Climate, the Milankovitch cycles, and man

Eccentricity. Obliquity. Precession. The Earth’s climate is influenced by both natural long-term cycles and recent human activities. The Milankovitch cycles refer to the long-term variations in the Earth’s orbit and rotation that change how much sunlight our planet receives at different locations. The cycles are predictable changes that act as a natural pacemaker for the Earth’s climate, triggering the transition between ice ages (glacials) and warm periods (interglacials) over thousands of years. Named after the Serbian mathematician and geophysicist Milutin Milankovitch, who calculated them by hand in the early 20th century, these cycles describe the collective effects of the Earth’s movements on its climate over tens to hundreds of thousands of years.

The eccentricity cycle refers to the shape of the Earth’s orbit around the sun as it shifts from a nearly perfect circle to a more squashed ellipse or oval in approximately 100,000 years. When the orbit is more elliptical, the distance between the Earth and the sun varies more significantly throughout the year, making temperature changes, daylight fluctuations, and rainfall patterns vary more extremely. The obliquity cycle refers to the angle of the Earth’s tilt relative to its orbital plane as it shifts back and forth between roughly 22.1 degrees and 24.5 degrees in about 41,000 years. A greater tilt makes hotter summers and colder winters, while a smaller tilt leads to milder seasons. Milder summers allow winter snow to survive and build up into massive ice sheets.

The precession cycle represents the Earth’s wobble like a spinning top that is slowing down, changing the direction its axis points in space over approximately 23,000 years. Precession affects the seasonal distribution of solar radiation that the Earth receives. It shifts the strength and location of tropical monsoons, influencing regional rainfall patterns.

The Milankovitch cycles create the initial conditions for climate shifts, which are often amplified by natural feedback loops of warming and cooling mechanisms. These cycles attest to the Earth’s natural climate variability. While they are powerful natural drivers of climate, they operate over thousands of years, and predict that, by today, the Earth should be in a long-term cooling trend approaching a new ice age. Rapid human-induced (anthropogenic) greenhouse gas emissions have, however, currently overpowered these natural rhythms, causing global warming that is happening roughly 10 times faster than past natural cycles. Since the industrial revolution, anthropogenic sources have trounced natural climate variability by introducing greenhouse gases at a rate and volume that exceed the Earth’s natural capacity to balance them. While natural factors like volcanic eruptions and the Milankovitch cycles still influence climate, their total contribution to global warming since 1850 is estimated at less than ±1 degrees Celsius, whereas human activity is responsible for nearly all observed heating.

Human activities have altered the Earth’s energy balance. The burning of coal, oil, and gas (for electricity, heat, and transportation) accounts for the highest percentage of global greenhouse gas emissions. These activities have raised atmospheric carbon dioxide levels higher than any observed in at least 2 million years. Cutting down forests releases stored carbon and removes natural sinks that absorb heat. Replacing natural landscapes with buildings and cemented roads creates urban heat islands where hard surfaces absorb and retain more heat than rural areas. Modern farming is a major source of potent heat absorbers like methane and nitrous oxide. Animals like cows and sheep produce large amounts of methane during digestion. The use of nitrogen-based fertilizers in farming releases nitrous oxide, a greenhouse gas.

Current global warming is occurring roughly 10 times faster than the average rate of recovery following previous ice ages. While an increase in solar energy would warm the entire atmosphere, observed patterns show the lower atmosphere warming while the upper atmosphere cools. This is a clear fingerprint of the greenhouse effect trapping heat near the surface. With certainty, it is human influence that is the principal driver of changes observed across land, air, ocean, and the biosphere. Human activities are the primary driver of global climate change, primarily through the emission of greenhouse gases that trap heat in the atmosphere. This is the anthropogenic greenhouse effect that overpowers natural climate cycles. This phenomenon disrupts the Earth’s natural climate rhythm.

Our role is pivotal in addressing this challenging global climate problem. Curbing global climate change requires a multipronged approach that involves reducing emissions (mitigation), building resilience (adaptation), and systemic change (collective advocacy). It is the power of collective action that genuinely celebrates Earth Day!

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Dr. Joel Tiu Maquiling may be reached at jmaquiling@ateneo.edu