What climate tipping points should we be looking out for?
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…
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 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.
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.
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.
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.
An atmospheric CO2 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.
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 CO2 sequestration.
As Professor Joachim Schellnhuber, Germany’s climate advisor and Director of the Potsdam Climate Impacts Institute, has said:
We’re simply talking about the very life support system of this planet.