Positive Feedbacks and Non-Linear Response
The invention of an efficient steam engine (by James Watt, 1736-1819) brought profound changes to the way humans live on the planet, as did the invention of efficient internal combustion engines (by Gottlieb Daimler, 1834-1900 and Rudolf Diesel, 1858-1913). The discovery of petroleum spawned an enormous chemical industry, and made whales obsolete as a source of oil. None of these developments could have been foreseen by the most brilliant philosophers thinking a century ahead of their time.
In analogous fashion, we can be quite sure that climate physicists thinking ahead a hundred years (aided by their computers whose programs they wrote using current knowledge) will be unable to predict events that are fundamentally unexpected. Does climate do unexpected things? The simple answer is Yes, it does. The reason is two-fold: (1) positive feedback and (2) non-linear response. As was discussed in Lesson 1, positive feedback occurs when a small change in the system produces additional change in the same direction. For example, warming in high latitudes melts snow and ice, which in turn darkens the ground (or the sea), allowing absorption of more sunlight and producing additional warming. A positive feedback tends to enhance the response of the system to changes in forcing. In contrast, a negative feedback tends to mute the response of the system to changing forces.
(A) The structure of methane ice
, whereby a molecule
of methane sits in a cage-like structure made of water molecules and (B) a photo of an actual piece of methane ice (the scale on the bottom is in centimeters). From: USGS
Feedback mechanisms also introduce nonlinear response, a phenomenon whereby the strength of the response depends on the previous history of the system. For instance, there may be thresholds where the response changes drastically, like when permafrost melts and the arctic forest on it dies, thus changing albedo irreversibly in ways difficult to guess before the event. What kind of surprises may be lurking ahead, monstrous or not, no one can say. The only way to increase our awareness of strange possibilities is to study past climate change. The further back we look, the more unusual events we are confronted with, challenging our understanding of climate dynamics in situations well out of bounds of experience or imagination.
Methane Ice & Other Uncertainties
There is evidence, from the carbon isotope record in deep-sea sediments of the early Tertiary, that large amounts of methane were released to the ocean (and the atmosphere) on short notice on at least one occasion. The possibility that such releases happened sporadically, upon warming a previously cold ocean, definitely exists and is being looked into by a number of geologists. The mechanism that is thought responsible is the melting of methane-bearing ice on the sea floor that goes by several names including "methane ice," "methane clathrate" or methane hydrate. Such ice forms where organic matter is being fermented by bacteria, and where the ocean is cold enough (and the pressure high enough) to make ice in the presence of high methane concentrations, ice that has an open structure which can accommodate the gas . Once methane ice is destroyed on a large scale, the greenhouse effect resulting from injecting the methane into the atmosphere could result in much additional warming, generating more release of methane. This would be a classical run-away positive feedback.
The total amount of methane in gas hydrates on the sea floor is not known. A general ballpark estimate is 10,000 Gt of carbon (1 gigaton is 1015g), more than ten times all the carbon in atmosphere, and more than the readily recoverable coal and petroleum.
Another major worry is a rapid rise of sea level, from collapse of a major glacier on Greenland, or the removal of support from glaciers in the western Antarctic. The support is from grounded ice sheets, locked on knobs on the sea floor. If the sea level rises, the support weakens. Many geologists think the sea level was somewhat higher 5000 years ago, and that it could easily go up by several meters.