Runaway Climate Change - Feedbacks

Feedbacks

The core of the concept of runaway climate change is the idea of a large positive feedback within the climate system. When a change in global temperature causes an event to occur which itself changes global temperature, this is referred to as a feedback effect. If this effect acts in the same direction as the original temperature change, it is a destabilising positive feedback (e.g. warming causing more warming); and if in the opposite direction, it is a stabilising negative feedback (e.g. warming causing a cooling effect). If a sufficiently strong net positive feedback occurs, it is said that a climate tipping point has been passed and the temperature will continue to change until the changed conditions result in negative feedbacks that restabilise the climate.

An example of a negative feedback is that radiation leaving the Earth increases in proportion to the fourth power of temperature, in accordance with the Stefan-Boltzmann law. This feedback is always operational; therefore, while it may be overridden by positive feedbacks for comparatively small temperature changes it will dominate for larger temperature changes. An example of a positive feedback is the ice-albedo feedback, in which increasing temperature causes ice to melt, which increases the amount of heat that Earth absorbs. This feedback only operates in a restricted range of temperatures (those for which ice exists, and does not cover the whole surface; once all the ice has melted, the feedback ceases to operate).

Climate feedback effects can be from:

• The same cause as the forcing (e.g. rising methane levels causing more methane to be released)
• Another greenhouse gas (e.g. CO₂ causing methane release)
• On other variables (e.g. ice-albedo feedback)

Without climate feedbacks, a doubling in atmospheric carbon dioxide (CO₂) concentration would result in a global average temperature increase of around 1.2°C. Water vapor amount and clouds are probably the most important global climate feedbacks. Historical information and global climate models indicate a climate sensitivity of 1.5 to 4.5°C, with a best estimate of 3°C. This is an amplification of the carbon dioxide forcing by a factor of 2.5. Some studies suggest a lower climate sensitivity, but other studies indicate a sensitivity above this range. Partly because of the difficulty in modeling the cloud feedback, the true climate sensitivity remains uncertain.