Glaciers melting in the Himalayas. Sinking shorelines in Tuvalu. The mercury reaching record highs just about everywhere.
The visual signs of climate change are all around us. And for a team of researchers from Chiba University and other Japanese institutions, such indications of warming even extend to something that you wouldn’t normally think of as being visible at all: the air we breathe.
Using enormous amounts of satellite data and imagery, the team has effectively turned aerosols into tracers that can track the effects of warming on wind patterns in East Asia. The team's study, published in August in Science of The Total Environment, determined that the pathways of human-made aerosols arriving in Japan from China have shifted over the last 20 years as the planet warms.
These aerosols have major impacts on cloud formation, precipitation, solar radiation and human health, making them an important area of study as the climate crisis unfolds.
The scientists see widespread applications for the research that could help shape public opinion and government policymaking on climate change, while offering yet another visual — and, crucially — early clue that warming is affecting the planet in obvious and not-so-obvious ways.
A human-caused issue
Aerosols, or tiny particles of water or solids suspended in a gas, might not be the easiest things to detect, but they aren’t invisible.
In fact, they are quite good at reflecting the sun’s rays, leading to an uncomfortable reality: The better humans get at minimizing pollution and improving air quality, the hotter the planet will get.
Anthropogenic aerosols can be made up of substances like sulfates, nitrates and black carbon, and primarily come from the burning of fossil fuels.
It’s likely of little surprise that all are harmful to human health. Studies have shown that exposure to fine particles — known as PM2.5 and small enough to penetrate deep into the lungs — can have a devastating impact on the cardiovascular and respiratory systems.
The effect aerosols have on weather, however, is perhaps less well known.
Aerosols have direct and indirect impacts on cloud formation, a hot topic in this field of research according to Yanda Zhang, a researcher at Columbia University and an expert on aerosols and climate change, and that naturally has a knock-on effect for precipitation.
“When we increase the aerosol concentration, generally there will be more clouds,” Zhang says. But that doesn’t mean more precipitation, and in fact, the opposite is often true. “As aerosol concentrations rise, this leads to smaller and more numerous cloud droplets, which can inhibit precipitation.”
It’s long been known that winds and weather patterns move from west to east in Japan — a pattern that is being altered by climate change and a Japan team’s use of high-tech satellites has helped quantify that change.
Climate change tracers
Twenty years might feel like a long time, but when it comes to scientific observations of something complex like wind patterns, it’s not enough to draw major conclusions.
Hitoshi Irie, an associate professor with Chiba University’s Department of Earth Sciences who led the study on aerosol wind patterns, was quick to emphasize that point and the research paper concluded with a proposal for long-term observation using satellite data.
What Irie and his team did find, however, tracks with Japan’s sweltering summer this year: the country is becoming tropical.
Using observations from satellite data and imagery, which can detect aerosols due to the sunlight they reflect back into space, the researchers created a new metric to detect aerosol pathways across national borders. They found that pollution coming from China over the sea southwest of Japan toward the Pacific Ocean followed a consistent west-to-east pattern in the winter and spring, but that winds pushed aerosols northward in the summer and fall. The probability of transboundary pollution being pushed far eastward was therefore becoming low, the study concluded.
Such changing wind patterns, Irie says, could be an indicator that, from a climate perspective, Japan is shifting from the temperate zone to the tropics.
The research also suggests that wind patterns are shifting to the north, Irie says.
“Simply, the tropical region is expanding and we are now kind of at the edge of the tropical region,” he says.
Zhang, who was not involved in the study, appreciates the methods and findings of Irie’s team, but agrees that more observation is needed in order to draw clear conclusions about climate change.
“I think this metric will be already very useful to look further at, for example, the transportability of aerosols and air pollution,” Zhang says. “But I think there are some limitations to use this metric to look at climate change because climate change is complex.
“I think the researchers will need to further look into and consider different influences and impacts from different climate change factors.”
Still, in the interim, Irie hopes the methods could aid climate modeling, giving the public — and more importantly, policymakers — more accurate tools and forecasts for the crisis in the years ahead.
“Our data based on observation and our evidence obtained from observation can be used for validating climate models,” Irie says. “So if a model fails to produce the situation of the aerosol shifting, then there may be a (fault)” in the model itself.
Irie notes that the study results do not mean that transboundary pollution from China is getting worse — in fact, the opposite is true, as China has made significant strides in clearing a pollution problem that was once one of the world’s worst.
The unfortunate consequence of those positive efforts by China and other major polluters is that removing pollution from the atmosphere lowers aerosols’ cooling effect, exacerbating warming.
The environment’s catch-22
The idea that reductions in harmful aerosol pollutants were intensifying warming became a major talking point in 2023 as the planet endured its hottest year on record.
While it’s difficult to remove a reduction in pollution from other causes of warming, there’s no doubt that aerosols’ ability to reflect the sun’s rays has an impact. That’s led some, including Columbia University professor and prominent climate scientist James Hansen, to propose controversial steps to geoengineer our way out of the climate crisis.
“Highest priority is to phase down emissions, but it is no longer feasible to rapidly restore energy balance via only GHG (greenhouse gas) emission reductions,” Hansen and other authors wrote in a paper last year, adding that purposeful injection of atmospheric aerosols will be needed to mitigate the worst effects of warming.
The idea remains hotly debated and researchers, including Irie and Zhang, warn of unintended consequences, which they say could be harmful to everything from human health to food security.
“I hope that people can return to the original Earth, without pollutants and without materials which humans have produced. That is our final goal,” Irie says. “Emitting aerosols ... does slow down climate change but it does not solve climate change.”
Zhang recognizes the gravity of the climate crisis and the catch-22 with aerosols, but the researcher believes that this only adds impetus for the world to rapidly decarbonize and reduce emissions.
He compares greenhouse gasses to a blanket, while aerosols are like a parasol. At the end of the day, you’re far better off removing the blanket than keeping it on and relying on a parasol to keep you cool.
“Hopefully governments will change policies and realize that, when we decrease air pollution, which is necessary for public health, there will be more dramatic warming,” Zhang says. “We will need to make more efforts to reduce greenhouse gas emissions. It will be harder than what we expected.”
With your current subscription plan you can comment on stories. However, before writing your first comment, please create a display name in the Profile section of your subscriber account page.