The sensitivity of the climate system is largely determined by feedback mechanisms. Feedbacks occur within the system that can either dampen or amplify the response to external factors affecting the climate. In other words, the effect of an initial process causes changes in a second process, which then influences the initial one. Negative feedbacks dampen the original process, and positive feedbacks amplify it.
Often a system can be ‘self-regulating’ as negative feedbacks reduce processes enough to create stability within the system. This is called a negative feedback loop. An example of this stabilising loop is a Thermostat. Suppose you set the target temperature to 18 °C. The higher the target, the greater the temperature gap (between the actual room temperature and the target room temperature). The greater the gap, the more heat is used to warm the room. As the room warms up the temperature gap goes down. The gap keeps going down until it is zero, at which point the room has reached the target temperature.
There are a few of these negative feedbacks within the climate system. One is the solubility pump of the ocean. Carbon dioxide (CO2) in the atmosphere can be regulated by the oceans as it dissolves in water. Currently, about 33% of CO2 emitted to the atmosphere is “absorbed” by the oceans. But this regulating process has a limit; absorption can take centuries and the efficiency is dependent on ocean circulation. Another important feedback relates to Earth’s outgoing radiation. As the temperature of the Earth increases, more infrared radiation is emitted back into space. This is well understood and based on physical laws, so is easily incorporated into climate models.
An example of positive feedback is world population. As more children are born, more grow up into adults and become parents. This in turn produces more children, and so on. So if the birth rate stays constant over time (in other words, the parents have the same number of children in each generation), then the total population of each generation will become larger than the previous.
In the climate system, positive feedbacks will accelerate a response, to make the climate much warmer or colder. An important positive feedback is that of water vapour. Although water vapour is a greenhouse gas, it has very little effect on the external factors controlling the climate, unless “pushed” from within. If the atmosphere starts to warm, the amount of water vapour in the atmosphere will increase. This will then slowly increase the greenhouse effect, reducing the amount of heat able to escape from Earth. The atmosphere warms further, enabling more water vapour to be held in the atmosphere, and so on in an accelerating positive feedback loop.
Another such feedback also includes a greenhouse gas. Methane is a gas trapped in frozen peat bogs (mostly in Siberia) and as “water ice” (clathrate) under sediment on the sea floor. If the atmospheric temperature and ocean temperature increases, the peat bogs and clathrates will thaw and release methane. This will in turn cause greater warming of the atmosphere, similar to water vapour and CO2, and further methane will be released.
Other positive feedbacks include ice-albedo, forest dieback, and clouds. The accelerating effects of positive feedback loops can be susceptible to irreversible tipping points, and these are outlined below.
Generally speaking, a climate tipping point is a critical threshold. It is a concept that describes non-linear changes to the climate, whereby changes are not steady and predictable. At a particular moment in time, a small change within a global climate system can transform a relatively stable system to a very different state of the climate. Similar to a wine glass tipping over, wine is spilt from the glass as the tipping event occurs and standing up the glass will not put the wine back; the state of a full wine glass becomes a new state of an empty glass.
Identifying what phenomena are capable of passing tipping points can be tricky. “Tipping elements” is used to describe large-scale components of the Earth System which may be subject to tipping points. Below are potential tipping elements in the climate system identified by Lenton et al. (2008).
Arctic sea-ice – Sea-ice provides a large white surface, reflecting solar radiation away from Earth, known as “albedo”. As sea-ice melts it exposes the much darker ocean surface, which absorbs radiation rather than reflects it. This, in turn, amplifies warming. Studies of the Earth’s energy balance suggest different stable states of sea-ice cover are possible, however large Arctic sea-ice loss in the summer may reach a critical, irreversible threshold.
Greenland and West Antarctic Ice Sheets – Ice sheets produce a similar albedo effect as sea-ice. They are much thicker than sea-ice though, being up to several kilometres thick and penetrating into the colder, higher altitude atmosphere. As ice sheets melt at their edges, their albedo is reduced, but they can also get thinner. The Greenland Ice Sheet is thinning and as it thins its surface lowers, subjecting it to warmer temperatures at lower altitudes. In West Antarctica, the ice sheet is largely resting on rock below sea level. Ocean water is able to melt floating ice but also undercut ice sheets, forcing more ice to float and therefore melt. Palaeo-records show that both Greenland and West Antarctic Ice Sheets have dramatically melted and collapsed in the past, indicating they are probably susceptible to tipping points.
Atlantic Deep Water formation – Cold, deep water is produced in the North Atlantic as part of the thermohaline ocean circulation. To maintain the ocean “conveyor belt” which transports warm ocean water towards Britain, heavy salty water must sink in the north. However, when ice sheets in the north (such as the Greenland Ice Sheet) melt, they release freshwater into the Atlantic. An input of freshwater makes the ocean less salty and less heavy, reducing the amount of Deep Water produced and slowing down the ocean conveyor belt. As ice sheets are susceptible to rapid melt, it means Deep Water formation and ocean circulation are probably vulnerable to a critical tipping point as well.
El Nino Southern Oscillation (ENSO) – ENSO is the name given to a certain state of an ocean-atmospheric relationship. It originates over the Pacific Ocean and influences temperatures and rainfall across neighbouring Asia and the Americas. The occurrence and strength of ENSO vary over years and decades. Climate models currently indicate that if the global climate was to pass into a different state, the ENSO may become much stronger. But it is not yet certain when a threshold might be reached, or whether this tipping point would be gradual or rapid. Similar relationships between ocean and atmosphere can significantly influence the strength and location of monsoons. Palaeo-records indicate that both the Indian Summer Monsoon and West African Monsoon have shifted in the past, causing large changes in rainfall and vegetation.
Amazon Rainforest – Rainfall in the Amazon Basin is largely recycled from moisture within the rainforest. It is also influenced by the ENSO mentioned above. If the strength and occurrence of the ENSO increases, the Amazon will be subjected to prolonged dry periods. Increased drying will cause higher dieback of the rainforest. A tipping point could arise where a certain amount of dieback stops the effective recycling of precipitation to the rest of the rainforest, resulting in rapid dieback. Another susceptible forest is the Boreal Forest. This is a sub-Arctic ecosystem which stretches across Europe and Siberia. The growth and stability of the forest is largely controlled by the physiology of the trees, the frozen ground (permafrost) and fire. Intense summers will lead to water stress on the trees, causing trees to die and making the forest susceptible to fire. Dieback of forests will, in turn, potentially create a tipping point for CO2 in the atmosphere as there will be less of the biosphere to take up CO2.