21^st century climate change affects the entire globe: every ecosystem, every mountain range, all the glaciers, all the land, the oceans. But of all continents it’s often said that Africa will face the biggest impacts.

African climate change is far from a uniform process though. Large geographical differences occur across the continent, that together form a pattern, as for all the African regions changes in temperature and precipitation are directly influenced by changes in the general circulation of the atmosphere.

While it’s clear that some regions will get really dry, others may get really wet, and some may warm faster than others; it is climate impacts on Africa’s tropical heartland that are perhaps most uncertain, as different climate models have a hard time capturing both the present and future convective precipitation that the Congolese rainforests depend on – rainforests that in turn are hugely important for the health of our planet as they are Africa’s densest biodiversity hotspot and a carbon store weighing in at a 60 gigatonne significance to the global climate system.

Current range of African forest elephant, down from historic. Of this elephant species an estimated total of 100,000 individuals remain – down from a historic population of 2 million, when these creatures could walk freely from the Guinean Forests of West Africa to the entire Congo Basin of Central Africa, where today just two pockets remain (shown in red, mostly in Gabon). The remaining Congolese rainforests are still a true biodiversity hotspot, and the single home of many other iconic but in some cases critically endangered species like the mountain gorilla (just 880 left, IUCN 2016) and our close relative the bonobo (29-50 thousand, IUCN 2016). The Congolese forests also hold a huge carbon store in place, of some 60 billion tonnes (equating to 220 billion tonnes of CO2). So it’s worrying that these forests in the heartland of tropical Africa show a drying trend – raising the important question: will this trend continue? [Image credit: The Elephant Listening Project, Cornell Lab at Cornell University.]

Understanding Congolese climate change – today: context of Africa; tomorrow: context of tropics…

Therefore, to develop a broader understanding, in this article we first take a look at the wider context of African climate change, and from there include a quick peek at some of the regionalised models that zoom in on the Congo Basin.

Tomorrow (in part 28 of this series) we use a different approach and try to see what tropical climate change elsewhere on the planet might mean for the future of Africa’s rainforests, as often it’s actually the lateral connections in the atmosphere that are most important – especially around the equator, where Earth’s daily spin reaches the maximum surface speed*.

[*) 465 m/sec if you’re interested, decreasing to zero at the northern and southern tip of Earth’s axis (close to the poles). Therefore trade winds – that essentially blow straight towards the equator on both hemispheres under the force of high to low surface air pressure within the Hadley Cell – are bent westward by the Coriolis effect. So in fact trade winds ‘don’t blow to the West’, but rather Earth spins at an increasing speed to the East underneath air molecules that are drawn towards the low pressure of the Intertropical Convergence Zone, an atmospheric trough close to the equator. This air (in fact the entire Hadley Cell) also moves to the East due to the forces of gravity, but is, while approaching lower latitudes, essentially outpaced by Earth itself. Yes, we’re a science website – these things matter to us. On all accounts West-East atmospheric connections are important, in both directions. For more on this see our next article! General circulation image credit: Kaidor, CC BY-SA 3.0]

IPCC: Africa warms relatively fast, and fastest over arid regions

The latest IPCC report (AR5, 2014) uses broad terms to describe the consequences of anthropogenic climate change for Africa, calling it the most vulnerable continent for two main reasons: relatively strong warming, and relatively small adaptive capacity [especially if you translate to possibly declining net agricultural productivity and also add human demography to the equation – or in simpler terms: ‘more people, less food’].

And although the most rapid warming will likely occur in inland arid regions, the effects of warming may be larger in wet, tropical regions – because natural climatic variation is smaller there, therefore the threshold of natural adaptive capacity may be sooner breached. On page 1206 this is stated as follows:

“Temperatures in Africa are projected to rise faster than the global average increase during the 21st century. Global average near surface air temperature is projected to move beyond 20th century simulated variability by 2069 under Representative Concentration Pathway 4.5 (RCP4.5) and by 2047 under RCP8.5. However, in the tropics, especially tropical West Africa, these unprecedented climates are projected to occur 1 to 2 decades earlier than the global average because the relatively small natural climate variability in this region generates narrow climate bounds that can be easily surpassed by relatively small climate changes.”

“Ensemble mean changes in mean annual temperature exceed 2°C above the late 20th-century baseline over most land areas of the continent in the mid-21st century for RCP8.5, and exceed 4°C over most land areas in the late 21st century for RCP8.5. Changes in mean annual temperature for RCP8.5 follow a pattern of larger changes in magnitude over northern and southern Africa, with (relatively) smaller changes in magnitude over central Africa.”

Witnessed and projected temperature rise over Africa, according to IPCC AR5 (2014). Under RCP8.5 emissions parts of northern and southern Africa will experience an average warming of over 6 degrees, compared to 1986-2005 climate average(!!)

Does mitigation policy make a difference? Yes: under the ambitious RCP2.6 global emissions scenario ensemble model projections for climate change in Africa keep 21^st century warming below 2 degrees on average. But please note the IPCC uses a different baseline* here(!) It’s not +2 or +4 against the preindustrial climate – the baseline that’s used in UNFCCC context, but an end-20^th century climate baseline, so total anthropogenic climate warming is higher.

[*) Then again no one uses a real preindustrial baseline – but instead, for global database quality, the 1880-1909 global temperature average. Yes, it’s very easy to get confused. In Africa’s case you can add approximately +1 degrees Celsius to the above numbers to get to full anthropogenic warming, so you can state that limiting climate change below the official UN targets of either 2 or 1.5 degrees is – for Africa – impossible, if global emission reductions are your only tool.]

Tropical West Africa is identified as a hotspot for climate change, the IPCC authors write, with CMIP5 general circulation model forecasts for RCP4.5 and RCP8.5 ranging “between 3°C and 6°C above the late 20th century baseline”. This is similarly strong warming as expected over the Southwestern part of Africa, including the Kalahari region – that’s another hotspot, as especially under high global emission scenarios strong precipitation declines are projected, possibly leading to large-scale desertification.

Now when we get to talk about actual precipitation projections it gets much trickier: “Precipitation projections are more uncertain than temperature projections and exhibit higher spatial and seasonal dependence than temperature projections” – although two drying zones clearly stand out: Mediterranean North Africa (essentially northward expansion of the Sahara desert) and central southern Africa, including ecologically vulnerable areas like the Karoo and Kalahari.

Climate models however have a much harder time projecting changes in tropical rainfall. On page 1210 of IPCC AR5 we read:

“West African precipitation projections in the CMIP3 and CMIP5 archives show inter-model variation in both the amplitude and direction of change that is partially attributed to the inability of GCMs to resolve convective rainfall. Many CMIP5 models indicate a wetter core rainfall season with a small delay to rainy season by the end of the 21st century. However, Regional Climate Models (RCMs) can alter the sign of rainfall change of the driving GCM, especially in regions of high or complex topography. There is therefore low to medium confidence in the robustness of projected regional precipitation change until a larger body of regional results become available […]”

Regionalised models have difficulty capturing Congo Basin climate – both current and future

We find a similar uncertainty of climate models if we try to zoom in on the area of key interest for forest preservation, the Congo Basin. In 2013 a research group led by Richard Washington of the Climate Research Lab of Oxford University described in the journal Philosophical Transactions of the Royal Society B how climate models already have a hard time to replicate current rainfall over the Congolese forests.

In their publication, aptly titled ‘Congo Basin rainfall climatology: can we believe the climate models?’ they explain how, when it comes to distribution and quantity of rainfall in the Congo Basin precipitation datasets in some seasons differ by an order of magnitude – while some datasets place the maximum precipitation in the far eastern margin of the basin, and others on the far western edge.

And then the regionalised climate models that do attempt to forecast precipitation changes over the Congo Basin show opposing results, another research group, led by Fahad Saeed of the German Climate Service Centre, writes in that same year in the journal Atmosphere: ‘Representation of Extreme Precipitation Events Leading to Opposite Climate Change Signals over the Congo Basin’:

“Three REMO simulations following three RCP scenarios (RCP 2.6, RCP 4.5 and RCP 8.5) are conducted, and it is found that the opposite signals, with REMO [regional climate model] showing a decrease and MPI-ESM [global climate model] an increase in the future precipitation, diverge strongly as we move from a less extreme to a more extreme scenario.”

Climate change over Africa, IPCC precipitation projection: southern Africa and Mediterranean North Africa become drier, parts of the southern Sahara and the naturally variable Horn of Africa become wetter. For the Sahel and tropical West and Central Africa the signal is more mixed.

In other words, if we listen to the IPCC and if we look at regionalised climate models, all we know for sure is the Congo Basin, like all of Earth, is projected to get warmer. How rainfall will respond remains relatively uncertain…

More tomorrow!

© Rolf Schuttenhelm | www.bitsofscience.org