**3. Movements of heat away from the Tropics**

The fact that the bulk of the solar radiation arrives on the surface of the Earth along the zone between the Tropics of Capricorn and Cancer results in a tremendous imbalance of heat distribution between the Equator and the Poles. The amount of solar heating of the polar latitudes throughout the\year varies greatly, with the polar latitudes receiving considerably more solar energy in summer than in the winter when they receive no solar heat at all. As a result, in the winter hemisphere, the difference in solar heating between the equator and that pole is very large. This causes the large-scale circulation patterns observed in the atmosphere. The difference in solar heating between day and night also drives the strong diurnal cycle of surface temperature over land.

The seasonal imbalance results in about 30% of the heat absorbed in the Tropics currently moving towards the polar regions each year to partially make up the difference. As will be shown below, the effectiveness of the processes carrying out this transfer depends on the arrangement of the land masses and oceans as well as the connections between these two contrasting regions. Antarctica is situated over the South Pole and has a circular land mass with few indentations. The main exception is the Antarctic Peninsula that projects northwards into the path of the Antarctic Circumpolar Drift (**Figure 3**). This contrasts with the North Pole situated in a

*Causes and Mechanisms of Global Warming/Climate Change DOI: http://dx.doi.org/10.5772/intechopen.101416*

#### **Figure 3.**

*Ocean currents, red being warm and blue being cold. The Gulf Stream/North Atlantic currents move the greatest amount of warm water, with the Kuroshio current moving the second-most amount. Both move north towards the north pole, but there are no comparable currents heating the shores of Antarctica.*

generally frozen sea that is connected by north-south extending seas (gateways) to the tropical oceans ([14], **Figure 1**). In the northern hemisphere, there are large continents separated by oceans from one another. This distribution of land and sea causes a tremendous difference in solar warming of both land and sea as well as the transport of heat towards the poles. Without the ocean gateways, the northern polar areas would be as cold and inhospitable as Antarctica.

#### **3.1 Agents of heat transfer around the Earth**

There are two mediums for the transfer of large quantities of surface heat around the Earth, *viz*., the water in the oceans and the air in the atmosphere. However, the thermal properties of the water make it far more effective in moving heat towards the higher latitudes. Dry air is not nearly as effective in moving heat, but it can transport heat over the surface of land masses. Hot, humid air is intermediate in transporting power since it contains up to 5% water vapor in extreme cases.

#### *3.1.1 Transport of heat by water*

Although water is confined to the lower levels of the globe except for lakes, its thermal properties make it a very important transporting medium for heat, *e.g.,* in bringing heat from central boilers to the houses in many cities in Russia and for cooling engines in automobiles, etc. Water has a very high heat capacity (4.187 mJ/ m3 K) so that it can store or transport large quantities of heat in a given volume of water [25]. In addition, it absorbs over five times as much heat as soil or rock since it is translucent. Currents, convection, and wave action mix the water whereas transmission into a rock or sediments must be by conduction. Thus, ocean currents transport an enormous amount of water polewards (**Figure 3**), primarily in the northern hemisphere where the Gulf Stream and North Atlantic currents transport heat to north Greenland. Note the five ocean gyres at tropical and subtropical latitudes in the Atlantic, Pacific, and Indian Oceans. These accumulate large quantities of heat in the upper layers of the seas, spawning the monsoons and tropical cyclones that move towards the poles.

**Figure 4.** *The thermohaline circulation (THC) and the salinity of the surface waters (NASA).*

Research during the past decades [26, 27] has shown that the North Atlantic Current is part of a vast system of fast-moving, deep thermohaline currents (THC) that moves heat down to the southern hemisphere and forms a global thermohaline circulation system (**Figure 4**). Periodically, the cold surface waters northward water off the coast of North America goes south to be replaced by cold water from Antarctica. Cold Antarctic surface waters must move north to replace this southerly current. This is believed to result in rapid cooling in Greenland and the Arctic regions whereas gradual warming take place in Antarctica [28–30]. This exchange was named the "bipolar see-saw" by Broecker [4].

The deep-sea thermohaline current goes south to the sea surrounding Antarctica via the Atlantic and its effects are discernible all the way to the North Pacific. This circulation pulls warm surface seawater north via the Atlantic because more water is needed to replace the dense, increasingly saline seawater that has sunk towards the ocean floor and subsequently participated in the conveyor belt. The total flow of the larger north-south exchange system is thought to be about 16–20 Sv, where Sv is a unit of ocean current flow, 1 Sv = 1 million m3 /s (million cubic meters per second).

During studies of the apparent warming on land in northeastern North America, it was found that about 60% of the energy increase was being stored in the adjacent seas. Levitus *et al*. [30] summarized the evidence showing that the warming was extending down to about 2000 m. Suddenly, the warm water disappeared [31], but was subsequently found having sunk and moved [32].

For the seawater to reach the great density required for it to become deep-sea water, the surface seawater must increase its salinity. This occurs when cold, dry Arctic air moves over the Arctic seas and sea ice, evaporating water, so concentrating the salts in the remaining waters. Deep-sea thermohaline water that forms in the northern oceans flows south over the ridge between Greenland and Iceland, Iceland and the Faroes, and between the Faroes and Scotland (**Figure 5**). Examination by a deep-sea array of sensors shows that the location of the sinking of the saline waters is no longer close to its source area near Newfoundland but has moved further east, perhaps due to an increase in movement of the cold Labrador current southwards [32]. The new path is shown in **Figure 5**. The second array of sensors at 25°N is showing a slowing of the Meridional flow at that location [33]. Galaasen *et al.* [34]

*Causes and Mechanisms of Global Warming/Climate Change DOI: http://dx.doi.org/10.5772/intechopen.101416*

#### **Figure 5.**

*New location of the saline North Atlantic water [32]. Blue is the return flow of deep warm water while the warm sinking waters are in orange.*

reported that there was a rapid reduction in North Atlantic deep water (NADW) during the peak of the Last Interglacial Period (Eemian), and this may be what is happening again now.

Examination of the oxygen isotope variations in the skeletons of foraminifera accumulating on the sea floor shows an intricate pattern of change during the last 3.5 Ma B.P. (**Figure 6**). There are over 100 changes representing over 50 major cyclic fluctuations of ocean surface temperature. However, there are far fewer major cold events on land during that period [38]. This means that there must be at least two different temperature cycles operating simultaneously. The first is this oceanic heating and cooling cycle. This marine cycle produces glaciations with a periodicity of about 100 ka during the last 800 ka B.P. [39] although the exact height of the individual peaks varies somewhat. From c. 800 ka to 2.6 Ma, the cycles occurred every 41 ka but were of lower amplitude [24]. From 2.6 Ma until at least 3 Ma, this cycle was even smaller in amplitude and more frequent during the much warmer climates. These cycles may be related to the bipolar seesaw [4, 40]. Currently, we appear to be at or near the top of a warming cycle in the North Atlantic.

These cycles must be accompanied by rising and falling sea levels due to the expansion and contraction of the seawater due to their temperature changes and there would be the associated degassing of carbon dioxide into the air during the warming phases, however, the gas would re-enter the water during the cooling phase of these cycles. This would result in fluctuating contents of carbon dioxide in the air, which would change in tandem with the air temperature except for a minor delay. The exact cause of these marine temperature cycles and their fluctuations over time is nonproven but likely to be related to the export of warm (c. 18°C) North Atlantic deep water (NADW) in the high-speed hydrothermal bottom currents (THCs) to the South Atlantic (Antarctic Ocean) to be replaced by cold surface water from that area. The Antarctic cold water would cool Greenland and the Arctic regions in the north very quickly so that glaciers are thought to have developed in eastern Greenland within 12 years of the exchange. This contrasts with the warm water, which would warm the main Antarctic ice cap very slowly [28]. Heat would build up in the surface layers of the NW Atlantic Ocean until the next exchange, hence the name "bipolar see-saw" [4].

#### **Figure 6.**

*Oxygen isotope palaeotemperature record [24, 35] and geomagnetic polarity timescale [36]. Black and white areas are normal and reversed polarity respectively. The arrow at the top indicates the mean Holocene oxygen isotope value. Numbers on the peaks and troughs are the isotope stages (modified from [37]).*

The third set of cycles is seen in the ice cores from both the Greenland and Antarctic ice sheets. Seventeen of the smaller temperature cycles occurred in these between 65 ka and 5 ka B.P., spaced irregularly between 1.2 ka and 5 ka B.P. apart [41] during the first 14 peaks in **Figure 6**. They show an abrupt temperature change in Greenland cores but a gradual adjustment in the corresponding Antarctic ice cores, which also fits with the bipolar seesaw hypothesis. Thus it appears that the heating and cooling cycles experienced in the environs of the North Atlantic Ocean and eastwards through western Europe are part of the movement of part of the net solar energy from the Northern Hemisphere to the coastal regions of North America, the western European land mass and the Southern Hemisphere to partly warm those regions, preventing them from becoming more frigid than at present.

## *3.1.2 Transport of heat in the atmosphere*

Dry air has a heat capacity of 0.00125 MJ/m3 K and a thermal conductivity of 0.024 W/m K [25]. However, it contains variable amounts of water, *e.g.*, 2–3% on average at latitudes 50–60°N in the eastern Cordillera of the Rocky Mountains, under 1% in deserts, and up to 6% in Monsoons and tropical cyclones. It is the water content that determines the ability of the air to carry substantial quantities of heat, which it can subsequently unload as rain or snow. When an air mass moves over a water body such as a sea, it will increase its moisture content until the relative humidity reaches 100%, leaving the seawater more saline. Wet air changes temperature more slowly than dry air when rising over mountains since the heat given out by the condensing water vapor prevents the air from cooling as fast as unsaturated air. Dry, descending air warms more quickly as it descends producing the chinook/ foehn effect, tending to dry and heat the ground over which it passes.

Unlike seas, the air masses can move in any direction over land or water bodies to places with lower air pressure. Wind speed depends mainly on the pressure gradient, which is also influenced by its temperature. Heavier gases such as carbon dioxide tend to collect in depressions. Lighter gases such as helium, hydrogen, and methane have low molecular weights and become lost to space with time. Those with the lowest molecular weights are lost most rapidly. The air becomes colder and decreases in pressure with altitude by expanding except where inversions occur. At higher elevations, there are fewer gas molecules to absorb incoming radiation.

The rotation of the Earth results in weather systems moving eastwards around the globe except at the Equator. It also causes moving air and water to slowly swing right in the Northern Hemisphere and left in the Southern Hemisphere due to the coriolis force.

There are several main climatic systems that transport large amounts of heat onto the nearby land areas, *viz*., tropical and subtropical monsoons, hurricanes/ typhoons, and air masses.

#### *3.1.2.1 Tropical and subtropical monsoons*

Monsoons are one of the dominant modes of heat transport in the Tropics [42]. The tropical monsoons originate over seas with surface temperatures above 27°C. They are the classic monsoons that bring enormous quantities of heat from the ocean gyres near the equator onto tropical landmasses during part of the year. The climate of these landmasses includes a marked dry season when the deciduous trees drop their leaves and the bare ground heats up enormously. These have a separate climatic regime to that moving heat to the Antarctic Seas. The main one is the Indian Monsoon that brings enormous amounts of rain to the Indian subcontinent in June–September. Large areas of western and central India receive more than 90% of their total annual precipitation during the period, while southern and northwestern India receive 50–75% of their total annual rainfall from the monsoons. Kathagat *et al.* [17] point out that the variations in the Indian Monsoon precipitation and air temperature are closely correlated with the success of the various Indian empires over the last 4600 years, with the higher precipitation correlating with periods of success. The main controlling factors are the degree of heating of the surface waters of the Gyre in the Indian Ocean and the presence or absence of either La Niña, which produces more precipitation, or El Niño, correlated with shorter periods of precipitation ending in drought [42]. Borah *et al.* [43] report that a cold anomaly in the North Atlantic can cause drought by a steep decline in late-season rainfall in India. The Indian monsoon cools the lower land but brings heat and moisture to the upper part of the Himalayan Mountains. The hot high-pressure cell over Tibet in summer aids the upper levels of the Monsoon to cross those mountains to deposit

snow on the high peaks and ridges and then the air descends 4000 m like a chinook to the deserts of North China. Its main effect is to bring dry heat from just south of the equator northwards into the semi-deserts and deserts of south-central Asia.

An offshoot of the Indian monsoon is found in North Australia and represents the southward movement of Monsoon weather during the Australian summer. It is the main source of moisture for northern Australia and is related to the eastern section of the Indian Monsoon affecting the area west of Borneo and north to the Philippines. It represents a method of limited relocation of heat energy southwards over the adjacent seas rather than a substantial poleward movement of energy, unlike the situation in Tibet.

The second tropical monsoon is the West African Monsoon affecting primarily the west coast of Africa south of the Sahel deserts, starting at c. 10°N–18°S. The heat comes onshore from the seasonal shifts of the Intertropical Convergence Zone (ITCZ) and produces the great seasonal temperature and humidity differences between the Sahara and the equatorial Atlantic Ocean. The ITCZ migrates northward from the equatorial Atlantic in February, reaches western Africa on or near June 22, then moves back to the south by October. Various factors control the monsoon variability including the variability of ocean sea surface temperature, continental land surface conditions, and atmospheric circulation. It does not move energy polewards but offsets the variations in the extent of the Sahara.

The East Asian summer monsoon is subtropical and develops as the trade winds modified by the Coriolis Force pick up moisture from the China Sea and the western Pacific Ocean. It provides moisture to southern Japan and the eastern shores of China (**Figure 7**). It ceased during the last glacial maximum (30–19 ka, [37, 45]) due to the lowered sea level leaving much of the South China Sea as land [46]. It is affected by the Tibetan Plateau high-pressure cell in Spring, drawing the moist, hot air upslope onto the NE shoulders of the Qinghai-Tibet Plateau [47]. Currently, the temperature increase on the adjacent land is decreasing the extent

**Figure 7.** *Area affected by the East Asia monsoon (from [44]).*

#### *Causes and Mechanisms of Global Warming/Climate Change DOI: http://dx.doi.org/10.5772/intechopen.101416*

of this precipitation [48]. It brings moisture to the coastal areas and both heat and moisture to the higher land areas, which are otherwise semidesert.

The other subtropical monsoon is the North American Monsoon affecting the western and southern margins of the desert areas of Southwest USA and centered in northern Mexico [49]. It is a pattern of pronounced increase in thunderstorms and rainfall over large areas of the southern Cordillera, typically occurring between July and mid-September. During the monsoon, thunderstorms are fueled by daytime heating and build up during the late afternoon-early evening. Typically, these storms dissipate by late night, and the next day starts out fair, with the cycle repeating daily. The monsoon typically loses its energy by mid-September when much drier conditions are reestablished over the region. Lachnet *et al*. [50] found that it was much weaker during the last glacial maximum but strengthened after about 11 ka B.P. It is currently becoming more extreme but with fewer thunderstorms [51].

In summary, monsoons generally bring moisture onshore to areas that would otherwise be deserts or semideserts. Only on the north slope of the Tibetan Plateau does it bring heat energy northwards.
