What are aerosols?
When we talk about aerosols in the climate sciences, we're not talking about "aerosol sprays" like those used in deodorants. Aerosols are a technical term for small particles that can be found just about anywhere, suspended in the air around us and in the atmosphere above us. "Small particles" really does mean tiny: they're about the size of a red blood cell, so they're invisible to the naked eye. Yet they are omnipresent in our atmosphere, and like greenhouse gases, have a crucial influence on our climate.
Illustration of the size of aerosols, represented here by the small blue and red particles. Source : US EPA.
These tiny particles come from a variety of sources, which can be divided into two categories: natural sources and those linked to human activity. For example, small dust particles from the Sahara, or salt particles blown from the ocean by the wind, 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 high CO2 emissions, also produces carbonaceous aerosols. Industry, transport and agriculture are major 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 : NASA GOES-5 Nature run.
It is estimated that aerosol emissions linked to human activity are responsible for almost 7 million dead every year. In particular, the smallest particles, commonly referred to as "fine particles", have the ability to penetrate our lungs, increasing the risk of developing respiratory infections and cardiovascular disease. Morocco is no exception, with airborne concentrations of fine particles exceeding more than 5 times World Health Organization recommendations in major cities like Casablanca.
In Casablanca, the concentration of fine particles is estimated to be 5.2 times higher than recommended by the World Health Organization, and is responsible for almost 13,000 deaths every year. Source : Breathe Life 2030. Source : Breathe Life: Breathe Life 2030.
Why are they important for the climate?
As well as posing a threat to health, aerosols also have a fundamental impact on the climate. As we all know, greenhouse gases such as CO2 and methane help to warm the climate. Aerosols, on the other hand, contribute to cooling the climate, or more precisely, to masking some of the warming. And that's not necessarily good news.
There are two main modes of action by which aerosols have a cooling effect on the climate. The first is called the direct effect Aerosols suspended in the atmosphere act directly to reflect the sun's rays. As a result, this solar energy is sent directly back into space, without ever reaching the earth's surface, and therefore does not contribute to global warming.
The second, and more important, mode of action is called the indirect effectand is linked to... clouds. Yes, as we all know, clouds are water vapor, but they're also aerosols! In fact, these tiny particles act as nuclei around which water accumulates to form clouds: without aerosols, there's no accumulation, and therefore no clouds.
The more aerosols suspended in the atmosphere, the brighter, larger and longer-lasting the clouds formed. And bigger, brighter clouds reflect more of the sun's rays. Consequently, an increase in aerosols in the atmosphere encourages the creation of more reflective clouds, preventing more solar energy from warming the earth's surface. This indirect effect is the main component of aerosols' role in global climate.
The direct effect of aerosols (left) is to directly reflect the sun's rays. The indirect effect of aerosols (right), which is the most important, is to encourage the formation of clouds, which then reflect the sun's rays. Illustration adapted from figure 7.3, IPCC WG1 Chapter 7 Clouds and Aerosols.
These two modes of action are responsible for the cooling effect of aerosols on the climate. Consequently, without the presence of aerosols in the atmosphere, the manifestations of global warming we observe today would be far more pronounced. So, aerosols help mask some of the warming caused by greenhouse gases. But by how much?
It is estimated that the mitigation of warming attributable to aerosols linked to human activity could be as much as -1°C. However, it is also possible that this mitigation could be almost negligible. This large uncertainty as to the extent to which aerosols mask warming is one of the major questions currently being asked in climate science, and a fundamental issue in understanding the challenges of global warming.
Breakdown of the different contributions to the 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 : Figure 1, FAQ3.1, IPCC AR6 WG1.
Risk of accelerated warming
Why is it so important to reduce uncertainty about the magnitude of aerosol masking? Because a significant proportion of aerosol emissions result from human activities (industry, transport, agriculture), which also emit greenhouse gases. The reduction in greenhouse gas emissions needed to tackle climate change will inevitably lead to a reduction in aerosol emissions. As a result, the masking effect of aerosols will also diminish, potentially leading to higher temperatures!
This would accelerate the global warming trend, as the reduction of aerosols in the atmosphere would no longer mask part of the warming. This phenomenon has already could be observed in 2020The introduction of new regulations on shipping pollution led to a reduction in maritime traffic, and therefore a reduction in aerosol emissions and the cooling effect they exerted.
If aerosol masking is closer to -1°C than 0°C, we need to be prepared for the reduction in aerosol emissions to amplify global warming by around 1°C.
Aerosols to stop climate change?
At a time when climate inaction seems to be leading us straight towards a +2°C world, it's legitimate to ask whether aerosols could play a role in mitigating the consequences of global warming. More precisely, could the cooling effect of aerosols be used to offset the warming induced by greenhouse gas emissions linked to human activity?
As dystopian as it may sound, controlling the climate by injecting aerosols into the atmosphere is a technological solution being seriously considered by several countries in the global North with high-carbon economies, such as the United States and Japan. United States and the Chinaand largely financed by billionaires, their philanthropic organizations and oil companies.
However, this technology raises several major problems. Firstly, it does not address the real causes of climate change, thus constituting a form of techno-solutionism that allows polluting countries to delay the inevitable need to reduce their greenhouse gas emissions. It's a perfect excuse for climate inaction. The potential to use this technology as a climate weapon is also extremely worrying. The ability to control the climate could become a a geopolitical weapon for world powers capable of deploying itreinforcing existing neo-colonial structures and perpetuating global inequalities and injustices.
What's more, without waving the flag of a dystopian scenario à la Snowpiercer, major uncertainties persist as to the consequences this climate manipulation could have on ecosystems, air pollution, the water cycle, food safety and public health. In fact, since aerosols have an effect on cloud formation, studies based on climate simulations suggest that injecting aerosols into the atmosphere could disrupt the water cycle and cause droughts in Africa and AsiaThis threatens access to water and food security for billions of people.
Image from the Netflix sci-fi series "Snowpiercer" (adapted from the comic book "The Snowman") set in a dystopian future where a failed attempt at geo-engineering to counter climate change plunges the world into an ice age.
The concept of "termination shock" underlines another concern: for this technology to be effective, aerosols need to be injected into the atmosphere on a very regular basis. In the event of an abrupt halt, and without any real reduction in our greenhouse gas emissions to limit the rise in temperatures, we could see rapid warming in a matter of minutes, having a catastrophic impact on biodiversity. This situation would be far worse than if no stratospheric injection initiative had been undertaken in the first place.
Aerosols to remedy the drought?
As mentioned above, aerosols are an essential component in the creation of clouds. They act as small nuclei around which water accumulates in the atmosphere to form clouds. When enough water has accumulated around an aerosol, the water droplet thus formed becomes too heavy to remain suspended in the cloud and falls to the ground: it starts to rain. But could we intervene in this mechanism to speed it up and trigger the rain?
This is the idea, straight out of science fiction, behind cloud seeding technologies. By artificially introducing certain types of aerosols into clouds to accelerate the droplet formation process, cloud seeding not only triggers rainfall, but also increases the volume of water precipitated. This can be achieved either by air, by releasing aerosols directly into clouds from aircraft, or by land, by releasing aerosols into carrier winds reaching clouds. This meteorological modification technique imagined in the United States towards the end of the Second World War was used by the American army during the Vietnam War to prolong the harvest and destabilize the Vietnamese troops.
Illustration of the 2 main modes of cloud seeding: on the left, aerosols are released from the ground and carried by updrafts towards the cloud; on the right, aerosols are released directly above the cloud by air. Illustration wikimedia.
In Morocco, between the persistent drought in 2024 leading to increased water stress on populations, and the drop in rainfall from 20 to 40% expected by the end of the century with climate change, the prospect of being able to control rainfall is obviously a tempting one. And it's a desire that Morocco has been trying to realize since the 80s, through the Al Ghait" cloud seeding program initiated in collaboration with the United States Agency for International Development (USAID).
After first tests over the Oum Er Rbia basin in the Beni Mellal region, the Moroccan government extended and operationalized this program, so that today the kingdom has a fifty or so sites mainly located near the High Atlas mountains. On average, one quarantine operations are carried out every year between November and April. The ambition announced by the Minister of Equipment and Water, Nizar Baraka, is to extend this project nationwide, with a budget of 160 million dirhams over the next 3 years.
1996 map showing the various ground seeding sites south of Béni Mellal. Source : archive extract from the 1996 international symposium on "Water: Managing scarcity"..
However, the effectiveness of cloud seeding raises a number of questions. Firstly, far from being a magic solution to drought, cloud seeding only encourages precipitation when clouds and weather conditions favorable to rain are already present. So it goes without saying that seeding a clear sky during a drought will not produce any results.
Secondly, even when the right conditions are in place, there is still considerable uncertainty as to the actual impact of seeding on rainfall and meteorology. Far from being a reliable and well-controlled technology, the impact of seeding on rainfall is highly variable: it is estimated that it would allow a increase in rainfall ranging from 0 to 20%. This manipulation of the natural water cycle can also have serious consequences. unforeseen consequences on precipitation and the downstream environment beyond Morocco's bordersraising ethical concerns about its impact on populations. In 2009, for example, a seeding operation to counter drought in China had the unexpected effect of cause a sudden drop in temperature.
Finally, one of the most popular aerosols for seeding is silver iodide, which contains the heavy metal highly damaging to soils. Although current studies suggest that the quantities used in seeding are minimal and do not represent a hazard when they fall to the ground, the repeated application of silver iodide agent in the atmosphere could present long-term risks. risks for agricultural soils and groundwater.
One thing's for sure: in view of the drought recorded between 2021 and 2023 despite almost 50 seeding operations, this is no miracle solution, and it would be preferable to rethinking our water management policies rather than persisting in believing in a salvific techno-solutionism.
Conclusion
Aerosols, tiny particles with many facets, represent a double challenge: on the one hand, they pose a major health risk for millions of people every year exposed to fine particle pollution emitted by human activity; on the other, they act as a masking agent for part of the temperature rise induced by climate change.
Understanding the role of aerosols in climatology is imperative to developing effective policies for action against climate change. However, it is crucial to remain vigilant against the temptation to turn this understanding into a climate control technology. Any climate change policy that does not include a drastic reduction in greenhouse gas emissions is a mirage designed to delay climate action.
The same applies to rainfall control. In an arid, drought-prone country like Morocco, it can be seductive to imagine being able to intervene in the water cycle and make it rain at will. However, this technological fantasy of the future tends to obscure the inadequacy of present-day water management policies. Before playing sorcerer's apprentice with water in the atmosphere, wouldn't it be more urgent to better control the water we have on earth?
For further information, we recommend the excellent interview with Sofia Kabbejmember of Defense and Climate Observatory and expert researcher in climate change security and the impacts of geoengineering.
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Article by Shahine Bouabid