EU ETS: what you need to know about the EU carbon market
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In this article, we'll explore the EU Emissions Trading System (EU ETS), a European initiative for reducing greenhouse gases through a cap-and-trade scheme.
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Earth’s climate has never been stable - it has changed dramatically over millions of years, shifting between ice ages and warmer interglacial periods. But what causes these long-term shifts? While factors like greenhouse gases and ocean currents play a role, one of the most significant forces behind Earth’s past climate changes is a set of natural cycles tied to our planet’s orbit: Milankovitch cycles.
First theorised by Serbian mathematician and astronomer Milutin Milanković in the early 20th century, these cycles describe how variations in Earth's movement around the Sun influence the amount and distribution of solar energy reaching the planet. Over tens to hundreds of thousands of years, small shifts in Earth's orbit, tilt, and axial wobble have triggered major climate events, including the advance and retreat of ice sheets.
Understanding Milankovitch cycles is essential for reconstructing past climate patterns, but it also raises an important question: if Earth's climate has changed naturally before, how do we know that today’s warming is different?
In this article, we’ll break down the three key Milankovitch cycles, explore how they’ve shaped Earth’s climate over time, and examine why they can’t explain the rapid climate change we see today.
What are Milankovitch cycles?
“Milankovitch cycles refer to long-term variations in Earth’s orbit and axial tilt that affect how much solar energy the planet receives. These changes happen over thousands of years and are responsible for the natural climate shifts that have driven ice ages and interglacial periods throughout Earth’s history.”
The theory was first developed by Serbian scientist Milutin Milanković in the early 20th century. Using complex mathematical calculations, he demonstrated how small variations in Earth's position relative to the Sun could alter global temperatures and trigger large-scale climate events. Although his ideas were initially met with skepticism, later geological and ice core evidence confirmed that these cycles played a crucial role in past climate fluctuations.
Milankovitch cycles operate on three key mechanisms:
- Eccentricity
- Obliquity
- Precession
Each of these influences Earth’s climate in different ways, but together, they determine how much sunlight reaches different parts of the planet over long timescales.
The three orbital variations
Eccentricity (100,000-year cycle)
- This refers to changes in the shape of Earth’s orbit around the Sun. Instead of a perfect circle, Earth’s orbit shifts between being more elliptical and more circular over a roughly 100,000-year cycle.
- When the orbit is more elliptical, the difference in solar energy between the closest (perihelion) and farthest (aphelion) points in Earth's orbit is greater, leading to stronger seasonal contrasts.
- This variation has been linked to the onset of glacial and interglacial periods, as ice ages tend to occur when Earth’s orbit is more elongated, reducing the intensity of summer warming.
Obliquity (41,000-year cycle)
- Obliquity refers to the tilt of Earth’s axis, which shifts between 22.1° and 24.5° over a 41,000-year cycle.
- A greater axial tilt increases seasonal contrast, making summers warmer and winters colder. While a smaller tilt reduces seasonal extremes.
- Higher obliquity leads to stronger polar warming, which can accelerate ice melt, while lower obliquity favors ice sheet growth by keeping summers cooler.
Precession (23,000-year cycle)
- Also known as the wobble of Earth’s axis, precession affects the timing of seasons relative to Earth’s position in its orbit.
- Over a 23,000-year cycle, Earth’s axis slowly shifts, altering which hemisphere experiences more intense seasons.
- This influences monsoon patterns and can determine whether a particular region receives more or less sunlight during certain periods of the year.
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Climate influence
“Milankovitch cycles do not change the total amount of solar energy Earth receives over time, but they redistribute it affecting how much sunlight reaches different parts of the planet and at what time of year. These subtle shifts in solar radiation, known as insolation, are enough to tip Earth’s climate toward either warming or cooling over thousands of years.”
Changes in solar radiation distribution
- Each of the three Milankovitch cycles alters the way sunlight is distributed across Earth’s surface.
- The most significant factor for triggering ice ages is the amount of summer solar radiation at high latitudes (especially in the Northern Hemisphere).
- When summers are cooler due to lower insolation, ice sheets can expand, reflecting more sunlight and further cooling the planet. On the other hand, when summers are warmer, ice melts, leading to warming.
Seasonal and latitudinal effects
- Eccentricity changes the distance between Earth and the Sun, increasing or reducing seasonal contrasts.
- Obliquity strengthens or weakens seasonal differences - a higher tilt leads to more extreme seasons, while a lower tilt results in milder seasons.
- Precession determines when seasons occur within the orbit, influencing monsoon patterns and potentially reinforcing cooling or warming trends.
The ice-albedo feedback loop
- One of the key ways Milankovitch cycles trigger climate change is through the ice-albedo feedback loop.
- When ice sheets expand during periods of lower summer insolation, they reflect more sunlight (high albedo), causing further cooling.
- When ice sheets shrink due to increased solar energy, darker land and ocean surfaces absorb more heat (low albedo), reinforcing warming.
Carbon dioxide and climate amplification
- While Milankovitch cycles initiate climate shifts, CO2 levels act as a feedback mechanism, amplifying warming or cooling.
- As ice sheets grow, CO2 concentrations decrease, reducing the greenhouse effect and causing further cooling.
- During warming phases, melting ice releases CO2 stored in oceans, further increasing temperatures.
| Milankovitch Cycle | Impact on Solar Radiation | Impact on Seasons | Impact on Ice Sheets | Other Climate Effects |
|---|---|---|---|---|
| Eccentricity (100,000-year cycle) | Alters the shape of Earth's orbit from circular to elliptical, changing the distance between Earth and the Sun. | A more elliptical orbit increases seasonal contrasts, while a more circular orbit leads to milder seasons. | More elliptical orbit → Weaker summer warming, allowing ice sheets to expand. More circular orbit → More even solar energy distribution, reducing glaciation. |
Linked to glacial and interglacial periods, as lower summer insolation can trigger ice ages. |
| Obliquity (41,000-year cycle) | Changes Earth’s axial tilt between 22.1° and 24.5°, affecting the angle at which sunlight hits the planet. | A greater tilt increases seasonal extremes (hotter summers, colder winters). A smaller tilt reduces seasonal differences, leading to milder seasons. |
Higher tilt → Stronger polar warming, accelerating ice melt. Lower tilt → Cooler summers, allowing ice sheets to grow. |
Determines the latitudinal extent of climate zones—higher tilt leads to greater temperature variations between equator and poles. |
| Precession (23,000-year cycle) | Causes Earth’s axis to wobble, shifting the timing of the seasons relative to Earth's position in its orbit. | Changes which hemisphere experiences more extreme seasons at a given time. | Can reinforce warming or cooling trends, depending on whether summers occur at perihelion (closer to the Sun) or aphelion (further from the Sun). | Influences monsoon patterns and regional climate shifts by altering seasonal solar intensity. |
Historical climate correlations
“Milankovitch cycles have played a major role in shaping Earth’s climate over millions of years. The strongest evidence for their influence comes from glacial and interglacial cycles - the alternating periods of ice ages and warmer climates that have occurred throughout the Pleistocene epoch (the last 2.6 million years).”
Milankovitch cycles and ice ages
Ice ages follow a roughly 100,000-year pattern, closely aligning with Earth’s eccentricity cycle. However, eccentricity alone isn’t enough to cause these shifts - it works in combination with obliquity and precession to control how much sunlight reaches high latitudes during summer.
The key factor in triggering glacial cycles is summer insolation at high latitudes (particularly in the Northern Hemisphere):
- When summers are weak (low insolation): Ice sheets grow, leading to an ice age.
- When summers are strong (high insolation): Ice melts, pushing Earth into an interglacial period.
Each ice age does not begin or end immediately with orbital shifts. Instead, these cycles act as a climate trigger, setting off larger feedback mechanisms, such as changes in carbon dioxide (CO2) levels, ice-albedo effects, and ocean circulation patterns, which further amplify the impact on the climate.
Evidence from ice cores and geological records
Milankovitch’s theory was initially met with skepticism, but strong scientific evidence has since confirmed the link between orbital cycles and past climate changes:
- Ice core data from Antarctica and Greenland show a clear correlation between temperature, CO2 levels, and Milankovitch cycles over the last 800,000 years.
- Sediment records from deep-sea cores reveal layers of plankton fossils and dust deposits that align with predicted Milankovitch cycles.
- Glacial deposits and erosion patterns on land match the expected timing of glaciations and interglacial periods.
“These findings confirm that Milankovitch cycles are a dominant force behind Earth’s past climate shifts. However, if these cycles explain ice ages, why can’t they explain today’s rapid warming? ”
Why Milankovitch cycles can’t explain global warming
“Milankovitch cycles have been the driving force behind past climate shifts, but they cannot explain the rapid warming observed in the last century. Unlike the slow, predictable changes triggered by orbital variations, today’s climate is warming at an unprecedented rate, and the cause is not natural cycles - it’s human activity. ”
If Earth were following the natural rhythm of Milankovitch cycles, we would currently be in a cooling phase. Based on the current position of Earth’s orbit, the planet should be very gradually moving toward another ice age over the next tens of thousands of years. Instead, global temperatures are rising at a pace 100 times faster than any natural warming cycle in the past.
Three key factors highlight why Milankovitch cycles are not responsible for today’s climate trends:
- Timescale mismatch: Past glacial cycles took thousands of years to unfold. The warming we see today has occurred in just 150 years.
- Greenhouse gas concentrations: Ice core records show that during past warming events, CO2 levels rose after temperature changes, acting as an amplifier. Today, the situation is reversed: CO2 levels have increased first, leading to warming.
- Direct measurements: Satellites and climate models confirm that the current energy imbalance is due to greenhouse gas emissions, not changes in solar radiation from orbital variations.
The role of human activity
Since the Industrial Revolution, human activities, like the burning of fossil fuels, deforestation, and industrial processes, have dramatically increased greenhouse gas concentrations. This has overpowered natural climate drivers, including Milankovitch cycles.
“While orbital variations continue to shape Earth's long-term climate, they are not the cause of today’s warming. Instead, the scientific consensus is clear: the rapid rise in global temperatures is driven by human influence. ”
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Milankovitch Cycles in the Solar System
Milankovitch cycles are not unique to Earth - similar orbital variations affect the climates of other planets in our solar system. While Earth’s cycles play a role in ice ages, the impact of orbital shifts on other planets can be even more extreme.
| Planet/Moon | Key Orbital Variations | Climate Impact |
|---|---|---|
| Mars | Axial tilt (obliquity) varies between 10° and 60° over millions of years due to lack of a stabilising large moon. Orbital eccentricity also changes significantly. | Extreme tilt variations cause major climate shifts. High tilt results in melting of polar ice caps and redistribution of water vapour toward the equator. Low tilt causes ice buildup at the poles, leading to colder conditions. |
| Saturn’s Moon Titan | Influenced by Saturn’s 29.5-year orbit around the Sun, along with minor axial tilt and orbital eccentricity variations. | Experiences seasonal changes in atmospheric temperature and methane rain cycles, similar to Earth's obliquity-driven seasons. Seasonal shifts affect Titan’s lakes, cloud formation, and wind patterns. |
| Venus & Mercury | Both have almost no axial tilt (Venus: 2.6°, Mercury: virtually 0°), meaning no significant obliquity-driven climate variation. | Very little seasonal variation. Venus' extreme greenhouse effect dominates its climate, while Mercury experiences extreme day-night temperature contrasts due to its slow rotation. |
| Exoplanets (Binary Star Systems) | Complex gravitational interactions with two or more stars can lead to irregular orbital changes, including varying eccentricity, obliquity, and precession. | Potential for extreme and unpredictable climate fluctuations, affecting planetary habitability. Some exoplanets may experience cycles of intense heating and cooling due to shifting distances from their stars. |
“Studying how Milankovitch-like cycles operate beyond Earth helps scientists understand the role of orbital mechanics in shaping planetary climates, both within our solar system and on distant exoplanets. ”
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