Between Greenhouse and Icehouse

Abstract

There is a celebrated Flemish painting by Pieter Bruegel the Elder in the Kunsthistorisches Museum in Vienna. It depicts the age-old battle between Carnival and Lent. Carnival—a time of high spirits, led in this vision by a fat man on a beer-barrel, carousing and brandishing a pig’s head on a spit—is opposed by Lent, deflating the happy excitement and bringing in a time of sobriety and abstinence. Bruegel’s understanding of these opposed rhythms of rural life in the sixteenth-century Netherlands was acute: he was nicknamed ‘Peasant Bruegel’ for his habit of dressing like the local people, to mingle unnoticed with the crowds, all the better to observe their lives and activities. Bruegel’s vision of the age-old rhythm of life, in the form of an eternal oscillation between two opposing modes, may be taken to a wider stage. From the late Archaean to the end of the Proterozoic, the Earth has alternated between two climate modes. Long episodes of what may be regarded as rather dull stability, best exemplified by what some scientists refer to as the ‘boring billion’ of the mid-Proterozoic, are punctuated by the briefer, though more satisfyingly dramatic, glacial events. This alternation of Earth states persisted into the last half-billion years of this planet’s history—that is, into the current eon, the Phanerozoic. If anything, the pattern became more pronounced, as if it had become an integral part of the Earth’s slowly moving clockwork. There were three main Phanerozoic glaciations—or more precisely, there were three intervals of time when the world possessed large amounts of ice—though in each of these, the ice waxed and waned in a rather complex fashion, and none came close to a Snowball-like state. Thus, these intervals often now tend to be called ‘icehouse states’ rather than glaciations per se. Between these, there were rather longer intervals—greenhouse states—in which the world was considerably warmer; though again, this warmth was variable, and at times modest amounts of polar ice could form. Of the Earth’s Phanerozoic icehouse states, two are in the Palaeozoic Era: one, now termed the ‘Early Palaeozoic Icehouse’ centred on the boundary between the Ordovician and Silurian periods, peaking some 440 million years ago; and a later one centred on the Carboniferous and early Permian periods, 325 to 280 million years ago.