76 / Basics of Environmental Science
Potassium-40, a radioactive isotope of potassium with a half-life of 1300 million years, also occurs
naturally (and, because of its presence in our food, is the principal source of our own exposure to
radiation). Most^40 K decays to^40 Ca, which cannot be used because calcium is so common, but about
11 per cent decays by a different route to^40 Ar (argon). This decay is used to date rocks more than
250000 years old.
A radioactive isotope of rubidium,^87 Rb, decays in a single step to strontium (^87 Sr) and this decay is
used to date certain rocks, especially those containing mica and potassium, but there is some doubt
about the half-life of^87 Rb. Two values are used: 4.88×10^10 and 5.0×10^10 years. A more recent method
uses the decay of samarium (^147 Sm) to neodymium (^143 Nd). Samarium-147 has a half-life of 2.5×10^11
years and this decay is used in studies of the formation of rocks in the Earth’s crust and mantle (and
can also be used on materials of extraterrestrial origin).
It is impossible to predict when an individual unstable atom will decay, but it is possible to calculate
the probability that the atom will decay within a certain period. This is called the ‘decay constant’
for the isotope, from which the half-life can be calculated as the time taken for the decay of half the
unstable atoms present. The process is exponential: half the atoms decay in the first half-life period,
half the remainder in the second period, half of that remainder in the third, and so on (e.g. 100; 50;
25; 12.5, etc.). Most of the decays used are based on half-lives much longer than the age of the Earth,
but it is not necessary to wait until a complete half-life has elapsed before calculating an age. What
matters is the ratio of isotopes.
Since radioactive decay involves only the nucleus of the atom, its rate is not affected by temperature,
pressure, or any other outside influence. This makes it a very reliable measure of the age of materials.
Radiometric dating has allowed scientists to reconstruct the history of the Earth in some detail.
20. Climate change
Milutin Milankovich (1879–1958) spent most of his career as a mathematician and physicist working
at the University of Belgrade, where he devoted thirty years to comparing the amount of solar radiation
received in different latitudes over the last 650000 years with the climates during that time. He
discovered a clear relationship between solar variability and the incidence of ice ages that is now
accepted by most climatologists. Presented as a graph, it is known as the Milankovich solar radiation
curve (geography.miningco.com/library/weekly/aa121498.htm).
We picture the Earth spinning on its axis and orbiting the Sun in a very regular fashion. So it does,
but within its regularity there are slow, cyclical variations. Milankovich identified three that affect
climate when they coincide to maximize or minimize insolation.
The first cycle, illustrated in Figure 2.26, concerns the Earth’s orbital path. Much exaggerated in the
diagram, this varies from almost circular to slightly more elliptical. In other words, the path stretches,
varying the distance between the Earth and Sun at perihelion and aphelion. Starting at any date, it
takes about 95000 years for the orbit to move through the full cycle and return to its initial path.
Clearly, a variation in the distance between the Earth and Sun affects the intensity of radiation
received at the Earth’s surface and, therefore, the climates of Earth.
The second cycle occurs because the axis wobbles, describing a circle, rather like a toy gyroscope
(see Figure 2.27). It is this wobble, due to gravitational attraction, mainly from the Sun and
Moon, that causes the position of the equinoxes to move westward, taking 25800 years to
complete one orbit. The phenomenon is called the precession of the equinoxes. At present