ADVERTISEMENTS:
In this article we will discuss about the external and internal forcing mechanisms of climate change.
External Forcing Mechanisms:
This section of the article discusses some of the various external forcing mechanisms operating over time scales of 10 years to 109 years.
i. Galactic Variations:
ADVERTISEMENTS:
The orbit of the Solar System about the centre of the Galaxy has been considered as a possible external climate forcing mechanism. During the course of a galactic year (now estimated at 303 million years), variations in the interstellar medium may influence the amount of solar radiation incident at the Earth’s surface, thus acting as a radiative forcing mechanism to induce climate change. Williams (1975a) also suggests that variations in gravitational torque induced by our Galaxy’s near neighbours, the Small and Large Magellanic Clouds, could have far-reaching consequences for the Earth’s climate.
Unfortunately, the enormous time scale associated with this forcing (and any hypothesised global climatic change) makes empirical confirmation of this premise exceedingly imprecise. Nevertheless, it is indeed possible that the ice age super-cycles during the last 700 million years could be the result of such galactic forcing mechanisms.
ii. Orbital Variations:
In the mid-19th century, Croll (1867a, 1867b) proposed an astronomical theory linking the Pleistocene (2 Ma to 10 Ka) ice ages with periodic changes in the Earth’s orbit around the Sun. Croll’s ideas were later refined and elaborated by Milankovitch (1941). The Milankovitch theory is the name given to the astronomical theory of climate variations. Since these ideas were put forward, much evidence has been found to support the theory. In this section, the forcing mechanisms involved with the Milankovitch theory are discussed.
The original Milankovitch theory identifies three types of orbital variation which could act as climate forcing mechanisms, obliquity or tilt of the Earth’s axis, precession of the equinoxes and eccentricity of the Earth orbit around the Sun. Each variation has its specific time period.
ADVERTISEMENTS:
Obliquity – Today the Earth is tilted on its rotational axis at an angle of 23.4° relative to a perpendicular to the orbital plane of the Earth. Over a 41,000 year time period, this angle of inclination fluctuates between 22° and 24.5°, influencing the latitudinal distribution of solar radiation.
Obliquity does not influence the total amount of solar radiation received by the Earth, but affects the distribution of insolation in space and time. As obliquity increases, so does the amount of solar radiation received at high latitudes in summer, whilst insolation decreases in winter. Changes in obliquity have little effect at low latitudes, since the strength of the effect decrease towards the equator.
Consequently, variations in the Earth’s axial tilt affect the strength of the latitudinal temperature gradient. Increased tilt has the effect of raising the annual receipt of solar energy at high latitudes, with a consequent reduction in the latitudinal temperature gradient.
ADVERTISEMENTS:
iii. Eccentricity:
The Earth’s orbit around the Sun is not perfectly circular but follows an elliptical path (see Figure 11.4). A second orbital variation involves the strength of the ellipse, or eccentricity. This parameter, e, is determined by Equation 1, which compares the two focal lengths, x and y in Figure 11.4.
e = {(x2 – y2)1/2}/x …(1)
When the orbit is circular, the lengths x and y are equal and e = 0. The Earth’s orbit has been found to vary from being near circular (e = 0.005) to markedly elliptical (e = 0.06) with two primary periodicities of approximately 96,000 and 413,000 years. The current value of e is 0.018.
Variations in eccentricity influence the total amount of solar radiation incident at the top of the Earth’s atmosphere. With maximum eccentricity, differences in solar radiation receipt of about 30% may occur between perihelion and aphelion (Figure 11.4).
iv. Precession:
The third orbital variation is that of precession. The Sun lies at one of the focal points of the Earth’s orbital ellipse. Due to the gravitational interaction of other planetary bodies in the solar system, primarily the Moon and the planet Jupiter, the perihelion (the point at which the Earth passes closest to the Sun) moves in space with a consequent shifting or precessing of the elliptical orbit. This phenomenon is known as the precession of the equinoxes, and affects the intensity of the seasons.
Precession has two components – an axial precession, in which the torque of the other planets exerted on the Earth’s equatorial bulge causes the rotational axis to gyrate like a spinning top; an elliptical precession, in which the elliptical orbit of the Earth itself rotates about one focus. The net effect describes the precession of the equinoxes with a period of 22,000 years. This term is modulated by eccentricity which splits the precession into periods, 19,000 and 23,000 years.
ADVERTISEMENTS:
Like obliquity, precession does not affect the total amount of solar energy received by the Earth, but only its hemispheric distribution over time. If the perihelion occurs in mid-June i.e. when the Northern Hemisphere is tilted toward the Sun, then the receipt of summer solar radiation in Northern Hemisphere will increase. Conversely, if the perihelion occurs in December, the Northern Hemisphere will receive more solar radiation in winter (see Figure 11.4). It should be clear that the direction of changes in solar radiation receipt at the Earth’s surface is opposite in each hemisphere.
v. Milankovitch Cycles and Ice Ages:
The three components of the orbital variations together affect both the total flux of incoming solar radiation and also the temporal and spatial distribution of that energy. These variations have the potential to influence the energy budget of the climate system, and can therefore be regarded as possible causes of climate change over a 104 to 105 year time scale. Being external to the climate system, they may be classified as external forcing mechanisms.
Milankovitch (1941) considered the changing seasonal (precession) and latitudinal (obliquity) patterns of incoming radiation to be critical factors in the growth of continental ice sheets and in the initiation of ice ages.
He hypothesised that when axial tilt was small (large latitudinal temperature gradient), eccentricity was large and perihelion occurred during the Northern Hemisphere winter (warmer winters and colder summers), such a configuration would allow the persistence of accumulated snow throughout the summer months in the Northern Hemisphere. Additionally, the warmer winters and stronger atmospheric general circulation due to the increased temperature gradient would increase the amount of water vapour at the high latitudes available for snowfall.
For long-term proxy temperature data, spectral analysis, which permits the identification of cycles, has shown the existence of periodicities of 100,000, 43,000, 24,000 and 19,000 year (see Figure 11.5), all of which correspond closely with the theoretical Milankovitch cycles.
Nevertheless, verification of a causal link between the orbital forcing factors and the climatic response is far from being proved, and significant problems remain. Firstly, Figure 11.5 shows that the strongest signal in the observational data is the 100,000 year cycle. This would be the result of eccentricity variations in the Earth’s orbit, which alone account for the smallest insolation changes.
Secondly, it is not clear why changes in climate appear to be global. A priori reasoning indicates that the effects of precession would cause opposite responses in each hemisphere. In fact, climate change is synchronised between Southern and Northern Hemispheres, with a growth of ice sheets during glaciations occurring in the Arctic and Antarctic.
Most crucially of all, however, it seems that the orbital forcing mechanisms alone, could not account for the observed climatic variations over the past 2 million years. In order to explain the magnitude of the observed climatic changes, it seems necessary to invoke various feedback mechanisms. Indeed, Milankovitch himself had expected the direct effects of variations in insolation to be magnified by feedback processes, such as, at high latitudes, the ice albedo effect.
vi. Solar Variations:
Although solar variability has been considered, a priori, to be another external forcing factor, it remains a controversial mechanism of climate change, across all time scales. Despite many attempts to show statistical associations between various solar periodicities and global climate cycles, no realistic causal mechanism has been proposed to link the two phenomena.
The best known solar cycle is the variation in the number of sunspots over an 11 year period. Sunspot cycles are thought to be related to solar magnetic variations, and a double magnetic cycle (approximately 22 years) can also be identified. What is of interest to a climatologist is whether the sunspot cycles are accompanied by variations in solar irradiance – the solar constant – which, potentially, could force climate changes.
The solar constant (approximately 1368Wm-2) is a measure of the total solar energy flux integrated across all wavelengths of radiation. Two decades of satellite observations reveal that the solar constant varies over time scales of days to a decade, and there does appear to be a significant relationship with the sunspot number cycle. At times of high sunspot number, the value of the solar constant increases.
Although sunspots are regions of cooler than average Sun surface temperature, their presence is accompanied by brighter (hotter) faculae which more than compensates for the increase in darker sunspot areas. This relationship can be extended back over time using the long sunspot record. Solar irradiance changes thus calculated are reproduced in Figure 11.6.
The difficulty in attributing any observed climate change to these variations in solar irradiance is that the latter are small in magnitude – a change of much less than 1% over the course of the sunspot cycle. Wigley (1988) stressed that with such small variations in the solar constant, the global climatic response would be no more than a 0.03°C temperature change.
Nevertheless, many climatic records (e.g. indices of droughts, temperature and total atmospheric ozone) do appear, at least statistically, to display periodicity linked to one or both of the sunspot cycles. It should be clear, however, that a statistical association between solar variability and climate change does not prove cause and effect.
It is of course possible that the approximate 11-year cycle identified in many climate records is caused by some unknown internal oscillation and not by external solar forcing. It is conceivable that, simply by chance, the phase of the oscillation could coincide with the phase of the solar variability. More plausibly, an internal oscillation can become phase-locked to the solar cycles, thus augmenting the climatic response by a kind of feedback mechanism. For the time being, therefore, the link between the sunspot cycles and climate change must remain a speculative one.
However, there are other solar periodicities, with longer time scales that could be considered as climate forcing mechanisms. It has been suggested that the long-term variation in the amplitude of the sunspot cycles may have an influence on global climate. Observations made with the naked eye reveal times when sunspot activity was very limited, including the Maunder Minimum (1654 to 1715) and the Sporer Minimum (1450 to 1534).
These events occurred during the period known as the Little Ice Age, and Eddy (1976, 1977) has hypothesised that the two may be causally linked. With the sunspot cycles, however, the evidence is largely circumstantial. Other solar variations include cycles of sunspot cycle length (between about 9 and 13 years), changing solar diameter and the rate of change of solar diameter. Although some of these long-term variations may involve larger changes in solar output, this is again mere speculation.
Proxy records of solar irradiance changes are needed when even longer time scales are considered. A number of scientists have used records of 14C in tree rings to investigate the relationships between potential solar forcing mechanisms and climate change. Changes in the output of energetic particles from the Sun (the solar wind) are believed to modulate the production of 14C in the upper atmosphere.
The magnetic properties of the solar wind change with the variation of sunspots, leading in turn to variations in the production of 14C. The effect of the solar wind is such that high 14C production is associated with periods of low sunspot number.
Relatively long and reliable 14C records are now available. Spectral analysis has revealed a number of solar periodicities including a 2,400 year cycle, a 200 year cycle, a 80 to 90 year cycle and the shorter 11 and 22 year cycles. The 14C records have also been correlated with a number a climate change indicators, including glacial advance- retreat fluctuations and annual temperatures for England. Episodes of low 14C production are associated with high sunspot activity and warmer climates.
It is certainly feasible that the climatic variations of the Holocene (last 10,000 years since the end of the last ice age), and the shorter fluctuations associated with the Little Ice Age have been forced by the interacting millennia and century scale cycles of solar activity. However, conclusive evidence of a mechanism linking cause and effect is again missing. In addition, numerical modelling of Wigley & Kelly (1990) seems to indicate that solar irradiance changes would not be substantial enough to bring about the observed climatic changes without invoking additional internal feedback mechanisms.
Internal Forcing Mechanisms:
This section of the article discusses some of the various internal forcing mechanisms operating over time scales of 1 year to 108 years. They may be either radiative or non-radiative forcing mechanisms.
i. Non-Radiative Forcing:
Any change in the climate must involve some form of energy redistribution within the global climate system.
Nevertheless, forcing agents which do not directly affect the energy budget of the atmosphere (the balance between incoming solar radiation and outgoing terrestrial radiation, are considered to be non-radiative mechanisms of global climate change. Such agents usually operate over vast time scales (107 to 109 years) and mainly include those which affect the climate through their influence over the geometry of the Earth’s surface, such as location and size of mountain ranges and position of the ocean basins.
ii. Radiative Forcing:
A process which alters the energy balance of the Earth-atmosphere system is known as a radiative forcing mechanism. These may include variations in the Earth’s orbit around the Sun, solar radiation, volcanic activity and atmospheric composition. Associating a particular cause with a particular change, however, is extremely difficult.
The interlinked nature of the climate system ensures that there are feedbacks; a change in one component leads to a change in most, if not all, other components. Before investigating some of the more important forcing mechanisms, both internal and external, there is one factor that needs elaborating – time scale.
a. Orogeny:
Orogeny is the name given to the tectonic process of mountain building and continental uplift. Such mechanisms operate only over tens or even hundreds of millions of years. The Earth’s outer surface, a layer known as the lithosphere (made up of the crust and upper section of the mantle), is broken up into about 12 different plates which are constantly adjusting their positions relative to each other.
Such movements are driven by the internal convective dynamics within the Earth’s mantle. When plates collide, one may either be subducted beneath another, or both are pushed continually together, forcing upwards any continental land masses, to form long mountain ranges. The Himalayas formed when the Indian plate crashed into Asia about 20 to 30 million years ago.
b. Epeirogeny:
Epeirogeny is the term used to describe changes in the global disposition of land masses, and like orogenic processes, these changes are driven by internal plate tectonic movements. Because the internal dynamics of the Earth are slow, continents move about the globe at a rate of several centimetres per year. However, over tens or hundreds of millions of years, both the size and position of land area can change appreciably.
At times in Earth history, there have been super-continents in which all the continental plates were locked together in one area of the globe. The last of these occurred about 250 million years ago, and is named Pangea. Since that time, the continents have gradually moved apart, the most recent separation occurring between Europe and North America, during the last 60 to 70 million years. What is now the Pacific Ocean used once to be the vast expanse of water, called the Panthalassa Ocean that surrounded Pangea.
A number of possible mechanisms which forced global climate to fluctuate between “greenhouse” and “icehouse” states have been explored. First, as the continental area occupying high latitudes increases, as a result of continental drift, so the land area with permanent ice cover may expand, thus raising the planetary albedo, forcing (radiatively) a global cooling (the ice-albedo feedback).
Second, the arrangement of continental land masses significantly affects the surface ocean circulation. Since ocean circulation is involved in the latitudinal heat transport regulating global climate, so the wandering of land masses may force (non-radiatively) climate change over times scales involving tens or hundreds of millions of years.
Such long term variations in ocean circulation as a result of continental drift, in addition to orogenic processes, may have accounted for the return to a global “icehouse” that has taken place over the last 40 million years.
Associated with continental drift is the tectonic process of sea floor spreading. New lithospheric plate material is formed at mid-ocean ridges, tectonic spreading centres that mark the boundary between two diverging plates. These sea-floor regions, for example the Mid-Atlantic Ridge, release large amounts of energy and associated greenhouse gases. At times of enhanced tectonic activity and sea floor spreading, elevated levels of greenhouse gas emissions may initiate or augment a “greenhouse” world.
As the newly formed plates diverge, they slowly begin to cool, and as the density of the exhumed rock increases, so the ocean crust begins to subside. During times of increased tectonic activity, spreading rates are faster and the ocean crust has less time to cool and subside. The resulting ocean bathymetry is shallower that it otherwise would be and causes an (epeirogenic) rise in sea level.
During Cretaceous times, mid-ocean ridges were indeed more active than they are today. Consequently, sea levels stood several hundred metres higher (due also to the absence of water-storing ice sheets), covering vast continental areas with shallow-level (epeiric) seas. Such a situation may have two important consequences. First, ocean circulation will be markedly affected, influencing global climate as illustrated above. Second, the large shallow seas, with relatively lower albedos than the land areas which they submerge, would be capable of storing considerably more energy, thus heating the Earth’s surface.
There is now little doubt that the presence of mountain ranges on the Earth can dramatically influence global climate, and that orogenic uplift can act as a non-radiative (internal) forcing mechanism. North-south orientated mountain ranges in particular have the ability to influence global atmospheric circulation patterns, which usually maintain a more east-west trend on account of the Coriolis Force.
Ruddiman & Kutzbach (1991) have proposed that the uplift of the Tibetan Plateau, the Himalayas and the Sierra Nevada in the American south-west may have induced a global cooling during the last 40 million years. Raymo & Ruddiman (1992) also suggest that increased uplift of these regions exposed more rock, thereby increasing the rate of physical and chemical weathering.
During chemical weathering, carbon dioxide is extracted from the atmosphere to react with the decomposing rock minerals to form bicarbonates. These bicarbonates are soluble and can be transported via rivers and other fluvial channels, finally to be deposited on ocean floors as sediment. In essence, carbon dioxide is sequestered from the atmosphere, thereby decreasing the Earth’s natural greenhouse effect, causing further cooling.
In view of this greenhouse feedback, mountain uplift seems to generate both non-radiative forcing (atmospheric circulation changes) and radiative forcing (greenhouse feedback). In such situations as described above, isolating a primary cause of climatic change from its secondary feedbacks, becomes ineffective. Mountain uplift may also increase the land surface area covered by snow the year round. The subsequent increase in planetary albedo will reduce the amount of energy absorbed at the Earth’s surface, initiating further cooling.
c. Variations in Atmospheric Composition:
The changing composition of the atmosphere, including its greenhouse gas and aerosol content, is a major internal forcing mechanism of climate change. As we have seen, the Earth’s natural greenhouse effect (involving an increase in the downward energy flux) plays an important role in the regulation of the global climate. Obviously, then, changes in the atmospheric concentrations of greenhouse gases will modify the natural greenhouse effect, and consequently affect global climate.
Changes in the greenhouse gas content of the atmosphere can occur as a result of both natural and anthropogenic factors, the latter which has received considerable attention in the last 20 years. Mankind, through the burning of fossil fuels, forest clearing and other industrial processes, has increased the amount of carbon dioxide and other greenhouse gases since the eighteenth century.
Natural changes in greenhouse gas concentrations can occur in numerous ways, most often in response to other primary forcing factors. In this sense, as with ocean circulation changes, such forcing should be more strictly regarded as secondary forcing or feedback. Changes in atmospheric CO2 and methane (CH4) have been associated with transitions between glacial and interglacial episodes. Much of the empirical evidence suggests that these changes lag behind the climate signal, and must therefore act as feedback mechanisms to enhance climate change rather than as primary forcing mechanisms.
Changes in the atmospheric content of aerosols, again both natural and anthropogenic can act as climate forcing mechanisms, or more usually secondary feedback mechanisms. Increases in atmospheric turbidity (aerosol abundance) will affect the atmospheric energy budget by increasing the scattering of incoming solar radiation. Atmospheric turbidity has been shown to be higher during glacial episodes than in interglacials, with a consequent reduction in direct radiation reaching the Earth’s surface. Such a situation will enhance the cooling associated with glacial periods.