OUR NATURALLY BURNING EARTH – THE EXTERNAL INFLUENCES
Natural changes to the Climate can be either external or internal. In Earth’s Temperature Cycles, it is clear to see when these influences change the Earth’s global temperature and to what extent. We are all likely to be aware of the internal influences, but what about external? These are on such a large scale that they don’t happen on Earth, they happen to the Earth itself. So what are these influences and are they responsible for Climate Change rather than our human influences?
So, natural changes are either external or internal. Internal changes are ones that we are all likely to be familiar with – volcanic eruptions and continental drift for example. External changes, maybe not. These relate to the Earth itself. In the 1920’s a Serbian Geophysicist and Astronomer Milutin Milankovitch proposed that the ‘long-term, collective effects of changes in Earth’s position relative to the Sun are a strong driver of Earth’s long-term climate, and are responsible for triggering the beginning and end of glaciation periods (Ice Ages).’ Since he proposed his theory a century ago, it has remained the most significant and as such, the cycles are known simply as the Milankovitch Cycles. What are the Milankovitch Cycles? There are three – The Eccentricity (the shape of the Earth’s orbit), the obliquity (changes in the angle that the Earth’s axis makes with the plane of the Earth’s orbit) and the precession (the change in where the direction of the Earth’s axis of rotation is pointing). In short, the internal changes are what happens on Earth, external changes affect the level of solar radiation reaching Earth. So, what do all these mean?
Figure 1a, CAE Lifestyle – The Milankovitch Cycles
THE ECCENTRICITY
The eccentricity relates to the movement of the Earth around the sun within our Solar System, known as the Earth’s Orbit. NASA defines an Orbit as ’a regular, repeating path that one object in space takes around another one’. So in the same way that we are orbiting the sun, our moon and human-made satellites are orbiting us – it’s like having an imaginary road circling the Earth, with the moon and satellites driving along it. This also applies to the Earth orbiting the sun. But how does this apply to the Earth’s Climate? Well, the shape of the Earth’s orbit affects the amount of solar radiation (insolation) that the Earth receives. The Earth’s orbit is not always a near perfect circle. Over time, the pull of gravity from Saturn and Jupiter causes the Earth’s orbit to change, from circular to more elliptical (more like an oval). These changes occur within a longer cycle of around 413,000 years and another cycle of around 100,000 years, corresponding to the cycles of Ice Ages and Inter-Glacial periods. The level of radiation reaching the Earth can affect the energy and temperatures that the receiving areas experience, which (alongside the other cycles) can affect the general melting and ice sheets, for example.
The eccentricity is also the reason why our seasons are different lengths. Lets forget about the changes in the shape of the orbit for a moment and just think of it as one constant shape – the orbit is never a prefect circle, which means that the distance between the Sun and Earth will not be the same the whole way around the orbit. The Earth is closest to the sun at the beginning of January and furthest away at the begging of July, which means that more solar radiation reaches Earth in every January than it does every July (around 6.8% currently). This is the case every year, but when the orbit changes, so does the furthest point between the Earth and the Sun. When the Earth’s orbit is at its most elliptical, there can be around 23% more solar radiation reaching the Earth when its closest to the sun, than when its at its furthest point. The impacts this has on the seasons and the Climate for varying locations, is further influenced by both the obliquity and the precession.
Figure 1b, CAE Lifestyle – The Earth’s Orbital Eccentricity
THE OBLIQUITY
The obliquity relates to the angle of the Earth’s axis – this means how much it tilts as it travels in orbit. The tilt is why we have seasons and their intensity is determined by the degree of tilt. When the tilt is higher, both hemispheres experience more solar radiation during their summers as they are tilted to the sun and less during their winters, when they are tilting away from the sun. Therefore, the more incoming solar radiation, the greater the heat and the more glaciers and ice sheets melt. The tilt is always changing between its maximum and minimum angle (thought to be 22.1° and 24.5°) in a cycle of around 41,000 years. So how does this relate to the Climate? Well our Earth’s axial tilt is currently decreasing, meaning year on year we are heading towards milder seasons of warmer winters and cooler summers, leading up to a period of advancing snow cover at higher latitudes (as summer temperates aren’t as high to cause vast melting). The greater the snow cover, the greater the amount of incoming solar radiation that can be reflected back into Space, thus creating more cooling.
However, there are a few problems. What we need to remember this is on a large scale and will take hundreds of years to change – the tilt is estimated to reach its minimum in just under 10,000 years, so this is isn’t something that’s going to happen in our lifetime. That being said, the same goes for the changes in our seasons that we are currently seeing. For example February 2020 was the warmest it has been since February 2016. This change shouldn’t be happening in 4 years as a result of the obliquity, there must be another factor playing a part. Summers are also a problem, as the temperatures are increasing, when in theory they should be doing the opposite (though there will of course be anomalies, but anomalies at this level?).
Figure 1c, CAE Lifestyle – The Earth’s Obliquity
THE PRECESSION
The precession relates to the direction that the Earth’s axis of rotation is pointing. The Earth is always rotating, both on a larger and smaller relative scale. The Earth spins on its axis, taking just under 24 hours to complete the rotation. This is why we have days. However the axis also spins – no matter what the degree of tilt. This is known as a wobble or precession. One spin (or precession) takes around 26,000 years, so is on a much larger scale. Why is there a wobble? Well, as a result of the Earth spinning on its axis, its shape is not a sphere, its more of spheroid (stretched sphere). This creates the equatorial bulge, which becomes the cause of the precession. This is due to tidal forces, which themselves are a result of ‘gravitational influence from the sun and moon’. So, the sun and moon are the ‘nudge’ that causes the axis to spin. The effects of the precession are similar to those of the obliquity change – the seasons. NASA explains the precession as making ‘seasonal contrasts more extreme in one hemisphere and less extreme in the other.’ As mentioned previously with the obliquity change, the tilt means that both the Northern Hemisphere and Southern Hemisphere will be closest to the sun during either their summer or winter. Whilst the obliquity changes only a few degrees and the spins on its axis daily, it is the axis rotation that causes the tilt to change to face towards the sun or away from the sun. This change happens extremely slowly however, as it takes 26,000 years to complete a full rotation – we are not talking about year on year drastic change (as we are currently seeing in some areas of the globe), gradual change takes hundreds of years.
So how does this relate to the Climate? When either Hemisphere is closest to the sun during their winter, the winters will be warmer as more solar radiation is reaching the Earth, whilst the summers will be cooler. This reduces the variations between seasons within the whole of the Northern and Southern Hemispheres. This can have global implications, as it can trigger a glacial period. But how? In the winter snow and ice will accumulate, but as the summer temperatures are cooler and are closer to the warmer winter temperatures, this snow and ice will not melt completely in the summer that follows. This means the snow and ice cover will expand over time and thus, bring about a glacial period – this will be global. Whilst the seasonal variations will be less extreme (as just explained) in one Hemisphere, the opposite will be occurring in the other – the temperature variation between summer and winter will be bigger. This is all experienced over thousands of years, so the change in variation is not yearly.
Currently, the Northern Hemisphere is closest to the sun during winter, meaning we should be experiencing smaller variations in temperature between our seasons. This will be the case for about another 13,000 years, slowly bringing about a glacial period (what’s also known as an Ice Age). In 13,000 years the axial precession will cause these variations to flip, with the Southern Hemisphere having warmer winters and cooler summers.
Figure 1d, CAE Lifestyle – The Earth’s Axial Precession
It’s seems clear that the Milankovitch Cycles are not short-term changes, they are long-term changes – influencing temperatures over thousands of years. Therefore, external influences cannot be responsible for the rapid change in Climate that we are currently experiencing.
