The Earth’s climate has no doubtedly changed throughout it’s 4.6 billion year history, and we understand those historic changes in the same way we understand current climatic shifts. However, something is very different about climate change today that does not follow the natural cyclicality of the past. To understand WHY anthropogenic (human-induced) climate change is different from the natural variability measured in the past, we must first understand the natural processes at work, and it begins with the relationship between our Sun and Earth.
“Something is very different about climate change today that does not follow the natural cyclicality of the past.”
At an incredible 93,000,000 miles away (known as an Astronomical Unit) from the Sun, the Earth receives just the right amount of energy (sunlight and heat) from the Sun to sustain an atmosphere capable of evolving life. However, Earth does not simply orbit the Sun in the same way millennia after millennia. In fact, Earth slowly changes it’s proximity to the Sun over time affecting the amount of total solar radiation it receives. The astronomical processes which shift the Earth position in space are called Milankovitch cycles, and these cycles are the primary driver of Earth’s natural climate variability
What are Milankovitch Cycles?
Milankovitch cycles are three orbital parameters, which were named after the Serbian geophysicist and astronomer Milutin Milankovic, who in the 1920’s, theorized that global climate was influenced by variations in eccentricity, obliquity, and axial precession. As these orbital patterns modulate over time, so does the amount of incoming solar radiation, changing how much sunlight reaches different latitudes, ultimately affecting global temperatures and seasonality. When eccentricity, obliquity and axial precession position the Earth in closer proximity to the Sun it is known as Perihelion. During perihelion when the northern hemisphere, and most of the Earth’s land mass faces the Sun, global temperatures increase. The opposite is known as Aphelion, where eccentricity, obliquity, and axial precession position the Earth and northern hemisphere furthest from the Sun, causing global temperatures to decrease. These three parameters control how radiation from the Sun interacts with the Earth, so lets explore each a little closer to understand exactly what is changing.
Eccentricity
Eccentricity is the longest cycle, and refers to the shape (from circular to elliptical) of our Earth’s orbit around the Sun, which changes every 400,000 to 100,000 years. As the orbit becomes more elliptical, the Earth will spend less time near the Sun. This decrease in time spent near the Sun leads to a reduction in the amount of radiation received annually by the Earth, causing global temperatures to be slightly cooler. During periods when the Earth’s orbit changes shape and becomes more circular, it spends a substantially longer amount of time closer to the Sun, therefore increasing global temperatures.
Obliquity (Axial Tilt)
Obliquity is also known as axial tilt, or the angle of the Earth along an imaginary vertical axis that runs from the north to the south pole. Axial tilt is typically around 23.5° (degrees), but can vary from 22° – 24.5° (degrees). This cycle changes every ~41,000 years, and with an angle that varies from 22° – 24.5°, the Earth can tilt as much 2.5° degrees over that time. 2.5° degrees may sound insignificant, but when considering the size of the Earth (approximately 12,700 km in diameter), this seemingly small variation can greatly affect global climate. As obliquity increases, leaning the Earth towards the Sun, global temperature warm, while the opposite deceases Earth’s proximity to the Sun, lowering global temperatures.
Axial Precession (Axial Wobble)
Precession works in the same way as a spinning top that begins to wobble just before it topples over. This wobbling about the Earth’s axis has a cyclicality of approximately 23,000 years, and is driven by the gravitational pull of our Sun and Moon (much like our tides). Precession creates differences in polar seasonal variations, such that: the hemisphere at perihelion (facing the Sun) will experience a warmer summer but a cooler winter, while the hemisphere in aphelion (facing away from the Sun) will experience a warmer winter, but cooler summer.
Present Anomaly (The Smoking Gun)
Skeptics of climate change often claim that our Earth goes through natural cycles of warming and cooling, and the current trend we have been measuring is following a period of warming, which is unrelated to human activity. However, this is just not true. While the Earth does indeed go through natural cycles of warming and cooling, presently, neither eccentricity, obliquity, or axial precession place Earth at a closer proximity to the Sun to cause the observed increase in global temperatures. Understanding that Earth’s natural climate variability is largely bound by changes in the Milankovitch Cycles, is the first step to understanding our current climatic anomaly.
As scientists realized that our Earth’s natural climate cycle should be cooling, they began looking for answers. If astronomical forcing is not currently changing our climate, what is? Could the Sun be more active, is there a data bias created by a surplus of more accurate measurements, has geologic activity (volcanic/hydrothermal/weathering) increased, is human activity driving the change? These are all relevant questions, along with many others, which have been exhausted trying to find the smoking gun. In short, scientific observation and repeated experimentation lead us to conclude that human activity is the single largest contributor to climate change today.
Sources:
Open Source Systems, Science, Solutions, 2017, Milankovitch Cycles: http://ossfoundation.us/projects/environment/global-warming/milankovitch-cycles (accesses December 2016)
Physical Geology by Steven Earle used under a CC-BY 4.0 international license.
Zachos, J., Pagini, M., Sloan, L., Thomas, E., Billups, K., 2001, Trends, Rhythms, and Aberrations in Global Climate 65 Ma to the Present: Science, v. 292, p. 686 – 692