The climate over the Congolese rainforests (in the central tropics of Africa) seems to show a drying trend over the last four decades. And although this deviation is smaller than multi-annual variation, the average decline in precipitation does lead to forest degradation in the world’s second-largest remaining tropical rainforest.

Average decline of precipitation over the rainforests in the Congo Basin in the heartland of tropical Africa. As the graph shows the rainfall decline is small, less than 1 millimeter per day. (For comparison, under 21^st century climate change daily average rainfall in Central American rainforests may decline by 3-6 millimeters.)

This is concluded by an international research group led by Liming Zhou of Albany University, who published their findings in Nature in 2014.

The team used earlier findings by Yadvinder Malhi and George Wright from 2004 in Philosophical Transactions of the Royal Society B, called ‘Spatial patterns and recent trends in the climate of tropical rainforest regions’ – a study into background climate trends over the last four decades of the 20^th century for all tropical rainforests on Earth.

They note a particularly strong drying trend in northern tropical Africa, of 3-4% per decade. About the Congo rainforest Malhi & Wright wrote the following:

“The strong drying trend in Africa suggests that this should be a priority study region for understanding the impact of drought on tropical rainforests. We develop and use a dry–season index to study variations in the length and intensity of the dry season. Only African and Indian tropical rainforests appear to have seen a significant increase in dry–season intensity. In terms of interannual variability, the El Niño–Southern Oscillation (ENSO) is the primary driver of temperature variations across the tropics and of precipitation fluctuations for large areas of the Americas and southeast Asia. The relation between ENSO* and tropical African precipitation appears less direct.”

[*) If you’re interested in that relationship, you may also want to read our article about the influence of El Niño and La Niña further to the East – the Horn of Africa, where strong droughts are naturally recurring.]

This recommendation was taken at heart by two other scientists, Salvi Asefi-Najafabady and Sassan Saatchi, who published a follow-up study in that same journal in 2013, looking into (more recent!) precipitation trends for tropical Africa specifically: ‘Response of African humid tropical forests to recent rainfall anomalies.’

They find the downward trend accelerates over the first decade of the 21^st century:

“Using a combination of rainfall observations from meteorological stations from the Climate Research Unit (CRU; 1950–2009) and satellite observations of the Tropical Rainfall Measuring Mission (TRMM; 1998–2010), we show that West and Central Africa experienced strong negative water deficit (WD) anomalies over the last decade, particularly in 2005, 2006 and 2007. These anomalies were a continuation of an increasing drying trend in the region that started in the 1970s.”

They add that despite the large-scale tropical African droughts and negative rainfall anomalies, there have been no significant impacts on rainforests of West and Central Africa during the same period.

Declining ‘greenness’, worst sign for any forest

But then the team of Zhou and colleagues took an overview look on Central Africa to find out if really it was not possible to measure any vegetation changes to correlate to the now-four decade precipitation decline.

They could, from space, using the ‘colour’ measurements [rather optical, thermal, microwave and gravity sensors] in several satellite datasets – as they wrote in their 2014 Nature letter, titled: ‘Widespread decline of Congo rainforest greenness in the past decade’.

Several climatically defined forest ecoregions in the Congo Basin that together hold about 25 percent of the world’s remaining rainforests. Sadly, although deforestation happens more slowly here than in Southeast Asia and the Amazon, together the Congolese forest ecoregions are below the 90% ‘biodiversity intactness’ that some use as a safe limit for cascading ecosystem decline. Source: Global Forest Atlas.

Where forest goes from green to brown, you have a clear indication that photosynthesis goes down. And that’s one of the strongest indicators you can have of general forest degradation.

This is of course very important for many reasons. After the Amazon rainforest the Congo basin holds the world’s largest remaining tropical rainforest. It’s very important for the global climate as a carbon store, and even more important for Life on Earth as Africa’s densest biodiversity hotspot, that includes a wealth of iconic species, like the bonobo, the western and the now-endangered eastern gorilla and the African forest elephant (not to be mistaken with the African savanna elephant – a totally different species).

But what’s more: we thought the Congo rainforest was relatively resilient against the effects of climate change. We clearly have to take a better look. Of course then we also have to assess if indeed the drying trend is likely to continue – or to reverse, as global temperatures continue to rise and effects on Earth’s general circulation become ever more complex.

We’ll do that soon, as of course our series continues. For now what we’ve learned: if droughts in tropical Africa would increase further – that probably will affect the remaining forest.

© Rolf Schuttenhelm | www.bitsofscience.org