Anthropocene

The notion that humankind has changed the world is not new. However, with terms such as anthropozoic, psychozoic, and noosphere proposed over the last century, the idea of humans as a new global forcing agent can be considered recent and closely linked with the proposition and dissemination of the term anthropocene. It was coined more than a decade ago by Paul Crutzen, one of the three chemists who shared the 1995 Nobel Prize for discovering the effects of ozone-depleting compounds.

The term anthropocene first appeared in the paper titled “The Anthropocene,” published in the International Geosphere–Biosphere Programme Newsletter in May 2000, where Crutzen and his colleague Eugene Stoermer—a professor at the University of Michigan—noted that many forms of human activity are now capable of undermining the natural environment. For instance, in the case in carbon and nitrogen cycles, the amount that is fixed synthetically competes with the amount that is fixed by the planet’s vegetation, land, and oceans.

In their paper, Crutzen and Stoermer argued for the appropriateness of the term anthropocene to characterize the current geological epoch, emphasizing the human footprints on the planet that have been seen in geology and ecology over the last decades. Two years later, in 2002, Crutzen restated the argument in a concise article titled “Geology of Mankind” in the prestigious journal Nature. According to him, the anthropocene could have begun in the latter part of the 18th century, based on analyses of air trapped in polar ice that showed the beginning of increasing concentrations of carbon dioxide (CO2) and methane at the global level.

After these two groundbreaking articles, the term gained popularity and began to appear more often in the scientific literature of a range of disciplines and topics, leading to a more profound and careful consideration of the term. Many consider that the term is not only vivid—as much for the public as for scientists—but that it was also coined at a time of growing realization that human activity was changing the Earth on an unprecedented scale, with changes that are now seen as permanent, even on a geological time-scale. These attributes influenced scientists to increasingly use the term anthropocene to denote the current interval of time, one dominated by human activity. However, many argue that the term remains informal and is not precisely defined.

Antarctic Ice Sheet

Ice covers much of the polar regions of Earth and is a critical component to the planet’s climate. An ice sheet is defined as a mass of ice that is greater than 31,000 mi. (50,000 km) in area, such as those of Antarctica and Greenland. Ice sheets should not be confused with ice caps, which are masses of ice covering less than 31,000 mi. (50,000 km) in area. It is estimated that approximately 90 percent of the Earth’s total ice mass is located in the Antarctic Ice Sheet, or 6.4 million cu. mi. (27 million cu. km). The Ross Ice Shelf and the Filchner–Ronne Ice Shelf are the world’s largest, both found in Antarctica. Scientists are concerned that increasing temperatures of air and sea will cause significant melting of the world’s ice, including the Antarctic Ice Sheet, and cause sea-level rise.

Ice sheets, glaciers, ice caps, permafrost, snow, and sea ice are all components of the Earth’s cryosphere (portions of the Earth’s surface that are frozen over land or water). In the Southern Hemisphere, the Antarctic Ice Sheet covers approximately 98 percent of the Antarctic continent and is the single largest mass of ice on the planet. Ice sheets form as snow and frost build up in an area, compressing the previously fallen snow into ice.

The total area of ice sheets is changed regularly by melting, primarily where the ice comes into contact with water or warmer dry land at its base; and by calving, or the falling off of large pieces of the ice sheet, which become icebergs. The Antarctic Ice Sheet covers the major landmass of the continent in the eastern Antarctic and extends over the ocean in western Antarctica, where the ice sheet is as deep as 2,500 mi. (4,023 km) below sea level. The Antarctic continent is cold year-round and is as dry as a desert, with little to no annual precipitation.

Historically, the Antarctic Ice Sheet has experienced very little melting from the surface. The oldest portions of the Antarctic Ice Sheet are estimated to be 15 million years old. Typically, the seasonal fluctuation that is experienced by the ice sheet is focused on the northern Antarctic peninsula and the northeastern regions of the ice sheet. Most ice from the Antarctic Ice Sheet is lost by calving of glaciers from the protruding ice shelves of the sheet. The total volume of the ice in this region is estimated to be 7.5 million cu. mi. (31 million cu. km).

Antarctic Circumpolar Current

The Antarctic Circumpolar Current (ACC), also known as the West Wind Drift, is the only current that flows completely around the globe, unimpeded by continents. Famous explorers have often referenced the ACC in their navigational logs, including Edmond Halley (the first to note the ACC in a voyage from 1699 to 1700), James Cook, James Clark Ross, Sir Francis Drake, and James Weddell. The ACC is notably the roughest sea crossing for navigators, particularly the 497 mi. (800 km) wide Drake Passage extending around Cape Horn and the Antarctic Peninsula. The role of the ACC as “mixer of the deep oceans” also has a significant impact on global climate.

The ACC, as the name implies, flows around the continent of Antarctica in an eastward direction driven by westerly winds through the Atlantic, Indian, and Pacific oceans. The ACC is as deep as 6,562–13,123 ft. (2,000–4,000 m) and as wide as 1,243 mi. (2,000 km), accounting for the vast transport of waters, despite its relatively slow eastward current. It is estimated that some of the seawater of the ACC travels the entire circumference of the globe (24,900 mi.) in a mere eight years. For comparison, the ACC carries 150 times more water around Antarctica than flows through all of the world’s rivers combined.

While flow of the ACC is not blocked by any landmasses, it is severely constrained by them. The borders of the ACC are further defined by convergence fronts with significant temperature and salinity variability. The greatest temperature change is north of the ACC in the Subtropical Convergence (Front), where the average sea surface temperature decreases from 54 degrees F (12 degrees C) to 45–46 degrees F (7–8 degrees C) in the ACC and salinity decreases from 34.9 or greater to 34.6 or less.

The southern boundary of the ACC is defined by the westward flowing Antarctic Coastal Current with a surface temperature around 30 degrees F (minus 1 degree C). Mean ACC temperature ranges from 41 to 30 degrees F (5 to minus 1 degree C). Climate change and ocean warming will likely have a significant effect on the ACC, because of this typically narrow temperature range. Any otherwise small increases in sea surface temperatures may induce dramatic effects on the system.

Albedo

Albedo originates from a Latin word albus, which means white. Albedo is the amount of sunlight (of all wavelengths) that is reflected back from an object or a substance. The more the amount of light reflected back, the brighter the color of the object. A lesser amount of light is reflected back from darker objects. The albedo of an object varies from 0 to 1. Black objects have zero albedo, while white objects have an albedo of one. Sometimes, it is also expressed in terms of percentage, 1–100. An ideal white body thus has an albedo of 100 percent, while an ideal black body has an albedo of zero percent. Some standard amounts of sunlight reflected from certain objects.

Usually, albedo is used in the field of astronomy to describe reflective properties of planets, satellites, and asteroids. There are two types of astronomical albedo: normal and bond albedo. Normal albedo is a measure of a surface’s brightness when illuminated and observed vertically, while bond albedo is defined as the fraction of total solar light reflected back to space and is a measure of a planet’s energy balance. The value of bond is defined over the entire range of wavelengths.

Surface reflectance values vary across the globe, mainly because of variation in the presence or absence of snow, ice, or clouds, which increases albedo values in those areas. The presence of ice and snow, for example, at the poles and the absence of snow and ice at the equator reflects the difference in albedo values at the poles and the equator.

The reflectance value (and hence the albedo value) changes with the change in dust concentration, thickness of the clouds (or amount of cloud cover), and zenith of sunlight falling in that zone, which is also reflected in seasonal variation in albedo value for the same region. This can be observed best at higher altitudes, where in winter the surface is covered significantly by snow (or ice), thus increasing the surface reflectance values; while in spring, when most of this snow (or ice) melts, the surface (bare soil) absorbs a lot more sunlight, thus decreasing the albedo values for the same place.

Albedo is an important concept in climatology. When albedo is expressed in percentages, snow has an albedo of 90 percent and charcoal has an albedo of 4 percent. When seen from a distance, the ocean surface has a low albedo as do most forests, while desert has one of the higher albedo values.

The role of the concept of albedo in climate change can be seen in the following example: Ice reflects back more sun radiation than water; with the snow cover getting smaller and the water in lakes (and seas and oceans) increasing, the amount of sunlight absorbed (and, hence, heat retained) is increasing, leading to further increases in the temperature of lake, sea, and ocean water. On the other hand, if more snow is formed, a cooling cycle starts. The amount of sunlight (radiation) absorbed or reflected back causes fluctuations in temperature, wind, ocean currents, and precipitation. In a way, the hydrological cycle changes with the fluctuations in temperature (which is related to how much evapotranspiration takes place). Also, the climate system equilibrium is dependent on the balance between the amount of solar radiation absorbed and the amount of terrestrial radiation emitted back to space.

Agulhas Current

The Agulhas Current is the major western boundary current of the Southern Hemisphere. It completes the anti-cyclonic gyre of the South Indian Ocean, and because the African continent terminates at a relatively modest latitude, it becomes a mechanism for the climatologically important inter-ocean exchange between the Indian and Atlantic oceans. The southwestward flowing Agulhas Current only becomes fully constituted along the east coast of southern Africa at a latitude somewhere between Durban (South Africa) and Maputo (Mozambique). It increases in speed and volume flux downstream. On average, its volume flux is 70 × 106 m3/s, with only small temporal changes. Its depth, by contrast, can vary from 6561 ft. (2,000 m) to the sea floor at 9,842 ft. (3,000 m) over a period of months. It is underlain by an opposing undercurrent at a depth of 3,937 ft. (1,200 m), with a maximum velocity of about 0.2 m/s and carrying about 4 × 106 m3/s equatorward.

An offshore profile of the surface speed of the current shows a peak of about 1.5 m/s close inshore, slowly tapering off to about 0.2 m/s at a distance of roughly 62 mi. (100 km) offshore. The temperature of its surface waters is about 11 degrees F (6 degrees C) higher than ambient waters and decreases from 80 to 71 degrees F (27 to 22 degrees C) from summer to winter.

Radar remote-sensing instruments

Any of a range of active REMOTE-SENSING instruments that propagate radio or microwave radiation and measure the BACKSCATTERING (echoes) that returns sometime later. Such systems can provide two types of information. By measuring the time taken for each echo to return to the sensor, distance (range) from the sensor can be determined.

At the same time, the intensity and POLARISATION of backscattered radiation may also be detected. There are three generic types of spaceborne RADAR instruments: altimeters, scatterometers and imaging radars. Altimeters (or nadir [downward] looking radars) are used to derive elevation profiles under the orbit track. These profiles are constructed by accurately measuring the time delay for a radar echo (pulse) to propagate to the surface and return back to the sensor. This has applications in mapping surface TOPOGRAPHY and in OCEAN and GLACIER monitoring. 

Scatterometers measure the radar cross section of a target (surface reflectivity), which is a function of how the target interacts with microwave radiation, and are typically used to measure wind speed and direction over water or detect rainfall. Imaging radars, such as synthetic aperture radar (SAR), are used to acquire high spatial resolution (a few metres to a few tens of metres) images, measuring range, intensity and sometimes polarisation of backscattered radiation over large areas. SAR instruments emit radar pulses as the platform moves and, by correcting the pulses for the transmission and reception times, an aperture can be synthesised that has a much greater size than the physical size of the antenna, which results in a finer spatial resolution. Radar instruments are generally composed of several parts: A transmitting source; an antenna, which shapes the transmitted energy into a beam pointing in a certain direction and collects energy from this direction; and equipment for processing and storing the data.

Wave Creation

As the wind moves across the surface of the water, some of the wind’s energy gets transferred into the water. The more energy the wind transfers to the water, the bigger the waves will be.

• The faster the wind blows (wind speed), the more energy the wind has, and the bigger the waves it can generate.

• The longer the length of time the wind blows (wind duration), the greater the amount of time it can transfer energy to the water, and the bigger the waves it can generate.

• The greater the distance over the water that the wind travels (fetch), the more opportunity there is for air–water interaction, and the bigger the waves that can be created.

Thus, high winds blowing across a long length of water for a long time can transfer a lot of energy, which will move as very large waves. Once waves are generated, they move across the surface of the water until they encounter resistance.

Waves

Waves are one of the most important processes in the coastal zone, especially on open shorelines. A wave is simply movement of energy. And it is the delivery of energy to the shoreline via waves that makes them so important. The word “wave” can be applied to a wide range of phenomena: radar waves, microwaves, earthquake waves, light waves, radio waves, sound waves, shock waves.

All of these “waves” are energy moving from one place to another. The energy that moves as a wave across water is almost certain to have come from the wind. However, it is possible for other sources of energy to produce a wave. Boats make waves called wakes; earthquakes can generate tsunamis; throwing a rock into a pond will produce ripples. Nevertheless, the moving air in the atmosphere is responsible for almost all the transfer of energy into the water.

Dangerous Semicircle

This term, now somewhat archaic, is given to that portion of a tropical cyclone in which the winds and rain are most intense. Primarily used by mariners, the term is derived from the practice of dividing a hurricane, typhoon, or cyclone into dangerous and navigable semicircles, or right and left halves, based upon the system’s forward motion. In the Northern Hemisphere, where tropical cyclones spin in a counterclockwise direction, an observer facing into the winds of an approaching hurricane will find the dangerous semicircle on the right, or eastern side, and the navigable semicircle on the left, or western side. The reverse is true, of course, in the Southern Hemisphere, where the clockwise spinning of a cyclone will yield a dangerous semicircle on the left, or western side, and a navigable semicircle on the right, or eastern half.

While in actuality both halves of a tropical cyclone are dangerous, the half that finds itself strengthened by both the forward speed of the system’s steering current and the storm’s own forward velocity will possess significantly faster winds and higher seas. For this reason, those mariners who have divided an oncoming hurricane, typhoon, or cyclone into dangerous and navigable semicircles have been better able to guide their vessels away from the storm’s most furious aspects, thus greatly improving their odds of surviving it.

Covalent Bonds

Recall that an atom is chemically stable when its outermost energy level is full. A state of stability is achieved by some elements by forming chemical bonds. A chemical bond is the force that holds together the elements in a compound. One way in which atoms fill their outermost energy levels is by sharing electrons. For example, individual atoms of hydrogen each have just one electron. Each atom becomes more stable when it shares its electron with another hydrogen atom so that each atom has two electrons in its outermost energy level.  How do these two atoms stay together? The nucleus of each atom has one proton with a positive charge, and the two positively charged protons attract the two negatively charged electrons. This attraction of two atoms for a shared pair of electrons that holds the atoms together is called a covalent bond.

Molecules A molecule is composed of two or more atoms held together by covalent bonds. Molecules have no overall electric charge because the total number of electrons equals the total number of protons. Water is an example of a compound whose atoms are held together by covalent bonds. The chemical formula for a water molecule is H2O because, in this molecule, two atoms of hydrogen, each of which need to gain an electron to become stable, are combined with one atom of oxygen, which needs to gain two electrons to become stable. A compound comprised of molecules is called a molecular compound. Polar molecules Although water molecules are held together by covalent bonds, the atoms do not share the electrons equally. The shared electrons in a water molecule are attracted more strongly by the oxygen atom than by the hydrogen atoms. As a result, the electrons spend more time near the oxygen atom than they do near the hydrogen atoms. This unequal sharing of electrons results in polar molecules. A polar molecule has a slightly positive end and a slightly negative end.

Ions

Sometimes atoms gain or lose electrons from their outermost energy levels. Recall that atoms are electrically neutral because the number of electrons, which have negative charges, balances the number of protons, which have positive charges. An atom that gains or loses an electron has a net electric charge and is called an ion. In general, an atom in which the outermost energy level is less than half-full — that is, it has fewer than four valence electrons — tends to lose its valence electrons.

When an atom loses valence electrons, it becomes positively charged. In chemistry, a positive ion is indicated by a superscript plus sign. For example, a sodium ion is represented by Na+. If more than one electron is lost, that number is placed before the plus sign. For example, a magnesium ion, which forms when a magnesium atom has lost two electrons, is represented by Mg2+.

An atom in which the outermost energy level is more than half-full — that is, it has more than four valence electrons — tends to fill its outermost energy level. Such an atom forms a negatively charged ion. Negative ions are indicated by a superscript minus sign. For example, a nitrogen atom that has gained three electrons is represented by N3‒. Some substances contain ions that are made up of groups of atoms—for example, silicate ions. These complex ions are important constituents of most rocks and minerals.

Isotopes

Recall that all atoms of an element have the same number of protons. However, the number of neutrons of an element’s atoms can vary. For example, all chlorine atoms have 17 protons in their nuclei, but they can have either 18 or 20 neutrons. This means that there are chlorine atoms with mass numbers of 35 (17 protons + 18 neutrons) and 37 (17 protons + 20 neutrons). Atoms of the same element that have different mass numbers are called isotopes. The element chlorine has two isotopes: Cl-35 and Cl-37. Because the number of electrons in an atom equals the number of protons, isotopes of an element have the same chemical properties.

Scientists have measured the mass of atoms of elements. The atomic mass of an element is the average of the mass numbers of the isotopes of an element. Most elements are mixtures of isotopes. The atomic mass of chlorine is 35.453. This number is the average of the mass numbers of the naturally occurring isotopes of chlorine-35 and chlorine-37.

Radioactive isotopes The nuclei of some isotopes are unstable and tend to break down. When this happens, the isotope also emits energy in the form of radiation. Radioactive decay is the spontaneous process through which unstable nuclei emit radiation. In the process of radioactive decay, a nucleus can lose protons and neutrons, change a proton to a neutron, or change a neutron to a proton. Because the number of protons in a nucleus identifies an element, decay changes the identity of an element. For example, the isotope polonium-218 decays at a steady rate over time into bismuth-214. The polonium originally present in a rock is gradually replaced by bismuth.