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The Relationship between the Earth’s Journey around the Sun & Natural Climate Change
The Role of the Sun
To understand the relationship between the Earth’s journey around the Sun and natural climate change, we must first establish if the amount of solar energy the Earth receives from the Sun is constant or varies over time. Since the Sun is the principal source of the radiant energy the Earth receives from space, it is useful to know if the solar energy flux or solar irradiance, the amount of energy arriving per unit time, varies and if so, then to what degree, in what way, and over what timescales (e.g., are there specific periods or cycles of variation at least over timescales since the rise of human civilization).[1]
The Solar Constant
The value of the radiant energy received from the Sun is called the ‘solar constant’. It is a measure of the mean solar electromagnetic radiation arriving per unit area that would be incident on a plane perpendicular to the Sun’s rays, at a distance of one astronomical unit (AU) from the Sun. Because the Earth’s orbit is not circular, there are times in the year when the Earth is closer to the Sun that at other times, hence the AU is a measure of the mean distance.[2]
The solar constant includes all types of solar radiation, not just visible light. However, the solar constant is not a physical constant such as the speed of light, which in physics is considered to be an absolute constant in accordance with Einstein’s Theory of Relativity. In reality, the solar constant is an average of a varying value that for some 400 years of observations has not changed by more than 0.2 percent.
So, why does it vary? The primary reason for this small variation over the past four centuries is attributable to minor changes in the amount of solar energy received at the Earth because of sunspot cycles. Sunspots are darker regions of the Sun relative to its general brightness. The darker regions reduce the amount of radiant energy coming from the Sun as they are cooler than most of the solar surface. The cycle varies from no spots to many spots to no spots in an approximately 11-year cycle. The current and previous two cycles are illustrated in Exhibit 1.
Exhibit 1
[1] The solar irradiance integrated over time is called solar irradiation, solar exposure or insolation.
[2] An AU is the mean distance from the Sun to the Earth over the course of a year, the time it takes the planet to travel around the Sun. In 2012, an Astronomical Unit was defined as a specific length, that being 149.597870700 million km or for practical purposes 150 million km.
Data show that the current cycle, called Cycle 24, is the weakest in the past 100 years. The reason for this is not understood. In effect, this significant drop in sunspots is tantamount to increasing the solar constant.
There are additional cycles in the strength and duration of this basic 11-year pattern, however their cumulative apparent influence on the solar constant is far less that the 0.2 percent as measured since 1700 and inferred from the record to date.
The Orbital Motions of the Earth
Now, let us turn to the Earth and its annual journey around the Sun. There are three factors related to the motion of the planet that modulate the amount of solar energy that arrives at the top of the atmosphere over periods of time measured in thousands of years. These are known as the Milankovitch Cycles, after an early 20th Century Serbian scientist who formulated this Sun-Earth relationship.
The first thing to note about Earth's orbit and its effect on climate change is that Milankovitch Cycles occur over tens of thousands of years, consequently the climate trends that orbital patterns characterize are long-term events.
Eccentricity
As noted above, the Earth does not orbit the Sun in a circle; but rather in an ellipse. Consequently, the Earth’s distance from the Sun varies over the course of the year, thus the energy received at the Earth varies with its distance from the Sun. The measure of the shape of the Earth’s orbit around the Sun is referred to as the Earth’s eccentricity. However, over a period of some 100,000 years the shape of the orbit moves through a range from a near circle, with virtually zero eccentricity to an ellipse with roughly a five percent deviation from a circle.
These oscillations, from less elliptic to more elliptic to less elliptic, are of prime importance to understanding natural climate change. Why? Because the changing distance between the Earth and the Sun increases and decreases the amount of solar radiation received at the Earth's surface in different seasons over the years. It is the amount of solar energy received in the atmosphere (i.e., the troposphere) and at the surface that modulates temperature; with less energy leading to cooler temperatures and more energy to warmer temperatures. These temperature changes then cause changes in precipitation, winds, snow and ice cover, and ocean currents, among other factors.
The full range of the eccentricity of the Earth’s orbit is illustrated in Exhibit 2, although this is not to scale.
Exhibit 2
Today a difference of only about three percent occurs between aphelion, the Earth’s furthest point from the Sun and perihelion, the closest point. In other words, the Earth is currently in nearly a circular orbit. This three percent difference in distance means that Earth experiences a six percent increase in received solar energy in January than in July.
However, due to changes in ellipticity this six percent range of variability also changes. For example, when the Earth's orbit is at its most elliptical shape the amount of solar energy received at perihelion would be in the range of 20 to 30 percent more than at aphelion. Consequently, over time these continually altering amounts of solar energy received by the planet result in significant changes in the Earth's climate, such as acting as a driver of glacial and interglacial periods. At present the orbital eccentricity is nearly at the minimum of its approximate 100,000 year cycle with the Earth still being in an interglacial period; but perhaps nearing its natural end as a new ice age commences in the coming centuries. This pattern is well established over the last several million years.
Axial Tilt/Obliquity
Axial tilt, also known as obliquity, is the inclination of the Earth's axis in relation to its plane of orbit around the Sun. Oscillations in the degree of Earth's axial tilt occur with a periodicity of approximately 41,000 years and range from 22.1 to 24.5 degrees. The tilt is currently at 23.5 degrees and is decreasing. It is expected to be at its minimum in about 11,000 years. This range in tilting is illustrated in Exhibit 3.
Exhibit 3
The current tilt largely accounts for our seasonal climate patterns globally. Because of the periodic variations of this angle the degree of severity of the Earth's seasons change.
Earth's axis remains tilted in the same direction with reference to the background stars throughout the year. This means that one pole and the associated hemisphere of Earth will be directed away from the Sun at one side of the orbit, and half a year later this pole and hemisphere will be directed towards the Sun. Presently, summer occurs in the northern hemisphere when the north pole is directed toward the Sun. The current trend of decreasing tilt will promote warmer winters and colder summers, as well as an overall cooling trend.
Precession
Precession is the Earth's slow wobble as it spins on its axis. Precession is synonymous with a child’s spinning top as it starts to wobble. In this case the wobble takes 26,000 years to make a round. It is sometimes referred to as climatic precession due to the impact of precession on the Earth’s climate.
In terms of the background stars today, the precession of Earth’s wobble ranges from pointing at Polaris, the North Star, to pointing at the bright star called Vega. When this gradual shift in the axis is at its limit the north pole will be pointing at Vega, some 12,000 years from now. Vega would then be seen as the North Star. This is illustrated in Exhibit 4.
Exhibit 4
Due to this wobble significant climatic change takes place relative to today. When the axis is tilted towards Vega the positions of the northern hemisphere winter and summer solstices will coincide with the aphelion and perihelion, respectively. This means that the northern hemisphere will experience winter when the Earth is furthest from the Sun and summer when the Earth is closest to the Sun, the opposite of today’ situation. This change will result in greater seasonal contrasts, primarily because there is significantly more landmass in the northern hemisphere. Areas of landmass are much more susceptible to dynamic changes in climate than ocean areas due to a number of factors such as the differences in albedo and heat capacity between land and sea.
Collective Interaction of the Milankovitch Cycles
The primary cause of the Milankovitch Cycles is due to the influence of gravitational interactions between the Earth, Moon, Sun, Jupiter and Saturn. A familiar example of gravitational influence on the Earth are the ocean tides, which are primarily caused by the gravitational dance between the Earth-Moon system. The Milankovitch Cycles are far more subtle and take place over thousands of years as compared to the daily tides. The impact of these cycles on climate is often referred to as the Milankovitch Hypothesis.
These three orbital characteristics interact in complex ways. The timescales of interaction are none repeating in exact terms due to the complex dynamics caused by the gravitational dynamics previously mentioned. Consequently, the cycles are classified as aperiodic in nature. Exhibit 5 illustrates these aperiodic cycles over a 300,000 year timespan, with the present marked and the past to the right. Because the planetary dynamics are predictable within some limits, one can see here a forecast up to 100,000 years into the future. The scales of measurement in Exhibit 5 are related to standard indicators used for each cyclic pattern.
Exhibit 5
Exhibit 6 combines the time series of Exhibit 5 with modes of the phenomenon being measured.
Exhibit 6
The time lines illustrated above show the rates of change across the range of behaviours of the three components of the Milankovitch Cycles. These are further illustrated in diagrammatic form by showing the physical pattern of what happens to the Earth as it moves through each of these cycles.
Orbital Inclination
The inclination of Earth's orbit drifts up and down relative to its present orbit with a cyclical period of about 70,000 years. Milankovitch did not study this three-dimensional movement. More recent researchers have noted this phenomenon and that the Earth’s orbit also moves relative to the orbits of the other planets.
The so-called ‘invariable plane’, the plane that represents the angular momentum of the solar system is approximately the orbital plane of Jupiter. It turns out that the inclination of the Earth's orbit has a 100,000 year cycle relative to the invariable plane. This 100,000 year cycle closely matches the 100,000 year pattern of ice ages as seen in the Milankovitch Cycles. The last five interglacials and their associated glacial periods are shown in Exhibit 7. The data were derived from the 2,083 metre Vostok ice core extracted from East Antarctic. The 100,000 year cycle is clearly visible in these data.
Exhibit 7
A study of the chronology of Antarctic ice cores using oxygen to nitrogen ratios in air bubbles trapped in the ice, which appear to respond directly to the local insolation, concluded that the climatic response documented in the ice cores was driven by northern hemisphere insolation as proposed by the Milankovitch hypothesis.[1] This is a further validation of the Milankovitch hypothesis by a relatively novel method, but it is inconsistent with the ‘inclination theory’ of the 100,000-year cycle. Due to variations in the Earth's orbit, the amount of insolation varies with periods of approximately 21,000, 40,000, 100,000, and 400,000 years, the so-called Milankovitch Cycles. Variations in the amount of incident solar energy drive changes in the climate of the Earth, and are recognised as a key factor in the timing of starting and ending periods of glaciations. However, the Milankovitch Hypothesis of orbital forcing refers to a discrepancy between the reconstructed paleo-temperature record and the reconstructed amount of incoming solar radiation, or insolation over the past 800,000 years. This constitutes one of several outstanding problems.
Problems
Because the observed periodicities of climate fit so well with the orbital periods, the orbital theory has strong support among the scientific community. However, there are several problems in reconciling theory with observations. Four such issues are briefly touched on here.
100,000 Year Problem
The 100,000 year problem is concerned with the fact that the eccentricity variations have a significantly smaller impact on solar forcing than the effects of precession and obliquity; thus, resulting in the view that this cycle produces the weakest climatic effects. However, observations show that during the last one million years, the strongest climate signal is the 100,000 year cycle. In addition, despite the relatively large 100,000 year cycle, some have argued that the length of the climate record is insufficient to establish a statistically significant relationship between climate and eccentricity variations. Furthermore, some models can reproduce the 100,000 year cycles as a result of non-linear interactions between small changes in the Earth's orbit and internal oscillations of the climate system. This problem remains a research issue in spite of the weight in favour of the Milankovitch Hypothesis.
400,000 Year Problem
The 400,000 year problem is concerned with eccentricity variations having a secondary cycle that appears as a 400,000 year cycle. This cycle is only clearly present in climate records with time series longer than a million years. If the 100,000 year variations of Milankovitch are having such a strong effect as suggested above, then these variations might also be expected to be apparent in influencing the 400,000 year cycle. This cycle is also known as the Stage 11 problem, after the interglacial period in marine isotopic Stage 11, covering the time interval from 374,000 to 424,000 years ago. In other words, a 400,000 year cycle does not appear to be present in the Stage 11 data, derived from many marine cores. However, the relative absence of this periodicity in marine isotopic records may be due, at least in part, to the response times of the climate system components involved — in particular, the carbon cycle. Again, this problem calls for further research.
Stage 5 Problem
The Stage 5 problem refers to the timing of the second last interglacial, based on well-established marine isotope records known as Stage 5, which provides a paleo-temperature record between 80,000 and 130,000 years ago. Based on marine isotopes the interglacial appears to have begun some ten thousand years in advance of the solar forcing hypothesized to have been caused according to the Milankovitch Hypothesis. This is sometimes referred to as the ‘causality problem’. Further research is required.
Effect Exceeds Cause
The argument here is that some cyclical variations are believed to be due to variations in the intensity of solar radiation upon various parts of the globe (e.g., significant variations in cloudiness). However, observations show climate activity is much more intense than the calculated variations. Various internal characteristics of climate systems are believed to be sensitive to such insolation changes, causing amplification, a positive feedback and damping responses, a negative feedback. The jury is still out on this issue.
There are a number of additional issues about cycles and forcing functions regarding climate change in the more distant past. The Milankovitch Cycles seem to describe the glacial-interglacial cycles of the last million or so years; however, the mechanisms by which orbital forcing influences climate is still not well understood.
Present Conditions
The amount of solar radiation or insolation received in the northern hemisphere at 65° N seems to be related to the occurrence of an ice age. Astronomical calculations show that 65° N summer insolation should increase gradually over the next 25,000 years, and that no declines in 65° N summer insolation sufficient to cause an ice age are expected in the next 50,000 to 100,000 years. The relatively low eccentricity of the present orbit also results in a 6.8% difference in the amount of solar radiation during summer in the two hemispheres.
As mentioned above, at present perihelion occurs during the southern hemisphere's summer, and aphelion during the southern winter. Thus, the southern hemisphere seasons should tend to be somewhat more extreme than the northern hemisphere seasons.
Conclusion
There is a relationship between the Earth’s journey around the Sun and natural climate change. In periods of high eccentricity, solar radiation exposure on Earth can fluctuate more widely between periods of perihelion and aphelion. Those fluctuations are likewise far milder in times of low eccentricity. Currently, the Earth's orbital eccentricity is near zero, thus it is closer to being at its most circular.
The eccentricity, axial tilt, and precession of the Earth's orbit vary in several patterns that have resulted in 100,000-year ice age cycles of glaciation over the last few million years. Moreover, the Earth's axis completes one full cycle of precession approximately every 26,000 years. At the same time, the elliptical orbit rotates, more slowly, leading to a 21,000 year cycle between the seasons and the orbit. In addition, the angle between Earth's rotational axis and the normal to the plane of its orbit moves from 22.1 degrees to 24.5 degrees and back again on a 41,000 year cycle. Currently, this angle is 23.44 degrees and is decreasing. Each of these cycles has a signature in past temperature measurements inferred from proxy data such as oxygen isotope ratios in ice cores and marine sediment cores as well as oxygen/nitrogen isotope ratios in gas bubbles contained in ice cores.
It is important to note that the Earth's current warming trend is happening in spite of a relatively cool orbital phase. Given this trend, the implication from all of the above discussion is that there must be more to the Earth's average temperature than can be explained by Milankovitch Cycles. Furthermore, arguments about anthropogenic global warming, which climate scientists overwhelmingly believe is the prime cause of the current warming trend, is powerful enough in the short-term to counteract a relatively cool orbital phase. This is a fact that should give us pause to consider the profound effect that humans can have on climate even against a backdrop of Earth's natural cycles. At the same time, more research is required as there are a number of ‘problems’ as discussed regarding the Milankovitch Hypothesis.
[1] Source: Kawamura et al, Nature, 23 August 2007, vol. 448, p 912-917