The « flying rivers » essential factor of the global climate

Translation in English of an article from Reporterre

Les rivières aériennes de vapeur issues de la forêt amazonienne jouent un rôle capital pour le climat. CC BY-NC-ND@Neil Palmer/Ciat/Cifor

To avoid climate collapse, it will not be enough to reduce the concentrations of greenhouse gases in the atmosphere. It will also be necessary to protect forests and soils from deforestation and intensive agriculture, because the water cycle is also very important for climate regulation. We are just beginning to understand that.

Shortly after the New York Convention (1992), which launched the process of conferences on global warming, scientific publications drew attention to the “air rivers” which are steam or water vapour currents very active for rainfall. Initially called tropospheric rivers, these «aerial rivers» are created by the evaporation of the oceans and by the evapotranspiration of the great forests of Amazonia, Congo, Siberia.

In 1992, in "Tropospheric rivers? – a pilot study" Reginald Newell and other scientists indicated that the flow of air currents of water vapor was very powerful: in Amazonia it approached the flow of the Amazon River, or 165.000 m3 per second.

About fifteen years later, in 2006 and 2007, Victor Gorshkov and Anastassia Makarieva, from the University of Saint Petersburg, formulated the principle of the biotic pump, “a mechanism through which natural forests create and control winds from the ocean to the land, bringing moisture to all life on earth.” They thus showed the pivotal role of forests and trees on climate: to the rivers of water vapor created by the evapotranspiration of large forests, one had to add the humidity sucked up from the ocean and transported over thousands of kilometers of distance.

Picture of flying rivers
UNEP’s Foresight Brief 25@Stefan Schwarzer, UN Environment/GRID Geneva

Continuing their work, Antonio Donato Nobre, a researcher from the Earth System Science Center of the National Institute of space research (INPE), added to this idea in 2014 in 'The Future Climate of Amazonia'. He pointed out that the aerial rivers of steam from the Amazonian forest play a key role for the climate of all of South America (east of the Andes Cordillera, which forms a dam) and even of part of North America, and are responsible for rain.

Halting deforestation immediately

But Antonio Donato Nobre also denounced the devastating effects of deforestation: it reduces the amount of water vapour resulting from the evapotranspiration of trees, and therefore the rain resulting from it, and disturbs the biotic pump, for it affects the forests close to the sea a lot, which suck its humid air. One of the dramatic consequences of this destruction could be the transformation of the Amazonia into a savannah – an area where the presence of trees would fall in favor of herbaceous plants. A savannah exists in Cerrado, in the south of the Amazon, and it is already 40% destroyed for the benefit of breeding and growing soybeans.

A wide quadrangle around and west of São Paulo (a large city in the south of Brazil where the majority of the country’s population and activity is concentrated) could also turn into a desert, as exists in Australia at a close latitude. A recent movie (2021) shown on Arte, The Mystery of the Flying Rivers of the Amazon, by Pascal Cuissot, shows that the process has begun: the Iguazú Falls, located in this quadrilateral (quadrangle?), on the Brazil-Argentina border, which formerly used to flow year-round on a three kilometer front now flows less than half that width during the dry season.

For its part, the United Nations Environment Programme (UNEP) recommended as early as August 2021 to “work with plants, soils and water to cool the climate and hydrate the Earth’s landscapes.” Its conclusion was clear: to avoid a climate catastrophe, one must not only immediately stop deforestation, but also engage in reforestation worldwide, and develop agricultural practices that regenerate the soil, as promoted by agroforestry.

Indeed, in the dominant agricultural model based on chemical fertilizers, the low residual organic matter in soils (humus) decreases the water retention capacity of soils, and the resilience of crops in case of drought. This has consequences on the evapotranspiration, which varies according to the plant cover – crops, meadows, hedges, trees.

“Agriculture can stop being destructive of the water cycle”

By ignoring the role of water in climate change, and by restricting the fight against climate change to the role of greenhouse gases, the Intergovernmental Panel on Climate Change (IPCC) went down a dead end. Even if we were able to stop the increase in the rate of greenhouse gases in the atmosphere, climate change due to deforestation and our land use patterns would continue, and entire regions could be turned into deserts.

Surely, the increase in greenhouse gases emission was the explanation for climate change when the institution was created (1988). But now, to avoid the drying out of the planet and an increase in desertification, it is crucial to take into account the role of water in the climate. We must urgently stop deforestation in large forests, start reforesting and undertake an agricultural revolution. For agriculture can stop its destruction of water cycles, and instead become a lever for change, with living soils able to keep moisture and sequester more carbon.

Author : Daniel Hofnung, (retired engineer) co-chair Coordination Eau Île de France