Hortensia de los Santos
A revealing connection
Here was a connection I found especially revealing: the remains of a prehistoric lake could influence the modern atmosphere.
Dust lifted from an ancient lakebed can cross the Atlantic, affecting radiation, clouds, and the transport of mineral nutrients. A climatic transformation thousands of years old continues to participate in processes occurring today.
The Earth remembers its past, not through consciousness, but through the physical consequences preserved in its landscapes.
Our discussion of the African monsoon naturally led to another system that had long interested me: the Indian monsoon.
Both monsoons depend on seasonal changes in atmospheric circulation, yet their geographical expressions differ. The Indian summer monsoon transports moisture from the Indian Ocean toward the subcontinent, while the West African monsoon carries tropical moisture northward into the Sahel.
Their behavior is influenced by continental heating, ocean temperatures, pressure gradients, and the migration of tropical atmospheric circulation.
But their importance extends far beyond meteorology.
Civilizations developed in environments whose water supplies depended on climatic systems that were never entirely stable.
I had been studying the ancient geography of the Indian subcontinent, particularly the river systems associated with the Indus civilization and the historical traditions concerning the Sarasvati. The relationship between ancient settlement patterns, changing rivers, and monsoon variability is of considerable interest.
The mature urban Indus civilization flourished during the third millennium BCE, but its urban organization underwent major transformations after approximately 1900 BCE. Paleoclimate evidence suggests that changing precipitation patterns and river dynamics contributed to the environmental pressures experienced by communities in the region.
Yet climatic influence must not be confused with climatic determinism. Civilizations are complex human systems. Their transformations cannot be adequately explained by assigning every historical change to a drought, a flood, or a weakening monsoon.
Nevertheless, the possibility that long-term atmospheric changes influenced the distribution of settlements, agricultural practices, and river systems is compelling.
It also invites a broader question: how many events that we interpret primarily through archaeology or written history were shaped, in part, by environmental processes operating beyond human perception?
The investigation had now carried us across several timescales, from the days and weeks of solar magnetic activity to the millennia of monsoon variability.
But one astronomical question remained particularly important to me.
If Earth's axial orientation changes cyclically, when was the last time our planet occupied approximately the same seasonal relationship to the Sun that it occupies today?
The distinction was essential. I was not asking when Northern Hemisphere summer last occurred near perihelion. I wanted to know when the relationship between the seasons and Earth's closest and farthest orbital positions had resembled the present arrangement.
Axial precession takes approximately 25,772 years to complete a cycle, but the orientation of Earth's elliptical orbit also changes. The resulting climatic precession cycles have dominant periods of approximately nineteen thousand and twenty-three thousand years.
A broadly comparable seasonal phase therefore occurred roughly twenty-one to twenty-three thousand years ago.
And what was Earth like then?
The answer was extraordinary.
That interval overlapped the Last Glacial Maximum. Immense ice sheets covered much of northern North America and extended across large areas of northern Europe. Global sea level was approximately 120 meters lower than today. The planet's average surface temperature was roughly six degrees Celsius below the preindustrial average.
Yet in one important astronomical respect, the seasonal relationship between Earth's orbit and the Sun resembled our present one.
This comparison revealed something fundamental.
The position of Earth within its orbital cycles does not, by itself, determine the planet's climatic condition.
Other factors must be considered: the extent of continental ice, the reflectivity of Earth's surface, ocean circulation, the distribution of vegetation, and the composition of the atmosphere.
It was the last of these that produced the most unexpected moment in our investigation.
During the Last Glacial Maximum, atmospheric carbon dioxide concentrations were approximately 190 parts per million.
Today, they exceed 420 parts per million.
I had known, of course, that atmospheric carbon dioxide had increased. But placing the two concentrations side by side, in the context of a comparison between two broadly similar phases of climatic precession, gave the difference a new significance.
The modern atmosphere contains more than twice the concentration of CO₂ present during the Last Glacial Maximum.
What did that mean physically?
Carbon dioxide absorbs and emits infrared radiation, influencing the rate at which Earth loses energy to space. Its radiative effect increases approximately logarithmically with concentration.
A widely used approximation expresses the change in radiative forcing as: ΔF = 5.35 ln(C/C₀)
Using 190 ppm as the reference concentration and 426 ppm as an illustrative modern value gives a forcing difference of approximately 4.32 watts per square meter.
That number is not a direct measurement of temperature change. Nor can the temperature difference between the Last Glacial Maximum and today be attributed exclusively to CO₂. The enormous ice sheets, changes in surface reflectivity, ocean circulation, and other greenhouse gases were also important.
Nevertheless, the calculation demonstrated that the atmospheric change was physically consequential.
A doubling of CO₂ produces approximately 3.7 watts per square meter of radiative forcing under the conventional approximation. The resulting temperature response depends on feedbacks and the timescale over which the climate system adjusts.
Water vapor generally amplifies warming. Melting snow and ice expose darker surfaces that absorb more sunlight. The oceans absorb heat and delay the full atmospheric response. Clouds introduce additional complexity.
The greenhouse effect is not a political interpretation. It is a physical process.
This realization brought me to a point I had not anticipated when I began examining the quiet Sun.
Acceptance
For many years, I had been skeptical of the manner in which anthropogenic climate change was presented in public discourse. Political rhetoric, exaggerated claims, and the tendency to attribute nearly every unusual weather event to global warming had made me reluctant to accept certain conclusions without closer examination.
But skepticism, if it is to remain intellectually meaningful, must apply equally to one's own assumptions.
The evidence concerning atmospheric CO₂ could not be dismissed simply because I objected to the political language surrounding it. The question was no longer whether natural climatic variability existed. We had just spent hours examining its extraordinary influence across geological and historical time.
The question was whether natural variability adequately explained the rapid warming observed during the modern period.
The answer required separating processes that are often carelessly combined.
The Interconnectedness of the Solar-Terrestrial System
Orbital cycles alter the seasonal and geographical distribution of sunlight over thousands of years. Solar magnetic cycles produce comparatively small variations in total solar irradiance over approximately eleven years. Ocean-atmosphere oscillations redistribute heat and influence regional weather over months to decades. Human activities, meanwhile, have substantially increased the concentration of greenhouse gases in the atmosphere.
These processes are not mutually exclusive. They operate simultaneously, but through different physical mechanisms and on different timescales. The existence of natural climatic change does not invalidate the possibility of human-caused warming. Conversely, the evidence for anthropogenic warming does not justify attributing every drought, hurricane, flood, or unusual temperature to human activity.
Scientific understanding requires more discrimination than either extreme permits.
I found myself reconsidering a position I had held for years.
It was not the language of catastrophe that persuaded me. It was the accumulation of evidence: the atmospheric measurements, the paleoclimatic record, the physics of infrared absorption, and the distinction between orbital forcing and greenhouse-gas forcing.
The conclusion was uncomfortable, but discomfort is not a valid reason to reject evidence.
Human Impact
Humanity has altered the atmosphere in ways that have measurable consequences for Earth's energy balance. The full implications are complex, but the underlying physical relationship is sufficiently established to demand serious consideration.
There was something almost ironic about the route by which I had reached that conclusion.
I had begun by wondering whether a quiet Sun might be influencing Earth's climate. I had then followed the relationships among solar activity, atmospheric circulation, tropical cyclones, monsoons, ancient deserts, orbital mechanics, glaciation, and atmospheric composition.
At every stage, the investigation revealed another layer of complexity.
Not one of these components operates in complete isolation.
The Sun supplies the energy that sustains the climate system, but the distribution of that energy changes with Earth's orbital geometry. The atmosphere determines how radiation is absorbed, emitted, and redistributed. The oceans store and transport heat. Ice sheets alter planetary reflectivity. Vegetation influences moisture exchange and surface energy balance. Human activity can modify the composition of the atmosphere and the character of the land surface.
Yet interconnectedness must not become an excuse for imprecision. It is not enough to say that everything influences everything else. Such a statement, however appealing, explains very little unless the connections can be identified, measured, and tested.
The purpose of scientific inquiry is to distinguish meaningful relationships from coincidental ones, to determine the strength of physical mechanisms, and to recognize the limits of what the available evidence can establish.
We had not demonstrated that the Sun's temporary quietness was responsible for the weakness of Atlantic hurricane activity. We had not established a direct causal chain linking the modern solar cycle to changes in the African monsoon. Nor had we completed the historical comparison necessary to determine whether the observed decline in Solar Cycle 25 was exceptional.
Those questions remained open.
Our investigation had accomplished something more important than confirming an initial suspicion.
It had revealed how a seemingly simple observation could lead to a reconsideration of the physical history of the planet.
The Sahara's vanished lakes, the changing strength of ancient monsoons, the transformation of river systems, the expansion of continental ice sheets, and the modern accumulation of atmospheric carbon dioxide belong to different chapters of Earth's history. Yet they can be understood within a common framework: the interaction of energy, matter, motion, and time.
The Earth is not a static stage upon which life and civilization unfold. It is an evolving physical system, continuously transformed by astronomical forces, internal feedbacks, and, increasingly, the activities of one of its own species.
The comparison with the Last Glacial Maximum remains especially striking to me. Approximately twenty-two thousand years ago, Earth occupied a broadly similar phase of climatic precession, yet its surface was dramatically colder and its atmosphere contained far less carbon dioxide.
The orbital resemblance did not produce climatic resemblance.
That fact alone is a powerful reminder that no single variable can adequately describe the climate system.
Perhaps the most valuable lesson of the investigation was not a particular conclusion about the Sun, the monsoons, or atmospheric carbon dioxide. It was the importance of following a question wherever the evidence leads, even when the destination differs from what one initially expected.
A quiet Sun had prompted the inquiry.
The changing Earth had supplied the answers—and, inevitably, many more questions.
Science advances through precisely this process: observation, curiosity, doubt, comparison, and the willingness to revise an explanation when the evidence requires it.
The universe does not owe us simple answers. Its processes are indifferent to our preferences, our convictions, and our political disputes.
Our responsibility is to understand them as faithfully as we can.
And sometimes, the most significant discovery is not something previously unknown about the world, but the recognition that our own understanding of it must change.
WHAT I BELIEVE
I believe that nothing within our solar system exists or operates in complete isolation. The Sun, the planets, their gravitational interactions, magnetic fields, atmospheres, oceans, and the physical processes governing them are all components of a vast, interconnected system. Changes in one component may influence others, sometimes through mechanisms that are not immediately apparent. For this reason, I question the adequacy of studying individual phenomena without considering the larger environment in which they occur. Solar activity, terrestrial climate, ocean circulation, and atmospheric dynamics should not be examined as unrelated subjects, but as interacting processes within a common physical framework. This does not mean that every event is causally connected to every other event, or that all influences are equally significant. Rather, it means that **we must investigate the connections before assuming their absence**.