The story behind this study is closely linked with two European Geosciences Union (EGU) General Assembly in Vienna, Austria. In 2017, I was a doctoral candidate at the Hong Kong Polytechnic University while seeking postdoctoral opportunities. A few weeks before I went to Vienna, together with my wife (Juanming) and our five-month-old child at that time, I sent my first and only postdoc application to Dr. Alfonso Saiz-Lopez, the leader of my current postdoc host department in the Spanish National Research Council (CSIC). I then had a very casual interview with Alfonso over a beer after his EGU halogen session. In 2018, I finished my PhD and joined CSIC as a postdoc. In 2019, I went to Vienna again to attend EGU with my colleagues (Alfonso and Carlos) and my family. After my oral presentation, Alfonso initiated this halogen-methane study over a cup of “worst ever coffee” (according to Carlos) and proposed that I lead this project.

As we all know by now, the whole world was soon shaken by the global pandemic of COVID-19. But it also provided me with a new perspective on the importance of our work. When humans encounter a life-threatening problem, should we sit back, pretend everything is fine, and when the problem is out of control, we regret that we should have done something? Or should we anticipate the probable outcome of the current situation, and make the best and knowledge-based decision before it is too late? I think that the answer is crystal clear.

In the geoscience field, climate change is one of the biggest life-threatening issues. Among all the characteristics of climate change, the uncertainty of its future evolution concerns me the most. Humans need to act before it is too late. To improve the capability of projecting the Earth’s climate, many scientists and engineers have been utilizing various methods, including field measurement, laboratory experiment, and computer modelling, in which global model prediction plays a central role in providing “a peek into the future”.

To project the probable evolution of Earth’s climate in the future, one needs to gain sufficient information about how our beloved and vulnerable planet works. Worldwide scientists are convinced that anthropogenic (human-induced) emissions, particularly greenhouse gases (GHG), dominate climate change since pre-industrial. Methane is the second-largest GHG and is actively participating in atmospheric chemistry, ergo with a relatively short lifetime (~10 years), while the other powerful GHGs have lifetimes >100 years. Note that the smaller the lifetime, the shorter the GHG stays in the atmosphere. Thus, an increasing interest has been developing to control methane as a short-term strategy. Most of the recent methane studies focus on its various sources with less attention on its loss (predominantly via hydroxyl radical but also chlorine atom).

Halogen species had long been recognized as the cause of stratospheric ozone hole. But in the past two decades, the role of reactive halogen species (RHS), with an atmospheric lifetime <180 days, in the tropospheric chemistry has been consistently reported to be significant, e.g., perturbating the budgets of both hydroxyl radical and chlorine atom.

Alfonso and his group members (including myself) have dedicated our research careers to enhancing the awareness of RHS in the broad geoscience field, but when we talk to researchers in climate science, air quality, Earth science, etc., we realize that somehow our message was not delivered and we still have a long way to go.

In the present study, we stepped out of our comfort zone (atmospheric chemistry) and quantified the significant influence of RHS on all critical methane parameters that are widely used in climate science, geoscience, etc., including methane loss rate, lifetime, burden, and radiative forcing. We also adopted a state-of-the-art Earth System model, Community Earth System Model (CESM), and forecasted the RHS impact on methane in the entire 21^st century. Our results show that the RHS effect on methane burden by the end of the century is similar to its increase in the past three to four decades, and that the RHS impact on radiative forcing is about 30% of the third largest well-mixed GHG (N₂O). With these significant results, we recommend that climate model projections of CH₄ should consider reactive halogen species and chemistry.

I am grateful to my co-authors: Rafael P. Fernandez, Ryan Hossaini, Fernando Iglesias-Suarez, Carlos A. Cuevas, Eric C. Apel, Douglas E. Kinnison, Jean-François Lamarque, and Alfonso Saiz-Lopez, for their detailed guidance and ample support. This study is funded by European Research Council Executive Agency under the European Union’s Horizon 2020 Research and Innovation Program. Computing resources, support, and data storage were provided by the Climate Simulation Laboratory at NCAR.

The detailed information about our halogen-methane study can be found in Nature Communications: https://www.nature.com/articles/s41467-022-30456-8