The Big Haze

Whenever there’s fire, there’s always smoke. From the black exhaust coming from a diesel truck’s tailpipe, to the aromatic gray and brown plumes from a wood fire, to the invisible pollutants from the blue flame of a fossil methane flare, fire always spreads chemical byproducts into the air in the form of gases or particles. Air pollution is one of the most harmful consequences of industry, potentially playing a role in shortening the lives of over 8 million people each year, who die prematurely from strokes, heart disease, or respiratory illnesses. Climate action must address air pollution as well, and eliminating combustion does both. We need to electrify everything and generate electricity from sources that don’t require burning of fossil fuels or biomass.

When there’s smoke, there’s always a haze. And there’s so much smoke created by industry that skies are noticeably dimmer across the globe. “Global dimming” refers to the decrease in sunlight reaching the surface in recent decades, caused by smoke, haze and more reflective clouds from tiny liquid or solid particulate matter. Scientists are refer to these particles suspended in air as aerosols. There’s so much pollution in modern skies that the light reaching the Earth’s surface has been less, enough to offset some of the warming of the planet induced by heat-trapping gases.

“Global dimming” requires sophisticated equipment called radiometers to measure sunlight with very high precision, and not many such devices have been in operation continually for decades. The radiometers that exist around the world have consistently measured dimmer skies since the mid-1970s as compared with the 1950s. This is because particulate matter from air pollution has reflected or absorbed sunlight before it strikes the Earth’s surface.

What’s the (particulate) matter?

Not all smoke is created alike. The deep black soot in smoke or left behind in ashes is made of black carbon, tiny amorphous blobs of stuck-together pure carbon spherules. When black carbon collects in freshly fallen snow, it can cause rapid melting by absorbing extra sunlight. This process is the basis for the EarthGames project Soot-Out at the 0^o C Corral.

Organic carbon is made of hydrocarbons that are released by wood burning or other combustion. Organic carbon is especially present in smoldering fires, as is often associated with forest fires.

Coal smoke contains a large amount of sulfur, which is particularly bad for the health of humans and ecosystems. Sulfates form extremely fine particles that can make their way into human lungs with ease. Sulfur dioxide also readily creates sulfuric acid in the atmosphere, which eventually falls as acid rain, damaging water and soils. The haze created by sulfates are quite reflective as well. Sulfates are the main cause of global dimming.

Pollution in the past

Evidence of early air pollution can be reconstructed by examining toxins in the layers of snow that have solidified in the ice sheets of Greenland or mountain glaciers. It’s perhaps not surprising that some the most prominent early instances of air pollution were associated with the development of coined money, itself closely related to the rise of militarism and imperialism.

Consider David Graeber’s “cartoon” explanation of the birth of coined money and taxation, presented in Debt: The First 5000 Years.

“Say a king wishes to support a stand­ing army of fifty thousand men. Under ancient or medieval conditions, feeding such a force was an enormous problem    unless they were on the march, one would need to employ almost as many men and ani­mals just to locate, acquire, and transport the necessary provisions. On the other hand, if one simply hands out coins to the soldiers and then demands that every family in the kingdom was obliged to pay one of those coins back to you, one would, in one blow, turn one’s entire national economy into a vast machine for the provisioning of soldiers, since now every family, in order to get their hands on the coins, must find some way to contribute to the general effort to provide soldiers with things they want. Markets are brought into existence as a side effect.”

The rise of coinage, in turn, led to significant amounts of air pollution in places like ancient Rome. Silver mining and smelting was extremely hazardous. Each gram of silver on average produced 300 grams of the highly toxic element lead. Some of the excess lead was put to use in pipes and jewelry, and miners as well as citizens suffered from exposure to the neurotoxin. Lead steals our cognitive capacity and impulse control, and despite knowledge of these effects, Rome continued its extraction and use.

Smelting of metals to make coins also led to a huge amount of air pollution. The remnants of this pollution from ancient Rome can be seen in layers of ice in Greenland. When the empire collapsed and militarism decreased, the pollution cleaned up quickly and significantly, as did use of coinage and slave labor. Two thousand years later, air pollution is one of the deadliest consequences of industry, contributing to millions of premature deaths and asthma attacks each year.

Particulate matter and health

It’s the smallest of the particles that can most easily reach deep into our lungs. PM2.5 refers to particulate matter that is less than 2.5 microns (2.5 micrometers) in diameter. For comparison, human hair ranges in diameter from 60-100 microns. There is substantial research that links PM2.5 with cardiovascular health. While PM10 is typically confined in the upper bronchus, PM2.5 can make it all the way into our alveoli.

There have been many cases where cleaning up PM pollution led to immediate, measurable health benefits. Mortality risk dropped across the US following the Clean Air Acts of 1990. After the city of Dublin banned coal sales in 1990, respiratory deaths fell by 15%, and cardiovascular deaths by 10%. After Tokyo placed restrictions on diesel vehicles, cardiovascular mortality fell by 11% and pulmonary mortality by 22%. Each of these cities experienced a 6% drop in total non-trauma deaths. A strike at the Geneva Steel mill in Vineyard, Utah in 1986-87 was associated with 50-75% decreases in the number of children’s asthma, pneumonia, and total respiratory admissions. Many of these admissions occurred within a small number of days that were especially polluted, when weather conditions and pollution both conspired to cause very bad air quality.

Indeed, pollution on a day-to-day basis is strongly affected by the weather. Many historic air quality disasters, like in Donora, Pennsylvania in 1948, London in 1952 and East China in 2013 were influenced by weather conditions. Sometimes near-surface air is unable to vent into higher elevations in the atmosphere, in a situation called an inversion. This happens when cold air pools near the surface, while warmer, less dense air is situated aloft. Other times air currents arrange themselves in a stagnant “blocking” pattern, where a particular air mass stays over one region for days. Each of these two situations can cause poor air quality near pollution sources, and even in places where there are not many local emissions sources. Air pollution respects no state or national boundaries.

You can and should keep an eye out for air pollution alerts, from sites like AirNow or local organizations like Puget Sound Clean Air Agency. Especially the elderly, children, and those with asthma or other health concerns are particularly susceptible to air pollution events. Active community members across the world check in on vulnerable neighbors on days when there are health risks from poor air quality. Risks can be mitigated by staying inside and using air filtration systems (including budget do-it-yourself systems with a box fan).

Environmental justice and particulate matter in the US

The racial and wealth disparities in pollution exposure in the United States and internationally remain large. Black and Latinx communities in the US experience a larger share of pollution from particulate matter, and it’s not because they consume more. In fact, those communities cause less pollution.

Air pollution is extremely harmful, and strongly disadvantages communities who experience the worse impacts. It’s not clear that there’s any safe level of PM2.5 pollution. Despite the fact that there has been a decrease in air pollution across this country, which has benefitted all ethnic groups, there remains large racial differences in pollution burdens. Areas that were most polluted before the Clean Air Acts are still the most polluted today. We need to make sure that climate solutions repair prior harm and help to ensure that everyone has clean air to breathe.

Global dimming and climate

Global dimming has two causes: hazier air, detectable in clear skies when clouds aren’t around, and more highly reflective clouds. Little pieces of pollution can give water droplets something to stick to, which can make clouds more reflective or longer lived.

Particulate matter is formed from industrial pollution as well as natural sources like dust, volcanoes and sea salt. Particularly sulfates from coal burning are known to be a large cooling influence on the planet, and are also the largest single uncertainty to radiative forcing.

Radiative forcing (in TW) from particulate matter since 1750, including aerosol-radiation interactions, aerosol-cloud interactions, and black carbon on snow. The 5-95% confidence interval is displayed (Forster et al 2024).

Cooling from particulate matter doubled between the late 1940s and 1980. From the 1980s to the 2000s, the pollution-induced cooling remained relatively steady, although different regions of the world increased or decreased their pollution much more than the global average. Clean Air Acts in the United States and Europe have been successful at reducing the asthma-inducing aerosol pollution in many, but not all communities. As the US and Europe reduced its pollution, countries like China and India saw pollution skyrocket, largely due to increased burning of coal for electricity and industry. After peaking in 2006, sulfur dioxide emissions in China have decreased substantially, leading to a reduction that is visible even in global emissions. In 2020, another substantial drop in sulfur dioxide emissions occurred in the transportation sector, partially from COVID-induced decreases in transportation, but also from a switch to lower sulfur fuel oil in the shipping industry.

The uncertainty in particulate matter forcing is huge, around 700 TW in recent years. We just aren’t sure how much effect the pollution is having on sunlight. This is in part because we don’t have a clean preindustrial atmosphere for comparison. The role of haze is one of the most important uncertainties in the projection of future climate.Global emissions of sulfur dioxide by sector. Data from CEDS. 

Because sulfate particles are reflective, they cool the planet. They also help clouds to form, which also reflect away sunlight. Both of these effects, known as the direct and indirect effect of aerosols, respectively, are important in causing the approximately 500 TW of cooling from pollution.

Global emissions of black carbon by sector. Data from CEDS. 

Some aerosols are absorbing, particularly soot (also known as black carbon). Included in the cooling figure above is an offsetting heating from absorption of sunlight by soot in the atmosphere, and after it falls on the ground into snow.

Attribution of global heating

The global temperature since 1880 is plotted along with the best scientific estimate of the total radiative forcing from industrial pollution and natural factors (volcanoes, which make the air hazier after large eruptions, and solar variability). The industrial-caused radiative forcing has increased sharply in recent decades. With the exception of a few large volcanic events, El Chichón in 1982 and Pinatubo in 1991, the radiative forcing has been positive since the late 1970s. A positive radiative forcing results in the Earth heating up until the planet gets back into energy balance.

Global temperature anomaly in units of K, and radiative forcing in units of PW = 1000 TW (NASA GISS, Forster et al 2023).

The radiative forcing is punctuated by large negative excursions due to volcanoes, and temperature tends to drop a little bit after these as well. Since the radiative forcing recovers quickly after a volcanic eruption, within about 2-3 years, so does the temperature.

Comparing the long-term trends in radiative forcing shows that the natural factors are tiny as compared with recent industrial-caused heat trapping. This is the most basic proof of how we know that global heating is caused by pollution. If you also consider the uncertainties in each forcing component (like air pollution aerosols in the previous section) and the potential for natural ups and downs of global temperature, we find that the industrial pollution explains somewhere between 70% and 130% of the warming. That is, natural factors are about equally likely to have caused cooling as they have warming. And the best estimate for the pollution-caused amount is 100%.

Have you ever heard the argument, “the climate is always changing, how do we know it’s fossil fuels now?” Let’s be clear about this: pollution is the cause. There’s no scientific evidence for any other cause of global heating except pollution.