Aerosols Enter the Equation
Aerosols Enter the EquationPrioritize...
After reading this, you should be able to:
- Define aerosols and explain how they can either cool or warm the atmosphere.
- Describe how aerosols falling onto a surface (particularly "white" surfaces, like snow) impact climate.
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When we consider the impact of human activities on global climate, it's important to recognize that not all outcomes have to lead to warming. An obvious example is the emission of aerosols into the atmosphere. The combustion of fossil fuels, while a primary driver of climate change, is also a major source of air pollution! This pollution comprises various gases and particulates, including sulfur compounds and an assortment of small solid particles like soot and ash. On a small scale, if you’ve ever lit a wood fire like the one below, you know all about this particulate matter that is emitted during the combustion process.
While these aerosols are harmful to air quality, they also have a complex impact on the Earth's climate. Remember when we talked about volcanoes ejecting sulfur dioxide (SO₂) into the stratosphere? Once there, it combines with water vapor to form sulfate aerosols. These aerosols act like tiny mirrors, reflecting incoming solar radiation away from Earth and reducing the amount of energy that reaches the surface. This reflection leads to a cooling effect.
But volcanoes aren’t the only source of sulfate aerosols. Sulfur gases released during combustion can also undergo chemical reactions in the atmosphere, producing aerosols. These tiny liquid droplets or solid particles often serve as cloud condensation nuclei—the seeds around which cloud droplets form. More aerosols in the atmosphere can lead to more clouds, and more clouds mean more sunlight is reflected back into space, increasing Earth's albedo. This is referred to as a "secondary" effect, since the cooling impact comes not directly from the aerosols themselves, but from their role in enhancing cloud formation and cloud reflectivity.
It’s also important to note that human-produced aerosols are more of a constant presence compared to those from volcanoes, which are sporadic. Scientists have observed that aerosols had a significant influence on the Earth's climate, especially during the first half of the 20th century. In fact, it’s estimated that the warming effect of greenhouse gases at the time was largely offset by the cooling effect of aerosols. Essentially, humans were pumping both warming and cooling pollutants into the atmosphere, and the overall impact was more or less a balancing act!
But by the mid-20th century, it became clear that aerosols weren’t just affecting the climate—they were also taking a toll on human health. Long-term exposure to aerosols and particulate pollutants was linked to respiratory issues and increased cancer risk. This led to the implementation of stricter air quality regulations, like the Clean Air Act of 1970 in the United States, which successfully reduced the amount of aerosols released into the atmosphere. While this has been a huge win for public health, it’s had some unintended climate consequences. As aerosol levels have dropped, so has their cooling effect.
The Clean Air Act: good for health, meh for climate
The Clean Air Act of 1970 was a landmark piece of legislation in the U.S. that set out to regulate air pollution and protect public health. At the time, cities were grappling with thick smog, acid rain, and rising rates of respiratory illnesses. The Act gave the federal government the authority to set and enforce limits on pollutants like sulfur dioxide (SO₂), nitrogen oxides (NOx), and particulate matter. This had an immediate and dramatic effect on air quality, reducing the pollutants that were causing health issues like asthma, lung disease, and even cancer.
But there’s more—by cutting down on aerosol emissions, the Clean Air Act also had a ripple effect on the climate. Cleaner air means fewer aerosols to mask the warming caused by greenhouse gases, which has led to a faster rate of warming. In short, it helped us breathe easier, but it also pulled back the curtain on the true extent of global warming. So, while we’ve made progress on air quality, the flip side is that the pace of global warming has picked up.
However, I should finish by noting that not all aerosols have a cooling effect on the climate. One key player that does the opposite is black carbon - also known as soot. This aerosol comes from incomplete combustion in sources like diesel engines, wood stoves, and the open burning of biomass. If you've ever been behind a big diesel tractor-trailer moving up a hill, you've probably seen black carbon emanating from its smokestacks. Unlike most aerosols that reflect sunlight, black carbon actually absorbs it. Think of it as the “dark side” of aerosols, absorbing incoming solar radiation and converting it into heat. In this way, black carbon warms the atmosphere, acting more like a greenhouse gas—but with a twist. While greenhouse gases absorb and emit longwave (i.e., infrared) radiation, black carbon primarily absorbs shortwave radiation from the sun while emitting and absorbing longwave radiation as well.
The impact of black carbon doesn't stop in the atmosphere. When these particles fall (or “deposit”) onto snow and ice, they create another problem. Typically, these bright surfaces reflect a lot of sunlight back into space, thanks to their high albedo (remember, white surfaces mean lots of reflection, black surfaces mean lots of absorption!). But when black carbon settles on them, it darkens the snow or ice, reducing its reflectivity. This causes more sunlight to be absorbed, leading to faster warming and accelerated melting. Imagine a thin layer of soot covering a pristine snowfield on a mountain—suddenly, that reflective surface is now a huge absorbing one of shortwave radiation.
Reducing black carbon emissions can actually help cool the climate system, which makes it an important target for climate action. In contrast to sulfate aerosols that cool the planet by reflecting sunlight, black carbon is a warming agent. So, cutting down on black carbon could be a win-win: improving air quality and slowing global warming at the same time.
