People who follow climate science will likely be well aware that the Amazon rainforest is particularly vulnerable to anthropogenic climate change – as the basin becomes increasingly prone to droughts under rising global temperatures. Much of the Amazon ecosystem, the largest terrestrial hotspot of biodiversity, may collapse, flipping to a barren savanna-like state (cerrado grassland and caatinga semi-desert).

Regulars of Bitsofscience.org may also be able to explain why this biome switch might happen and describe a worrying geographical phenomenon in which the Amazon rainforest is essentially being swiped off the South American continent into the Caribbean Sea – climate extinction on a truly massive scale, as the below image illustrates:

Beyond a certain threshold [a climate tipping point we’ll try to quantify later in this series!] anthropogenic climate change leads to a massive die-off in the Amazon rainforest. This process is likely to start in the South of the Amazon basin and then gradually move North towards the equator, where just a fraction of the rainforest might remain.

As we’ve explained in our previous article of this special series, one of the drought-promoting mechanisms lies directly in the ongoing Amazonian deforestation itself, as the total combined surface of dense rainforest vegetation acts as a convection pump, quite literally creating the daily rain cycle that the rainforest depends on. The fewer trees, the weaker this atmospheric pump, the less rain, the fewer trees – is what that downward spiral looks like. Here too, the southern part of the Amazon basin seems the most vulnerable, and sadly that’s also where the most active deforestation is taking place.

But there is also a direct climate change component to this story:

Rapid warming of the northern hemisphere pulls on the South-American monsoon rains

The main mechanism behind the South-to-North Amazonian rainforest die-off lies in an often overlooked global pattern in which anthropogenic climate change manifests itself: the warming caused by the continuous elevated greenhouse gas emissions is in geographical practice often far from ‘global average’. Instead a skewed pattern shows up, with much faster warming of the northern hemisphere* than the southern hemisphere. The result is no marginal effect. In fact in 2016 the northern hemisphere already breached the ‘preindustrial +2 degrees’ climate limit, warming almost twice as fast as the southern hemisphere.

[*) This is for two reasons: the northern hemisphere is where most of Earth’s land masses lie, and atmospheric warming happens faster over land than over the oceans (because oceans have a larger thermal mass) – and also because the North Pole, the centre of the northern hemisphere, is more strongly influenced by amplifying climate feedbacks (than Antarctica), with Arctic warming at least 2-3 times faster than the global average temperature rise.]

As we’ve discussed in several other articles the result of this ‘skewed global warming’ is a weaker northern hemisphere Polar Cell and a relatively strong poleward movement of both the Ferrell Cell and the Hadley Cell that together comprise the general circulation of the troposphere. As a consequence also the ITCZ (or ‘monsoon’), the zone of maximum tropical convergence and convection, and therefore the zone of maximum rainfall, tends to linger longer (during boreal summer) on the northern hemisphere – increasing the tropical dry season in the South* (while increasing rainfall North of the equator).

The pattern can be deduced from the paleoclimatological record, in which periods of rapid boreal warming during the Pleistocene are associated with precipitation decreases in the Amazon basin.

[*) The seasonal position of the Inter-Tropical Convergence Zone is of course also influenced by other local factors and therefore this climate change response may not be uniform across Earth’s tropics. In the case of East Africa for instance the monsoon is subject to multi-decadal oscillations, influenced by the ocean patterns (cause of recurring natural droughts in the Horn of Africa) and in the case of South Asia by direct meteorological effects of human air pollution (the ‘Asian Brown Cloud’ – blocking the monsoon on its way North over India, offsetting the above warming mechanism). Due to local geographical differences the effects of anthropogenic climate change on the ITCZ could be larger in South America than it is in Africa and Asia, as is addressed below.]

The onset of a new dry season in the Amazon, with forest fires

In South America at least the effect is more than theory, recent climatic data show: unseasonably strong Amazonian droughts (likelihood ≤’once in a century’) that are linked to anthropogenic climate change were first witnessed in 2005, then again in 2010 and then again five years later, in 2015. The timing in the year illustrates the ITCZ pattern – with droughts and increasingly large-scale forest fires at the end of the boreal summer.

This is what ‘positive climate feedbacks’ look like in reality: widespread Amazonian forest fires, following drought and record-high temperatures at the end of the boreal summer of 2015. This satellite image does not show smoke, but elevated CO2 concentrations from tropical forest fires.

(Again, as explained in part 20 of this series, these Amazonian droughts are worsened by deforestation in the southern part of the basin, as normally the dense rainforest vegetation is able to create its own daily rain cycle, even in the dry season – a capacity that decreases with the total rainforest surface.)

Complicating factor: geography. Now what do models say?

So there is a theory that is both supported by present observations and by paleoclimatic evidence. But can the situation be replicated by climate and vegetation models – and can we predict the future development of the Amazon rainforest under continued climate change?

We found an interesting model study in Journal of Climate from 2008 by two researchers of Cornell University, Kerry Cook and Edward Vizy. In this publication, called ‘Effects of Twenty-First-Century Climate Change on the Amazon Rain Forest’, the modelling challenges are expressed as being strongly linked to the local geographical situation of the Amazon basin, that makes the Amazon quite separate from other remaining tropical rainforests – the presence of the bordering Andes mountain range:

“Simulating South American climate is a challenge for coupled atmosphere–ocean GCMs. The Andes topography, which is known to be an important determinant of the continental climate, is so steep that the elevation of the surface is artificially lowered by 2 km or more at typical GCM resolutions (Lenters and Cook 1995). This problem is usually addressed in models by adopting envelope topography, which preserves the volume of the topography so it produces reasonable planetary-scale perturbations of the flow. But this does not serve the regional representation of temperature and, especially, precipitation on the continent well.”

‘Bolivia, Paraguay and Argentina lose all rainforest, Brazil and Peru lose most…’

These researchers find that 21^st century climate change causes a large-scale die-off of the Amazon rainforest, starting in the South of the basin. Coupling a vegetation model to a general circulation model they calculate that under a CO2 stabilisation scenario of 757 ppm [a concentration that comes closest to RCP6.0 – so roughly 3 degrees warming, under conservative climate sensitivity assumption] from the year 2080 the climate in over two thirds of the basin would no longer be able to sustain a rainforest ecosystem – and locally even desertification would occur:

“Compared with the present day simulation, the areal extent of the Amazon rain forest is reduced by 69%. A total loss of rain forest vegetation in the central and southern Amazon basin is simulated. Bolivia, Paraguay, and Argentina lose all of their rain forest vegetation, and Brazil and Peru lose most of it. The surviving rain forest is concentrated near the equator, with the rain forest extent in Columbia essentially maintained. Along the northern coast, Venezuela and French Guiana suffer relatively small reductions in rain forest extent, while the loss in Guyana and Surinam is 30%–50%. Much of the rain forest in the central Amazon north of about 15° S is replaced by savanna vegetation, but in southern Bolivia, northern Paraguay, and southern Brazil, grasslands take the place of the rain forest in the 2081–2100 simulation. (Some of this area is not rain forest today, having been replaced by cropland, but the present day climate in that region would support rain forest.) East of about 52° W over a large portion of Brazil, present day savanna is replaced by shrubland as the caatinga vegetation of the Nordeste region spreads westward and southward. In the heart of this region (from about 5° to 12° S, centered around 40° W), present day caatinga vegetation is replaced by barren land.”

Remaining questions: what is the critical global temperature? Just how stable is the northern rainforest?

Now of course this study is almost ten years old, so we’ll also have to see how it compares to more recent climate-vegetation model studies before we draw our own quantitative conclusions about the Amazon climate tipping point – specifically what global average temperature rise would trigger the described effect…

For now though we think it’s very useful to get a sense of the underlying patterns, to understand why the Amazon rainforest is so vulnerable to the effects of anthropogenic climate change.

Next up in this series, just to be sure, we’ll take a closer look at the rainforests of Central America and the northern part of the Amazon basin. Are we really safe to say that these are less vulnerable to the effects of climate change? Let’s hope.

Stay tuned.

© Rolf Schuttenhelm | www.bitsofscience.org