Excerpt from: “Climate Change and Social Adaptation: how the past can inform the future,” (2010). Author: Diane L. Douglas. Article in press, Ename Center, Ghent, Belgium. Based on a presentation given at the “Climates of Heritage Conservation Colloquim, March 2009. http://www.enamecenter.org/en/collo2009
Variation in earth’s energy balance occurs over long and short time scales in response to multiple variables including changes in: 1) the amount of incoming solar radiation, 2) earth’s reflectivity (albedo), 3) earth’s CO2 cycle, 4) tectonic and volcanic activity, 5) atmospheric and oceanic circulation, 6) celestial mechanics, and 7) solar cycles. The contribution of each of these systems to earth’s energy balance is discussed in greater detail below.
Earth’s Energy Balance
Life on earth would not be possible without the energy received from the sun and the life-giving air of our atmosphere. Incoming solar radiation, the energy received from the sun as short wave radiation (insolation), is in delicate balance with the energy emitted by earth as outgoing long wave radiation (Figure 1).
The average amount of energy received at the top of the atmosphere from the sun is approximately 1,366 watts per square meter (W/m2), with the highest levels of radiation received at the equator. A marked increase or decrease in insolation effects earth’s climate, and is the primary reason earth experiences ice ages (glacial periods) and interglacial periods, such as today. Approximately 19 % of the sun’s shortwave radiation is absorbed by the earth’s atmosphere, and 51% is absorbed by the land and water bodies (Pidwirny 2006a and 2006b); the remaining short wave radiation (30%) is reflected back to space by particles in the atmosphere, clouds, and earth’s albedo.
For earth’s heat budget to remain in balance, the amount of incoming solar radiation absorbed by the atmosphere and earth (70%) must be balanced by the same amount of outgoing longwave radiation (70%). Radiation emitted by the earth is called longwave radiation because it is cooler than the sun and emits longer wavelengths of energy. Approximately 70% of the longwave radiation emitted by earth passes through the atmosphere and escapes to space. The remaining 30% is trapped by water vapor and trace gases in the atmosphere and warms the earth—this is referred to as the greenhouse effect. The greenhouse effect makes life on earth possible, but changes in the amount of trace gases and/or water vapor in the atmosphere can have a marked effect on climate. Higher levels of greenhouse gases trap more longwave radiation and effectively heat earth’s surface; a reduction in the levels of greenhouse gases allows more long-wave radiation to escape, effectively cooling earth’s surface. Earth’s atmosphere is composed of nitrogen (78.8%), oxygen (20.95%), argon (0.93%), carbon dioxide (0.038%), various other trace gases and water vapor (1%).
Albedo refers to the absorptive and reflectivity properties of clouds, the ocean and various landforms to incoming solar radiation. Light colored surfaces such as snow reflect the greatest amount of radiation back to space, whereas dark surfaces like asphalt absorb a lot of solar radiation. Human land use practices affect earth’s albedo because development, deforestation, desertification, agriculture and grazing livestock (a few of many human activities) change the absorptive and reflective properties of earth’s surface. These activities therefore have a direct affect on earth’s energy balance.
Various cloud forms also have different reflective properties, with some absorbing and others reflecting short wave and long wave radiation. Stratocumulus clouds, which are low thick clouds, cool the earth’s surface by reflecting short wave radiation back to space. In contrast, high altitude cirrus clouds warm the atmosphere because they transmit incoming solar radiation to earth’s surface and also reflect outgoing long wave radiation back to earth (NASA 2007). There is seasonal and regional variability in the cooling versus heating effect of clouds on the global energy balance, and NASA scientists note that the effects of low and high altitude clouds on climate generally balance each other out; the cooling effect of low altitude clouds is slightly stronger than the warming effect of high altitude clouds (NASA 2007).
Geographic and temporal variation in the solar radiation received on earth is the primary driving force of atmospheric circulation on earth. Changes in pressure, affected by changes in incoming solar radiation, affects how warm air is distributed from the equator to the poles, as well as how warm and cold air is distributed across continents. In climatology, air density is measured by atmospheric pressure and at sea level mean atmospheric pressure is about 1013 hectopascals (hPa). On top of Mount Everest, roughly 8.85 kilometers (5.5 mi) above sea level, mean atmospheric pressure is about 340 hPa (West 1999). Atmospheric pressure varies temporally and spatially across the globe in response to variations in incoming solar radiation. These changes in atmospheric pressure cause changes in regional weather, including everything from droughts, thunderstorms and tornadoes to hurricanes and monsoon rains.
The majority of the sun’s incoming solar radiation is received at the equator. Warm air is less dense than cold air, and the warm air parcel at the equator rises and is transported pole-ward about 10-15 kilometers above earth’s surface. When the air cools high in the atmosphere, it becomes denser and sinks back to earth’s surface, warming as it descends. This circulation system is referred to as the Hadley cell, which extends from the equator to roughly 30 o N and 30 o S (Figure 2). The Hadley cell circulation pattern sustains a nearly 1000 km wide band of arid landscape in the Southern Hemisphere, spanning roughly 20 to 30 o S. Within this band, are found the great deserts of the Southern Hemisphere: the Australian deserts, the Namib and Kalahari of Africa, and the Atacama of South America (the Patagonia, a cold arid desert of South America, lies to the south of this band). In the Northern Hemisphere, a roughly 2700 km wide band of arid lands extends from roughly 10 to 35 o N. Within this band are found the great deserts of the Northern Hemisphere: the Sahara of Africa, the Thar and Kutch of India, the Arabian Desert of Saudi Arabia, and the Sonoran and Mohave of North America. Smaller, less well known deserts are distributed between these well known deserts of the Northern and Southern Hemispheres. As in South America, cold deserts are found in slightly higher altitudes and latitudes of North America and Asia. The high latitudes of the arctic and Antarctic are also arid and are considered polar deserts.
In addition to temperature differences between the equator and poles driving atmospheric circulation, physical forces associated with earth’s rotation on its axis also affects circulation. The force of earth spinning causes winds in the atmosphere to be deflected away from the equator (Coriolis force). Winds associated with high pressure systems are deflected clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere (NASA 2007). Winds associated with low pressure systems are deflected counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The Coriolis force also results in the formation of earth’s prevailing winds: the Westerlies, Polar easterlies, Doldrums, Horse latitudes and Trade Winds.
Cyclonic (high pressure) and anti-cyclonic (low pressure) circulation systems are driven by earth’s larger scale circulation systems and contribute significantly to the distribution of warm air toward the poles, and cold air toward the equator. Low pressure systems form when air diverges in the upper atmosphere, drawing in air from nearer earth’s surface: Air rushes in to fill the void created by a low pressure system, often creating powerful winds before a storm. As the air rises it cools and condenses creating clouds and precipitation (rain and at times hail). When extreme low pressure systems form over warm-ocean water, the surrounding warm, moist air spirals in generating hurricanes and cyclones, such as hurricane Katrina (Handwerk 2005) and tropical storm Kammuri (Hong Kong Observatory 2008).
Intertropical Convergence Zone (ITCZ)
Seasonal changes in the location of the solar zenith (affect of earth’s tilt causing the greatest amount of sunshine to be received in June in the Northern Hemisphere and in December in the Southern Hemisphere), causes a shift in earth’s large scale pressure systems (i.e. the Hadley cell). This shift is greatest near the equator creating the Intertropical Convergence Zone (ITCZ) (Figure 3).
The ITCZ is an area of low pressure that forms where the Northeast Trade Winds meet the Southeast Trade Winds near the earth’s equator. As these winds converge, moist air is forced upward—as the air cools it condenses and creates a heavy band of precipitation. In response to changes in the location of the solar zenith, the ITCZ shifts from approximately 25 o N in Northern Hemisphere summer (June-July-August) to 20o S in Southern Hemisphere summer (November-December-January); the shift in the ITCZ causes heavy rains in each hemisphere, and intensifies monsoon systems (Chao 2000).
Like the atmosphere, oceanic circulation distributes heat from the equator to high latitudes in currents driven by earth’s prevailing winds, the Coriolis force and differences in the salinity (density) of the water (Gross 1993). Because large scale ocean circulation is driven by differences in temperature and salinity it is referred to as thermohaline circulation. The upper 400 meters of the ocean is also driven by prevailing winds in the atmosphere and wind driven currents are critical to the movement of warm equatorial surface waters to higher latitudes. The Gulf Stream, originating in the Gulf of Mexico is the largest surface current on earth (NOAA 2009a). The Gulf Stream carries warmer and less saline water than the waters of the Atlantic Ocean along the eastern coast of North America to northern Newfoundland. At this latitude, the current is deflected east by the Coriolis force and flows to the mid-Atlantic ocean (Figure 4).
West of Ireland, the current splits with one branch flowing north and another south. The northern branch becomes a broad, shallow swath of warm water called the North Atlantic Drift Current (NADC), which travels to the North Atlantic and moderates the climate of Europe and western Scandinavia. The southern branch of the current joins the southward flowing Canary Current, which is a wide, slow moving current that flows south to Senegal where it is deflected west (Rossby 1996 and 1999).
A similar circulation system occurs in the Pacific Ocean. The Kuroshio Current is akin to the Gulf Stream; it originates near Taiwan and flows north along the coast of Asia carrying warm water to the North Pacific Ocean. North of Japan, the current is deflected and becomes a broad swath of warm water moving east. In the eastern North Pacific Ocean the current splits into a northern and southern branch. The northern branch, the Alaska Current carries warm water north and the southern branch, the California Current, carries cooler water south.
The surface and deep water currents of the oceans form the Meridonal Overturning Circulation (MOC) system. Surface currents are driven by wind, whereas deep ocean currents are driven by differences in temperature and salinity. In the North Atlantic, the warm surface water condenses when it encounters colder air causing seasonal fog, rain and snow in Europe and Scandinavia (a similar system exists in the North Pacific Ocean). The colder, highly saline water left behind is dense and sinks to depths of 1500 to 2500 m (4900 to 8200 ft), forming the North Atlantic Deep Water (NADW) current. The NADW flows south along the coasts of the Americas finally deflecting east near the tip of South America to join the Antarctic Circumpolar Current (ACC). The ACC flows east along the northern edge of Antarctica until it splits into two branches off the tip of South Africa. One branch flows north into the Indian Ocean and another continues east along the Antarctic coast until it reaches the Pacific Ocean. Here the current deflects north and flows northward off the New Zealand coast and ultimately into the North Pacific Ocean (Smith et al. 2008).
In the North Pacific, this cold, deep current gradually mixes with overlying warmer surface water and forms into a surface current that is deflected to the southwest and into the Indonesian Ocean. The current continues a westward path, with branches entering the Indian and Atlantic oceans. When it enters the Atlantic Ocean, the current is deflected north and ultimately returns to the North Atlantic Ocean where it once condenses when it contacts colder air and water, becomes more saline and sinks to join the NADW of the conveyor-belt cycle. Oceanic models indicate it takes approximately 1000 years for a parcel of water within the conveyor belt to complete one cycle from the North Atlantic Ocean to the North Pacific Ocean and back to the North Atlantic Ocean (Doney et al. 2006).
Changes in Atmospheric and Oceanic Circulation
Changes in the volume and temperature of the Gulf Stream and Kuroshio Current can cause significant changes in regional weather in the North Pacific Ocean and North Atlantic. Seasonal changes in regional sea surface temperature as well as shifts in the location and intensity of surface ocean currents and deep water upwelling cause changes in regional and global climate (Broeker 1999; Little et al. 1997; Doney et al. 2006; Randall et al. 2007; NOAA 2009b). These are referred to as coupled ocean-atmosphere circulation systems and shifts in these systems can have dramatic effects on the intensity and duration of droughts in some regions and storms in others. Systems that are tracked by NOAA include the: Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) (Nie et al. 2008, NOAA 2006c); El Nino/Southern Oscillation (ENSO) (Nicholls 1992, NOAA 2006c), Pacific North American (PNA) Oscillation and Antarctic Oscillation (AAO) (NOAA 2006c).
The location and intensity of weather patterns associated with each of these systems is driven by the location and intensity of the high and low pressure systems that drive them (Randall et al. 2007; NOAA 2009c), as well as the influence of surface currents and NADW (Schiller, Mikolajewicz and Voss 1997; Bischoff, Mariano and Ryan 2003). For example, the strength of the NAO is directly affected by the warmth and volume of the Gulf Stream current (Saunders and Qian 2002; NOAA 2009a). Variation in the NAO effects winter weather in Europe and the UK (Saunders and Qian 2002). A moderately negative NAO leads to colder, drier, and calmer winters than average in northwest Europe and warmer and wetter winters in southern Europe (Saunders and Qian 2002). A moderately positive NAO results in wetter, stormy winters than average in northwest Europe, and cold, dry winters in southern Europe (Saunders and Qian 2002).
Like the NAO, variation in ENSO can significantly influence weather patterns in different regions. Warm ENSO events (El Niño) occur when sea surface temperatures (SST) are unusually warm in the central and east-central equatorial Pacific (Figure 5).
El Niño events typically result in above average precipitation in California and Mexico and drought in eastern Australia (NOAA 2009c). Cool ENSO events (La Niña) occur when SST in the equatorial pacific are unusually cool and result in above average precipitation in the Northwest of North America, dry conditions in California and Mexico and increased rains in eastern Australia
Earth’s CO2 Cycle
Earth’s natural CO2 cycle is extremely complex, with multiple factors influencing the amount of CO2 in the atmosphere and ocean at any given time in earth’s history. These factors range from the amount of plant life available to convert CO2 to O2; changes the pressure differential between the atmosphere and the ocean; algae blooms; variance in volcanic activity (terrestrial and deep ocean); the release of CO2 from thermal vents (terrestrial and deep ocean); the release of CO2 along fault lines during an earthquake, among others. The influence of these factors on earth’s natural CO2 cycle is discussed below.
Photosynthesis (Land and Sea)
Energy from the sun drives photosynthesis in all land plants, algae and some types of bacteria (Whitmarsh and Govinjee 1999). Photosynthesis converts CO2 from the air and water (H2O) into O2 and carbohydrates (C6H12O6); O2 is utilized by all oxygen consuming organisms on earth (Whitmarsh and Govinjee 1999,11). Absorption of CO2 by planktonic foraminifera in the upper layers of the oceans removes approximately 2 x 1015 grams of carbon per year from the atmosphere (Whitmarsh and Govinjee 1999). The process is initiated by plankton photosynthesizing CO2 into organic carbon, as well as CO2 and H2O reacting to form HCO3 (carbonate ions) which is absorbed by marine organisms to form shells of calcium carbonate (CaCO3). As these organisms die, they sink to the bottom of the ocean where they settle into the sediment (Whitmarsh and Govinjee 1999). The ocean contains approximately 19 and 50 times more CO2 than the terrestrial biosphere and atmosphere, respectively (Sabine 2008). In Polar waters, CO2 absorption is highest during the summer when the Arctic regions are blanketed in flora; absorption decreases during winter because of ice cover (Tans, Fung and Takahashu 1990).
Atmospheric and Oceanic CO2 Pressure Differential
CO2 is absorbed by the upper layers of the ocean when the CO2 gas pressure (pCO2) in the atmosphere is higher than the p CO2 in the ocean’s upper layers (Sabine 2008). In high latitudes, CO2 flows from the atmosphere into the ocean, and in the tropics CO2 is released from the ocean to the atmosphere through evaporation (Glushkov, Khokhlov and Loboda 2005). Large shifts in CO2 concentrations in the oceans occurs horizontally and vertically as a result of the slow rate of mixing by water volume; the concentration of dissolved CO2 is approximately 10 % higher by volume in the deep ocean than it is at the surface (Sabine 2008).
The CO2 Balance: Tectonics and Volcanism
Shifts in the location of tectonic plates affects atmospheric and oceanic circulation which affects climate and the CO2 balance; small scale movements (earthquakes) also affect earth’s CO2 balance. Seismic slipping along fault lines can cause supersaturation of CO2 in the melted sediment/rock. Infrared analysis of friction induced melts in rock, caused by seismic slipping along the Nojima fault during the 1995 Kobe earthquake (Japan), indicate 1.8 to 3.4 thousand tons of CO2 were released by the earthquake (Famin et al. 2007). Similarly, geochemical analysis of water from springs located near active faults in the Gulf of Corinth, Greece (Pizzino et al. 2004) revealed high levels of dissolved CO2 for 1 to 10 months following earthquakes (Famin et al. 2007). Correspondingly, it may be anticipated that during periods when tectonic activity is high, CO2 is emitted into the atmosphere and oceans from the rapid release of gases from rocks and sediment during the event.
Volcanism also affects the amount of CO2 in the atmosphere, but volcanic explosions generally lead to cooling rather than heating because of large emissions of sulfur dioxide (SO2). When Mount Pinatubo erupted in 1991, 14 to 26 million tons of SO2 were released to the atmosphere. The SO2 combined with the haze created by the volcanic ash and dust cooled the atmosphere by 0.5oC for a period of 12 to 18 months.
Active volcanic regions, however, release CO2 to the atmosphere through volcanic vents (fumaroles) and porous soil, even when the volcano is quiescent (USGS 2008). For example, Ajuppa et al. (2004) studied rates of CO2 degassing on the flanks of the Somma-Vesuvius volcano and found that soil CO2concentrations ranged from 50 to 10,500 ppmv. In another study, Favara et al. (2001) analyzed the amount of CO2 degassing from soils and vents on Pantelleria Island, an active volcanic complex, to determine how much CO2 is emitted from the soils, water and fumeroles on the island. These scientists found that 1.79 million metric tones of CO2 was degassing from the soil, water and fumeroles on the island.
The emissions of SO2 and CO2 from terrestrial and marine volcanoes are significant sources of trace gases into the atmosphere. Because of their threat to human life, as well as their impact on climate, terrestrial volcanoes are extensively studied (USGS 2008). However, 70 % of volcanic activity occurs along deep sea rifts located over one mile below the surface of the ocean, and the effect these marine volcanoes have on earth’s climate is poorly understood (NOAA 2009d). Because deep sea volcanoes have a significant impact on the global chemical and heat balance, the NOAA established a program dedicated to studying deep sea hydrothermal systems: Pacific Marine Environmental Laboratory (PMEL NOAA 2009d). In 2004, PMEL scientists studying hydrothermal vents along the Mariana Arc in the North Pacific Ocean discovered a vent emitting a wide stream of effervescent bubbles (Butterfield 2005; Lupton et al. 2006). They captured a sample of the liquid in containers and brought these to the surface for analysis. Butterfield (2005) and Lupton et al. (2006) identified 2.3 moles (101g) of gaseous CO2 per liter of hot water recovered from the vent. The rate in which CO2 is released from one section of the vent over a period of one day may be comparable to the rate of flow from a water hose. It is estimated that 30 liters of water flows from an average backyard water hose (1.6 cm diameter) every minute. If the amount of CO2 emitted from a 1.6 cm2 section of the hydrothermal vent was constant for 24 hours this area would emit 4,300 metric tonnes of CO2 in one day. If a single 1.6 cm2 area of a vent released liquid CO2 year round, 9 million metric tonnes of CO2 would be released from this small area in one year. Given there are almost 22,000 kilometers (~12,000 miles) of volcanic arcs along the ocean floor that are not well studied, this system could be contributing significant levels of CO2 to the ocean and atmosphere.
Long Term Cycles of CO2 Variation
Throughout earth’s history, levels of CO2 in the atmosphere has varied, ranging from over 3000 parts per million by volume (ppmv) to about 100 ppmv (Pagani et al. 2005). Extreme variations in atmospheric CO2 have occurred in response to natural geologic and biological processes (e.g., Pearson and Palmer 2000; Berner and Kothavala 2001; Obzhirov et al. 2004; Cardellini, Chiodini and Frondini 2003; Pagani et al. 2005; Spence and Telmer 2005; Gurrieri et al. 2006). During the late Paleocene and early Eocene, roughly 60 to 50 million years ago, atmospheric levels of CO2 ranged from 1000 to 1500 ppmv, and during this period, temperate forests extended to the poles and tropical climates extended to 45o N latitude (Pagani et al. 2005). Today only herbs, grasses, forbs, sedges and stunted trees grow at high latitudes due to lower insolation levels, and arid grasslands and temperate forests grow where tropical forests once thrived. The high levels of atmospheric CO2 during this period are attributed to volcanism in East Greenland (Pagani et al. 2005) and potentially a massive release of methane from the disturbance of organic matter buried on the sea floor as the result of an earthquake or mudslide (Venere 2006).
This period in earth’s geologic history is known as the Paleocene-Eocene Thermal Maximum (PETM). The high CO2 levels of this geologic time in earth’s history, combined with increased CO2 levels during the PETM led to acidification of the ocean, a 6 o C to 7 o C rise in high latitude temperatures and contributed to the extinction of dinosaurs (Storey, Duncan and Swisher 2007). Over the next several million years, atmospheric CO2 gradually decreased with this trace gas reaching modern levels by the late Oligocene (33.7 to 23.8 million years ago). Retallack et al. (2004, 817) note that several factors may have contributed to this decrease in atmospheric CO2 including changes in oceanic circulation associated with movement of continental plates, weathering associated with mountain building, and the expansion of grassland and grazers into formerly forested areas. Others note that a massive bloom of water ferns (Azolla) and subsequent die off resulted in a drawdown of atmospheric CO2 from roughly 3000 ppmv to 650 ppmv, as the dead ferns sank to the sea floor and became incorporated into deep sea sediments—known as the Azolla event (Dickens, Castillo and Walker 1997; Pagani et al. 2005). The expansion of temperate forests during the Miocene (23.03 to 5.33 million years ago) resulted in a further drawdown of atmospheric CO2 from 650 ppmv to 100 ppmv. After this period, earth’s plates continued to shift and mountains formed in New Zealand and Europe affecting atmospheric and oceanic circulation. During the Miocene earth began to experience ice ages, but the extreme glacial events of the Quaternary period (starting 1.8 million years ago) would not occur for several million years. Atmospheric CO2 is typically lower during glacial periods than during interglacial periods because ice sheets cover areas that are blanketed with flora during interglacial periods.
The astronomical theory developed by Milutin Milankovitch in the early 1900s is widely accepted as the best theory for explaining glacial and interglacial cycles as well as changes in the seasons (Milankovitch 1941; Kerr 1987). Milankovitch identified seasonal and longer-term systematic variations in earth’s orbit around the sun based on meticulous measurements on the position of stars and equations that explained how the gravitational pull of other planets and stars affected earth’s orbit. Through his calculations, Milankovitch identified three primary celestial mechanisms that affect long-term cycles of cooling and warming on earth including eccentricity, precession of the equinox and obliquity (Figure 6).
Eccentricity refers to a systematic shift in earth’s orbit around the sun from an eclipse to a circular pattern and back to an eclipse every 95,000 to 136,000 years, averaging 100,000 years. The shape of earth’s orbit around the sun affects how close the earth is to the sun on its annual cycle. When earth’s orbit is circular, its distance from the sun is relatively steady throughout the year, minimizing extreme seasonal variation. When earth’s orbit is eccentric, seasonal variation in temperature is more extreme. In an eccentric orbit, the earth is closest to the sun at perihelion and farthest from the sun at aphelion. If perihelion occurs during northern hemisphere summer this accentuates the length and intensity of the summer season. If aphelion occurs during northern hemisphere summer, the season is shorter and less intense.
Precession refers to a wobble of the earth in its orbital path around the sun that occurs every 19,000 to 23,000 years. This wobble affects the point when earth’s orbit is nearest the sun (the location of perihelion), and shifts from the northern to the southern hemisphere approximately every 9,500 to 11,500 years. Changes in earth’s orbital cycle (eccentric or round) accentuate the affect of precession on hemispheric warming and cooling (Imbrie and Imbrie 1980; Bradley 1985; Kutzbach 1987).
Obliquity refers to the earth rotating on an axis that is tilted at an oblique angle to the plane of ecliptic; the angle of earth’s tilt varies from 22.3 o to 24.5 o every 41,000 years. This tilt is what causes the seasons; northern hemisphere summer occurs when the North Pole is tilted toward the sun, and southern hemisphere summer occurs when the South Pole is tilted toward the sun. Seasons can be accentuated or modified by eccentricity and precession.
Astronomical theory suggests that cyclic changes in earth’s orbital parameters accentuates seasons and contributes to the onset of glacial and interglacial periods. Examination of the mathematical components of eccentricity, obliquity and precession reveal several components that suggest periodic movement of a wave in a fixed frequency and wavelength (Imbrie and Imbrie 1980). As such, changes in earth’s orbital parameters can be calculated over the past several million years as well as into the future (Imbrie and Imbrie 1980). The mathematical certainty of earth’s orbital parameters allows climate scientists to use the calculations to reconstruct the timing of glacial and interglacial periods, as well as predict future climate driven by celestial mechanics (e.g., Ruddima, Vavrus and Kutzbach 2005; Stuckless and Levitch 2007). However, Imbrie and Imbrie (1980) caution that celestial mechanics explain only 25% of climate change, noting that other natural forcing mechanisms and feedbacks drive 75 % of climate change.
Long Term Climate Cycles
Cyclical changes in earth’s orbit around the sun are the primary forces that drive glacial and interglacial periods. However, several natural forcing mechanisms affect the timing and intensity of the onset and end of each glacial and interglacial period. Little et al. (1997) speculated that at different times in the past, intensified trade winds forced a higher volume of warm tropical and subtropical waters across the equator and into the Gulf Stream. Trade winds may become more intensive near the end of an interglacial as the temperature of the water near the equator increases, warming the overlying air and causing it to rise, creating storms that result in more precipitation at low latitudes. Several scholars have speculated that these dynamics would result in an influx of warm water into the North Atlantic; when this water cooled and condensed it would result in increased precipitation/snow in high latitudes, enhancing the growth of ice-sheets and potentially contributing to the onset of ice ages (Broecker 1999; Little et al. 1997; Clark et al. 2002; Manabe and Stouffer 1988, 1997; Manabe et al. 1991; Vellinga and Wood 2002; Stouffer and Manabe 2003). Further, the constant influx of less-saline, warm water from the Gulf Stream would eventually dilute the cold, highly-saline water of the North Atlantic, potentially causing the MOC to shut down and the waters of the Gulf Stream to pool. A concentration of warmer, less saline water would result in higher levels of precipitation and stronger storms in Western Europe and Scandinavia. This precipitation and cooler temperatures would enhance the growth of alpine glaciers and ice sheets in these regions. Additionally, because fresh water freezes at a higher temperature than salt water 0 o C versus -1.9 o C, respectively, sea ice could form more readily as a result of an influx of less saline water to the Arctic via the Gulf Stream (see for example Frey, Smith and Alsdorf 2003).
Sunspots are caused by the differential rotation of the sun: The sun’s poles rotate slower than the sun’s equator, causing its magnetic field to become distorted and twisted over a period of years (Bergman 2005). Distorted magnetic lines within the sun, twist like rubber bands and eventually break through the visible surface of the sun—the photosphere. When these magnetic lines break through, they appear as dark spots on the sun’s surface; these planet-sized spots are about 1,800 o C or 3,300 o F cooler than the surrounding surface of the sun (Bergman 2005). Magnetic activity around sunspots is extremely high and can result in the formation of solar flares and coronal mass ejections, or solar storms.
Sunspots can last from days to months, and vary in intensity: The number of sunspots that can be observed on the sun’s surface varies from year to year, following a cycle that averages about 11 years. This 11-year cycle is referred to as the Schwabe cycle, after the first scientist to publish his observations of sunspot cycles recorded from 1826-1843 CE (Bergman 2005). Sunspot activity is greatest near the sun’s equator, extending roughly to mid-latitudes (Hathaway 2010). During each 11-year cycle, sunspot activity is usually more intense in one hemisphere than the other, reversing in the subsequent 11-year cycle—this is often called the “butterfly effect,” and is referred to as the 22 year Hale cycle (Hathaway 2010).
In addition to the 11 and 22-year sunspot cycles, several years may pass when the amplitude and frequency of sunspots are higher or lower than average—these periods are referred to Grand Maxima and Grand Minima, respectively. These cycles may range from 70 to 300 years and are believed to have affected some of the more significant short-term climate cycles on earth. For example, the Medieval Warm Period (MWP) (ca. 950-1250 CE) is believed to have been caused by a roughly 300-year period of higher sunspot activity (Eddy 1977a; Hathaway 2010; Usoskin 2010). In the northern hemisphere, temperatures were 1 to 1.5 o C warmer during the MWP than temperatures of the mid to late 1900s (Keigwin 1996; Douglas 1998; Raymond, Hughes and Diaz 2003); alpine glaciers receded and much of the Arctic ice pack melted leaving the North Atlantic open to exploration (Lamb 1977; Grove 1990; Burroughs 2007). Conversely, the Maunder Minimum (ca. 1645-1715 CE) of the Little Ice Age (LIA) (ca. 1450-1850 CE) is believed to have been caused by a period of extremely low sunspot activity (Hathaway 2010; Usoskin 2010). Temperatures during this period were 1 to 1.5oC cooer than the mid to late 1900s, alpine glaciers advanced and the Arctic ice pack increased in its extent exploration (Lamb 1977; Grove 1990; Burroughs 2007).
Eddy (1977a and 1977b) compared the historic record of sunspot activity against a 7500-year long fossil tree-ring record and identified a significant correlation between sunspot activity, global temperature and glacial dynamics. Eddy identified 18 periods on earth when solar activity was directly correlated with either cooler or warmer temperatures on earth, like the MWP and LIA. In every instance, warmer periods and receding glaciers were correlated with periods of high solar activity. Similarly, periods of low solar activity exhibited a significant correlation with cooler temperatures on earth, and the advance of glaciers. Bergman (2005) notes that since around 1900 CE, the amplitude of the sunspot cycle has been higher than it was for preceding five centuries, indicating that the sun is in a Grand Maxima cycle similar to the MWP.
In an effort to understand how longer term cycles of climate change (driven by celestial mechanics) and shorter term cycles of climate change (driven by solar physics and other natural forcing mechanisms) affect different regions of the globe, climate scientists have developed complex models.
links to publicly available information on natural causes of climate change: