Hadley Cell Expansion and the Re-Greening of Africa
The above illustration shows what North Africa may have looked like during the Holocene Maximum, also called the Holocene Climate Optimum (9,000 to 5,000 years ago)
It is worth noting the use of the word ‘optimum’. As in ‘best’. It was warmer than it is now, and lasted for four thousand years.
This article is the next iteration on the themes developed in my last article titled Solar-Driven Hadley Cell Expansion: How the SAM Index Explains Global Brightening Since the Little Ice Age.
In this article, I will invoke some of the same ideas generated in this June 28th article, to propose a new hypothesis that links modern day observations from the African continent to the paleorecord that shows a radically different regional landscape than present.
A central objective of this article is to propose that the structure of the atmosphere, which made possible the African Humid Period during the Holocene Climate Optimum (HCO) was radically different then the structure of the atmosphere that emerged following the Great Aridity Event some 4,200 years ago.
In essence, I propose that the middle latitude Ferrel Cells in either hemisphere, came into existence at the onset of the current neoglacial sub-epoch of the Holocene Interglacial Period, which is commonly called the Great Aridity Event.
Note that this idea will also be expanded upon in further articles.
Figure 1 is a cross section of the Northern Hemisphere, showing the three great atmospheric circulation zones called the Hadley, Ferrel and Polar cells. Note how the Hadley cells straddle the near equatorial latitude(s), which is the zone of maximum global precipitation and upwelling low pressure cyclonic air masses – also referred to as the Intertropical Convergence Zone (ITCZ).
Figure 1. Current structure of atmospheric circulation, where the Ferrel and Hadley Cells converge and subside along the 30th parallel (approximately).
Note that both the Ferrel and Hadley Cells converge and downwell as anticyclonic air masses along the 30th parallel (on average). These anticyclonic air masses are low in specific humidity as they begin to descend from the tropical tropopause and as they approach the lower troposphere they begin to compress and increase in temperature.
These descending hot dry air masses act to maintain the global deserts that align with the 30th parallel (on average) and are commonly called the Horse Latitudes.
The Horse Latitudes today are characterized by an evaporation potential that greatly exceeds total precipitation and it is the combined subsiding masses of both counter rotating cells that acts to maintain the great global deserts, such as the African Sahara (north) and Kalahari (south).
As described in my last Substack, George Tselioudis et al and numerous other recent publications, have identified that on average, the Hadley Cells and their Horse Latitudes have been migrating poleward by 80 to 200 kilometers per decade since the late 1970s.
Satellite surveillance of zonal cloud formations show that low level clouds within the Hadley Cells are declining, while the Ferrel Cells are contracting as the Horse Latitudes expand poleward.
At this pace of change, the Horse Latitudes will be fully displaced from the African continent by the end of the 21st century.
These modern trends establishes a plausible mechanism explaining how Africa was virtually void of deserts during the Holocene Climate Optimum.
Figure 2 represents this end point, which I argue may have defined the structural state of the atmosphere pre-Great Aridity Event.
Figure 2. Hypothetical structural state of the atmosphere during the Holocene Climate Optimum.
Of course, this would imply that the Horse Latitudes are advancing northward into southern Europe and that as they do, Europe would experience warmer temperatures and reduced cloud cover.
This is indeed what we are observing in this climate regime, but that is for another article and another day.
Now that I have introduced the hypothetical model for atmospheric restructuring in the context of the African Humidity Period (AHP), it is time to dive into the evidence behind this well known body of paleoclimatological data.
The Holocene Climate Optimum (HCO) and the African Humidity Period (AHP)
First, let me emphasize that there have been over 230 African Humid Periods (AHP) over the past eight million years.
Such conditions are defined in large part by an absence of the Sahara Desert in the north, the Kalahari Desert in the south and an overall expansion of the tropics poleward. Furthermore, the AHP includes a mega Lake Chad extending northward from the Sahel region up into today’s Sahara region.
This massive body of fresh water is estimated to have been upwards of 400,000 square kilometers during the HCO versus the 1,500 to 2,000 square kilometers today.
As indicated in the cover image, today’s Sahara Desert was littered with many large bodies of water and an entirely different ecosystem during the HCO.
For longer term perspective, the landscape evolution of the African continent over a full interglacial period is represented in Figure 3. Pay close attention to the expansion of the tropics, together with the retreat and adance of deserts / grasslands along the north and south middle latitudes.
Figure 3. Landscape evolution across Africa over a full interglacial cycle.
Figure 4 is an illustration by Francesco S.R. Pausata et al, which shows estimated precipitation levels across North Africa during the last AHP. Note that the lowest precipitation levels corresponding to grassland ecosystems in the far north are on par with those seen here in southern Alberta and that woodland ecosystems covered half of today’s Sahara Desert.
Figure 4. Vegetation and estimated precipitation countors across Africa during the African Humid Period (AHP).
While mainstream models largely invoke orbital mechanics, such as changes in the Earth’s orbital precession, eccentricity and obliquity, to explain the changes observed in northern Africa over the Holocene, one of the shortcomings is the failure of these orbital centric models to describe the mirror image changes developing in the southern continent over the same time scales.
These symetrical changes in ecosystem evolution occurring along either middle latitude in Africa are best characterized using modern surveillance methods.
Modern Oscillations in African Humidity
Over the Satellite Era, ecosystem changes over Africa have been characterized through spectroscopic changes in outgoing reflected shortwave and longwave length radiation. In this article, we will limit discussion to Normalized Difference Vegetation Index (NDVI) data.
NDVI is a satellite-derived metric that quantifies vegetation health and density by measuring the difference between near-infrared and red light reflected by plants. It is widely used to monitor changes in ecosystem structure, such as greening or browning trends, desertification, deforestation, or agricultural expansion.
By analyzing long-term NDVI time series from satellites like MODIS or Landsat, researchers can detect shifts in vegetation cover, productivity, and phenology across large spatial scales. This makes NDVI an essential tool for tracking climatic changes, land use, and disturbances on terrestrial ecosystems.
In other words, NDVI data is a measure of landscape greenness.
The first NDVI data I will share is shown in Figure 5, where Rasmus Fensholt et al examine changes in greenness across global arid ecosystems, using satellite data obtained over the span of 1981 to 2007.
Figure 5. Satellite‑derived GIMMS NDVI correlations with GPCP precipitation (top) and multiple‑regression climate controls on NDVI using GPCP precipitation and air temperature (bottom).
The first map is created by statistically correlating annual integrated NDVI with annual summed GPCP precipitation at each grid cell to determine whether vegetation greenness responds positively or negatively to rainfall variability.
The second map uses multiple regression, combining GPCP precipitation and annual mean air temperature to quantify how much of NDVI variability can be explained by these climate drivers, expressed through adjusted r² values.
The two correlation plots are built from GIMMS NDVI, which stands for Global Inventory Modeling and Mapping Studies Normalized Difference Vegetation Index, a long‑term satellite‑derived measure of vegetation greenness generated from AVHRR sensors aboard NOAA satellites.
The precipitation data come from GPCP, the Global Precipitation Climatology Project, which blends satellite microwave/infrared retrievals with surface gauge observations to produce globally gridded annual rainfall totals.
These difference correlation plots effectively demonstrate the increased greenness along the African Horse Latitudes is accompanied by increased precipitation and temperature.
The second select body of evidence that I have chosen to illustrate the multidecadal growth in African greenness is the 2018 study by Z. S. Venter et al, which is shown in Figures 6a and 6b.
Examine closely the embedded caption in Figure 6a. The right hand (b) woody cover change (percentage) map shows that the expansion of brush and trees along the southern Horse Latitudes since 1986 to 2016 is just as intensive as that found along the northern middle latitudes.
Many regions along the limits of the continental Hadley Cells are showing a 25 percent expansion in woody cover in a mere 30 years. This means the Sahel region is expanding northward and the Kalahari Desert is being over run by high precipitation requiring brush and trees.
Figure 6a. Satellite‑derived maps show mean woody plant cover and its 30‑year rate of change across sub‑Saharan Africa, highlighting widespread woody encroachment outside masked forest, cropland, wetland, and urban areas.
Figure 6b strengthens the argument that African climate is trending toward conditions reminiscent of the AHP by showing that multiple continental‑scale drivers are shifting in directions that collectively favor increased woody vegetation and higher biological productivities.
The steady rise in woody cover over three decades mirrors the northward drift of the tropical rain belt associated with Hadley Cell expansion, which historically helped sustain greener landscapes during past humid phases.
Rising atmospheric CO₂ enhances plant water‑use efficiency (aka CO₂ fertilization), allowing woody species to thrive even under marginal moisture conditions, a dynamic also believed to have amplified vegetation during the mid‑Holocene.
Concurrent increases in herbivore density and reduced fire frequency highlight rising landscape productivities and increased wildfire resilience. The continentally averaged positive rainfall trend (+three percent per decade) aligns with a gradual strengthening of moisture availability in semi‑arid zones, echoing the hydrological shifts that characterized the AHP.
Taken together, these synchronized trends suggest that modern climate forcing is nudging African ecosystems toward a greener, more wooded state analogous to earlier humid epochs.
Figure 6b. Time‑series trends of woody cover (percentage), atmospheric CO₂, herbivore density, rainfall, fire frequency, and air temperature across Africa from 1985–2015.
Changing Cloud Morphologies and Precipitation Efficiencies within the African Hadley Cells
To start this final section, it is crucial to first identify that generalized climate models (GCM), using CO₂ as the sole external forcing, do not replicate the observed pace of expansion of the Hadley Cells.
Johanson & Fu (2009) conclude that observed Hadley Cell widening since 1979 far exceeds what climate models simulate.
Observations show a 2°–5° poleward expansion of the Hadley circulation over the last two decades of the 20th century, while CMIP3 GCMs produce only ~0.2°–0.6° of widening over comparable periods, meaning models underestimate the trend by a factor of five to ten.
Observations show strong, rapid Hadley Cell expansion; models show weak, slow expansion over the past 50 years. The discrepancy is large, robust across datasets, and unresolved – one fundamental reason is GCMs do not effectively represent reproduce cloud dynamics.
Across Africa and the subtropics, satellite records reveal a decisive shift in cloud‑type composition, with METEOSAT showing a multi‑decadal decline in large, short‑lived deep convective systems and a rise in smaller, longer‑lived systems that increasingly dominate the monsoon and equatorial cloud belts.
This reorganization occurs even though the deep‑cloud fraction has remained essentially constant, demonstrating that the climate signal lies in changing cloud morphology rather than cloud quantity.
These evolving cloud types exhibit longer lifetimes and more persistent organization, traits that enhance precipitation efficiency and help explain the observed increases in heavy‑rainfall indices across West Africa, East Africa, and parts of southern Africa.
Intense‑precipitation trends show that many equatorial and mid‑latitude regions have experienced rising multi‑day rainfall totals and more frequent very‑wet days, consistent with a convective regime that is becoming more productive.
The Atlantic Niño further reinforces these changes, with the eastern Atlantic Niño strengthening West African monsoon rainfall by intensifying zonal moisture convergence into the Gulf of Guinea.
In contrast, the central Atlantic Niño shifts rainfall patterns zonally but does not produce the same magnitude of wetting over West Africa, underscoring the sensitivity of regional precipitation to cloud‑type and sea surface temperature pattern differences.
Together, these observational records show that Africa’s rising precipitation and expanding greenness are tightly linked to structural changes in convective organization rather than simple increases in cloud cover.
General circulation models fail to reproduce these cloud‑type transitions, highlighting the need for observational constraints to improve simulations of African hydroclimate.
In other words, we must measure more and model less, and reverse the current practice of expecting people to believe computer models instead of observations.
See more here substack.com
Bold emphasis added