Climate Change & Anthropocene Extinction 20: Amazon tree transpiration crucial to keep rainforest wet

The individual trees in the Amazon rainforest play a crucial role in keeping the rainforest intact. Not just because the trees together create the forest, but also because – together – they create the climate (through something called the shallow moisture convection pump).

Take home message: in order to preserve the Amazon, deforestation really has to stop completely. A ‘meeting in the middle’ compromise does not work – as (amplified by global climate change) that promotes devastating droughts in the remaining part of the forest.

Amazon rainforest - artist impression of deforestation, WWF ad
If you cut away half of ‘the lungs of the Earth’, don’t expect the other half to keep breathing the way it did. That’s because the Amazon rainforest depends on a wet climate that is literally created by the trees, all of the trees together – stretching from the Andes in the West to (very importantly!) the Atlantic Ocean in the East. Image from 2011 WWF forests campaign ad.

As we’ve learned in this series, in face of the Anthropocene Extinction tree species are (probably) even more threatened than birds, amphibians and mammals.

That naturally brings us to the Amazon – the largest terrestrial hotspot of biodiversity, and an ecosystem with huge significance to the global climate system. It is also the ecosystem with the largest number on endemic-only tree species.

In fact you can’t talk about climate & biodiversity without talking specifically about the Amazon rainforest – that many species are unique to this ecosystem and that much carbon is stored in its biomass. So let’s start a subseries here. First one is about the trees, and the rain:

Why is the Amazon a rainforest? Because it creates its rain

Guarding tens of billions of tons of carbon in its trees and organic soil is not the only reason why the Amazon is crucial for the stability of the global climate system. Another very important reason lies in rainfall, fixation of precipitation patterns over much of South America, even influencing Africa and the wider tropics.

The simple model of why a rainforest creates its own rain is this: dense vegetation uses a lot of water – channelling it from through its roots from the soil towards leaves, where photosynthesis takes place. Therefore vegetation promotes evaporation and increases air humidity, promoting saturation, cloud formation …and rain.

But that’s only explaining a cycle. If there is no water in the soil to begin with, it can’t start up. So where does the water for the Amazonian rain come from, given (as explained in the small text below) that the actual rainy season (monsoon) cannot last a full year.

Well, it comes from the Atlantic Ocean, and it’s sucked in by the ‘Deep Convection Moisture Pump’ (DCMP) – and that is essentially created by the Amazonian trees themselves.

This we learn from a new publication in PNAS, by a team of eight scientists led by Jonathan Wright, an associate professor at the Department of Earth System Science of Tsinghua University, who studies the atmospheric water cycle.

In that publication it is stated that over the Amazon rainforest daily rainfall starts on average about two to three months before the arrival of the monsoon. But why? Well, ideas do circulate – but these would need to be tested, as the authors write the following:

“[…] wet season onset over the southern Amazon precedes the southward migration of the Atlantic ITCZ by ∼2–3 months and occurs without a reversal in the land–ocean surface temperature gradient. Conventional mechanisms therefore cannot explain wet season onset over the southern Amazon. An alternative hypothesis holds that late dry season increases in rainforest transpiration may increase surface air humidity and buoyancy. Lifting of this humid near-surface air by cold fronts moving northward from midlatitude South America could cause large-scale increases in deep convection and upper-level heating, thereby initiating moisture transport from the tropical Atlantic. Large-scale moisture transport reinforces the conditions that favor deep convection, ultimately leading to wet season onset. We refer to this transition mechanism as the deep convective moisture pump (DCMP).”

Well, enter ‘shallow convection’ – as many large things start small, and even the biggest tropical thunderstorms do in fact start with one tiny drop of rain, that was an even smaller drop of water inside a tree leaf before that…

The deep convection that is needed in the troposphere to produce the tropical rainfall is first triggered at a smaller scale, and that’s where ‘transpiration’ from the trees is a possible candidate, to explain the witnessed early onset rainfall:

“To clarify the mechanisms involved in activating the DCMP, the first question that must be answered is whether the late dry season increase in lower tropospheric humidity primarily derives from rainforest transpiration or advection from the ocean” – after which the researchers present evidence for the first mechanism, deduced from a slow cascade of processes, that starts with the transpiration from trees, gradually decreasing the stability of the troposphere (promoting deeper convection streams) and ending in a daily rain cycle:

“The early transition is characterized by increases in rainfall and rainforest photosynthesis, along with continued destabilization of the atmosphere. Surface air temperature plateaus, but shallow convection intensifies, as indicated by strong positive ∂θe/∂t within the lower troposphere and continued increases in low cloud cover. Conditional instability continues to grow, as increases in convective available potential energy (CAPE) and decreases in convective inhibition confirm the development of an increasingly favorable environment for convection. Moistening accelerates in the lower troposphere and begins to penetrate into the upper troposphere.”

Onset of rain over the Amazon - graph
The process where dry season Amazon transpiration kick-starts the rain season 2-3 months before the actual ITCZ arrives is illustrated in the beautiful graph above, which we’ll just admit we don’t understand.

Bit of background on the Intertropical Convergence Zone, or ‘monsoon’. You’ll need this to understand climate impacts…
The Intertropical Convergence Zone (ITCZ) is not a geographically fixed precipitation band, but an intricate part of the Earth’s general circulation [that is actually best explained in our background article on (Atlantic) hurricanes <- see image 1, the ‘Hadley Cell’]. That’s why it’s much better known under the name monsoon – a seasonal climate shift, the ‘wet season’. This is because it’s the ‘convergence’ that determines the position of the ITCZ, the upward motion of air in the troposphere. It’s engine? Heat. The source of that heat? As always on Earth: the Sun. Now the tropics of course are the zone with the highest solar irradiance, which is why they are (generally) the warmest climate zone. But the solar irradiance (for any point on the surface of our planet) is not constant; it fluctuates as the axis of our planet is tilted. This tilt is actually in a constant direction [relative to our solar system, not the universe] but Earth itself is not: as Earth slowly turns around the Sun in twelve months time and constantly spins around its axis in 24 hours, days lengthen and shorten – while alternately the northern and then the southern hemisphere have the highest sun exposure. The point of maximum solar irradiance moves from the Tropic of Cancer (Northern hemisphere Tropic) at the June solstice to the Tropic of Capricorn (Southern hemisphere Tropic), which it reaches around December 21. In between these dates the line of maximum irradiance crosses the equator twice, around September 21 (in southward direction) and on March 21 (moving back to the northern hemisphere). As this line of maximum solar irradiance drags along with in (with some delay) the zone of the highest temperatures, it also determines the position of the two atmospheric Hadley Cells, the trade winds, the Intertropical Convergence Zone – and perhaps most notably: the rain. Areas that are touched by the ITCZ once per year have a single monsoon, areas that have a ITCZ passing over may have two rainy seasons per year.

Climate change and the Amazonian monsoon – more bad news…

The researchers in the PNAS study focus on a large area in the Southeast of the Amazon basin – this is where deforestation and agricultural expansion are causing the most direct ecological damage. The loss of rainforest surface in this critical area also decreases the strength of the ‘shallow moisture convection pump’ as explained by the scientists.

This means less Atlantic water vapour is drawn in from the East, and less of that water is transported [by the typical Southeast to Northwest trade winds – trade winds that are declining, bad sign(!)] to the areas of rainforest that lie further inland, causing them to dry out too.

Deforestation in the Amazon decreases rain, PNAS study
Now sadly this same portion of the Amazon – shown above in the image from the PNAS study – is also the region that is most directly affected by anthropogenic climate change, further exacerbating (annual) droughts.

Yes, climate change increases evaporation, but for the Amazon rainforest something else is worse

Again – just like with the above-explained model of the rain cycle – there is a strong geographical component to this story, that makes it a bit more complicated, and a lot more severe.

The simple model of climate warming’s impact is that higher temperatures cause a general increase in evaporation that is not directly compensated by an equal increase in precipitation, because warmer air can contain a larger amount of water as gas – so condensation and cloud formation are delayed, and a higher portion of the available water is always in the air – not in the biosphere.

This very simple model is a weak effect only [becoming more severe during a drought!] and does not explain the possible rainforest-to-savanna tipping point effect of climate change – or for instance the already witnessed Amazon droughts of 2010 and Amazon forest fires of 2015. These are caused by a far larger disturbance, a disturbance of the general circulation – reason why we went to some length to first explain the ITCZ above.

When the Arctic ice melts, Amazon droughts increase

Anthropogenic climate change is more pronounced on the northern hemisphere, for two reasons: that’s where most of Earth’s land masses lie, and there the atmosphere warms faster than above the oceans, which have a much larger thermal mass. And due to albedo feedbacks (declining summer ice and snow cover) the warming is most pronounced in the Arctic region, the centre of the northern hemisphere.

Therefore the effects of climate change on Earth’s general circulation are skewed: the size of the Polar Cell over the North Pole decreases (due to a decrease in the temperatue gradient) and the whole circulation system is dragged northwards – also because the ITCZ favours the zone with the highest temperatures.

As a result the ITCZ is inclined to stall longer on the northern hemisphere and shorter in the south – a hypothesis that is supported by paleoclimatic evidence for the Amazon, showing the ITCZ and therefore much of seasonal rainfall, is dragged northward, in the direction of the Caribbean, promoting droughts in especially the southern part of the Amazon rainforest, precisely the region that is also most prone to deforestation-induced droughts, as the new PNAS study showed.

That’s double bad news – illustrating there really is a tipping point to this rainforest.

In order to save the remainder the rainforest what has to be saved first is the rainfall. And that means two things: stopping climate change – and leaving the trees standing as they are. Work to be done.

© Rolf Schuttenhelm |

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