The below graph comes from a new global temperature trend study that compares different established datasets for land and ocean temperature. The results emphasize an often-overlooked phenomenon: geographically ‘skewed warming’ – leading to planet-wide precipitation shifts. Possible effects not only include drying of both the Amazon Basin and Africa’s Congo Basin, but also possible greening of the Sahel and the bordering southern Sahara, in North Africa.

But African biodiversity, that has to take a loss in drying zones elsewhere on the rapidly warming continent, may stand little chance setting foot in areas that are greening. That’s because these climatic changes are outpaced by rapid human population growth and agricultural expansion – we learn from another recent study, that compared climatic trends to land use trends.

‘Skewed global warming’ – northern & southern hemisphere, land & ocean temperatures. As greenhouse gas concentrations keep rising Earth is steadily warming. This happens faster over landmasses than over oceans as oceans conduct atmospheric heat to their large thermal mass and therefore add thermal inertia to Earth’s climate system – in turn delaying (local) atmospheric warming. The extent of this effect is not only shown when comparing land and ocean temperature datasets (the first showing more rapid warming) but also when comparing temperatures per hemisphere, with the northern-hemisphere (on average) warming much faster than the southern hemisphere, because that’s where most of Earth’s continental masses lie. This geographically ‘skewed warming’ is strengthened further by Arctic amplification, the combined effects of positive climate feedbacks that lead to very rapid warming over the Arctic tundra and melting sea ice. Earth’s ‘minimal warming latitude’ is situated in the Southern Ocean, around Antarctica. This is because warming over Antarctica leads to a strength decrease of the Polar Cell, therefore a decrease of circumpolar winds that develop a broader radius (comparable to observed northern hemisphere ‘meandering jet stream’ – related to local winter cold air outbreaks) and therefore more dissipation of cold, polar air. It is such changes in Earth’s general circulation that also affect the tropics, as the Intertropical Convergence Zone (that carries the monsoon rains) is drawn towards the more rapidly warming northern hemisphere, possibly drying both the Amazon and Congo rainforest basins the South, and possible greening Africa’s Sahel and parts of the Sahara desert.

The new global temperature study, performed by Timothy Osborn, Phil Jones and Manoj Joshi of the Climatic Research Unit of the University of East Anglia, was published on Wednesday in the journal Weather. It contains different interesting graphs, comparing datasets and temperature studies for land and ocean, including for instance the Berkeley study – and ongoing measurements by the UK Met Office’s Hadley Centre, by NASA GISS and NOAA.

We’ve all seen these temperature graphs – they go up, setting new global temperature records every couple of years, and those that know a bit about climatology can tell you that even if we manage to stabilise atmospheric CO2 concentrations the warming will continue for decades, possibly centuries due to different forms of thermal climate inertia. [That’s why the goal should be to lower this concentration, not to stabilise it – physically the only route to the 1.5 degrees ambition, as climate warms logarithmically.]

What is new about the study is that the British research group has also compiled the temperature datasets in a latitude-dependent graph, showing the average warming per decade between 1960 and 2017. That graph is shown on top of this article, and we think it’s very interesting, because it’s a very strong illustration of the skewed nature of anthropogenic (20^th & 21^st century) climate change, with the northern hemisphere warming much faster than southern hemisphere.

For the careful readers also illustrated are the two main mechanisms – the fact that land warming is quicker than ocean warming (and most of Earth’s land masses lie on the northern hemisphere) plus strong climate feedback-induced Arctic amplification – the epicentre of global average temperature rise that lies in exactly the centre of the northern hemisphere.

Rapid northern hemisphere warming is one of three reasons why the Sahel region in North Africa is greening. (It is the only reason why this greening may also continue in the future.)

Now to one of the possible effects of this skewed warming: Sahel greening. Sahel greening is the combined effect of three different climate trends, that all point (or have pointed) in the same direction:

1. A cold-to-warm switch in North Atlantic waters
2. Decreasing aerosols and cloud cover over Europe
3. Relatively fast northern hemisphere warming (as explained above)

The first climate trend that is strongly linked with Sahel precipitation is a cold (roughly 1960-1995) to warm phase switch of the Atlantic Multidecadal Oscillation (AMO), leading to a cool-to-warm temperature anomaly switch for average sea surface temperatures in the North Atlantic Ocean (and thereby strengthening northern hemisphere warming). This pattern is well-described in a 2009 publication in Ocean Science by a research group led by Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute KNMI.

Precipitation over the Sahel region in North Africa (south of the Sahara desert) follows the cyclicity of the Atlantic Multidecadal Oscillation (AMO) quite closely [albeit seemingly with a delay of a decade or so]. The past dry phase may have been aggravated by a peak in European aerosol pollution, whereas the current wet phase may be amplified by global climate change and more specifically rapid northern hemisphere warming.

Judging by a publication in ‘Scientific Reports’ (a Nature group journal) from last September by three researchers of the University of Southhampton the current AMO warm phase may have already peaked (in 2014). Relatively warm North Atlantic water temperatures are likely to continue for years, but for future decades the AMO trend could actually be down again, not promoting further Sahel greening.

Another relevant climate trend, possibly a one-time effect, is a decrease in aerosol air pollution over Europe in recent decades, following stricter environmental regulation and decreasing the dominance of local high pressure blockades with low-altitude stratus clouds and again (further) drawing the centre of atmospheric warming (over Euro-African longitudes) to the North, dragging along the ITCZ monsoon rains.

The opposite state, at the height of aerosol pollution, has been linked to increased Sahel droughts, possibly exacerbating famines in the 1980s. Ever since the strengthening of aerosol pollution measurements in Europe for instance the climate in the Netherlands has seen a remarkable change, with less fog and more (spring) sunshine and 0.42 degrees Celsius warming per decade – more than twice the global land temperature trend.

Now possibly the only structural climate trend, that is likely to continue well into the climatically foreseeable future, is the pattern of hemispherically skewed warming, as average temperatures are likely to keep rising faster over land than over the oceans and also because the Arctic region is likely to keep warming faster than any other area on Earth.

Sahel greening may or may not progress, but ‘current greening’ is likely to stay

Now if we add these three climatic developments and try to speculate a bit, will Sahel greening continue into the future? As one of the trends was a one time development (restoration of a more natural aerosol environment over Europe), another has possible reversed (AMO warm phase peak) and only one trend is set to continue on a larger scale (average northern hemisphere warming, relative to southern hemisphere warming) all we can say is that’s uncertain. Possibly northward migration of the ITCZ is increased by some of its own effects, like Amazon and possibly Congo Basin drying (or rather the opposite: rainforest vegetation acts as a break, keeping the ITCZ in place).

Development of summer (June-October) rainfall over part of the Sahel, Sahara Desert and Congo Basin, according to the CHIRP rainfall dataset. Current climate trends show an increase of precipitation on the Sahel-Sahara border – and also a clear multidecade drying trend for the Congo Basin. Question is: will the trend continue, reverse, or will the current anomaly become the new norm? The last might still translate as some limited Sahel greening, when compared to droughts of recent decades.

What is also not certain is whether a Sahel precipitation increase will keep manifesting itself as ‘greening’ – as climate models predict very strong 21^st century warming over North Africa, increasing evaporation and heat stress to vegetation, therefore also possible dramatically decreasing agricultural productivity in the Sahel. What’s more, much of the original Sahel (tree) vegetation has not recovered from the droughts of past decades.

If we go by the established climate models’ precipitation change forecasts for Africa, then a West-East pattern keeps showing up, with the whole of tropical East Africa forecast to receive more rainfall on average (although this will remain a tropical region with relatively large natural climate variation). Therefore it has been assessed that if Sahel greening is going to be progressive, that will most likely be the case for the eastern Sahel region.

Does Sahel greening offer compensation for African nature?

Now it’s quite clear to everyone who follows climate science that Africa is going to be strongly affected – because the margins are small: many regions are already very dry, many regions are already hot. Where these trends worsen adaptive capacity can be breached, for human societies and the natural world alike. All this is underlined by the fact that continent-wide temperature forecasts lie above the global average.

Under model forecasts of 21^st century African climate change especially Mediterranean North Africa and southern Africa look prone to increasing droughts – the latter possibly reaching as far North of the central African Congo Basin, where a drying trend over the past decades [that can be linked to Sahel greening!] has led to forest degradation, exacerbating biodiversity loss of hunting and active deforestation.

So, it’s clear that African ecology is under pressure – in fact most of the continent is below what’s considered a biodiversity ‘safe limit’. Now as some regions become wetter and others drier, the borders of large-scale African biomes might want to migrate – for instance the border between rainforest biome and savanna or savanna and desert.

But migration of biomes is only possible if species can freely disperse – and do so as intact ecosystems. Among many other things (like the speed of climate change, which has to be kept very low) this requires one very important element: space. Space for a seed to drop, a root to grow, a tree to rise up and a tree bird to fly to. Together with all the other species they depend on.

Sadly, climate change is never a single pressure. And even in Africa, where it manifests itself strongly, it is rapidly outpaced by another trend: human population growth – driving human land use expansion.

Land use trend outpaces climate trends: population increases about 2000% in 150 years time

Africa’s human population has increased 5.5 times since the mid-20^th century, from 230 million in 1950 to 1,268 million people in 2017.

And this rapid population growth is forecast to continue throughout the 21^st century, with Africa being the only continent that is not predicted to experience a peak population, but rather continued growth, to (medium variant) 2.53 billion in 2050 and (medium variant) 4.47 billion people in the year 2100 – according to the UN Population Division’s* World Population Prospects 2017 Revision that is (PDF).

[*) An organisation that has a remarkable history of annually raising its global forecasts, so Earth’s future will be very different still if that trend continues too.]

Now what can we say about the combined effects of vegetation changes and human population changes over Africa? Yes, it’s been researched:

A recent satellite study by a Danish-Catalan research group, led by the University of Copenhagen compared both the climatic and human population trends, assessing their combined effects on woody vegetation cover. The study, titled ‘Human population growth offsets climate-driven increase in woody vegetation in sub-Saharan Africa’, was published in March this year in Nature Ecology & Evolution, a new subsidiary of the publishing group centred around the journal Nature. And you’ve guessed it, the image on the left with maps a for vegetation change and b for speed of human population growth comes from that publication.

Now firstly the study shows not one, but two broad areas of greening: a band overlapping much of the Sahel region, and southwestern Africa. We chose to ignore the latter, because this is an area with a strong projected future drying trend as a consequence of global climate change – so a likely reversal of this greening.

Map b shows that although everywhere on the African continent the human population is on the rise (no negative values) there are centred hotspots of very rapid growth, some relatively close to or in both the eastern and western Sahel, for instance northern Nigeria. [These hotspot are also in line with another important demographic trend: rapid African urbanisation: 300% between 2010 and 2050.]

The researchers have tried to assess what quality of vegetation change is happening in different areas, specifically looking for woody vegetation – a sign of tree growth, and therefore natural vegetation (not cropland or grazed field).

The maps a and b share a clear pattern they write: “in particular, areas with a decrease in woody cover, and no relation to precipitation, coincide with a high population pressure.”

The authors note that in humid lands an increase of woody vegetation is possible, as it’s also helped by another trend: CO2 fertilisation, enabling a temporary increase of forest carbon stocks. This woody vegetation increase is also monitored outside forested areas, and therefore the authors think greening and drying regions in Africa balance each other’s effect on the carbon cycle. However, woody vegetation increase does not occur in greening regions with rapid population growth:

“The opposing trends in dry and humid zones have implications for our understanding of environmental change in sub-Saharan Africa. Whereas areas of high population growth, mostly in humid zones, on average experience a decrease in woody vegetation, areas with low population growth on average experience an increase in woody vegetation, mainly driven by changes in precipitation and CO2 concentrations.”

Now again if we try to look at decades ahead we know that two main trends will continue: warming and population growth, whereas precipitation changes are often partly cyclical and CO2 fertilisation is a one-time boost, sometimes with paradoxical net carbon cycle effects.

Africa is the only continent that is not projected to experience a population peak anywhere in the 21st century, driving global population growth. World Population Prospects, 2017 Revision.

So, in the long run, the areas that are still greening decades from now we fear are highly unlikely to be left to nature. There simply isn’t enough space. If there are areas where plants still grow well – that’s where humans will settle, the most adaptive of all species.

© Rolf Schuttenhelm | www.bitsofscience.org