What are aerosols?
In climate science, when we talk about aerosols, we're not referring to "aerosol sprays" like those used for deodorants. It's a technical term designating tiny particles that can be found suspended in the air all around us and in the atmosphere above us. "Tiny particles" really means minuscule: they are comparable in size to a red blood cell, and are therefore invisible to the naked eye. Yet they are omnipresent in our atmosphere, and like greenhouse gases, they have a crucial influence on the climate.
Illustration of aerosol size, represented here by the small blue and red particles. Source: u003ca href=u0022https://www.epa.gov/pm-pollution/particulate-matter-pm-basics#effectsu0022u003eUS EPAu003c/au003e.
These tiny particles come from various sources, which can be classified into two categories: natural sources and those linked to human activity. For example, small dust particles from the Sahara, or salt particles torn from the ocean by winds are aerosols of natural origin. Volcanic eruptions and forest fires are also natural sources, emitting significant quantities of aerosols into the atmosphere. Human activity, with its CO2 emissions, is also accompanied by emissions of carbonaceous aerosols. Thus, industry, transport and agriculture are important sources of aerosol emissions linked to human activity.
Visualization of aerosols emitted into the atmosphere. In red, mineral dust (e.g. desert dust), in green carbonaceous aerosols (e.g. forest fires, human activity), in white sulfate aerosols (e.g. industry, volcanic eruptions), in blue sea salt aerosols. Source: u003ca href=u0022https://svs.gsfc.nasa.gov/30017/u0022u003eNASA GOES-5 Nature runu003c/au003e.
It is estimated that aerosol emissions linked to human activity are responsible for nearly 7 million deaths each year. In particular, the smallest particles, commonly called "fine particles", have the ability to penetrate our lungs, thus increasing the risk of developing respiratory infections and cardiovascular diseases. Morocco is not immune to this reality, with a concentration of fine particles in the air exceeding more than 5 times the recommendations of the World Health Organization in major cities like Casablanca.
In Casablanca, it is estimated that the concentration of fine particles is 5.2 times higher than World Health Organization recommendations, and is responsible for nearly 13,000 deaths each year. Source: Breathe Life 2030. Source: u003ca href=u0022https://breathelife2030.org/flat-html/u0022u003eBreathe Life 2030u003c/au003e.
Why are they important for the climate?
Beyond the health threat they represent, aerosols also have a fundamental impact on the climate. We know well that greenhouse gases such as CO2 or methane contribute to warming the climate. Well, for aerosols it's the opposite: they contribute to cooling the climate, or more precisely, to masking part of the warming. And that's not necessarily good news.
Two main modes of action can be distinguished by which aerosols have a cooling effect on the climate. The first is called the direct effect: aerosols suspended in the atmosphere act directly by reflecting the sun's rays. Thus, this solar energy is sent directly back into space, without ever reaching the Earth's surface, and consequently does not contribute to warming the Earth.
The second mode of action, which is the most important, is called the indirect effect, and is linked… to clouds. And yes, we know clouds are water vapor, but they're also aerosols! In fact, these tiny particles actually serve as nuclei around which water accumulates to form clouds: without aerosols, there's no accumulation, and therefore no clouds.
Now, the more aerosols there are suspended in the atmosphere, the brighter, larger and longer-lasting the clouds formed will be. And larger and brighter clouds reflect more solar rays. Therefore, an increase in aerosols in the atmosphere favors the creation of more reflective clouds, thus preventing more solar energy from warming the Earth's surface. This indirect effect is the main component of the role of aerosols in planetary climate.
The direct effect of aerosols (left) consists of directly reflecting the sun's rays. The indirect effect of aerosols (right), the most important, consists of promoting the formation of clouds which will then reflect the sun's rays. Illustration adapted from Figure 7.3, u003ca href=u0022https://www.ipcc.ch/site/assets/uploads/2018/02/WG1AR5_Chapter07_FINAL-1.pdfu0022u003eIPCC WG1 Chapter 7 Clouds and Aerosolsu003c/au003e.
These two modes of action are at the origin of the cooling effect of aerosols on the climate. Therefore, without the presence of aerosols in the atmosphere, the manifestations of global warming we observe today would be much more pronounced. Thus, aerosols contribute to masking part of the warming caused by greenhouse gases. But how much?
It is estimated that the warming mitigation attributable to aerosols linked to human activity could reach as much as -1°C. However, it is also possible that this mitigation is almost negligible. This wide uncertainty about the magnitude of the masking of warming by aerosols is one of the major questions currently in climate science, and a fundamental question in understanding the stakes of global warming.
Breakdown of the different contributions to temperature anomaly in climate models. Greenhouse gases linked to human activity (red) contribute a warming effect while aerosols linked to human activity (blue) contribute a cooling effect. Source: u003ca href=u0022https://www.ipcc.ch/report/ar6/wg1/downloads/faqs/IPCC_AR6_WGI_FAQs_Compiled.pdfu0022u003eFigure 1, FAQ3.1, IPCC AR6 WG1u003c/au003e.
Risk of accelerated warming
Why is it so important to reduce the uncertainty about the extent of aerosol masking? Because a significant portion of aerosol emissions results from human activities (industry, transport, agriculture) that also emit greenhouse gases. Now, reducing greenhouse gas emissions, which is necessary to address climate change, will inevitably lead to a reduction in aerosol emissions. Therefore, the masking exerted by aerosols will also decrease, potentially resulting in an increase in temperatures!
This would contribute to accelerating the global warming trend because the reduction in aerosols in the atmosphere will no longer mask part of the warming. This phenomenon has already been observed in 2020, when the introduction of new regulations on maritime transport pollution led to a reduction in maritime traffic, and thus a reduction in their aerosol emissions and the cooling effect they were exerting.
If aerosol masking is closer to -1 °C than to 0 °C, we must be prepared for the reduction in aerosol emissions to amplify global warming by about 1 °C.
Aerosols to stop climate change?
While climate inaction seems to be leading us directly toward a world of +2°C, it is legitimate to ask whether aerosols could play a role in mitigating the consequences of global warming. More specifically, could the cooling effect of aerosols be used to offset the warming induced by greenhouse gas emissions from human activity?
What are aerosols?
In climate science, when we talk about aerosols, we're not referring to "aerosol sprays" like those used for deodorants. It's a technical term designating tiny particles that can be found suspended in the air all around us and in the atmosphere above us. "Tiny particles" really means minuscule: they are comparable in size to a red blood cell, and are therefore invisible to the naked eye. Yet they are omnipresent in our atmosphere, and like greenhouse gases, they have a crucial influence on the climate.
These tiny particles come from various sources, which can be classified into two categories: natural sources and those linked to human activity. For example, small dust particles from the Sahara, or salt particles torn from the ocean by winds are aerosols of natural origin. Volcanic eruptions and forest fires are also natural sources, emitting significant quantities of aerosols into the atmosphere. Human activity, with its CO2 emissions, is also accompanied by emissions of carbonaceous aerosols. Thus, industry, transport and agriculture are important sources of aerosol emissions linked to human activity.
It is estimated that aerosol emissions linked to human activity are responsible for nearly 7 million deaths each year. In particular, the smallest particles, commonly called "fine particles", have the ability to penetrate our lungs, thus increasing the risk of developing respiratory infections and cardiovascular diseases. Morocco is not immune to this reality, with a concentration of fine particles in the air exceeding more than 5 times the recommendations of the World Health Organization in major cities like Casablanca.
Why are they important for the climate?
Beyond the health threat they represent, aerosols also have a fundamental impact on the climate. We know well that greenhouse gases such as CO2 or methane contribute to warming the climate. Well, for aerosols it's the opposite: they contribute to cooling the climate, or more precisely, to masking part of the warming. And that's not necessarily good news.
Two main modes of action can be distinguished by which aerosols have a cooling effect on the climate. The first is called the direct effect: aerosols suspended in the atmosphere act directly by reflecting the sun's rays. Thus, this solar energy is sent directly back into space, without ever reaching the Earth's surface, and consequently does not contribute to warming the Earth.
The second mode of action, which is the most important, is called the indirect effect, and is linked… to clouds. And yes, we know clouds are water vapor, but they're also aerosols! In fact, these tiny particles actually serve as nuclei around which water accumulates to form clouds: without aerosols, there's no accumulation, and therefore no clouds.
Now, the more aerosols there are suspended in the atmosphere, the brighter, larger and longer-lasting the clouds formed will be. And larger and brighter clouds reflect more solar rays. Therefore, an increase in aerosols in the atmosphere favors the creation of more reflective clouds, thus preventing more solar energy from warming the Earth's surface. This indirect effect is the main component of the role of aerosols in planetary climate.
These two modes of action are at the origin of the cooling effect of aerosols on the climate. Therefore, without the presence of aerosols in the atmosphere, the manifestations of global warming we observe today would be much more pronounced. Thus, aerosols contribute to masking part of the warming caused by greenhouse gases. But how much?
It is estimated that the warming mitigation attributable to aerosols linked to human activity could reach as much as -1°C. However, it is also possible that this mitigation is almost negligible. This wide uncertainty about the magnitude of the masking of warming by aerosols is one of the major questions currently in climate science, and a fundamental question in understanding the stakes of global warming.
Risk of accelerated warming
Why is it so important to reduce the uncertainty about the extent of aerosol masking? Because a significant portion of aerosol emissions results from human activities (industry, transport, agriculture) that also emit greenhouse gases. Now, reducing greenhouse gas emissions, which is necessary to address climate change, will inevitably lead to a reduction in aerosol emissions. Therefore, the masking exerted by aerosols will also decrease, potentially resulting in an increase in temperatures!
This would contribute to accelerating the global warming trend because the reduction in aerosols in the atmosphere will no longer mask part of the warming. This phenomenon has already been observed in 2020, when the introduction of new regulations on maritime transport pollution led to a reduction in maritime traffic, and thus a reduction in their aerosol emissions and the cooling effect they were exerting.
If aerosol masking is closer to -1 °C than to 0 °C, we must be prepared for the reduction in aerosol emissions to amplify global warming by about 1 °C.
Aerosols to stop climate change?
While climate inaction seems to be leading us directly toward a world of +2°C, it is legitimate to ask whether aerosols could play a role in mitigating the consequences of global warming. More specifically, could the cooling effect of aerosols be used to offset the warming induced by greenhouse gas emissions from human activity?