Droughts are one of the most serious natural hazards faced by the world’s population. Only flooding represents a more serious threat in terms of impact.
In the 1995-2015 period, 1.1 billion people suffered the effects of a severe dry period.
By comparison, this represents almost twice as many people as those affected by all hurricanes combined.
An international team of researchers decided to look into the intensification of this phenomenon in South America, with the aim of both analyzing it and engaging with local government bodies to help them prepare for this aspect of climate change.
An increase in the frequency of droughts has in fact been observed over the past 30 to 40 years in South America, Australia, North America and in the Mediterranean region.
In February 2018, the Argentinian Ministry of Economy estimated that there had been loss of earnings of at least 3 billion dollars in the agricultural sector because of the current drought in Argentina’s central region.
Tracking drought expansion
The first fieldwork project, called Themes (for “The Mystery of the Expanding Tropics”) was carried out in February 2018 in the Argentinian Andes.
Nineteen scientists (including eight engineers and technicians and two PhD students) from France, Argentina, Chile, the UK and the US were involved in the project. A sampling project with the same goals as the Argentinian fieldwork also took place in Chile. Readings of measurement instruments will also be scheduled during the Austral Summer for the next two years.
The scientists are focusing on the expansion of the Hadley circulation, an element of atmospheric circulation the dynamics of which could provide a partial explanation for the droughts observed in the Central Andes and in numerous tropical areas.
The Hadley circulation explained
This is an atmospheric circulation – made up of cells that behave rather like conveyor belts 15km tall and almost 3 000km wide – which transfers heat at altitude from the Equator to the tropics.
Each “conveyor belt cell” begins its journey in the Equator region, where the air is heated and less dense and starts to rise. At altitude, depressurization and water vapor condenses cools it, which produces heavy rainfall. The “Hadley cell” mechanism comes into play when the dry air column separates into two masses and is driven towards the North or the South on either side of the Equator before plummeting towards the surface at a latitude of around 30°.
These downward flows of Hadley cells bring hot and dry air to the earth and are responsible for the dry weather in subtropical regions. The largest deserts on the planet (like the Sahara or the Atacama Desert) can be found at their latitude.
However, Hadley cells have been extending towards the poles over the past few decades, which has led to an expansion of subtropical regions, where the air plummets towards the ground. This effect, which has been particularly noticeable in the Southern Hemisphere, has resulted in an increase in the frequency and severity of droughts.
Several studies have suggested that this expansion could be linked to global climate change. The authors theorize that, in reducing the contrasts in pressure around tropopause (which marks the Hadley circulation’s upper boundary), the warming of the atmosphere has increased the stability of air masses and pushed the downward flow of dry air further towards the poles.
Researchers think that the Hadley cell expansion of 1 to 3° of latitude observed in both hemispheres over the past 40 years will probably continue throughout the 21st Century.
The impact in terms of the decrease in rainfall has already been significant in the Mediterranean region, in the South West of the United States, in Australia, in South Africa and in the South West of South America. This is precisely what seems to be happening in in the South American Andes from the Altiplano (from 16°S) to North Patagonia (43°S).
Natural archives provide further clues
Scientists exploring in the Argentinian Andes weren’t starting from a blank sheet: several studies suggested a link between an increase in droughts and the expansion of the Hadley cell as early as 2012.
However, the mechanisms and the long term scope of the Hadley cell expansion remain poorly understood. Gaining a better understanding of both these things would help better predict; it would also serve to mobilize civil society and governments in the countries affected, so as to secure water supply.
These gaps in our knowledge are due, at least in part, to the lack of large-scale historical data on variations in the tropical-subtropical atmospheric circulation. In this part of the world, direct measurements of rainfall are rare and don’t go back further than 50 years.
To counteract this problem, the Themes research project uses dendrochronology, which is the study of the data provided by tree rings year after year. These natural archives store many parameters for each stage of the tree’s growth and could extend our knowledge of variations in rainfall to several centuries (at least 400 years).
This is why the geography of South America offers it a unique advantage: trees with annual rings can be found in the Andean Mountains in a broad strip of land (extending from 20° to 60°S).
In addition, there is a comprehensive collection of data and wood samples in Argentinian and Chilean research laboratories. They will be examined and further sampling will take place, then be added to the archives.
The aim is to update and/or develop new dendrochronological series along the Andes, from the Altiplano to North Patagonia and via Central Chile.
Isotopes, a language that needs decrypting
Themes team members intend to combine a large number of indicators collated from tree rings as well as meteorological data to reconstruct past changes in the Hadley circulation and its localization.
They will be relying especially on the composition of tree rings in oxygen-18 and carbon-13 isotopes: these rare varieties of two common atoms in the natural world have a number of neutral particles (neutrons) in their nucleus that is different from that of the majority atom. The nucleus of oxygen-18 holds two neutrons more than oxygen-16 and carbon-13 one more than the classic carbon-12. The properties of these different “versions” are roughly similar to the majority atom but not strictly identical.
The difference can be detected when certain physicochemical processes occur: for instance, when water evaporates, oxygen isotopes with 16 neutrons (called 16O) tend to go into vapor more easily than isotopes with 18 neutrons (18O). Similarly, isotope 12C carbon is preferred by plants for photosynthesis, which leads to less sugar being formed in 13C compared to the atmospheric CO2 carbon.
Trees therefore make it possible to observe various reactions that impact on how isotopes are distributed – from water absorption (which provides oxygen atoms) to cellulose production. The intensity of these reactions depend on a range of factors, such as temperature and air humidity.
As a consequence, variations in the isotopic composition of cellulose, from one tree ring to another, mirror variations in a tree’s growth environment … Correlations can therefore be drawn between isotopic composition and temperature, humidity and insulation.
The procedure should lead to a robust and reliable reconstruction of fluctuations in past centuries. The data will then be used to calibrate current climate models, and then to attempt a prediction of future variations.
Models to predict the future
Climate models represent digital programs that incorporate physics equations governing matter and energy flows in the oceans and atmosphere. Computation power being finite, these models resort to simplified representations of the world, using initial conditions that researchers think are reasonable.
To test their validity, specialists run these models in time and space scales, the characteristics of which are already known. At the end of the modeling process, if the data calculated by the virtual model is in line with actual measurements, the parameters of the model can be taken to be correct. Only then do researchers undertake simulations for the future.
Finally, if the computation assumptions of theoretical models match field data, they will be used to estimate the probable trends in hydroclimatic variations that the region can expect for the rest of this century.
As this comparison-based validation stage is crucial, researchers need to increase the duration of measurement series. With that objective in mind, dendrochronology represents a particularly useful approach.
Following the sampling of several species of wood (Patagonian Cypress, Chilean Cedar, Araucaria) which took place in the Andes in February, analysis of the materials gathered is now starting, paving the way for a better understanding of the future.
Valérie Daux, Professor in Earth Sciences, Université de Versailles Saint-Quentin en Yvelines – Université Paris-Saclay and Olivier Aballain, Director of Studies, ESJ Lille (École supérieure de journalisme de Lille)
The original version of this article was published in The Conversation.