Hidden cost of lowering air pollution

Drop in air pollution has made addition to global warming called the 'clean air paradox' 

Scientists have been faced with a huge dilemma, as research reveals that reducing air pollution has increased global warming. While smog kills millions of people every year, it also whitens clouds – making them more reflective. So by slashing air pollution, we're inadvertently diminishing the brightness of clouds, which are key regulators of global temperature. Earth’s mirror is dimming over two of its largest ocean basins, and the lost shine is turning into heat. Marine cloud reflectivity, caused by cleaner air, has dropped by roughly 2.8% per decade across the North Atlantic and Northeast Pacific. Together, those regions span about one-seventh of the planet’s surface, making even small changes in brightness significant on a global scale. In the new study, researchers from the University of Washington analysed the clouds over the Northeastern Pacific and Atlantic oceans between 2003 and 2022. Their analysis revealed that the clouds have become less reflective. And according to the experts, approximately 70% of this change can be attributed to reductions in air pollution. 'When you cut pollution, you're losing reflectivity and warming the system by allowing more solar radiation, or sunlight, to reach Earth,' explained lead author Knut von Salzen. 'We may be underestimating warming trends because this connection is stronger than we knew. 'I think this increases the pressure on everyone to rethink climate mitigation and adaptation because warming is progressing faster than expected.'

The finding helps explain why recent warming has outpaced many forecasts. It points to a simple driver with complex physics: fewer air particles mean dimmer clouds and more shortwave energy reaching the sea surface. The study was led by Dr. Knut von Salzen, a senior research scientist at the University of Washington (UW) whose work explores how tiny airborne particles and cloud microphysics shape Earth’s heat balance. Clouds play a major cooling role by reflecting sunlight back into space, a property known as albedo, the fraction of light a surface reflects. Low clouds over cooler oceans handle much of this job. But satellite data now reveal a subtle shift. The cloud radiative effect, the net change in energy caused by clouds, has weakened as marine clouds brightened less and covered less area. The team zeroed in on the Northeast Pacific and North Atlantic, where ocean temperatures have risen rapidly over the past two decades. Measurements from NASA’s CERES EBAF dataset show a steady drop in reflected shortwave radiation in these regions. Together, these basins cover roughly 14% of Earth’s surface. Even a modest decline in cloud reflectivity can translate into a measurable boost in global warming.

While smog kills millions of people every year, it also whitens clouds, making them more reflective. So by slashing air pollution, we're inadvertently diminishing the brightness of clouds, which are key regulators of global temperature. Global aerosol declines didn’t happen by chance. Independent analyses show sharp drops in sulphur dioxide emissions from power plants in China during the 2010s, with similar pollution cuts seen across other nations. Yet even as the air grew cleaner, heat-trapping gases continued to rise. Long-term measurements from NOAA’s Mauna Loa Observatory show a steady increase in atmospheric CO2, adding to the planet’s overall heat gain. This progress carries a trade-off. Lower particle pollution brings major health benefits but also reduces the number of cloud condensation nuclei (CCN), the microscopic seeds that water vapour condenses on to form droplets. With fewer particles, clouds become less reflective and dissipate more easily. Aerosol particles form cloud droplets by attracting water vapour. Airborne aerosols have declined as pollution controls tightened. With fewer particles, droplets grow larger and clouds lose some brightness and tend to rain out sooner. “We may be underestimating warming trends because this connection is stronger than we knew,” said Dr. von Salzen. Two classic processes put numbers to that story. The Twomey effect, more particles make clouds brighter, weakens as air gets cleaner. The Albrecht effect, fewer particles can shorten cloud lifetime, also grows in a cleaner atmosphere.

With global warming accelerating faster than ever, scientists around the world have been scrambling to understand the factors contributing to this increase. The team focused on the impact of air pollution. Most previous studies have focused on the benefits of reducing smog. For example, a study published earlier found that up to 250,000 deaths from poor air quality could be prevented annually in central and western Europe by 2050 if emissions are drastically reduced. As a result, there have been global efforts to limit pollution, with a gradual transition from oil and gas to clean energy. Many Earth system models have not fully captured the observed changes. The new simulations improved accuracy by refining how particles activate into droplets and how droplet size influences drizzle and cloud cover. Those improvements allowed the model to reproduce both the magnitude and the geographic footprint of the reflectivity drop. They also linked most of the change to aerosol reductions rather than ocean temperature shifts alone. In the absence of aerosols, each cloud droplet carries more water but with aerosols, that water is dispersed between more droplets, impacting how reflective the cloud is and how long it lasts for. “We do not want to go back in time and take away the Clean Air Act,” said Sarah Doherty, senior research scientist at UW. “But we need to understand how cleaner air changes the planet’s energy balance.”

However, this reduction has had a surprising effect. 'Reductions in particulate air pollutants are contributing to accelerated warming,' said Sarah Doherty. For the study, the team analysed 20 years of satellite data, documenting cloud dynamics above the Northeastern Pacific and Atlantic oceans, which are warming faster than almost anywhere else on Earth. Satellite observations show a clear trend, fewer shiny droplets, larger droplet sizes, and faster drizzle. Together, those shifts make marine clouds less reflective and expose darker ocean surfaces to more sunlight. The physics form a positive feedback loop: as oceans warm, low clouds thin, allowing even more shortwave radiation to reach the surface and amplify the initial heating. In response, some researchers are exploring marine cloud brightening (MCB), a proposed method to restore reflectivity by spraying fine sea-salt particles into the air to seed brighter clouds. The concept aims to mimic nature by using salt rather than industrial pollution, but the science is still evolving. The analysis revealed that air pollution influences clouds in two ways. Firstly, small particles give water droplets something to cling to, and with a fixed amount of water, more aerosols means more small, reflective droplets in the clouds. By the same logic, reducing aerosols increases cloud droplet size. Secondly, large droplets are heavier, and quicker to fall to Earth as precipitation, which decreases cloud cover. Given the detrimental impact of air pollution on human health, the solution is clearly not to increase air pollution. Instead, the researchers say there are several interventions we could consider that aim to make the clouds shinier without polluting the air. This includes 'marine cloud brightening', a technique that involves spraying seawater into the air to make clouds more reflective. 

'You could think of it as replacing unhealthy pollutant particles with another type of particle which is not a pollutant, but that still provides a beneficial cooling effect,' explained Robert Wood, a professor of atmospheric and climate science at the University of Washington. However, before these methods can be implemented, more research is need to check they're safe, and don't have any unintended consequences. NOAA reviews highlight key unknowns, including how predictable and safe such interventions would be if tested on a large scale. The satellite image taken from ships, which emit sulphate aerosols that can be toxic to humans and the environment. Aerosols also make the clouds more reflective. Efforts to curb pollution have reduced the amount of sunlight Earth reflects and accelerated warming, a new University of Washington study shows. In a recent study, scientists from New York University warned that marine cloud brightening could wreak havoc on the weather, and even make climate change worse. Following are the emissions:-

Sulfur dioxide 

Sulfur dioxide (SO2) also primarily comes from fossil fuel burning, but can also be released from car exhausts. SO2 can react with water, oxygen and other chemicals in the atmosphere to cause acid rain. 

Carbon dioxide

Carbon dioxide (CO2) is one of the biggest contributors to global warming. CO2 concentration has fluctuated over the last 800,000 years between 180 to 280ppm, but has been vastly accelerated by pollution caused by humans. After the gas is released into the atmosphere it stays there, making it difficult for heat to escape, and warming up the planet in the process. It is primarily released from burning fossil fuels such as coal, oil and gas, as well as cement production. The average monthly concentration of CO2 in the Earth's atmosphere, as of April 2019, is 413 parts per million (ppm). Before the Industrial Revolution, the concentration was just 280 ppm. 

Carbon monoxide 

Carbon monoxide (CO) is an indirect greenhouse gas as it reacts with hydroxyl radicals, removing them. Hydroxyl radicals reduce the lifetime of carbon dioxide and other greenhouse gases. 

Nitrogen dioxide 

There is far less NO2 in the atmosphere than CO2, it is between 200 and 300 times more effective at trapping heat. The gas nitrogen dioxide (NO2) comes from burning fossil fuels, car exhaust emissions and the use of nitrogen-based fertilisers used in agriculture. 

Short-term forecasts may soon need to factor in dimmer clouds in cleaner air. As aerosol emissions continue to fall, the study points to a persistent push toward less reflective marine clouds over the North Atlantic and Northeast Pacific. The insight doesn’t change the urgency of cutting greenhouse gases. But it does refine climate projections by showing how cleaner air can unmask hidden warming once offset by pollution particles. Particulate matter refers to tiny parts of solids or liquid materials in the air. Some are visible, such as dust, whereas others cannot be seen by the naked eye. Materials such as metals, microplastics, soil and chemicals can be in particulate matter. Particulate matter (or PM) is described in micrometres. The two main ones mentioned in reports and studies are PM10 (less than 10 micrometres) and PM2.5 (less than 2.5 micrometres). Air pollution comes from burning fossil fuels, cars, cement making and agriculture. Scientists measure the rate of particulates in the air by cubic metre. Particulate matter is sent into the air by a number of processes including burning fossil fuels, driving cars and steel making. Particulates are dangerous because those less than 10 micrometres in diameter can get deep into your lungs, or even pass into your bloodstream. Particulates are found in higher concentrations in urban areas, particularly along main roads. Scientists are now watching several key indicators. One is radiative forcing, the energy imbalance that drives warming or cooling. In these ocean basins, reduced aerosols have boosted the sunlight absorbed at the surface, adding regional heat pressure. Another is the cloud radiative effect, which tracks how much energy clouds reflect or trap. Long-term satellite records help distinguish short-lived variations from lasting changes linked to cleaner air and shifting policy.

According to the WHO, a third of deaths from stroke, lung cancer and heart disease can be linked to air pollution. Some of the effects of air pollution on the body are not understood, but pollution may increase inflammation which narrows the arteries leading to heart attacks or strokes. As well as this, almost one in 10 lung cancer cases in the UK are caused by air pollution. Particulates find their way into the lungs and get lodged there, causing inflammation and damage. As well as this, some chemicals in particulates that make their way into the body can cause cancer. Around seven million people die prematurely because of air pollution every year. Pollution can cause a number of issues including asthma attacks, strokes, various cancers and cardiovascular problems. Women exposed to air pollution before getting pregnant are nearly 20% more likely to have babies with birth defects. Living within 3.1 miles (5km) of a highly-polluted area one month before conceiving makes women more likely to give birth to babies with defects such as cleft palates or lips, a study by University of Cincinnati found. For every 0.01mg/m3 increase in fine air particles, birth defects rise by 19%. Previous research suggests this causes birth defects as a result of women suffering inflammation and 'internal stress'. Air pollution can cause problems for asthma sufferers for a number of reasons. Pollutants in traffic fumes can irritate the airways, and particulates can get into your lungs and throat and make these areas inflamed. 

The Paris Agreement, which was first signed in 2015, is an international agreement to control and limit climate change. It hopes to hold the increase in the global average temperature to below 2°C (3.6ºF) 'and to pursue efforts to limit the temperature increase to 1.5°C (2.7°F)'. International carbon credits let nations continue emitting carbon while paying for trees to be planted elsewhere, balancing out their emissions. The UK government has announced plans to make the country carbon neutral by 2050. They plan to do this by planting more trees and by installing 'carbon capture' technology at the source of the pollution. Some critics are worried that this first option will be used by the government to export its carbon offsetting to other countries. In 2017, the UK government announced the sale of new petrol and diesel cars would be banned by 2040.  However,  MPs on the climate change committee have urged the government to bring the ban forward to 2030, as by then they will have an equivalent range and price. The speedy electrification of Norway's automotive fleet is attributed mainly to generous state subsidies. Electric cars are almost entirely exempt from the heavy taxes imposed on petrol and diesel cars, which makes them competitively priced. The Committee on Climate Change (CCC) has said there is a 'shocking' lack of Government preparation for the risks to the country from climate change. The committee assessed 33 areas where the risks of climate change had to be addressed, from flood resilience of properties to impacts on farmland and supply chains, and found no real progress in any of them. The UK is not prepared for 2°C of warming, the level at which countries have pledged to curb temperature rises, let alone a 4°C rise, which is possible if greenhouse gases are not cut globally, the committee said.