The concept of a “tipping point” – a threshold beyond which a system shifts to a new state – is becoming a familiar one in discussions of the climate.

Examples of tipping points are everywhere: a glass falling off a table upon tilting; a bacterial population hitting a level where it pushes your body into fever; the boiling point of water, or a cube of ice being thrown into warm water, where it rapidly melts.

The ice cube is a poignant example, because scientists now fear that West Antarctica’s ice sheets are also heading towards irreversible melting.

Likewise, the recent discovery of deep canyons beneath the Greenland ice sheet raises concerns regarding its stability.

The history of the atmosphere, oceans and ice caps indicates that, once changes in the energy level which drive either warming or cooling reach a critical threshold, irreversible tipping points ensue.

An example is a process called “albedo flip”, where a small amount of melting creates a film of water on top of the ice. The water absorbs infrared radiation and melts more ice, leading to runaway melting of ice sheet. The opposite process occurs where the freezing of water results in reflection of radiation to space, leading to cooling and freezing of more water.

Other examples are abrupt warming episodes during glacial states, termed “interstadials”, for example the “Dansgaard-Oeschger” warming cycles which occurred during the last glacial period between about 100,000 and 20,000 thousand years ago, which caused large parts of the North Atlantic Ocean to undergo temperature changes of several degrees Celsius within short periods. Other examples are points at which a glacial state ends abruptly to be replaced by rapid glacial termination.

Over the threshold

An increase in global temperatures can lead to a threshold representing the culmination and synergy of multiple processes, such as release of methane from permafrost or polar ocean sediments, retreating sea ice and ice sheets, warming oceans, collapse of ocean current systems such as the North Atlantic Thermohaline Current and – not least – large scale fires.

A major consequence of warming of ice sheets is the increase in supply of cold fresh melt water to adjacent oceans, such as the abrupt cooling of the North Atlantic Ocean inducing rapid freezing events (stadials), as represented by the “Younger dryas” event (12,900-11,700 years ago), or the rapid melting of Laurentian ice cap about 8500 years ago and related abrupt cooling events in Europe and North America.

The question is whether the post-18th century global warming trend may culminate in a major tipping point or, alternatively, is represented by an increase in disparate extreme weather events, as are currently occurring around the world.

A potential indicator of such tipping point may be represented by a collapse of the North Atlantic Thermal Circulation, which would lead to a sharp, albeit transient, temperature drop in the North Atlantic Ocean, North America and Western Europe. Evidence for a weakening of the North Atlantic deep water circulation by about 30% between 1957 and 2004 has been reported in Nature as well as by other researchers.

The question of tipping points is of critical importance since it affects future climate projections and adaptation plans. In this regard the latest Intergovernmental Panel on Climate Change report leaves the question of tipping points open.

The crucial question

So how likely is the current climate change trend to reach a tipping point, and if so of what magnitude and on what time scale?

General circulation climate models which attempt to delineate overall future climate trends are limited in their capacity to predict the precise timing, location and magnitude of abrupt climate and weather events with confidence.

Since the 19th century the rise in the energy level of the atmosphere has reached a level of more than 3 degrees Celsius when the masking effects of sulphur aerosols are discounted. This degree of temperature rise is just under the energy rise level associated with the last glacial termination between about 16,000 and 10,000 years ago.

The atmosphere-ocean system continued to warm following the peak El-Nino event of 1998. Most of the warming occurred in the oceans, whose mean temperature has risen by about 0.3C since 1950.

The current rise in atmospheric CO 2 of about 2 parts per million CO2/year, reaching 401.85 parts per million at the Mauna Loa Observatory in Hawaii in May 2014, exceeds rates observed in the geological record of the last 65 million years.

An atmospheric CO 2 level of 400 parts per million is estimated for the Miocene, about 16 million years ago, when mean temperatures have reached 3 to 4 degrees Celsius above those of pre-industrial temperatures. Economically available fossil fuel reserves, if used, are capable of returning the atmosphere to tropical state such as existed during the early to mid-Eocene prior to the formation of the Antarctic ice sheet about 32 million years ago.

The evidence indicates that, since the mid-1980s, the Earth is shifting from a climate state that favoured land cultivation since about 7000 years ago to a climate state characterised by mean global temperatures about 2-3 degrees Celsius above pre-industrial levels.

At this level, extreme weather events would render large parts of the continents unsuitable for agriculture. The accelerated melting of the Greenland and west Antarctic ice sheets could lead to conditions akin to those of the Pliocene, before 2.6 million years ago, when sea level were between 5 and 40 metres higher than at present, as estimated by the US Geological Survey.

The evidence indicates the climate may be tracking toward – or is already crossing – tipping points whose precise nature and timing remain undefined, depending on the extent to which ice sheet melting is retarded due to hysteresis. The increase in frequency and intensity of extreme weather events around the globe may represent a shift in state of the atmosphere-ocean system. There is no alternative to a global effort at deep cuts of carbon emissions coupled with fast-tracked CO 2 sequestration.

As Professor Joachim Schellnhuber, Germany’s climate advisor and Director of the Potsdam Climate Impacts Institute, has said: