Natural Climate Change:A Geological Perspective
Cathles, Lawrence M.
If the 4.6 billion years the earth has been in existence is one day of geologic time, all of recorded earth history corresponds to the last one tenth of one second of that day- hardly enough time to form an opinion of what is going on in the room in which you are sitting, and hardly a good basis for forming a perspective on climate change. Geology provides the needed perspective. From direct evidence (glacial striations, ice-rafted drop stones, changes in pollen, ice cover, and sea level) and indirect evidence (changes in the isotopic composition of sediments and ice, and in the dust content of polar ice) geologists infer that the earth may have been alternatively completely frozen (-50C) and very warm (+50C) in Late Proterozoic time (800-600 million years ago). Thereafter the earth was warmer than present except for "ice ages" in Pennsylvanian and Permian time (~320-250 million years ago) and in the Quaternary (last 4 million years). Before the last decline in global temperature, England had a climate similar to that of Indonesia. Fossils of crocodiles and broad leaf tropical plants are preserved in the Eocene (40 million year-old) clays there. Continental glaciers developed in Antarctica ~34 million years ago and in Greenland ~8 million years ago. Starting about 2 million years ago continental glaciers periodically covered North America and Europe. Each glacial cycle was ~120,000 years in duration: 100,000 years of ice growth (ice-house conditions) and 20,000 years of warmer, interglacial, green-house conditions. The end of each interglacial was abrupt. The last glacial cycle started about 130,000 years ago. Ice reached Ithaca, New York about 45,000 years ago. At its maximum extent the ice reached to Long Island, and there was ~ 1 mile of ice over Ithaca. The ice age ended abruptly ~12,000 years ago and the last ice melted in Canada ~5,500 years ago. Our interglacial has probably been more uniform in temperature than previous interglacials, but non-the-less there have been significant and rapid climate changes within it. It was warmer than present during the Holocene Maximum (7000-4000 years ago) and during the Medieval warm period (1000-1400 AD), but colder that present during the Little Ice Age (1400-1860) when the Dutch skated on their canals and Washington and Jefferson commuted to Washington D.C. by sleigh. Recent geological studies suggest that recent minor climate changes (last 12,000 years) are caused by changes in solar energy output. These natural climate changes provide a context for discussing changes that might be caused by humans and for what can be meant by "sustainable". Of course climate change is real (40 million years ago England had the climate of Indonesia, but in the last few million years it has been subject to ice ages interspersed with interglacials). Human activity may be a factor, but other factors also operate. If we are to take out insurance (by controlling greenhouse gases, for example), we should consider what we wish most to insure against (another ice age or global warming) and how much we are willing to pay. My recommendation is to make commitments deliberately, wait for scientific clarification (which will come quickly), and avoid politicizing science. Objective science will be our best long-term protection.
A presentation to the Seminar on Sustainable Development NBA 573, BEE 673 Sage Hall B-11
Internet-First University Press
Earth and Atmospheric Sciences Department; Cornell University; Departmental History; Global Warming