Keywords

Vegetation plays an important, often neglected role in regulating the climate. Imagine the difference you feel when, on a hot summer day, you stand either in an open field or in a dense forest. It is self-evident that the major change brought about by the conversion of forests into farmland or urban areas greatly affected the climate.

Of the solar radiation that hits an area densely covered with vegetation, only 1% is used for photosynthesis (Fig. 1), between 5% and 10% heats the air (“sensible heat”). More than 70% of the radiation is used by plants for transpiration, converting liquid water into water vapor, which is a very energy-intensive process (“latent heat”). Including unplanted areas and water surfaces, about 50% of the solar energy reaching the Earth’s surface is used for evaporation and transpiration of water (“evapotranspiration”Footnote 1) (Pokorny et al., 2010; Jasechko et al., 2013). As air masses rise, the water vapor condenses and releases the same amount of energy that was consumed at ground level, with some escaping into space. The resulting clouds reflect incident solar radiation and are the source of new precipitation.

Fig. 1
An illustration of solar energy incident on vegetation. The heat from the Sun enters the atmosphere out of which 10% is reflected, 1% is used for heat input biomass, 1% is used for photosynthesis, 5 to 10% is the sensible heat, 70 to 80% is the latent heat, and 5 to 10% is the heat input soil.

Distribution of solar energy incident on vegetation (Latent and sensible heat are types of energy that are released or absorbed in the atmosphere. Latent heat refers to phase changes among liquid, gaseous, and solid. Sensible heat refers to temperature changes of a gas or object without a phase change) (Pokorny et al., 2010)

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On land surfaces, annual precipitation amounts to about 120,000 km3, of which around 50% originate from the oceans and 50% from land (Fig. 2) (Ellison et al., 2019). About 60–80% of this land-sourced atmospheric moisture derives from plant transpiration (Wei et al., 2017), which shows the important role of vegetation in the precipitation cycle as well as in the transfer of energy from the soil to the upper atmosphere.

Fig. 2
An illustration of the global water cycle. It marks the origin of water by ocean, land, transpiration, and evaporation. It has the interconnectedness of the key components with the movement of water between land and ocean.

Global water flows. Of the 120,000 km3 of precipitation over land, 50% comes from the oceans and 50 % from land areas. Of the latter amount, about 70% comes from plant transpiration and 30% from water bodies and soils. 32,000 km3 of evapotranspiration on land returns to the ocean via atmospheric moisture; 40,000 km3 are discharged into the oceans via rivers. (Data from van der Ent et al. (2010))

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It was assumed until recently that human influence on atmospheric water vapor is negligible, as the focus was mainly on industrial processes. It is now known that anthropogenic changes in land cover make this influence substantial (Kravčík et al., 2007; van der Ent et al., 2010; Mahmood et al., 2014)—with the main factor being deforestation which, since the beginning of agriculture, has wiped out nearly half of the world’s forests.

Trees Provide Cooling and Generate Water Vapor

Every tree in the forest is a fountain that with its roots sucks water from the ground, pumps it through its trunk, branches, and leaves, and releases it through its leaves into the atmosphere as water vapor. On a normal sunny day, a single tree can evaporate several hundred liters of water, providing its surroundings with 70 kWh of cooling per 100 liters, which is equivalent to the cooling effect of two air-conditioners running for 24 h (Pokorny, 2012; Ellison et al., 2017). Billions of trees together generate huge rivers of water in the air (“flying rivers”)—rivers that form clouds and generate precipitation hundreds or even thousands of kilometers away (Nobre, 2014; Weng et al., 2018).

Globally, 50% of precipitation falling over land comes from moisture produced by evapotranspiration over land, mainly from transpiring trees (Eltahir & Bras, 1994; van der Ent et al., 2010; Keys et al., 2016; Ellison et al., 2017; Staal et al., 2018). In some regions of the world, the share is 70% (or more) of precipitation (van der Ent et al., 2010), with higher shares inland (Fig. 3).

Fig. 3
A study area map of the world with a precipitation recycling ratio. The share is higher for the central sections of the continents of South America, Africa, and Asia, from 0.6 to 1.

Average continental precipitation recycling ratio (1999–2008). The higher the number, the more precipitation comes from land evaporation (van der Ent et al., 2010; van der Ent, 2014)

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Tropical evergreen deciduous forests occupy only about 10% of the Earth’s land surface but contribute 22% of global evapotranspiration, (Wang-Erlandsson et al., 2014) which underscores their importance for the trans-regional hydrological cycle. The typical distances that moisture evaporated from land travels in the atmosphere before falling back onto land are in the order of 500–5000 km; the typical time scale ranges from 8 to 10 days (van der Ent & Savenije, 2011; van der Ent & Tuinenburg, 2017). For example, moisture evaporating from the Eurasian continent is responsible for 80% of China’s water resources (van der Ent et al., 2010). The main source of rainfall in the Congo Basin is moisture evaporated over East Africa, while the Congo Basin is in turn an important source of rainfall in the Sahel (van der Ent et al., 2010). The condition of the West African rainforest is especially important for precipitation in the Ethiopian highlands and thus in turn for the runoff of the Nile (Gebrehiwot et al., 2019). This explains why even in large river basins such as those of the Amazon, the Congo, and the Yangtze, rainfall is more influenced by land use changes outside than inside the basins. Even in river basins that do not span several countries, runoff has been significantly affected by land use in other countries (Wang-Erlandsson et al., 2018).

Altered Heat Flows, Altered Atmospheric Patterns

Models show that local conversion of forests or grasslands to cropland can reduce annual terrestrial evapotranspiration by 30–40% (Sterling et al., 2013). On a global scale, land cover change between 1950 and 2000 resulted in a reduction in terrestrial evapotranspiration by 4–5%, or 3000–3500 km3, and a 6.8% increase in surface water runoff (Gordon et al., 2005; Sterling et al., 2013). On the other hand, scientists have found that vegetation has a cooling effect due to increased efficiency in the vertical movement of heat and water vapor between the land surface and the atmosphere (Chen et al., 2020).

Satellite observations indicate that forests have a major influence on cloud formation, not only in the tropics but also in temperate zones: disappearance of forests can lead to a substantial decrease in local cloud cover and thus in precipitation (Teuling et al., 2017). Model calculations have shown that large-scale global deforestation between 1700 and 1850 has led to a decline in monsoon rainfall over the Indian subcontinent and southeastern China, and a consequent weakening of the Asian summer monsoon circulation (Takata et al., 2009). In the tropics, deep cumulus convection has changed significantly due to land use changes (especially the conversion of forest to cropland). This affects not only local precipitation, but also has long-distance impacts through processes known as remote effects (or “teleconnections”). These can affect higher latitudes, significantly altering the weather in these regions (Sheil & Murdiyarso, 2009; Gebrehiwot et al., 2019). Even relatively small disturbances in land cover in the tropics can lead to effects at higher latitudes (Chase et al., 2000); for example, changes in Amazonia can lead to effects in the north-west of the United States (Medvigy et al., 2013). Forest disappearance can also lead to lower rainfall and longer dry seasons, as reported from Rondônia in Brazil (Coe et al., 2017) and from Borneo, where water catchment areas with the greatest forest loss were found to have suffered a 15% decline in rainfall (McAlpine et al., 2018). In India, patterns of decreasing rainfall during the monsoon were accompanied by changes in forest cover, due to reduced evapotranspiration with subsequent decline in the recycled rainfall component (Paul et al., 2016).

Back Radiation from Bare Ground

Normally, more than 50% of the solar radiation hitting the Earth’s surface is converted into latent heat by evapotranspiration, which in turn enters the atmosphere, feeds the precipitation cycle, and partially radiates back into space.

On bare surfaces, such as fallow fields, dry meadows (in summer and after the hay harvest) as well as on concrete or asphalt surfaces, the ground absorbs more incident solar radiation, thus heats up more, generates sensible heat, and releases thermal energy to the atmosphere which, according to the Stefan-Boltzman law, increases in proportion to the fourth power of its absolute temperature (Fig. 4). The differences in surface temperature between these bare areas and the forested areas can be up to 20 °C on Central European summer afternoons (Fig. 5) (Hesslerová et al., 2013). On the Indonesian island of Sumatra, temperature differences between forested and bare areas of up to 10 °C have been observed, which in turn can be explained by the evaporative cooling effect of the forests, which outweighs the albedo heating effect resulting from forest areas being darker (Sabajo et al., 2017). Local biophysical processes triggered by forest loss can therefore cause a net increase in summer temperatures in all regions of the world (Alkama & Cescatti, 2016).

Fig. 4
A chart of evapotranspiration. The forest has 80 to 90% latent heat and 5 to 10% sensible heat. The factors are reflection of the clouds, heat emission into space, cloud formation, and precipitation. The field and meadow has 10 to 20% latent heat and 70 to 80% sensible heat. The factors are cloud formation and precipitation.

Evapotranspiration lowers ground temperature and it increases cloud albedo, radiation to space during the condensation process, cloud formation, and thus precipitation. Removal of vegetation increases ground temperature, radiates exponentially increasing amounts of heat energy as ground temperature increases, creates high-pressure areas that impede the passage of low pressure areas (and thus potential precipitation bringers), reduces cloud formation potential and thus precipitation. (Data from: various textbooks on climatology, own design)

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Fig. 5
2 satellite images of a mixed landscape of the physical geography and temperature. Asphalt is at 49, sparse vegetation at 36 degrees Celsius, harvested meadow at 42.5 degrees Celsius, wet meadow at 29 degrees Celsius, alder stand at 28.8 degrees Celsius, and water at 26 degrees Celsius.

Surface temperature distribution in a mixed landscape (Hesslerová et al., 2013; Ellison et al., 2017)

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Historical deforestation has actually reduced latent heat flow on land and increased sensible heat on the ground (Bounoua et al., 2002; Brovkin et al., 2006). Deforestation has caused significant warming from 2003 to 2013, averaging up to 0.28 °C in tropical areas and up to 0.32 °C in temperate regions of the southern hemisphere (Li et al., 2016). At the current rate of deforestation, the loss of tropical forests could increase global temperatures by 1.5 °C by 2100, not taking into account other human-induced temperature increases (Mahowald et al., 2017).

Between 1950 and 2000, surface temperature increased by 0.3 °C worldwide due to changes in land cover (Sterling et al., 2013). Disturbances in the surface energy balance caused by vegetation changes between 2000 and 2015 have led to an average 0.23 °C increase in local surface temperatures (Duveiller et al., 2018). The average warming due to land cover changes could explain 18–40% of current global warming trends through the reduction in evapotranspiration that overcompensates for the increase in surface albedo (Ban-Weiss et al., 2011; Alkama & Cescatti, 2016; Wolosin & Harris, 2018).

Biogenic Aerosols for Cloud Formation

In addition to the importance of forests for energy flows and as sources of precipitation, large forests also appear to be biogeochemical reactors in which the biosphere and atmospheric photochemistry generate nuclei for cloud and precipitation formation, thus maintaining the hydrological cycle. Trees produce volatile organic compounds and release microorganisms, bacteria, fungal spores, pollen, and other biological particles, which get into the air after rainfall from leaf surfaces, especially of trees. Once in the atmosphere, they contribute to the formation of condensation nuclei, which in turn affect cloud formation and precipitation. Biogenic aerosols can also help increase the freezing temperature by forming ice nuclei, without which freezing would occur only at cloud temperatures of −15 °C or below. Facilitated by such ice nuclei, freezing can occur at temperatures close to 0 °C, which enables efficient cloud formation and promotes (local) rain events.

Policy Implications

Vegetation, fertile soils, and water retention must be recognized as key regulators of the water, energy, and carbon cycles. Some of the policy implications are listed below and should be implemented if possible.

  • We should be aware of the positive feedback loops. As explained earlier, deforestation makes land areas and the climate drier and warmer. This leads to conditions that increase the risk of forest and vegetation fires, which in turn release CO2 and lead to further deforestation, creating a vicious cycle. Climate change, deforestation, drought, and forest fires form a triple cycle of reinforcing feedbacks (Fig. 6).

  • In view of the long-distance effects of large forest ecosystems, these should be seen as a global goods. For example, the REDD+ mechanism developed under the UNFCCC could provide a model for recognizing and financing the international water and energy services that such forests provide.

  • Especially important and sensitive forest areas should be protected and managed accordingly.

  • It is of utmost importance to stop deforestation and to increase reforestation worldwide.

  • Agricultural practices should focus on soil building, year-round plant cover, and use of agroforestry methods.

Fig. 6
A diagram of interconnectedness of deforestation, drought, forest fires, and climate change. Deforestation leads to drought and forest fires, which in turn release C O 2 and contribute to climate change. More droughts result in less precipitation, less photosynthesis, less carbon sinks, and higher temperatures.

Triple positive feedbacks between deforestation, drought, forest fires, and climate change. (Modified from Peduzzi (2012))

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Conclusion

It is important to understand that the carbon, water, and energy cycles on land are closely interlinked. Restoring the atmospheric and terrestrial moisture cycles in vegetation, soils, and the atmosphere is paramount to cooling the planet and safeguarding precipitation patterns globally. The penalty for failure is the drying-out of terrestrial landscapes.

Stopping deforestation, increasing reforestation, and introducing agroforestry practices are essential if we are to avoid climate catastrophe. A systemic approach is needed to understand and harness the underlying patterns of rain formation. To restore rainfall to areas like the Sahel, trees must be planted not only in the region, but also on the coast to draw moist air from the ocean onto land (Ellison & Speranza, 2020).

Another important approach to supplying water and energy cycles involves increasing soil fertility, water retention, and soil protection through the practice of the regenerative organic movement, such as year-round vegetation cover through intercropping and undersowing or introduction of agroforestry. Finding ways to build up additional soil organic matter is one of the keys to success for large areas of the world which are currently under cultivation.

In general, we need a paradigm shift that values the hydrological and climate-cooling effects of vegetation, and particularly of forests, in addition to their carbon sequestration potentials. We must pay more attention to the beneficial effects that cover with vegetation, especially trees, has on the local, regional, and continental climate.