Biology & The Water Cycle

Water is the source of life, the most abundant molecule in any living organism and it happens to be the most important molecule for climate regulation due to its thermal properties. Not surprisingly, water is the key ingredient of our planet's cooling system. Ocean currents, atmospheric cells and atmospheric rivers are all well known phenomena in the regulation and distribution of heat throughout our planet.

The movement of water molecules within our planet in its various states, liquid, vapour and solid, is what is called the water cycle. The water cycle is often described as the circulation of water from the oceans evaporating into the atmosphere where clouds are formed and rain occurs. But this process is much more complex and there are other key factors and actors in play. 

From the geological and physical point of view water from the oceans in the atmosphere weakens exponentially as we move inland. This means that sufficient rain to support life would only occur within a relatively narrow band along the ocean leaving the remaining part of the continent as arid land. However, studies have shown that along the basins of the rivers Amazon in South America, Yenisei in Asia and Congo in Africa, rain does not weaken as we move inland. A non-geological or physical mechanism ensures that rain is distributed along these basins many kilometres away from the ocean (up to 2 500 km away from the ocean). This mechanism is driven by forests.

The role of biology in the water cycle

Plants and trees release a great amount of water vapour, mainly through their leaves, in a process called transpiration. Plants and trees absorb large amounts of water through their roots, water is used to obtain and transport nutrients as well as for photosynthesis. The excess water, approximately 90% of the water absorbed, is released back into the atmosphere through transpiration.

Studies have shown that water vapour released by plants and trees greatly contributes to the atmospheric vapour required for cloud formation. It is estimated that, as an average, between 40-60% of the rainfall on land comes from land based evaporation and of that percentage between 60-80% comes from vegetation transpiration. In certain regions of the world such as the Amazon forest, the Congo rainforest or the Yenisei river basin, these figures reach much higher percentages. 

Transpiration not only contributes to water vapour in the atmosphere, it also moderates temperatures. The transpiration process is biology equivalent to an air conditioning unit. Water at the plant leaf draws energy from the immediate environment and turns into vapour. This has a cooling effect on the environment. Water vapour rises to higher parts of the atmosphere taking with it the energy as latent heat. In the upper atmosphere vapour condenses to form clouds releasing the latent heat. This biological air conditioning system utilises the energy of the sun and the water cycle to cool the environment. In fact, the daily cooling effect of an average tree is equivalent to two domestic air conditioning units running for 24 hours. The effect of large concentrations of vegetation in forests is substantial in moderating temperature.

Biology and rain

But the presence of vapour in the atmosphere alone does not always result in rain. There are two necessary steps for rain to occur. The first one is that water vapour needs to condensate into water droplets to form clouds in a process called cloud formation. The second one is that cloud droplets need to crystallise to form precipitation droplets. These two steps are triggered by the presence of certain aerosols, very fine particles, in the atmosphere. 

A range of different aerosols have been found to have the ability of cloud formation. However, biologic aerosols, mainly organic compounds released by vegetation, have been found to be one of the primary aerosols in this process in most parts of the planet.  

Similarly the presence of nucleating particles is critical for the formation of rain drops, and here biology also plays an important role. Biological aerosols, especially bacteria and fungal spores, are one of the most efficient types of nucleating particles for the formation of precipitation ice crystals, as they trigger ice formation at relatively warm temperatures. Only particles of biological origin can initiate ice nucleation at temperatures above -15 degrees Celsius. These types of bacteria and fungi are found on plant leaves. These microorganisms are released into the atmosphere mainly in water vapour and organic compounds and are returned back to the surface of the earth through precipitation. These microorganisms have developed a transport mechanism that provides the necessary rain for life to flourish.

Similarly to how cyanobacteria paved the way for life on land millions of years ago by releasing oxygen into the atmosphere through photosynthesis, forests are paving the way to life on continents by bringing water to areas away from the coastal regions.

The soil

Life below ground also plays an important role in the water cycle. Floods are the result of intensive rain but more importantly the failure of the surfaces to infiltrate the rainfall.  The hard surfaces of our urban environments and the depleted and compacted soils of our degraded landscapes are the main contributors to flood events. Floods are not existent in healthy forest ecosystems.

We need healthy and spongy soils to infiltrate rain in order to support plants, microorganisms and replenish our aquifers. The sponginess of the soil and therefore the capacity of the soil to infiltrate and retain water depends on the amount of humic compounds, organic matter and microorganisms present, especially fungi. Carbon, which is extracted by plants from the atmosphere through photosynthesis and deposited in the soil in the form of root exudates, decomposing organic matter and ultimately soil microorganisms, is the main component of the humic compounds responsible for the soil spongy structure.

Humans have been altering the natural water cycle for thousands of years with our land use management. This has resulted in many, once fertile landscapes becoming deserts. Deforestation, large-scale agricultural practices and soil depletion are one of the main reasons for the increase in desertification and extreme climatic events. We have removed the biological climate regulation machine in many parts of the world. With limited biological moderation we are seeing weather extremes. But the solution is in our hands, we need to let nature do what it has been doing for millions of years, paving the way for all life on earth to thrive.

References:

Udo Schneider, Peter Finger, Anja Meyer-Christoffer, Elke Rustemeier, Markus Ziese and Andreas Becker: Global Precipitation Climatology Centre (GPCC), Deutscher Wetterdienst, Offenbach 63067, Germany, 2017

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Cindy E. Morris, Samuel Soubeyrand, E. Keith Bigg, Jessie M. Creamean, and David C. Sands: Mapping Rainfall Feedback to Reveal the Potential Sensitivity of Precipitation to Biological Aerosols https://doi.org/10.1175/BAMS-D-15-00293.1, 2017

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Shu Huang a, Wei Hu a, Jie Chen b, Zhijun Wu b, Daizhou Zhang c, Pingqing Fu a: Overview of biological ice nucleating particles in the atmosphere PMID: 33271442  DOI: 10.1016/j.envint.2020.106197

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