Science File -

                        Figure 1:  A simple mechanical analogy (Source NAS, 2002)

A simple mechanical analogy is shown above.  The mechanism consists of a curved track balanced on a fulcrum. If a ball is placed on the track it is free to move until it reaches one or other of the extremes.  This system has two stable points denoted (a), and (c) in the top row of Figure 1. The middle point (b) is unstable: if the ball is moved slightly to one side or another, the displacement will progress until the ball lands in one extreme or other and far from its original position. If the ball in contrast is at the extreme points and is moved slightly it will rock back and forth a little until settling back into the original position. 

If the arm is pushed down in a small displacement as shown in (a₁) and released, the ball will roll back and forth a bit and then settle into the original state.  The system is stable for small perturbations.

If the arm is pushed further (a₂) the ball will roll over the fulcrum and slide down into the other extreme.  The system has passed a threshold and cannot return to the original state unless pushed past the threshold in the other direction. 

Climate has multiple equilibria (multiple (b) points) with constant, and more or less eccentric, departures.  Which is a third possibility in the simple mechanical analogy.  Hitting the track hard enough can cause the ball to jump out and roll under the sofa to join with the dust mites and old potato crisps in plotting an overthrow of the prevailing domestic regime entirely.  Such a possibility, metaphorically speaking, has prompted Wally Broecker of the Scripps Institution of Oceanography to characterise human greenhouse gas emissions as poking a stick at an angry and unpredictable climate beast.

Jonathon Adams, of the Environmental Sciences Division of the Oak Ridge National Laboratory, and colleagues suggest that abrupt climate change is ‘a disturbing scenario to be borne in mind when considering the effects that humans might have on the climate system through adding greenhouse gases. Judging by what we see from the past, conditions might gradually be building up to a 'break point' at which a dramatic change in the climate system will occur over just a decade or two, as a result of a seemingly innocuous trigger.’

The classic example of abrupt climate change in the paleoclimatic record is the period of the Younger Dryas occurring around 12,800 to 11,500 years ago. 

       Figure 2:  Temperature and Ice Accumulation in Greenland Spanning the Period of the Younger Dryas to Modern Times (Source NAS, 2002)

There was a sudden shift out of glaciation some 15,000 years ago.  This was followed by an almost equally rapid cooling that culminated in the extreme cold period of the Younger Dryas lasting for 1300 years.  The consequences of a modern descent into a similar climate mode, over as little as a decade, are unthinkable.  Surface temperatures drive evaporation over oceans – as temperature cools aridity increases and there is less snow accumulation.   

A review by Adams et al of abrupt paleoclimatic change can be seen at the Quaternary Environment Network website.  ‘It is still unclear how the climate on a regional or even global scale can change as rapidly as present evidence suggests. It appears that the climate system is more delicately balanced than had previously been thought, linked by a cascade of powerful mechanisms that can amplify a small initial change into a much larger shift in temperature and aridity. At present, the thinking of climatologists tends to emphasize several key components.

• North Atlantic circulation as a trigger or an amplifier in rapid climate changes.
• Carbon dioxide and methane concentration as a feedback in sudden changes.
• Surface reflectivity (albedo) of ice, snow and vegetation.
• Water vapour as a feedback in sudden changes.
• Dust and particulates as a feedback in sudden changes.
• Seasonal sunlight intensity as a background to sudden changes.’

 I would add some decadal sub-systems to the mix.  Such as changes in solar UV intensity causing ozone heating and cooling and influencing ice cloud formation above the poles.  This in turn influencing downwelling in the polar vortices which shows up in storm tracks and therefore lower latitude temperature and precipitation in both the southern and northern hemispheres.  Downwelling in the Antarctic Vortex also feeds into sea surface flows up the Antarctic Peninsula to the west coast of South America thus influencing the thermal evolution of the El Niño Southern Oscillation and therefore the strength of Walker and Hadley cell circulation and thereby global temperatures, global precipitation and extratropical cloud formation.  There are many more possible sub-systems including convection, oceanic phytoplankton, land surfaces, atmospheric waves, mountain updrafts, adiabatic cooling, cosmic rays and cloud formation and obscure shifts in sea surface temperature.