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14 August 2016

If Aerosols Change, the Climate Changes

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With respect to climate and its changes, aerosols are solid or liquid particles in suspension within the earth’s atmosphere. It is clear that these particles are involved in the balance of radiation in the SUN-EARTH system, and that this intervention plays an important role in the exchange of energy. However, for years, this role has not been sufficiently taken into account when studying the climate, and above all its changes.

Studies at the Desert Research Institute in Nevada (USA) have demonstrated that 15 of the 16 coldest summers between the years 500 B.C. and A.D. 1000 were caused by injection into the atmosphere of large quantities of aerosols from major volcanic eruptions.

The definition of the aerosols themselves and of the structure of the clouds tells us that we can consider clouds as bags of aerosols in liquid or solid drops that form around the “condensation nuclei”. Also aerosols are those that contain the smoke from fires or the burning of products of any type, agricultural, forestry, etc., or the dust that rises and is carried by the wind from deserts or areas that are in the desertification phase.

Other examples of aerosols are the particles from volcanic eruptions, from pollution produced by industry, or the condensation trails (contrails) of aircraft (in fact, we talked about them in a previous article), which often extend, giving rise to cirrus clouds; or those from many other activities, whether natural or not.

As you can see, aerosols come from many different sources and they have clearly been on the increase from the 19th century. But it took a long time for the presence of the aerosols in the atmosphere to be given the importance that they really have for the climate of the planet. In fact, in studies on climate change it was recognized for many years that the level of scientific understanding, in other words knowledge of the role of aerosols, was “VERY LOW“.

Even beyond the effects that could be assigned to aerosols in terms of their role to absorb or disperse incident solar radiation (from above down) or emerging terrestrial radiation (from the ground into space), doubts arose about what the result of these influences would be. Would the Earth cool or heat up? These doubts persisted for most of the first half of the 20th century, so according to the physical and mathematical model used, opposing predictions were obtained with respect to the future of the climate.

The results of the models used by the different groups of climate experts differed to such an extent that an eminent scientist, the Soviet climatologist Mikhail Budyko (1920 – 2001) considered that , in any event, we were facing a “climate catastrophe” whose trend is very uncertain, given that in the case of warming, the temperature of the planet would rise steeply, melting the ice and making the Earth a permanently hot and arid planet, but in the case of cooling, the temperature would fall brusquely, until the Earth reached a stable state of total glaciation. This was the divergent and ambiguous vision of the future climate of our planet shared by a large number of climate experts in the middle of the 20th century.

The way out of all this ambiguity was discovered in Mars, when in 1971 the Mariner 9 space probe observed the planet, finding that large masses of dust (aerosols) frequently enveloped the whole planet, strongly altering the climate conditions of the “red planet“.

This led researchers to think that atmospheric dust, and by extension all aerosols, played a much more important role in the climate of the planet Earth than had been thought before then.

From then on, and throughout the 1990s, the equations that served as a basis for climate models were updated, including in them terms that took into account the increasing presence of aerosols. As a result, the results obtained by the models were much more in line with what was being observed.

Most of the atmospheric aerosols contribute to protect us from the planet’s warming, given that they constitute a filter for the entering solar energy, so that all the mechanism that nature deploys to inject aerosols should in principle be welcome from the point of view of helping in the fight to put a stop to global warming.

The sun is the referee that must control the result of the competition between the two contenders (greenhouse gases and aerosols), and try to maintain a balance

As I said before, volcanic eruptions are a very important source of aerosols, capable of injecting millions of tons of particles into the atmosphere in a relatively short space of time. These eruptive particles, launched frequently above the Earth’s tropopause into the stratosphere, acquire enormous importance in their role of “atmosphere coolers“.

One example of this was the eruption in 1991 of Mount Pinatubo in the Philippines, which gave rise to the injection of more than 20 million tons of sulphur dioxide into the stratosphere, affecting the global climate for years, and giving rise to global cooling that 18 months after the eruption was nearly half a degree centigrade, which is a significant figure within the framework of average climate values.

The eruption of Pinatubo was an exceptional opportunity for testing whether the changes made to the climate models were correct or not in terms of including the effect of aerosols. The models suggested that within a few months a cooling of 0.3º C would take place as a result of the enormous eruption, and the measures carried out in hundreds of reference observation stations, distributed across the world, gave rise to a result of… 0.3º C .

The precision obtained by the models since the inclusion of aerosols has given rise to greater confidence about their predictions on global warming associated with the battle for supremacy between greenhouse gases, which increase warming, and aerosols, which reduce it. But given that the future values of these two climate drivers (greenhouse gases and aerosols) have a natural component of around 50%, the other half is the result of human activity, and so their future values will depend on what humanity does. To study their possible effects you therefore have to work with “scenarios“, in other words estimates of what humans can do.

The referee that has to control the result of the competition between the two contenders (greenhouse gases and aerosols) and try to maintain a balance is the sun, and more specifically all that part of solar radiation that reaches the ground and the radiation that emerges from it. So we have two galactic teams and a star referee , but we also need the public, the so-called 12th man, whose attitude, which is important in any match, is essential here, above all given that the stadium capacity, which in this case is the whole planet Earth, is getting on for 10 billion active spectators, so their role will be absolutely key. We hope that the fans behave during the match.

You can see the original article published in i-ambiente here.

Adolfo Marroquín

PhD in Physics, Geophysicist Industrial Technical Engineer, Meteorologist and Climatologist

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