According to a recent research, mapping tree density at the global level, there are 3.04 trillion trees covering 87% of ice-free land on Earth, with a ration of trees-to-people of 422:1. The mainstream opinion about climate change claims that the vegetation on the planet has been reduced by human activity and it is true that since the beginning of human civilisation around 45.8% of trees have fallen with 15.3 billion of trees cut for a total area of 192000 km2 every year.  However, surprisingly, the scientific findings reveal that there is an ongoing opposite trend defined global greening.

It is a gradual but large increase in trees and leaves on plants, particularly significant in northern extratropical land surface, which in the last 35 years covered an area equivalent to 2 times continental United States. Natural forces and internal climate variability are not enough to explain such a huge phenomenon. Conversely, the research carried out by T. W. Crowther, H. B. Glick and M. A. Bradford showed that it can rigorously be attributed with high statistical confidence to anthropogenic forcing. More specifically, 70% of it depends on the atmospheric concentration of greenhouses gas, 9% to nitrogen deposition and the remaining to other causes like rural to urban migration transforming abandoned lands in efficient industrial-scale agriculture or in marginal green areas, the green revolution spreading ecological techniques, and the regulated reduction of deforestation.

As the first cause is concerned, many studies investigated the so-called CO2 fertilisation determining human-induced greening. It is estimated that every year 10 billions of carbon emitted remains in the atmosphere and it promotes photosynthesis spurring plants and leaves growth. This peculiar fertilising effect begins with enhanced photosynthetic CO2 fixation, where non-structural carbohydrates, such as starch, soluble carbohydrates and polyfructosans, tend to accumulate in leaves and in other organs. Then, in the process of growth, photoassimilates are allocated to the vegetative shoots, root system or reproductive organs, but in some cases, more photoassimilate of CO2-enriched plants is partitioned to the root system than to the shoots. Above ground, more photoassimilate usually goes into stems and supporting structures than into leaves. This phenomenon may not be an inherent response to elevated CO2, but may be a by-product of the larger size of plants often found in CO2-enriched atmospheres, especially by species that produce branch stems along the aerial mainstems (Allen et al., 1991). As a result, reproductive biomass and vegetative biomass growth are induced by elevated CO2 concentration and produce the global greening effect.

The immediate impression given by this news is generally positive, but there are controversial consequences.    First of all, there is a considerable impact on local temperatures as 60% of biomes are benefiting from the mitigating effect of global greening: warm areas are cooling with a reduction of temperatures by 14% thanks to the condensation of plants evapotranspiration, while cold areas are warming with an increase in temperature by 10% because the green coverage obstacles sunlight reflection back from the surface. However, it has drastic effects on extreme weathers, like in the case of Sahel. It is a transition zone between the arid Sahara and the (sub-)humid tropical savannas in the south, which presents a marked seasonality with a long dry season and a short humid season in the northern hemispheric summer. Between the 1960s and throughout the 1980s the Sahel region was affected by a series of unprecedented droughts, but between 1980s and 2003 there was a reversion of this trend, with an unprecedented greening, requiring a complete change and adaptation as lifestyles and agricultural practices are concerned.       Secondly, it is possible to witness a reduction of CO2 concentrations in the atmosphere, thanks to plants respiration, but this is happening only partially as so far it is registered a diminishing only from 50% to 40% and this beneficial effect will decrease over time due to plants adaptation.     Finally the FAO show that CO2 fertilisation gives hope about increasing food production to fight hunger in the world, but it is important to consider all the sides effects involving lands impoverishment, air quality deterioration and so on.   As F. Bazzaz and Wim Sombroek suggest in “Global climate change and agricultural production”, to take advantage of the increasing concentration of CO2 produced by fossil fuels consumption, adaptation and mitigation actions for agriculture could include:
  1. Selection of plants that can better utilize carbohydrates produced with elevated CO2 concentration;
  2. . Selection of plants producing less structural matter and more reproductive capacity under CO2 enrichment;
  3. Search for germplasms that are adapted to higher temperatures to improve flowering and seed set;
  4. Optimise planting periods and other crop management procedures under new climatic conditions;
  5. Select species that have more stable production under high temperatures or drought;
  6. Where needed or possible, develop irrigation systems for crops.

However, to balance the positive and negative effects of human footprints, it is fundamental to look not only at economic benefits maximisation, but also at political and social consequences, tackling in a complete way the phenomenon of global greening after spreading awareness about it.


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