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THEMES: what can the growth rings of trees teach us on climate change?

As part of its Climate Initiative program, the BNP Paribas Foundation is supporting the THEMES project. Its goal? Better understanding the interaction between climate warming and the Hadley cell (a large-scale atmospheric movement that redistributes heat from the equator to the tropics), which is expanding the subtropical dry zones in the Southern hemisphere.

No other extreme climate event affects as many people around the world as drought. Over the past 30 to 40 years, drought has become increasingly frequent in subtropical regions. This increase is possibly linked to the expansion of the Hadley cell. This circulation of air is a large-scale movement by which heat is distributed from the equator to the tropics, on opposite sides of the globe. 

At the equator, warm moist air rises and cools producing clouds and heavy precipitations. At 10-15 km above the surface, the air breaks into two branches flowing northward and southward towards higher latitudes. In each hemisphere, it sinks to produce high-pressure regions, around 30 degrees latitude.The world’s major deserts (the Sahara, the Atacama, etc.) are located in the area underneath the descending branches,. 

“ Since the 1980s, the Hadley cells have expanded  of 0.5 - 1 degree poleward per decade in each hemisphere ”, explains Valerie Daux of the Laboratoire des Sciences du Climat et de l’Environnement (CNRS/CEA/University of Versailles Saint Quentin). The underlying mechanisms of this expansion remain uncertain. Tackling this question will be a team of  European, North American and South American researchers headed by Valerie Daux.

“ These trees along the Andes will help us reconstruct episodes of drought in the past and better characterise the current drying trend. ”

Valérie Daux

Laboratoire des Sciences du Climat et de l’Environnement (CNRS/CEA/UVSQ) 

Thousand-year-old trees

The expansion of the Hadley cells may be an effect of natural climate variability, but it can also be linked to current global warming. 

In an attempt to draw the line between these two views, researchers want to get the trees of the Andes mountains to talk. “This is the only place in the world with a continuum of trees from the tropics to the high latitudes”, the French dendro-isotopist explains.

Hence the idea to travel from Bolivia to Patagonia to meet with these trees whose wood is a record of their growth conditions and the stress they have endured.

This is the only place in the world with a continuum of trees from the tropics to the high latitudes.

Daux’s team will analyze the isotopic compositions of the carbon and oxygen present in the wood of the different growth rings: they should say something about temperature and humidity variations in particular. Another two teams, in Chile and Argentina, will study growth rates by looking at the size of growth rings, while a third team from Scotland will focus on wood density. Daux explains that “each approach will give us clues on the growth conditions of these trees”. The researchers are hoping to go back as far as 1,000 years or more, as some of the trees – namely the Patagonian Cypress- are dated at over 3,000 years old.

get the trees of the Andes mountains to talk.

Climate sensitive Austrocedrus tree in northern Patagonia - Tree core ©R. Villalba, IANIGLA

Back to the futur

After the sampling campaigns, between autumn 2017 and spring 2018, the researchers will date the collected core samples and analyze them at the partner laboratories. “This painstaking work will allow us to carry out several-century long reconstructions of past temperature and rainfall variations and other hydro-climatic indicators (drought). We’ll be able to reconstruct the evolution of the sizeand intensity of the Hadley cell over time and characterize the current drying taking place”, Daux concludes. This should provide insight into the underlying mechanisms of the expansion that has been observed.

In parallel, a team of modellers will work on the meteorological data recorded in this region of the world since 1950, and on climate model outputs. The combination of hydro- climatic reconstructions based on tree rings, meteorological data and modelling results will shed light on the links between the variations in rainfall, the expansion of deserts and of the Hadley cell. The collective work will also be used as input for the simulation models with different future greenhouse gas emissions scenarios, and allow for more accurate predictions of hydro-climatic factors in this part of the world.

THEMES : The Mystery of the Expanding tropics

Since about forty years, the intensity and the duration of period of drought increases in subtropics. These regions are dependent on Hadley's Cell(Unit) ( atmospheric movement) which has just increased. The THEMES project is going to allow to redraw the climatic evolution during the last millennium and to improve the climatic simulations on the horizon 2100. The project is directed by Valérie Daux (LSCE - CNRS/CEA/Université de Versailles Saint Quentin).

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