The Congolese rainforests are the world’s second largest remaining tropical rainforest expanse and a 60 gigatonne carbon store. Although climate models have a hard time predicting rainfall changes over the Congo Basin and despite a multi-decade drying trend, these forests in the heartland of tropical Africa are generally thought to be relatively resilient to the effects of climate change – at least when compared to the Amazon, a well-researched climate tipping point.

But ironically the possible large-scale drying and disappearance of the Amazon rainforest might also be the climatic Achilles’ heel of the Congolese rainforests.

This global precipitation forecast from NOAA shows how tropical precipitation patterns might shift. The high-precipitation zone over South America may get more narrow and shift (annual average) to the Northwest and also over the tropical Atlantic Ocean the ITCZ’s average position may move further North of the equator. That will have consequences for Africa too. Shown by this model is partial drying of the Congo Basin. (Also shown is how climate change leads to drying of the Central American rainforests, and how climate change hurts what’s left of the Brazilian Atlantic Forest.)

Yesterday, in our background piece about the effects of 21^st century climate change in Africa, we saw how the continent on average is warming very rapidly. And although around the equator this temperature rise may be less pronounced than in Africa’s subtropics (and ‘northern and southern tropics’), its effects can paradoxically be bigger there, as Central Africa is not used to large seasonal temperature shifts. Therefore a relatively small warming can have major ecological effects.

Also, more importantly, while most equatorial regions will get wetter, some may become drier. And there’s one ecosystem that depends on that coin drop: rainforests. Therefore, as part of our series about the ecological consequences of climate change, we have a special interest in climate projections for the Congo Basin, home of the world’s second largest remaining tropical rainforest.

Today we take a look at general precipitation changes for Earth’s tropics – and try to see how these might connect across the Atlantic Ocean, between the Amazon of South America and the Congo Basin of Africa.

Wet regions wetter, dry regions drier – on average yes, locally no

If you’re one of those people that’s into climate communication, you’ve probably heard the ‘wet regions will get wetter & dry regions will get drier’ story and perhaps shared it with your somewhat less climate-literate friends.

Well. You may have to get back on that. Because it’s not always true. Climate change can (slightly) relocate the Intertropical Convergence Zone (ITCZ) – and one of the keys to that shift lies in the Amazon (and another as far North as the Arctic).

As explained in yesterday’s background article about 21^st-century climate change in Africa the easterly trade winds, defining for tropical climates, are part of the Hadley Cell, that is in turn part of the general circulation of the atmosphere. Climate change is more pronounced on the northern hemisphere, and therefore the model to the left becomes skewed, with a relatively smaller Polar Cell over the Arctic and an average position of the Intertropical Convergence Zone (ITCZ) slightly further North of the equator. If also the Amazon rainforest disappears (and as a consequence convective Amazonian rainfall decreases), meandering shifts of the Intertropical Convergence Zone may amplify that effect for Africa – possibly fueling a drying trend for the Congo Basin.

The wet tropics may get wetter, but as a climate zone it also becomes narrower

One of the most pronounced effects of climate change (following increased heat absorption in the troposphere due to ever-increasing concentrations of greenhouse gases) is an intensification of the general circulation – the pattern of rising and subsiding air currents, caught in different cells from the poles to the equator.

Therefore both high pressure-dominated climate zones (the arid subtropics) and low pressure-dominated climate zones get more pronounced. This justifies concerns of desertification in for instance the Kalahari region on the southern hemisphere and a possible expansion of the Sahara desert across the Mediterranean.

But as the circulation intensifies, it also slightly changes shape – as illustrated by the below projection by NOAA. The left image shows total annual precipitation (climate average between 1950 and 2000) for Earth’s latitudes – and the right image shows the predicted precipitation change for the 21^st century as a consequence of anthropogenic warming. If you look carefully you see for instance that the brunt of drying occurs at a higher latitude than the centre of the current dry zone and that the tropical precipitation increase is actually a relatively narrow zone:

One other detail you may notice from the above projection is that Earth as a whole seems to get wetter. But please remember that all that extra rain is a product of something else: an increase in evaporation. So it’s quite possible to have an increase in warming-induced droughts in apparently ‘wet’ climate zones – especially if you take into account seasonal changes (like summer heat waves – that even lead to tundra droughts, and tundra wildfires).

The Intertropical Convergence Zone is of course itself also not seasonally fixed. That’s why the ITCZ is more commonly known as the monsoon, or ‘rainy season’ – a band of often-heavy convective rainfall that crosses the equator four times per year en route to either its boreal or its austral summer maximum. Locally it tends to linger though as part of natural fluctuations like ENSO (then often-leading to floods) and generally the seasonal ITCZ migration is larger in the directions of continental land masses. Here is how the monsoon season ‘normally’ crosses the African tropics:

Current climate average seasonal ITCZ migration, leading to monsoon seasons across Earth’s tropics. For rainforests especially the annual average ITCZ position is relevant. Where it passes quickly you may have seasonal forests are savanna, where it passes often or somehow lingers longer, you may have rainforests. Please note that the average position of the ITCZ is currently already just North of the equator. That’s because the northern hemisphere is ‘naturally warmer’, mainly due to surface heat transport to the North-Atlantic Ocean by the Gulf Stream. Land warming and Arctic amplification as a result of anthropogenic climate change will further emphasize this, first threatening the Amazon rainforest from the South – and in turn possibly later also the (southern) Congolese rainforests. Image source: BBC.

If the Amazon rainforest disappears (from the South, or the Southeast) then that may have major consequences for Africa too, as it’s not just the rainforest that grows where the ITCZ flows, but also the other way around (due to dense rainforest vegetation creating a convection pump, that sucks in the ITCZ). In other words, the ITCZ preference to increasingly linger North of the equator as a direct consequence of anthropogenic climate change may be amplified by climatic effects of the Amazon rainforest to savanna biome shift.

Although we have to emphasize that we are speculating here, it does make sense that this amplified northward migration of the ITCZ over South America will also influence the average position of the ITCZ over the tropical Atlantic – and in turn also over Africa.

Part of the semi-arid Sahel may get wetter, while the Congo Basin may actually dry out – and probably also from the South. Models meanwhile also show an East-West difference arising in Africa: East Africa will get more rain (although with large seasonal fluctuation) – while West Africa becomes more prone to droughts.

Back to the climate models, is back to one certainty: higher emissions, more drastic changes in tropical precipitation

But because science writers shouldn’t speculate too much, let’s end this one with what the actual climatologists say about tropical precipitation changes. In 2015 four researchers of the UK Met Office’s Hadley Centre published a study in Nature Climate Change, comparing different climate models.

They conclude that although all models show significant changes, they still differ in their geographic directions. However one thing is sure, they add: the total surface area of the tropics that will be subject to unprecedented changes, either drying or wetting, increases with the total anthropogenic greenhouse gas emissions.

The tropics are in for big changes, and for any ecosystem, whether rainforest, savanna, or anything else, change is never good.

© Rolf Schuttenhelm | www.bitsofscience.org