Changes in Shipping Emissions As a Natural Analogue for Climate Intervention: Detecting and Attributing Changes Due to Specific Human Activities As a Testbed for Future Controversies

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Geoengineering News

Jun 16, 2024, 8:29:17 AMJun 16

Daniele Visioni, Ilaria Quaglia


In 2020, new regulations from the International Maritime Organization (IMO) have resulted in a substantial reduction in the amount of SO2 emitted by vessels crossing the oceans, particularly over the Atlantic and Pacific oceans (Watson-Parris et al., 2022). Studies published before the regulations went into effect had already postulated that they would have an effect on cloud formation and direct forcing from the lack of sulfate aerosols produced, with an overall small but non-zero global impact (Jin et al., 2018). In the meantime, greenhouse gases concentrations keep rising, and there is a growing perception amongst the general public and the climate community that the latest extreme events observed have grown over the last few years: 2023 has a high likelihood of being the warmest year ever on record, while there have been record fire seasons in both Canada and Hawaii and many regions in the Northern Hemisphere have observed anomalously high sea surface temperature (SSTs), coupled with (and partially driven by) a strong positive phase of ENSO. Discussions in the news about whether some particular factors have contributed to this extreme year are growing, and many have focused on the reduction in sunlight-reflecting aerosols as a potential culprit.

Such questions are strongly tied with those around the opportunity to study Sunlight Reflection Methods (SRM) as a climate intervention strategy that may ameliorate the effects of climate change by reducing incoming sunlight, perhaps using a thin layer of aerosols in the stratosphere, where they last longer and are not as harmful to people. There is robust agreement over the cooling potential of aerosols in the climate system: at the same time, there is similar agreement over the health benefits of reducing aerosol concentrations near human centers to improve public health. SRM by means of stratospheric aerosol injections (SAI) might be a proposal that ties both considerations, but in order to consider it seriously far more research is needed to reduce uncertainties and understand how the climate system would respond.

In this study, we use both a range of observation spanning surface air temperatures (SAT), sea surface temperatures (SST) and top-of-atmosphere Earth Energy Imbalance (EEI) and a large ensemble of simulations performed with the Community Earth System Model (CESM2) (Simpson et al., 2023). Firstly, we will use the CESM2 LENS to understand the detection of the global signal and to attribute specific changes in regional climate, coupling available simulations with new ones where the aerosol emissions from shipping are quickly removed, as the LENS uses the Shared Socioeconomic Pathway (SSP) 3-7.0, where shipping emissions continue (Fig. 1). The comparison of the two scenarios will provide a needed counterfactual that a simple analysis of observational datasets, considering internal variability, does not allow. However, the comparison of multiple observational datasets (such as the Berkeley temperature record and various reanalysis products such as ERA5) will also allow for a discussion of statistical significance of the detected signal based on inherent uncertainties in our knowledge of the climate system and internal climate variability.

Secondly, we will expand this by simulating similar scenarios, but in which the aerosols are not removed completely but moved to the stratosphere in order to understand the projected differences between tropospheric and stratospheric aerosols in terms of regional climatic impacts. This will allow us to make more generalized conclusion around the issue of Climate Intervention (CI), in particular the combined health and climatic benefits of both removing aerosols from the troposphere, but adding them in the stratosphere so that they can cool without impacting negatively on surface air quality. This way of thinking about climate intervention will highlight the concept that CI should be thought of not as an addiction but as a vertical re-distribution of a fraction of the tropospheric aerosols, so that they can keep their benefit and reduce their negative impacts, and will help the community gain a better understanding of the limits of detectability for CI, which can better inform future governance discussions around outdoor tests.

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