UNDER THE MICROSCOPE:WE HEAR A lot these days about climate change, all in the context of a perceived man-made forcing of climate change. But looking at the bigger picture, we see that the earth has always experienced, and will continue to experience, natural cycles of climate change. There have been at least four major ice ages in the planet's history (including those of 2.5 billion years ago, 700 million years ago and 450 million years ago). We are presently living in a warm interval in an ice age, but severe cold will surely return – the question is how soon?
The term “ice age”denotes a long-term reduction in earth’s temperature, with the expansion of continental and polar ice sheets and alpine glaciers. Within an ice age, colder periods (glacials) and warmer periods (interglacials) occur regularly. Glacials last about 100,000 years, separated by intervening interglacials, each lasting about 12,000 years. We are now living in an interglacial period in an ice age that began about 2.6 million years ago. The last glacial period ended about 10,000 years ago, which means that severely cold weather should soon return. However, some geologists argue that the next glaciation will not occur for another 12,000 years.
Much evidence that ice ages have occurred comes from geology and palaeontology. Geological features such as the scouring and scratching of rock by moving ice-sheets, valley-cutting, glacial moraines and drumlins all point to past ice ages. Palaeontology shows changes in the geographical distribution of fossils that one would expect from ice ages – cold-adapted organisms spread into lower latitudes while warm-adapted organisms become extinct.
What causes ice ages and their associated glacial and interglacial periods? The consensus is that several factors are involved, including atmospheric composition, changes in earth’s orbit around the sun, tectonic plate movement, variable solar output, earth/moon dynamics, and volcanic activity. There is evidence that greenhouse gases (carbon dioxide, methane) fell at the start of previous ice ages and rose at the end, but it is hard to know what is cause and what is effect. Some argue that our current artificial pumping of greenhouse gases into the atmosphere is beneficial as it will prevent a sudden onset of the upcoming glacial. But it would be very unwise to allow such thinking to persuade us to ignore greenhouse-gas emissions. The next glacial may not occur for thousands of years, whereas disastrous consequences may result within 100 years from a continuation of current greenhouse-gas emissions.
Continents move on the surface of the globe on great tectonic plates. Ice ages seem to begin when continents are in positions that block/reduce circulation of warm water from the equator to the poles, allowing ice sheets to form. The ice reflects solar radiation, reducing absorption of heat, causing the ice sheets to grow faster, reflecting more solar radiation – a positive feedback loop. The spread of ice reduces weathering of rocks and photosynthesis in oceans (both a great “sink” for atmospheric carbon dioxide), allowing carbon dioxide to build up in the atmosphere. Eventually the enhanced greenhouse effect ends an ice age.
Long-term climate data show a strong correlation between earth’s glacial and interglacial periods and three astronomical cycles known as Milankovitch (after Serbian mathematician Milutin Milankovitch) cycles. These cycles include the tilt of the earth’s axis in its plane of orbit around the sun (it varies over a 41,000-year period), the shape of the earth’s orbit around the sun (which cycles between being more or less elliptical over a period of 100,000 years), and the Precession of the Equinoxes, also known as the earth’s wobble, which rotates the direction of the earth’s axis over a period of 26,000 years (like the wobble of a spinning top as it runs down). These cycles reinforce each other at times and partially cancel each other out at times. The net effect doesn’t change the amount of sunlight received by the earth but redistributes it, affecting the intensity of seasons. Thus, ice sheets grow when summers become too cool to melt off all the previous winter snowfall. The extra ice reflects more sunlight back into space, further cooling the earth and encouraging further ice growth.
The sun is gradually emitting more energy – an estimated 10 per cent increase every one billion years. This cannot be causing ice ages. But there are short-term variations, the best-known of which are sunspot cycles. These cycles may also help to explain the cycles of glacials and interglacials.
During the last glaciation much of northern Europe and North America lay under ice. Ireland was entirely covered by ice, up to 1km thick in places. A glacial period will return and perhaps soon. Now and then we should take a break from worrying about global warming and think how we might cope with a massive ice-sheet.
William Reville is associate professor of biochemistry and public awareness of science officer at UCC – understandingscience.ucc.ie
