Climate Change Vocabulary: Aerosols

Here’s a quick climate change riddle. What can warm and cool the Earth, cause or prevent it from raining, makes for pretty sunsets and comes from sources as disparate as campfires, oceans, and smog? Aerosols of course. In spite of all this knowledge, though, there’s still some certainty as to their exact effects on climate change.

Aerosols can be explained most simply as particles in the atmosphere. Dig a little deeper, though, and there’s a wealth of differences which account for the uncertainty of their effects.

Where They Come From

Aerosols have many sources, both natural and manmade. On the natural end of the spectrum, the two most common aerosols are salt and dust. Salt aerosols come from the sea. They get transferred from ocean to atmosphere via the foam on whitecap waves and evaporation.
Dust generally gets whipped up from deserts. Dust plumes can spread for thousands of miles from their source. The image on the right shows a dust cloud spinning off the western coast of Africa.

In the middle of the spectrum we have semi-natural sources of aerosols. Campfires and wildland fires both release smoke. This smoke contains a type of aerosol called black carbon, which is unique in the world of aerosols.

On the far end of the spectrum, manmade pollution creates aerosols. Smoke stacks, car tail pipes, and other forms of combustion all create aerosols as a side effect. Of course, these processes also generate carbon dioxide, which isn’t an aerosol but a greenhouse gas with a definite warming effect on the climate.

What They Do

Each type of aerosol has a different effect depending on its source, size, and color. Many aerosols both natural and manmade have an overall cooling effect on the Earth. This is because they tend to reflect incoming solar radiation back into space. The major exception is black carbon, which tends to absorb incoming solar radiation.

If it lands on snow when it falls out of the atmosphere, the effect can be like wearing a dark shirt on a sunny day except instead of you just getting hot, it can melt the snow at a faster rate. Black carbon is particularly worrisome for place like the Arctic which are already experiencing accelerated climate change.

Aerosols also indirectly affect rainfall. Larger (though still microscopic) aerosols like dust and salt can increase rainfall by making it easier for water droplets to grow in clouds. Most manmade aerosols are smaller help more water droplets form overall. However, these droplets are smaller in size and so are less likely condense into actually rain. Pollution can also cause local precipitation patterns to shift.

Manamde aerosols also alter the local environment. On the downside, particulate matter can exacerbate health problems like asthma. If you live in an area prone to red air alert days, you can thank aerosols for that. The elderly and young are particularly at risk. Estimates of the number of people who die from aerosol-aggravated issues are anywhere from 20,000-63,000 annually in the US.

An upside to manmade aerosols is that they can create spectacular sunsets. This happens because manmade aerosols are small enough that they scatter the shorter wavelengths of light like violet and blue. As the sun gets close to the horizon, the light has to travel further, giving those particles more of a chance to intercept the short wavelengths. That leaves a lot of red left by the time it makes it to your eye, which leads to sunsets like the photo to the left which I caught in Seattle a few days ago.

Lesser understood effects of aerosols also include how the alter cloud lifetime and cloud height, both of which can have significant impacts on the magnitude of climate change. The exact amplitude of how much warming black carbon causes is also a source of uncertainty when it comes to aerosol science.

Understanding their effects in full matters, though, if scientists are to get a more accurate picture of what climate change will look like in the future.

Photo credits: NASA/courtesy Amato Evan, NASA Goddard, and the author