Transpirational cooling (biological)
Excess heat generated from solar radiation is damaging to plant cells and thermal injury occurs during drought or when there is rapid transpiration which produces wilting.
[1] Green vegetation contributes to moderating climate by being cooler than adjacent bare earth or constructed areas.
As plant leaves transpire they use energy to evaporate water aggregating up to a huge volume globally every day.
The Earth’s energy budget reveals pathways to mitigate climate change using our knowledge of the efficacy of how plants cool.
[4] In the tallest trees, such as Sequoia sempervirens, the water rises well over 100 metres from root-tip to canopy leaves.
Each gram molecule (mole) of condensing water will bring about a marked 1200-fold plus reduction in volume.The simultaneous release of latent heat will drive air from below to fill the partial vacuum.
The energy required for the surrounding air to move in is readily calculated from the small (one-fifteenth of latent heat) reduction in temperature.
In addition to receiving water from the roots and creating the raw materials for photosynthesis, they also have a large internal surface area to enable the exchange of gases.
In many plants, this is achieved in a structure thin enough to be semi-translucent, to enable some light to pass through to neighbouring leaves.
The water that becomes raw material for sugar production, also cools the leaf and supports its structure through the pressure of turgidity.
Fred Pearce states that “a single tree transpiring a hundred litres of water a day has a cooling power equivalent to two household air-conditioning units”[7] (p. 29).
Of the 150 kWh falling on the crown, 1% is used for photosynthesis, 10% reflected as light energy, 5 to 10% as sensible heat with the remaining 79 to 84% entering the process of transpiration.
[3]Extrapolated to a hectare, the cooling power of a closed canopy is 35,000 kWh a day.Cities with constructed surfaces and devegetation are typically warmer than adjacent countryside.
[citation needed] There were about six trillion trees on the planet, but human activity has destroyed roughly half.
Using satellite imagery, the impact of regeneration processes restoring vegetation in arid areas is visible from space and can tracked over time.
The movement of heat embodied in water vapour as it leaves vegetation is not well understood given the complexity of the dynamics.
The image of the Coronation Reserve shows an areas of turf and the margin of native forest separated by a herbicide strip.
Understanding these dynamics provides more pathways to moderate the climate than simply relying on emissions reductions and sequestration alone.
This is achieved through a whole system approach including regenerating the soil carbon sponge, protection of existing forests, reafforestation, and restoring the biotic pump.
A study of 12,000 years of population data found that “three quarters of terrestrial nature has long been shaped by diverse histories of human habitation and use by Indigenous and traditional peoples”.
[22] With rare exceptions, current biodiversity losses are caused not by human conversion or degradation of untouched ecosystems, but rather by the appropriation, colonization, and intensification of use in lands inhabited and used by prior societies.
[22]This calls on us to unlearn some of the assumptions embedded in Western epistemologies and the decolonisation of knowledge as a foundation for more effective climate action.
