Geoscience Reference
In-Depth Information
Circles and cycles
It is in the lowest 15 kilometres of the atmosphere, the troposphere, that most of the
action takes place. This is where weather happens. It's where clouds form and disperse and
where winds blow, transferring heat and moisture around the planet. In a dynamic planet,
everything seems to go round in circles, flows of energy. And here, close to the surface,
these cycles are driven by solar power. There are the obvious cycles of day and night as
the Earth spins on its axis and the ground alternately heats and cools, and the annual cycle
of the seasons as the Earth orbits the Sun, presenting first more of one hemisphere then
more of the other to the sunshine. But there are longer cycles too, such as the wobble of the
Earth's axis over tens of thousands of years.
Just as the Earth orbits the Sun, so the Moon orbits the Earth. It takes about 28 days to
complete an orbit, giving us our months. As the Earth spins on its axis, the Moon's gravity
pulls a bulge in the oceans around the planet, creating tides. This also acts as a brake on
the rotation of the Earth, slowing down the days. Daily growth bands in fossil corals 400
million years old suggest that their days were several hours shorter than our own.
The Moon helps to stabilize the orbit of the Earth and hence the climate. But there are far
longer cycles at work too. The Earth's orbit around the Sun is not a perfect circle, but an el-
lipse, with the Sun at one focus. Hence the distance of the Earth from the Sun varies during
its orbit. In addition, the degree of variation itself changes over a 95,800-year period. Also
the Earth's rotation axis slowly wobbles or precesses like a spinning top off balance. Over
a period of 21,700 years the planet's axis traces out a complete cone. At present, the Earth
is nearest to the Sun during the northern hemisphere winter. The inclination of the Earth's
spin axis with that of its orbit around the Sun (the obliquity) also changes on a 41,000-year
period. These so-called Milankovich cycles add up over tens or hundreds of thousands of
years to affect climate. They have been blamed for such phenomena as the ice ages that
have affected the Earth over the last three and a quarter million years. But the reality is
probably even more complex, with their effects amplified or reduced by factors such as
ocean circulation, cloud cover, atmospheric composition, volcanic aerosols, the weathering
of rocks, biological productivity, and so on.
Solar cycles
Cycles of change are not restricted to the Earth. The Sun can change too. Over its 5-billion-
year history, the Sun has been getting progressively warmer. Surface temperatures on Earth
have remained more constant, however, as levels of greenhouse gases have fallen over the
same time. This was largely due to the effects of life, as plants and algae consumed carbon
dioxide that acted like a blanket to keep the young Earth warm. There have been other sol-
