In his latest book Heaven and Earth (ConorCourt, 2009) Ian Plimer attempts to put a rigorous scientific foundation under the denailist case on anthropogenic global warming (AGW). His book has to date sold about 30,000 copies in Australia, and is due to be picked up by foreign publishers. I think it is set to become the bible of denialists world-wide, and a best-seller. So it deserves to be taken seriously, if only for that reason.
In the course of developing his case, Plimer also denies the existence of ‘tipping points’ in science, which I suppose means his personal recognition of them. “A popular catastrophist view”, he says, “is that as the climate warms, less and less CO2 will be dissolved in the oceans, and a ‘tipping point’ will be reached when the Earth enters a runaway greenhouse. Another ‘tipping point’ is that the oceans will become acid. Permanently. In fact there is no such thing as a ‘tipping point’ (or even a ‘precautionary principle’) in science. The use of these words in the popular media and by political advocates immediately advertises non-scientific opinions…” (Heaven and Earth p.338)
Plimer’s claim is that any acidification of the world’s oceans due to dissolved CO2 will be countered by the calcium carbonate and other minerals in abundant contact with the sea water. However, as he cites no references to show his sources of the above claims allegedly made by the unnamed catastrophists, I can venture no further comment. But I have yet to see any ‘alarmist’ or ‘catastrophist’ document predicting permanent ocean acidification.
(On the subject of the carbonic acid in the oceans being naturally neutralized, it depends on what time scale we are talking about. The most reactive base in the ocean is the carbonate ion of the calcium carbonate, much of which is in the skeletons of mollusks, corals and other invertebrates. They cannot be expected to remain unaffected if their hard parts are neutralising the acid. Nor does the reaction between carbonic acid and carbonate minerals or shells decompose or otherwise eliminate the carbonate. CO2 as gas, dissolved or airborne simply continues on. The olivines, pyroxenes and feldspars found in many rocks react very slowly with carbonic acid to form clay. A simple test: get a bottle of soda water (carbonic acid) put in some road gravel or other rock with unweathered faces, and watch it slowly weather before your eyes. Allow say, a hundred years for it to become clear that reaction is happening, and don’t hold your breath.)
So I beg to differ with him, and will shortly publish a series of articles offering a more detailed critique of his book. I will in the course of that return to the Precautionary Principle as science knows it.
But back to tipping points: While there may be none of them in Plimer’s science, there are nonetheless plenty in nature. It is called metastability  and covers phenomena as diverse as phase changes in matter, electronic circuitry and chemical systems and well as some phenomena found in nuclear physics and quantum mechanics. In a metastable system, quantitative change proceeds to a point beyond which further such change is irrelevant, because a completely new regime has arisen. This often happens catastrophically; particularly for those used to the former state. The concept has long been known, and is well summed up in the old Arab phrase ‘the last straw that broke the camel’s back.’ 
Consider the often-seen event suggested by the term itself: the dumping of a load of road fill or gravel by a tipper truck. As the tray of the truck is lifted out of the horizontal, the load commonly does not slide at first or otherwise move. But once a critical angle is exceeded, the load slides out with a rush. A tipping point has been passed. That point is created by three physical facts: (1) that the force of friction between the load and the tray diminishes as the tray rises, while (2) the down-slope weight or gravitational vector component of the load itself progressively increases (non-linearly) with the sine of the angle the tray makes with the horizontal, as the slope becomes ever steeper. At the point where its weight vector component overcomes friction, the load starts to slide; and (3) once doing so experiences a reduced frictional force opposing the sliding, and so accelerates under the net down-slope force. The truck operator has created an avalanche, a phenomenon also involving mountain terrain and snowfield tipping points. 
As has long been recognized, the dynamic friction between the surfaces of two bodies, say a book like Heaven and Earth and the table it rests upon, is less than the static friction between the same two. For example, if one is attempting to push start a car with a dead battery, it is easier to keep it rolling along a flat road than to get it rolling in the first place.
Or consider the eruption of a volcano. For a period of time, the strata overlying the magma chamber are able to resist whatever force is being exerted upwards from below. Then over a usually much shorter period they fail and give way, often with spectacular consequences. My favourite example is Paricutin in Mexico:
On the afternoon of February 20, 1943, Dionisio Pulido, a farmer in the Mexican state of Michoacan, was readying his fields for spring sowing when the ground nearby opened in a fissure about 150 feet long. “I then felt a thunder,” he recalled later, “the trees trembled, and is was then I saw how, in the hole, the ground swelled and raised itself 2 or 21/2 meters high, and a kind of smoke or fine dust gray, like ashes began to rise, with a hiss or whistle, loud and continuous; and there was a smell of sulphur. I then became greatly frightened and tried to help unyoke one of the ox teams.”
Virtually under the farmer’s feet, a volcano was being born. Pulido and the handful of other witnesses fled. By the next morning, when he returned, the cone had grown to a height of 30 feet and was “hurling out rocks with great violence. “During the day, the come grew another 120 feet. That night, incandescent bombs blew more than 1,000 feet up into the darkness, and a slaglike mass of lava rolled over Pulido’s cornfields.
See this link,  where a photo of the mountain that formed so rapidly in Pulido’s field can be seen. I hope Pulido was somehow compensated for the loss of his field.
Huge natural dams holding glacial meltwater may have collapsed in the past triggering rapid and global climate change. “Meltwater dams in far northern Canada collapsed 8,500 years ago, which was followed by a 500 year period of intense cold windy glacial climate.” This is from Plimer himself (Heaven and Earth p 48). In other words a series of tipping points were passed, with sudden catastrophic change as consequence. It is indeed strange that in the same book in which this is cited, Plimer claims there are no tipping points “in science’.
We are in one regime until weight, pressure or some other quantity exceeds a structural limit in the situation, and we transit to a new regime. We see another tipping point when a piece of wood or a pane of glass is distorted to the point where it suddenly breaks. Hooke’s Law, says that within the elastic limit, stress is proportional to strain. Take a small spring, such as a paper clip, and gently distort it. On release, it springs back into its former shape. Distort it too far, and it will no longer do so, because a tipping point (operating at the level of inter-crystalline and inter-atomic forces) has been passed.
Springs are also components of light switches. A light switch commonly has two positions, known to the householder as on and off, and to the physicist as separate metastable states. It takes work to get the switch over a potential energy hump set up by the force of resistance of the spring, and to make it pass from one state to the other. Whatever state the switch is in, the spring will oppose its going to the other: until the tipping point is passed going either way; then the spring helps the change. A common concern amongst climatologists is that the Earth’s climate system may have its own analogous metastable states, and that right now we may be passing from one to another: from a better state to a worse one, from which return may not be quick, be easy, or for that matter, be possible at all.
In the 1950s such a question was posed by a young geochemist, Wallace Broecker, who saw how cores drilled from ocean beds seemed to indicate that sea temperatures could change abruptly – in as little as a thousand years:
“Does the ocean-atmosphere system have more than one stable mode of operation?” Broecker’s question was already on the mind of computer modelers concerned with future climate change. Even before Broecker published his ideas, Kirk Bryan and others had been working up numerical simulations that included changes in ocean salinity as well as wind patterns. What they found was troubling. A 1985 study suggested that if the level of atmospheric CO2 jumped fourfold, the ocean’s thermohaline conveyor belt circulation could cease altogether. Another study found that even small perturbations could give rise to radically different modes of ocean circulation. In particular, a spurt of fresh water suddenly released from a melting continental ice sheet — the kind of event that some thought might have triggered the Younger Dryas — could switch the circulation pattern in as little as a century.
These studies were no more than suggestive, for the models of the mid 1980s were still extremely limited. The planet might be represented in the computer as, to take one example, three equal continents and three equal oceans, extending from pole to pole like the segments of a grapefruit, with the oceans all of uniform depth and the continents without mountains. To keep computation time within reason, Bryan had to hold the cloudiness constant, although he knew clouds would interact with climate change in crucial feedbacks. Along with all that, as Bryan remarked, “uncertainties abound concerning the interaction of the ocean circulation and the carbon cycle.”
Spencer Weart, Ocean Currents and Climate, at The Discovery of Global Warming 
Back to the library: Take a book, such as Heaven and Earth, and stand it upright on a table. It has a certain stability, but if given a gentle and continuous enough push will fall over. (A too gentle push will see it return to standing upright.) It will be seen from the simple geometry of the situation that the centre of gravity of the book has to rise slightly before it can fall. In other words, work has to be done on the book, and its gravitational potential energy has to be slightly raised. As the book falls, it may hit a second book similarly standing, and that book may hit a third, and so on: the familiar domino effect. Dominoes can also be set up so that the first knocks over two, each of those two knock over a further two and so on, to give a cascading effect which can be arranged to finish with hundreds of dominoes falling in the last step of the overall event.
Such a cascade of falling dominoes is routinely invoked as a model for a chemical or nuclear chain reaction, resulting in energy release far greater than that which triggered it, and in practice impossible to reverse. When a chemical or nuclear bomb goes off, that is what happens: a huge number of molecular or atomic tipping points have been crossed.
And from an email by Dr Andrew Glikson:
…Tipping points refer to the critical threshold at which a minor perturbation of the climate system can qualitatively alter the state or development of a system. In principle, early warning systems could be established to detect the proximity of some tipping elements.
Major consequences of the trend include warming in the north Atlantic and sea level rise. Other potential tipping elements include ):
1. Melting of the West Antarctica ice sheet.
2. Collapse of the Atlantic thermohaline circulation, regional cooling and southward
3. Shift of the inter-tropical convergence zone.
4. Intensification and rise in frequency of the El-Nino Southern Oscillation, with consequent droughts in SE Asia and elsewhere.
5. Retardation of the Indian summer monsoon and consequent droughts.
6. Extension of the Sahara/Sahel and West African monsoon one positive outcome of global warming.
7. Reduction of the Amazon rainforest and biodiversity loss
8. Reduction of the Boreal forest
9. Reduction of circum-Antarctic ocean vertical circulation (bottom water formation) associated with ocean warming, reducing ocean CO2 storage.
10. Increase in Tundra tree cover
11. Reduction in permafrost and in marine methane hydrates, threatening methane and CO2 release.
12. Oceanic anoxia and marine life extinction
13. Reduction in Arctic ozone and increase in UV radiation at the surface.
It’s the Sun: From Galileo to Fielding
Earth and paleoclimate research
Australian National University
One of the worst possible (non-climate) calamities in the offing is the expected collapse of the Cumbre Vieja volcano on the island of La Palma in the Canaries. According to Swiss researchers, there is potential for half a trillion tonnes of rock to avalanche into the Atlantic, which would in turn send a wave “650 metres high (2,130 feet) that would spread out and travel across the Atlantic at high speed.
“The wall of water would weaken as it crossed the ocean, but would still be 40-50 metres (130-160 feet) high by the time it hit land. The surge would create havoc in North America as much as 20 kilometres (12 miles) inland.” 
Similarly a collapse of the West Antarctic Ice Sheet (WAIS) would raise global sea levels by 3.3 metres (11 ft) according to one study. “Just how rapid the collapse of WAIS would be is largely unknown. If such a large mass of ice steadily melted over 500 years, as has been suggested in earlier studies, it would add about 6.5 millimeters or a quarter of an inch per year to sea level rise — about twice the current rate due to all sources.”
Needless to add: that would go on to change a lot of other present arrangements. 
But, then again, it could be worse.
About 74,000 years ago, the Toba volcano of Sumatra blew enough ash into the atmosphere to darken the sky around the whole planet, plunging temperatures by as much as 21 Celsius degrees at higher latitudes, and creating such devastation of the human population of the time that only a few thousand individuals appear to have survived world-wide. It was apparently the greatest genetic bottleneck our species has ever experienced. 
Another supervolcano lies dormant at the moment under Yellowstone National Park in the US. In 2001 geologists studying it reported the opinion that the next eruption could blanket half the United States in ash up to a metre deep. These explosions are apparently on a 600,000 year cycle, and it has been about 620,000 years since the last one.
Supervolcanoes have provided us with the greatest explosions ever to have happened in recorded history: Krakatoa in the late 19th C and the Greek island of Thera in the middle of the second millenium BC. But the greatest explosions we know about in the Universe are those of the supernovae: stars whose internal dynamics cross a tipping point resulting in a release of their thermonuclear energy not in the usual timespan of billions of years but over a few weeks or months. 
Notes and links.