Climate History and the Great Geophysical Experiment

Climate history is an important resource in the study of the Great Geophysical Experiment being performed on the planet, that is, the large-scale release of greenhouse gases (carbon dioxide and methane, and also others). Climate history is crucial in establishing that man-made global warming is an historic fact. The probability that recent warming has occurred independently of greenhouse gases derived from human activity is very small. The task is to make the best possible assessment of potential effects of global warming on regional agriculture and public health. The Great Experiment is a test of the response of the climate system to disturbance. It is also a test of the ability of human society to deal with environmental problems on a global scale.In history there is a rich store of experience concerning climate change, which we need to mobilize to assess the risks of the future. Information can be derived on many different time scales: from the last 1000 years (containing the “Little Ice Age”), from deglaciation (containing the enigmatic “Younger Dryas” cold spell) and even from periods millions of years ago. Climate history is not supposed to deliver analogs, but to expand our thinking and make us ask pertinent questions. Perhaps the worst of the bad-case scenarios associated with global warming is the notion that ocean warming can lead to large-scale release of methane from the sea floor. Apparently it did in the distant past, at the end of the Paleocene, 55 million years ago. Venting of methane is proceeding right now. The question is, will it accelerate.Traditional historians are reluctant to admit a strong role for climate in human history. However, as climate is becoming a product of human activities, it automatically becomes part of history; climate has entered the narrative of what humans do to each other.