Hortensia de los Santos
It began with an observation that seemed almost inconsequential: the Sun was unusually quiet.
For several days, I had been examining reports of solar activity, following the behavior of sunspots, solar flares, and the magnetic disturbances that occasionally reach Earth. The Sun, ordinarily so restless in its magnetic activity, appeared to have entered a period of relative tranquility. There were fewer significant flares than I expected, and I wondered whether this apparent inactivity was simply another expression of the natural solar cycle or something deserving closer examination.
The question was not whether the Sun could become quiet. Of course it could. Its approximately eleven-year cycle of magnetic activity has been documented for centuries. What interested me was whether the behavior I was observing differed substantially from that of previous cycles, particularly those of the late twentieth century.
I wanted to compare the present decline in solar activity with the corresponding stages of Solar Cycles 21, 22, 23, and 24. A comparison of isolated sunspot numbers would not be sufficient. The cycles differ in strength, duration, and the timing of their maxima. To determine whether the present interval was genuinely unusual, we would need to compare equivalent phases of successive cycles, taking account of their different amplitudes.
That investigation remained incomplete, but it raised a more interesting question.
What, precisely, does a quiet Sun mean for Earth?
The apparent simplicity of the question concealed several complications. Solar flares are only one manifestation of solar magnetism. Coronal holes can release streams of high-speed solar wind capable of disturbing Earth's magnetosphere even when the Sun produces relatively few major flares. A quiet solar disk does not necessarily mean a quiet environment in near-Earth space.
Nor does a decrease in solar magnetic activity necessarily imply a comparable decrease in the energy reaching Earth's surface. Total solar irradiance varies by only about one-tenth of one percent across a typical eleven-year cycle. The effect is measurable, but much smaller than one might imagine from the dramatic changes in sunspot numbers.
The distinction mattered. If I wanted to understand the relationship between solar behavior and terrestrial climate, I could not simply equate sunspot activity with temperature. I needed to consider the mechanisms connecting them.
And so the investigation moved from the Sun to Earth.
At the time, another phenomenon had attracted my attention: the extraordinary weakness of Atlantic hurricane activity during the 2026 season, despite the persistence of very warm ocean waters.
The apparent contradiction was intriguing. Tropical cyclones derive much of their energy from warm ocean surfaces, yet the availability of heat does not guarantee that storms will develop. Why, then, should an ocean capable of supplying considerable energy produce relatively little organized tropical activity?
Could solar conditions be involved? Could atmospheric circulation explain the discrepancy? Were the oceans and atmosphere responding differently to some larger climatic influence?
The first possibility was tempting, but temptation is not evidence. Solar variability can affect Earth's upper atmosphere and certain aspects of atmospheric chemistry, yet there was no adequate basis for attributing a weak hurricane season directly to the temporary quietness of the Sun.
A more immediate explanation lay in the organization of the tropical atmosphere.
Warm water supplies energy, but a developing hurricane also requires favorable atmospheric conditions. Strong vertical wind shear can disrupt the circulation of a tropical disturbance, separating its convection from the center of rotation and preventing the system from becoming vertically organized. Dry air can interfere with sustained convection. Even the location and intensity of the tropical rain belt influence where disturbances form and whether they survive.
The El Niño–Southern Oscillation offered another important connection. During El Niño, changes in tropical Pacific temperatures and atmospheric circulation frequently increase vertical wind shear over portions of the Atlantic. Under such conditions, exceptionally warm Atlantic waters may coexist with an atmosphere unfavorable to hurricane development.
The paradox was therefore less mysterious than it initially appeared, although it remained scientifically interesting. The oceans and atmosphere were not acting independently; rather, they were participating in a system whose components could reinforce or oppose one another.
That realization led me toward Africa.
The Sahara, the West African monsoon, and the atmospheric disturbances that travel westward from the African continent all influence conditions over the tropical Atlantic. African easterly waves provide the initial disturbances from which many Atlantic hurricanes develop. The Saharan Air Layer can transport warm, dry, dust-laden air across the ocean, sometimes creating conditions hostile to tropical cyclone formation.
But what interested me increasingly was not merely the present relationship between Africa and the Atlantic. It was the history of that relationship.
The Sahara had not always been the immense desert we know today.
There had been a time when much of North Africa supported grasslands, lakes, rivers, and wetlands. The existence of that ancient landscape had fascinated me for years. I was not thinking of the prehistoric Tethys Ocean, whose geological history belongs to a vastly different timescale. I was thinking of the relatively recent period when a substantial portion of the Sahara was green.
How could such an extraordinary transformation have occurred?
The answer led us back to the Sun, but in a manner entirely different from the solar activity with which our discussion had begun.
The African Humid Period, which extended broadly from approximately 14,500 to 5,000 years ago, was associated with a much stronger northward penetration of the African summer monsoon. Regions that now receive little rainfall supported extensive vegetation and freshwater systems. Lake Mega-Chad occupied an enormous area, while other basins contained lakes and wetlands that have long since disappeared.
The physical explanation involved the seasonal distribution of solar radiation.
Earth's orbit and rotational axis are not fixed in their geometrical relationships. The planet's axis is inclined, its orbit is slightly elliptical, and its orientation changes slowly under gravitational influences. These variations alter the amount of sunlight received at particular latitudes during particular seasons.
The consequences can be immense.
Approximately eleven thousand years ago, Northern Hemisphere summer occurred much closer to Earth's perihelion, the point at which the planet is nearest the Sun. Today, Northern Hemisphere summer occurs near aphelion, when Earth is farthest away.
The difference is not a trivial astronomical curiosity. Solar radiation varies inversely with the square of the distance from its source. Under the present orbital configuration, Earth receives approximately seven percent more solar radiation at perihelion than at aphelion. Although the planet's annual global energy budget changes much less than its seasonal distribution, the geographical and seasonal consequences of orbital variations can be substantial.
During the early Holocene, enhanced Northern Hemisphere summer insolation intensified continental heating and helped strengthen the African monsoon. Moisture penetrated farther north, sustaining vegetation and freshwater environments across areas that would eventually become desert.
The transformation was reinforced by the landscape itself. Vegetation altered surface reflectivity, moisture exchange, and the interaction between the land and atmosphere. A wetter environment could help maintain conditions favorable to additional rainfall.
The Sahara was therefore not simply a passive recipient of climatic change. Its vegetation and surface conditions participated in the climatic system that sustained them.
As the orbital configuration gradually changed, summer insolation declined in the relevant northern latitudes. The monsoon weakened, rainfall contracted southward, and large regions began their transition toward aridity. Vegetation loss could reinforce the drying process, although the timing and abruptness of the transition varied geographically. The ancient lakes disappeared or contracted. Their exposed sediments became part of a new environment.
One particularly striking consequence survives in the Bodélé Depression of northern Chad, where sediments associated with the former Lake Chad system contribute to one of the world's important sources of atmospheric dust.