Mauna Loa, on the island of Hawaii, has been well known for generations as the largest active volcano in the world, averaging one eruption every 3 1/2 years. Its gentle slopes tower to 13,681 feet above sea level, and the mountain itself covers more than half the island.
But among the meteorological community, Mauna Loa has become widely known in recent decades for quite a different reason; its fame rests on a small diagram which appears in virtually every learned article on climatic change, and which has no doubt been prominently displayed in recent days at the climate conference in Japan.
Simplistically, the diagram is just a wavy line heading upwards on a graph at an angle of about 30 degrees to the horizontal. But this is an important line; it shows the significant increase which has taken place over the last 30 years or so in carbon dioxide levels in our atmosphere.
The readings are taken at Mauna Loa Observatory, perched precariously on the lava-laden slopes of the great mountain, well above the tree line. It is of particular interest to scientists for two reasons: it is far removed from sources of industrial air pollution, and it provides one of the longest continuous and reliable records available of the atmospheric concentration of carbon dioxide.
The graph shows some interesting patterns. The upward slope illustrates a simple fact of which everybody now is well aware - even if we do not remember the precise numbers: that carbon dioxide concentration in the atmosphere has increased from 315 parts per million (ppm) in 1958 to its present value of around 360. But the graph is not a straight line.
It proceeds upwards in a very regular zig-zag pattern, each zig and zag, as it were, corresponding to the annual cycle of atmospheric carbon dioxide.
This annual rhythm is associated with seasonal changes in vegetation over the land masses of the northern hemisphere. During the summer, when there is an abundance of plants to "breathe in" the carbon dioxide, the proportion of the gas in the atmosphere steadily decreases; from November to April the atmosphere is required to absorb more carbon dioxide than can be removed - so the concentration increases again.
And if the record at Mauna Loa is examined very closely, a marked diurnal - or daily - variation can be seen as well. Day-time levels of carbon dioxide drop below 300 ppm, because the process of photosynthesis allows green plants to draw in carbon dioxide. By contrast, the concentration after dark often exceeds 400 ppm, as a result of respiration by animals and plants and the absence of sunlight to facilitate photosynthesis.