Ocean acidification may be increasing the intensity of lightning over the oceans

The anthropogenic increase in atmospheric CO2 is a significant driver of both global warming and ocean acidification. Previous research has highlighted the impact of acidification on various aspects of biogenic carbon cycling in ocean surface waters. This study explores a previously unconsidered consequence: the potential influence of ocean acidification on the intensity of lightning strikes over the oceans. The existing literature on lightning primarily focuses on charge formation, distribution, and accumulation within clouds, with models often simplifying the ground's electrical characteristics. While some studies predict increased lightning frequency due to global warming, others suggest a potential decrease. A recent report established an experimental correlation between lightning flash intensity (LFI) and the salinity of the water into which the lightning is discharged, offering a partial explanation for the observed higher lightning intensity over oceans compared to land. This difference was previously attributed to variations in updraft currents, charge buildup, and distribution in thunderstorm clouds. However, the higher conductivity of seawater compared to soil is likely the primary factor contributing to this difference. Given that acidity also affects conductivity, this study investigates the relationship between seawater acidity and LFI.

The existing literature on lightning flash intensity largely focuses on the atmospheric processes involved in charge generation and discharge, often neglecting the influence of the ground's electrical properties. Previous studies have shown differing predictions regarding the effect of global warming on lightning frequency, with some suggesting an increase and others a decrease. The authors' prior work demonstrated a positive correlation between LFI and seawater salinity, providing a possible explanation for the greater intensity of oceanic lightning compared to terrestrial lightning. However, the influence of ocean acidification on LFI has not been previously explored. This study builds upon these existing studies by considering the effect of another key physicochemical property of seawater—acidity—on lightning intensity.

LFI measurements were conducted using a laboratory setup previously described by Asfur et al. (2020). This setup involved generating electrical sparks between an electrode above the water's surface and a submerged electrode, using a Boost Step-up power converter to achieve high voltage. The LFI, represented by the integrated emission spectra (150–1050 nm) of the ionized gases, was measured using an optical fiber spectrometer. Experiments were performed using freshly sampled and filtered Mediterranean seawater (initial pH ~8.2, salinity ~39 PSU). Seawater pH was adjusted in two ways: (1) incremental addition of a strong acid (HCl) to achieve pH values ranging from 4.8 to 8.2; and (2) gentle bubbling of CO2 gas to reduce pH from 8.2 to 5.6. After each pH adjustment, LFI was repeatedly measured. Simultaneous measurements of pH, temperature, and conductivity were taken, and total alkalinity was measured at the beginning and end of each experiment. Calculated carbonate system parameters (HCO3-, CO2, and pCO2) were determined using the CO2sys2.1.xls spreadsheet, employing relevant thermodynamic constants. The data were analyzed to determine the relationship between LFI, pH, and pCO2, and a logistic model was fitted to the data to predict LFI changes based on different CO2 emission scenarios (RCPs).

The results showed a strong positive linear correlation between LFI and decreasing seawater pH for both acid addition and CO2 bubbling treatments. However, the rate of LFI increase with decreasing pH was significantly greater (factor of 2.6) in the CO2 bubbling treatment compared to the strong acid addition. This difference is likely attributable to the impact of alkalinity, which decreases with acid addition but remains unchanged during CO2 bubbling. The response of LFI to calculated seawater pCO2 also showed a stronger effect in the CO2 bubbling treatment up to ~5000 ppm. Above this level, the acid addition treatment showed a greater rate of LFI increase. These changes occur when the CO32- concentration becomes negligible compared to HCO3- and Cl- concentrations. This suggests that changes in ionic speciation, induced by changes in pH and alkalinity, influence the solution's conductivity and LFI. Within climatically relevant ranges (pH 7.5-8.2, pCO2 200-2700 ppm), LFI increased by a factor of 1.5. Comparing the pre-industrial level of pCO2 to the current level, LFI increased by 10%, while a comparison to predicted levels of pCO2 in the next century (1000 ppm under RCP8.5) suggests a 40% LFI increase. Using a logistic model fit to the experimental data and predicted pCO2 values from different RCPs, the study projected a 30 ± 7% increase in LFI by 2100 under the worst-case RCP 8.5 emission scenario, and smaller increases under less severe scenarios (3±1% to 10±2%). Additionally, the authors note that the increase in LFI observed in their experiments in response to decreased pH may be augmented by the concurrent effects of seawater warming on conductivity and therefore LFI.

The findings indicate a strong link between ocean acidification and increased lightning intensity over the oceans. The observed increase in LFI with decreasing pH is likely mediated by changes in seawater conductivity caused by altered ionic speciation. The greater impact of CO2 bubbling compared to direct acid addition underscores the role of alkalinity in this process. The projection of a significant increase in LFI under future ocean acidification scenarios suggests a positive feedback loop, where increased CO2 leads to ocean acidification, which in turn increases lightning intensity. This positive feedback could have significant implications for atmospheric chemistry and climate, as lightning is a source of NOx emissions, which influence ozone production and other atmospheric processes. However, it is crucial to recognize that the model used is based on experiments conducted at a constant temperature and salinity, and thus doesn't account for other factors that can influence lightning intensity in real-world scenarios.

This study provides strong evidence suggesting that ocean acidification may significantly increase the intensity of lightning strikes over oceans. The projected 30% increase in LFI under the RCP 8.5 scenario highlights the potential for a positive feedback mechanism between CO2 emissions, ocean acidification, and lightning activity. Further research should investigate the implications of this finding for atmospheric chemistry, climate modeling, and the broader understanding of ocean-atmosphere interactions. Future studies should also consider the combined effects of ocean acidification and warming on LFI, as well as the influence of other factors that can affect thunderstorm activity in a natural environment.

The study's main limitation lies in its reliance on laboratory-based experiments. While the experimental setup reasonably simulates the electrical processes involved in lightning, it does not fully capture the complexity of natural thunderstorms. Factors such as cloud dynamics, updraft currents, and charge distribution within clouds, which are known to influence lightning intensity and frequency, are not directly considered in the experimental design. The model is built using data from a specific seawater salinity and temperature (39 PSU and 25°C) and thus may not be directly extrapolatable to other regions or conditions. Additional research is necessary to validate these findings in the real world and incorporate other factors that influence lightning generation and intensity in thunderstorms.