Isolating the impact of North American and European anthropogenic aerosol emissions since the early instrumental period

Anthropogenic aerosols have been identified as an important driver of global and regional climate. Globally, aerosols are estimated to have offset much of the positive forcing due to greenhouse gases; regionally, their effect can be dominating, and can potentially drive climate anomalies far from the emission sources due to changes in the atmospheric circulation. Aerosols emitted from North America (NA) and Europe (EU) dominated the global aerosol loading until the late twentieth century. Despite recent progress, our knowledge of the climate imprint of NA and EU aerosols is still incomplete, especially regarding the decades before the mid-twentieth century, in which emissions were still lower and did not yet change as rapidly as later, but might have been more effective due to non-linearities in the aerosol-cloud interactions. The overarching goal of this work is thus to determine robust features of the impact of NA and EU aerosols on regional and large-scale climate and to advance current understanding of the underlying mechanisms, compared to those generated by other forcing agents as well as aerosols from other geographical regions. The study focuses mainly on the period of increasing sulphur dioxide (SO2) emissions -precursor of sulphate aerosols, the most abundant anthropogenic aerosol species- from NA and EU sources (1850-1975), and on identifying the aerosol impact over the Atlantic and Eurasian domain, where North American and European aerosols are presumed to have relevant impact. Along with observations, existing historical simulations from a range of coupled climate models are studied and complementary experiments performed and analysed. First, the boreal summer climate response to North American and European (NAEU) anthropogenic aerosol emissions during the twentieth century is characterised using a suite of models from the Coupled Model Inter-Comparison Project 5 (CMIP5). Supported by the co-variability of aerosol optical depth and near-surface climate, long-term variations in aerosol-only and all-forcing simulations are attributed to NAEU aerosol forcing if they undergo a significant reversal coinciding with the peak in NAEU SO2 emissions, measured by inter-model agreement on the sign of linear trends before and after 1975. Regionally, robust aerosol impact is found on Eurasian near-surface temperature, pressure, and diurnal temperature range; remotely, robust aerosol impact is found on the Inter-Tropical Convergence Zone (ITCZ) position and the subtropical jet stream. The contribution of anthropogenic aerosol forcing to the forced component of simulated inter-decadal climate variability of European-mean near-surface temperature is furthermore estimated to be more than a third throughout the twentieth century. Observed variations also of European-mean sea level pressure and diurnal temperature range tend to agree better with simulations that include aerosols. These findings highlight significant aerosol impact on Eurasian climate already in the first half of the twentieth century. The aerosol impact on observed West African and South Asian monsoon precipitation is then investigated by using a detection and attribution (D&A) approach. The aerosol source regions (NAEU, South Asia, or China) which are most important for explaining the observed 1920-2005 changes are identified. For this, fingerprints of the response to regional-aerosol forcing are derived from historical simulations with the GFDL-CM3 model along with CMIP5 simulations. It is found that in precipitation observations for West Africa, the only anthropogenic forcing which can be detected are NAEU emissions. In precipitation observations for South Asia, in contrast, local emissions are the only external forcing detected. Changes in West Africa are related to a meridional shift in the ITCZ due to aerosol-induced changes in the inter-hemispheric temperature gradient. Changes in South Asia, in contrast, are associated with a weakening of the monsoon circulation, driven by the increase of remote NAEU aerosol emissions until 1975 and since then by the increase in local emissions offsetting the decrease in NAEU emissions. These findings show for the first time that the aerosol forcing from individual emission regions is strong and distinct enough to be detected in the presence of internal variability. Finally, the dynamical impact of NA and EU sulphate aerosol emissions is fully analysed in the coupled Community Earth System model (CESM1-CAM5), focusing on the Atlantic. For this, multi-member ensemble simulations covering the period 1850- 1975 are performed, and the response to emissions from NA and EU is contrasted. The results show that sulphate aerosols from either source cause a long-term cooling of North Atlantic sea-surface temperatures (SSTs), with the patterns a combination of atmospheric aerosol effects and an aerosol-induced strengthening of the Atlantic Meridional Overturning Circulation (AMOC). The North Atlantic response to NA emissions is larger than that to EU emissions, with stronger indirect aerosol effects due to a wider aerosol spread over the Atlantic and collocation with climatological cloud cover. A southward shift of the ITCZ, affecting tropical precipitation globally, is also found. The (multi)decadal variability components of Atlantic SSTs and of the AMOC are furthermore both found to be externally forced. A suppression of Atlantic Tropical Hurricane frequency and a north-eastward shift of Atlantic extra-tropical storms in response to both NA and EU emissions are finally shown. The analysis provides novel insights into the mechanisms of aerosol impact on the Atlantic. Overall, the results from this work represent a significant contribution to advance our understanding of the historical impact of anthropogenic aerosols over the entire twentieth century and in particular that of aerosols from NA and EU by finding robust signals across models, using statistically rigorous methods to detect forced impact in observations, and analysing new model experiments. The findings emphasise the importance of historical anthropogenic aerosol emissions already before the late twentieth century and shed light on differences in the climate response to aerosols depending on their emission region, which will also be relevant for understanding future patterns of change related to further emission reductions

Values in climate modelling: testing the practical applicability of the Moral Imagination ideal

There is much debate on how social values should influence scientific research. However, the question of practical applicability of philosophers’ normative proposals has received less attention. Here, we test the attainability of Matthew J. Brown’s (2020) Moral Imagination ideal (MI ideal), which aims to help scientists to make warranted value-judgements through reflecting on goals, options, values, and stakeholders of research. Here, the tools of the MI ideal are applied to a climate modelling setting, where researchers are developing aerosol-cloud interaction (ACI) parametrizations in an Earth System Model with the broader goal of improving climate sensitivity estimation. After the identification of minor obstacles to applying the MI ideal, we propose two ways to increase its applicability. First, its tools should be accompanied with more concrete guidance for identifying how social values enter more technical decisions in scientific research. Second, since research projects can have multiple goals, examining the alignment between broader societal aims of research and more technical goals should be part of the tools of the MI ideal

How do value-judgements enter model-based assessments of climate sensitivity?

Philosophers argue that many choices in science are influenced by values or have value-implications, ranging from the preference for some research method’s qualities to ethical estimation of the consequences of error. Based on the argument that awareness of values in the scientific process is a necessary first step to both avoid bias and attune science best to the needs of society, an analysis of the role of values in the physical climate science production process is provided. Model-based assessment of climate sensitivity is taken as an illustrative example; climate sensitivity is useful here because of its key role in climate science and relevance for policy, by having been the subject of several assessments over the past decades including a recent shift in assessment method, and because it enables insights that apply to numerous other aspects of climate science. It is found that value-judgements are relevant at every step of the model-based assessment process, with a differentiated role of non-epistemic values across the steps, impacting the assessment in various ways. Scrutiny of current philosophical norms for value-management highlights the need for those norms to be re-worked for broader applicability to climate science. Recent development in climate science turning away from direct use of models for climate sensitivity assessment also gives the opportunity to start investigating the role of values in alternative assessment methods, highlighting similarities and differences in terms of the role of values that encourage further study

Detectable Impact of Local and Remote Anthropogenic Aerosols on the 20th Century Changes of West African and South Asian Monsoon Precipitation

Signo y Seña es la revista especializada del Instituto de Lingüística de la Facultad de Filosofía y Letras de la Universidad de Buenos Aires. Convoca y está destinada a investigadores del lenguaje formados y en formación tanto de la Argentina como del exterior. Es una publicación semestral electrónica y de libre acceso que tiene como objetivo fundamental promover el debate, el intercambio de ideas y la difusión de investigaciones recientes de todas las áreas de la lingüística. Signo y Seña incluye en cada número: una sección temática o monográfica a cargo de reconocidos especialistas en el área,una sección general de resultados investigaciones de todas las áreas de la Lingüística y una sección de reseñas y novedades referentes a la disciplina. En sus páginas se publican trabajos originales e inéditos evaluados con el sistema de pares externos doble ciego en español y en portugués

Learning from the 2018 heatwave in the context of climate change: Are high-temperature extremes important for adaptation in Scotland?

To understand whether high temperatures and temperature extremes are important for climate change adaptation in Scotland, we place the 2018 heatwave in the context of past, present, and future climate, and provide a rapid but comprehensive impact analysis. The observed hottest day, 5-day, and 30-day period of 2018 and the 5-day period with the warmest nights had return periods of 5-15 years for 1950-2018. The warmest night and the maximum 30-day average nighttime temperature were more unusual with return periods of >30 years. Anthropogenic climate change since 1850 has made all these high-temperature extremes more likely. Higher risk ratios are found for experiments from the CMIP6-generation global climate model HadGEM3-GA6 compared to those from the very-large ensemble system weather@home. Between them, the best estimates of the risk ratios for daytime extremes range between 1.2-2.4, 1.2-2.3, and 1.4-4.0 for the 1-, 5-, and 30-day averages. For the corresponding nighttime extremes, the values are higher and the ranges wider (1.5->50, 1.5-5.5, and 1.6->50). The short-period nighttime extremes were more likely in 2018 than in 2017, suggesting a contribution from year-to-year climate variability to the risk enhancement of extreme temperatures due to anthropogenic effects. Climate projections suggest further substantial increases in the likelihood of 2018 temperatures between now and 2050, and that towards the end of the century every summer might be as hot as 2018. Major negative impacts occurred, especially on rural sectors, while transport and water infrastructure alleviated most impacts by implementing costly special measures. Overall, Scotland could cope with the impacts of the 2018 heatwave. However, given the likelihood increase of high-temperature extremes, uncertainty about consequences of even higher temperatures and/or repeated heatwaves, and substantial costs of preventing negative impacts, we conclude that despite its cool climate, high-temperature extremes are important to consider for climate change adaptation in Scotland

Machine Learning Approach to Investigating the Relative Importance of Meteorological and Aerosol-Related Parameters in Determining Cloud Microphysical Properties

Aerosol effects on cloud properties are notoriously difficult to disentangle from variations driven by meteorological factors. Here, a machine learning model is trained on reanalysis data and satellite retrievals to predict cloud microphysical properties, as a way to illustrate the relative importance of meteorology and aerosol, respectively, on cloud properties. It is found that cloud droplet effective radius can be predicted with some skill from only meteorological information, including estimated air mass origin and cloud top height. For ten geographical regions the mean coefficient of determination is 0.41 and normalised root-mean square error 24%. The machine learning model thereby performs better than a reference linear regression model, and a model predicting the climatological mean. A gradient boosting regression performs on par with a neural network regression model. Adding aerosol information as input to the model improves its skill somewhat, but the difference is small and the direction of the influence of changing aerosol burden on cloud droplet effective radius is not consistent across regions, and thereby also not always consistent with what is expected from cloud brightening