The ambiguous role that forests play in relation to climate

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Temps de lecture : 4 minutes  

Forests are a place of exchange—with the atmosphere—energy, water, heat, and chemical compounds, and therefore at the center of the climate system. How can one assess its role in today’s global warming context? Models do exist and are improving; however they are based on rare and fragmented observations. Other unknown fact: the action of man, who cultivates the land and uses the wood.

Assessing climate effects is a difficult task: one has to model the biosphere, from leaf respiration to the migration of plant species. How will they react to climate change?

There is one popular solution: until the 1990s, reforestation was presented as a way to limit global warming. However, ten years after Kyoto, this position has changed. In its latest report published last May, Group III of the Intergovernmental Panel on Climate Change (IPCC) concluded that various actions such as reforestation in addition to changes in silviculture practices, and the increased use of energy and construction wood could prevent the emission of 1.7 to 18.2 billion tonnes of CO2 each year by 2030. The bracket is large, but the challenge is considerable: these values represent a significant part of the CO2 currently emitted each year in the world, the equivalent of 34 gigatons. Nevertheless, it is important to note that this estimate by the GIEC is a “basic” calculation of the flux of greenhouse gases, and under no circumstances can it be considered as an estimate of a potential decrease in atmospheric warming, expressed in degrees. This is due to the fact that the estimate only takes into account forest actions in the context of atmospheric greenhouse gases, while ignoring all the other roles forests plays in the climate system. In reality, beside carbon dioxide, forests exchange energy, water, heat and other chemical compounds with the atmosphere at any given moment. A change in these exchanges affects atmospheric circulation, and thus the climate. However these forest interactions are not all taken into account in the climate change models. Therefore, the real climate evolution could have some surprises in store for us.

Too many time frames

It is important to note that the system that takes into account the soil, vegetation and atmosphere is as difficult to calculate as the atmospheric and ocean circulations, which make-up the core of current climatic models. First, the time scales to consider are extremely varied: photosynthesis and respiration are instantaneous, while tree growth and leaf seasons are measured in yearly cycles. Migration, genetic adaptation, and the interaction between plant species are measured in centuries, and the storage and release of soil carbon in millenniums. This is without taking into account the essential integration of the agronomical choice of human societies: the ways the soil is used, levels of fertilization and irrigation, types of silviculture, and seeding and harvesting dates, etc.

It has never been easy to integrate vegetation into climatic situations. The first attempt was made by the Japanese Suki Manabe, who at the time worked for the United States NOAA geophysical laboratory, constructed the first simple model of the continental surface in the 1970s. He chose to represent the soil as a vessel fills up with rain, empties again through evapotranspiration, and overflows when full. Vegetation is considered in relation to two factors: albedo (capacity of the surface to reflect solar energy, which makes it possible to quantify the absorbed energy) and “ruggedness” (irregularity of this surface). The continental surface is then divided into approximately ten large types of vegetation or biomes, of which we attribute these two characteristics. This model may appear simplistic yet it is still used in a significant number of climate models. Its major flaw is that it cannot take evolution into account, such as deforestation for example, which was not a main concern during Manabe’s time.

Ten years later, one would like to see a more complete overview of vegetation. […]

Therefore, the biosphere is seen as a purely physical and static entity by climatologists. […]

Plants that adapt

This vision started to change at the beginning of the 21st century as physicists progressively introduced biogeochemical cycles (particularly carbon) into their models. For example, they began calculating the photosynthesis process, which enables plants to absorb carbon dioxide. The calculations also include the distribution of the absorbed carbon by trees by the leaves, branches, androots, with the most sophisicated models simulating a competition between different plants. They simulate a climatic change effect by allowing not only temporal variations in the functioning characteristics of a given plant type, but also movement of these plants towards a more favorable location.

In any case, are these models a reliable representation? […]

It is important to recognize that we still only have limited tools in the collection of precise data through the observation of the dynamics of ecosystems. Ground observation systems, in particular forest inventories, making it possible to estimate growth and biomass, are a relatively recent phenomenon (approximately 20 years) and one that only exists in the world’s richest countries. […]

Extreme events

Thus, the predictions based on these plant models are still unreliable. Even if most of them predict potential shifts in species ranges towards colder areas, how these shifts will occur is still relatively unknown. […]

The years 1976 (drought), 1999 (storms), and 2003 (heatwave and drought) have shown us that extreme events play a crucial role in the evolution of ecosystems. However, they are poorly or not at all represented in the current climatic situation. […]

In addition, there is also man, who changes landscape and colonizes open regions. The occupation of these areas by certain species will not only depend on their capacity to disperse but also on the spatial organization of the land. Few models include these constraints because one has to manipulate the explicit landscape representations to the very last detail. However, the proportion of hedges for example is a decisive factor in the migration potential of species.

Dangerous predictions

Contrary then to our naïve preconceptions, the climate is under strong biological control. The uncertainties regarding the role the biosphere will play on evolution further emphasizes that man must to reduce greenhouse gas emissions instead of trying to stockpile these compounds once they’ve been produced biological compartments that are still questionable.

Le rôle ambigu des forêts dans le climat

Nathalie de NOBLET-DUCOUDRE et Jean-Luc DUPOUEY

La Recherche no. 414

December 2007

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