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.