also highlights the existing gaps, which can only be bridged with concerted modeling and laboratory efforts.

Solar variability and its effects on climate

Variations in the Sun's output, either on decadal (the 11-year sunspot cycle) or on millennium (Maunder Minimum) time scales, can have a profound effect on the climate system. However, an accurate understanding of how the atmosphere responds to irradiance changes is still elusive (see review paper by Gray et al., 2010)

My research aims to improve our understanding of the effects of solar variability on climate. I quantified the portion of decadal stratospheric variability that can be unambiguously attributed to solar variability (see Chiodo et al., 2014). I have also explored the possible effects of a future solar minimum on boreal winter projections in the Northern Hemisphere (Chiodo et al., 2016). Most notably, I have revisited the link between North Atlantic climate and solar variability; this research revealed that quasi-decadal variations in the North Atlantic Oscillation are due to internal variability, and not the solar cycle, in contrast to a large body of literature claiming a solar/NAO link (see Chiodo et al., 2019).

Arctic Amplification and feedbacks

Changes in energy fluxes are the primary pathway for feedback processes driving climate change both globally and locally. Currently, I am involved in research activities investigating the time-dependency of feedbacks and external forcings, and their role in causing Arctic Amplification. For example, I was involved in studies which revealed that ozone depleting substances (CFCs) substantially contributed to Arctic Amplification over the second half of the 20th century (Polvani et al., 2020), and this effect is due to Arctic feedbacks rather than radiative forcing (Chiodo and Polvani, 2022). I was also involved in recent work showing that Arctic Amplification is inherently a rapid response to radiative forcing (Previdi et al., 2020). Lastly, by means of large ensembles from CESM, we have proven that feedbacks initiating Arctic Amplification start much earlier than the melting of sea-ice, and that surface heat fluxes are responsible for the rapid warming of the Arctic (Janoski et al., 2023).