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Carbo Europe

Improving Carbon Flux Models

Last April, in southwestern France, researchers measured the carbon exchanges between biosphere and atmosphere. The goal is to have an accurate trace, in the different ecosystems, of the sinks and sources of this harmful gas.

improving carbon

© E. Ceshia, CESBIO/Univ. Toulouse

Site for measuring fluxes of heat, humidity, and CO2 on a wheat field west of Toulouse.


 

This bustle of activity is quite unusual for the traditionally quiet region of the Landes. Planes are flying over the forests, people are perched up in the trees, measuring apparatus is spread out in the fields... and for once, gastronomy is not on the program, “though the region is excellent for that,” admits Joël Noilhan, from the GAME laboratory.1 These thirty or so scientists from all over Europe have a very specific objective: to measure and acquire better knowledge of the carbon dioxide exchanges between biosphere and atmosphere. Indeed, understanding the net carbon flux is an essential component of climate change research. Between April and September 2007, the second part of the Carbo Europe2 mission will refine the measurements made during a previous scientific study in 2005 in this part of southwestern France. Although there is worldwide concern today about an increase in atmospheric carbon dioxide and the predicted global warming of 2° to 4°C by 2100, scientists still don't fully understand the details of CO2 flux, and particularly the roles that some ecosystems play in absorbing or emitting this gas.

On Earth, some ecosystems act as CO2 sinks–they absorb the gas in certain conditions, which compensates for a portion of man-made emissions. Oceans constitute one such sink. Plants are also usually considered to be carbon sinks, but when observed individually and in close quarters, scientists have found that different systems behave very differently. Pasture, forest, and agricultural land are not equal carbon absorbers. “Our 2005 results have shown that non-agricultural land (meadows and forests) tends to absorb CO2 whereas farmland behaves more as a source of carbon dioxide.”

To investigate this phenomenon, the researchers are using all the means at their disposal: three planes that can haul measuring instruments over large distances; three towers high above the tree line (40 to 50 meters above ground); and flux stations about 30 cm above the crops, that measure the CO2, temperature, humidity, radiation, and wind direction.

After the first campaign and several months of data processing, the team has already noted great variations in CO2 flux depending on the type of agriculture and location. This is due to different levels of carbon assimilation by various plants. For example, large differences in emission have been found between forests and winter crops. The team also demonstrated the existence of a carbon flux from the ocean, brought in by sea breezes during the day. This is because the solubility of carbon dioxide in water varies according to daytime temperatures. But there are also other more subtle exchanges between ground and air, which should be better understood after the Carbo Europe program ends in 2008. For example, ploughed land emits more carbon dioxide than cultivated areas, and different plants don't seem to emit the same quantities of CO2. There are also seasonal variations, which is why it was decided to have two measurement campaigns this year, one in spring and one in fall.

In the long run, researchers are hoping to learn a lot more about carbon exchanges in the region. And according to Joël Noilhan, this knowledge will also be useful at a European level. “We're trying to optimize measurements and atmospheric transport models to devise a method for estimating CO2 sinks and sources all over Europe at a resolution of ten kilometers or so.” This new climate model is much more accurate than the present one, which represents carbon variations on “a grid of nearly 10,000 square kilometers”–i.e., at a resolution 10 times lower. At this scale, it is practically impossible to distinguish the roles played by agriculture and forests. The new map will be able to pinpoint the cultivated parts of meadows. Add to that a map of carbon emitted by urban areas, particularly Toulouse and Bordeaux–the two largest cities in the region–and authorities will have a useful instrument for making decisions about agriculture and urban planning.

 

Azar Khalatbari

Notes :

1. Groupe d'étude de l'atmosphère météorologique (CNRS / Météo France–Centre national de recherches météorologique).
2. Carbo Europe is a EU-funded framework program (2004-2008) which includes 61 research centers from 17 European countries. Its aim is to understand and quantify the terrestrial carbon balance of Europe and the associated uncertainties at local, regional, and continental scales. www.carboeurope.org

Contacts :

Joël Noilhan
Game, Toulouse.
joel.noilhan@meteo.fr


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