Detecting climate change using aerosols

Researchers analyzed long-term aerosol satellite observation big data focusing on the Pacific Ocean downwind of China. Using a newly developed metric that considered aerosols as tracers, they detected altered atmospheric transport patterns associated with climate change. They observed that the distance of transboundary air pollution moving east from China had shortened. Thus, long-term satellite-based Earth observations are crucial for early climate change detection and accurate evaluation of this trend.

Climate change is one of the most significant environmental challenges of present times, leading to extreme weather events, including droughts, forest fires, and floods. The primary driver for climate change is the release of greenhouse gases into the atmosphere due to human activities, which trap heat and raise Earth's temperature. Aerosols (such as particulate matter, PM2.5) not only affect public health but also influence the Earth's climate by absorbing and scattering sunlight and altering cloud properties. Although future climate change predictions are being reported, it is possible that the impacts of climate change could be more severe than predicted. Therefore, it is necessary to detect climate change accurately and as early as possible.

Building on these insights, a research team from Japan, led by Professor Hitoshi Irie from the Center for Environmental Remote Sensing at Chiba University, utilized long-term observational data to study the effect of climate change on transboundary air pollution in the downwind area of China by using aerosols. They utilized a completely unique perspective on how aerosols impact climate and developed a new metric to detect climate change by considering aerosols as tracers.

"The significance of this study lies in the fact that most of its results are derived from observational data. In natural sciences focused on Earth studies, the ultimate goal is to piece together highly accurate data obtained from observations to quantitatively understand the processes occurring on Earth and to pursue immutable truths. Therefore, the more observational data we have, the better. With the continued Earth observations by Japan's major Earth observation satellites (such as the GCOM series, GOSAT series, Himawari series, and ALOS series), we aim to complement these efforts with numerical simulations and data science methodologies to achieve a safe and secure global environment that mitigates the impacts of the climate crisis." explains Prof. Irie.

The research team included Ms. Ying Cai from the Graduate School of Science and Engineering, Chiba University, Dr. Alessandro Damiani from the Center for Climate Change Adaptation, National Institute for Environmental Studies, Dr. Syuichi Itahashi and Professor Toshihiko Takemura from the Research Institute for Applied Mechanics, Kyushu University, and Dr. Pradeep Khatri from Faculty of Science and Engineering, Soka University. Their study was made available online on May 23, 2024, and published in Science of The Total Environment on August 20, 2024.

China is a major contributor to air pollution in East Asia. The downwind area of China analyzed in this study is a unique open ocean area with minimal human interference yet an important zone of transboundary air pollution pathways, making it an ideal location for studying meteorological variations due to climate change.

In their study, the researchers analyzed aerosol optical depth (AOD) datasets derived from satellites, reanalysis datasets, and numerical simulations focused on the Pacific Ocean in the downwind area of China, over 19 years from 2003 to 2021. AOD, a measure of the amount of sunlight blocked by aerosols, is a key factor is analyzing aerosols and their impact on climate change.

The researchers developed a new metric called RAOD which utilized the potential of aerosols as tracers to evaluate the impact of climate change on transboundary air pollution pathways. Using RAOD the researchers were able to quantify significant temporal variations in aerosol transport. They discovered that long-term changes in RAOD due to climate change were outweighed by larger year-to-year variations in the meteorological field. Moreover, seasonal trends showed that aerosols moved west to east during spring and winter, and northward in summer. They concluded that the probability of aerosols from China to be transported far eastward was low, highlighting a shift in transboundary pollution pathways due to global warming. In this study the authors successfully detected climate change using long-term satellite observational data, in contrast to most existing studies that tracked transboundary air pollution using model simulations.

"These results suggest that RAOD is a valuable metric for quantifying the long-term changes in transboundary air pollution pathways due to climate change. These results are particularly significant because most of them are derived from observational data," says Prof. Irie, highlighting the importance of the study. Sharing the future implications of their study he concludes, "The effects of climate change could be more severe than currently predicted. This study will help verify climate change predictions from an unconventional perspective of 'aerosol observation,' enabling a more accurate understanding of climate change progression and implementation of rational countermeasures."

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Forest fires linked to low birth weight in newborns

Women exposed to smoke from landscape fires during pregnancy are more likely to give birth to babies with low or very low birth weights, according to findings published in eLife.

The study is the first to report a link between low birth weight and exposure to fire smoke in low and middle-income countries (LMICs), where 90% of low birth weight infants are born and landscape fires are prevalent.

Landscape fires, such as wildfires, tropical deforestation fires and agricultural biomass burning, play an important role in maintaining terrestrial ecosystems. Yet, landscape fire smoke is triggering a costly and growing global public health problem, causing recurrent episodes of pollution mostly affecting LMICs.

Previous studies have shown that exposure to fire smoke during pregnancy is linked to low birth weight, which itself is a public health problem in LMICs. Reducing the risk of low birth weight is one of the World Health Organization's global targets for 2025.

"Babies with low birth weights are at higher risk of a range of diseases in later life compared to normal weight newborns," explains co-first author Jiajianghui Li, a PhD student at the Institute of Reproductive and Child Health, School of Public Health Science Centre, Peking University, China. "Several studies have shown the effects of landscape fire smoke on acute lung and heart conditions, but the health impacts of these pollutants on susceptible pregnant women are not well known. We wanted to explore the association between birth weight and exposure to fire source pollution across several countries and over a long time period."

The researchers conducted a case-control study in 54 LMICs where they matched 108,137 groups of siblings to their mothers. They used surveys conducted by the US Agency for International Development between 2000 and 2014 to find out information about sibling birth weights and other health and demographic factors. They then assessed exposure to landscape fire pollutants using data on fire emissions from the Global Fire Emission Database and a model that converted this data into ground-surface concentrations of particulate matter in different regions.

Their analysis showed that an increase in exposure of one microgram per cubic metre of fire-sourced particulate matter was associated with a 2.17-gram reduction in birth weight. "The effect was even more pronounced when we looked at whether exposure to fire smoke was linked to low or very low birth weight; for every microgram per cubic metre increase in particulate matter exposure, the risks of low and very low birth weight increased by around three and 12 per cent, respectively," says co-first author Tianjia Guan, an assistant professor at the Department of Health Policy, School of Health Policy and Management, Chinese Academy of Medical Sciences and Peking Union Medical College, China.

The researchers found that very low birth weight was most strongly linked to the pollution. To find out why, they developed a model that looked at the average birth weight of infants within single families. Newborns in families that had lower birth weights on average were more susceptible to the risks of fire smoke pollution than those who had moderate baseline birthweights. "This suggests that other factors affecting maternal and foetal health, such as nutrition or maternal employment status, might make mothers and their developing infants even more susceptible to the risks of pollution," says co-first author Qian Guo, a PhD student at the School of Energy and Environmental Engineering, University of Science and Technology, China.

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Californian smoke drifted as far as Europe in 2020 and caused heavy clouding of sun

The smoke from the extreme forest fires on the US West Coast in September 2020 travelled over many thousands of kilometres to Central Europe, where it continued to affect the atmosphere for days afterwards. A comparison of ground and satellite measurements now shows: The forest fire aerosol disturbed the free troposphere over Leipzig in Germany as never before. An evaluation by an international research team led by the Leibniz Institute for Tropospheric Research (TROPOS) revealed an extraordinary optical thickness on 11 September 2020, which attenuated sunlight by a third. The study, published in Geophysical Research Letters, is the first publication to show that ESA's novel Aeolus satellite can not only reliably measure global wind profiles but also aerosols in the atmosphere as it was shown by comparing Aeolus measurements with lidar measurements from the ground. The Centre National de Recherches Météorologiques (CNRM) of the University of Toulouse, the German Aerospace Center (DLR) and the European Space Agency (ESA) were involved in the study.

Since August 2018, a new type of research satellite has been orbiting the Earth, named after a Greek wind god -- Aeolus. The aim of Aeolus is to actively measure wind from space and thus improve weather forecasting. On board of this satellite of the European Space Agency (ESA) is the "Atmospheric Laser Doppler Instrument" (ALADIN), a high-performance laser. ALADIN is the first instrument in space that can actively measure vertical profiles of wind speed. It uses the principle of a light radar (short: Lidar from "LIght Detection And Ranging"). A signal is emitted and the reflection provides information about location and distance. The Doppler effect is then used to measure the wind speed at different heights in the atmosphere. To validate the laser measurements in space, they are compared with laser measurements from the ground. Several research groups from Germany are involved in this effort within the framework of the EVAA initiative (Experimental Validation and Assimilation of Aeolus observations). TROPOS, for example, measures with its lidar devices every Friday evening and Sunday morning when the Aeolus satellite flies over Leipzig. The data from ground and space can then be compared. On 11 September 2020, this resulted in the rare constellation that the extraordinary plume of smoke from the Californian forest fires could be measured over Leipzig simultaneously from ground and from space.

"Using revolutionary laser technology, Aeolus is currently the only satellite in the world that can measure profiles of horizontal wind speed as well as the backscatter and extinction of aerosols and clouds independently. The satellite thus provides valuable information on the radiative properties of these smoke aerosols," emphasises Dr Sebastian Bley of TROPOS, who has been involved in the Aeolus project at the European Space Agency's (ESA) ESRIN research centre for the past three years. "It is expected that this unique configuration will contribute to improved predictions of such global smoke dispersion but also of weather in general."

In September 2020, the heat from the extreme forest fires on the US West Coast transported the smoke to high altitudes. Once high up, it was then transported with the jet stream across North America and the Atlantic to Europe. In Leipzig, Germany, the smoke layer appeared at an altitude of around 12 kilometres on the morning of 11.09.2020 and sank to an altitude of around 5 kilometres in the course of the day. This is shown by the data from the PollyXT lidar at TROPOS. Lidar measurements in Leipzig confirmed the strong attenuation of the direct sunlight on this Friday: "It was -- measured by the Aerosol Optical Thickness (AOT) -- the strongest influence of forest fire aerosol on the free troposphere above Leipzig ever observed since the beginning of regular lidar observations in 1997," reports Dr Holger Baars from TROPOS, "The free troposphere is the region of the atmosphere in which the weather takes place but the direct influence from the ground is low. We were able to estimate an average mass concentration of forest fire aerosol of 8 micrograms per cubic metre between 4 and 11 km altitude. At the peak it was even 22 micrograms per cubic metre -- that's quite remarkable for these altitudes." Saturday and Sunday were hazy days despite cloudless skies. The UV index of the Federal Office for Radiation Protection (BfS), among others, also showed how strongly the smoke layers dampened solar radiation in Saxony: the TROPOS station in Melpitz near Torgau registered about a quarter less UV radiation at noon on 12 September 20 than would have been possible under clear sky. The unusual state of the atmosphere was particularly striking at sunset with a distinctive milky-yellow light.

The researchers were able to confirm the origin of the smoke using a computer model: The backward simulation proves that the air masses that arrived at noon on 11 September at an altitude of 8.5 km above Leipzig originated from the west coast of North America, where intense fires took place days before. The frequency and intensity of fires in California continued to increase during the first week of September as satellite images show. Slightly weaker fires were observed in Oregon, Washington and Montana. "Due to the prevailing winds, the travel time of the smoke from the US West Coast to Europe was only about 3 to 4 days. The air masses even made the approximately 3000 kilometres across the Atlantic Ocean between Newfoundland and Ireland at high speed in only one day (9 September)," explains Martin Radenz from TROPOS.

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