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Climate change occurs when the global energy balance between incoming energy from the Sun and outgoing heat from the Earth is upset.
However, the global climate is also affected by other flows of energy which take place within the climate system.
This climate system is made up of the atmosphere, the oceans, the ice sheets, living organisms and the rocks, which all affect, to a greater or less extent, the movement of heat around the Earth's surface.
Within such long term cycles there exist much shorter climatic fluctuations over tens and hundreds of thousands of years, driven by changes in the Earth's orbit around the Sun.
Over the shortest time scales of centuries, decades and even individual years, global climate is influenced by solar variability, aerosols emissions and changes in atmospheric greenhouse gas concentrations.
An abrupt climate change occurs when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause.
An abrupt climate change takes place over a few decades or less, persists for at least a few decades, and causes substantial disruptions in human and natural systems.
Abrupt climate changes may occur over a region, a hemisphere, or the entire globe.
For a visual analogy of an abrupt climate change, imagine a landscape with two valleys and a ball sitting in one of these valleys (animated below).
A gradual push (due to forcing mechanism) is given to the ball and it begins to roll up the hill. If the push is not strong enough, the ball stops midway up the hill and rolls backward to its original stable position. This is called climate variability
With a stronger push, the ball rolls up the hill and, suddenly, the ball tops the hill (unstable state) and rolls down the other side into the second valley. An abrupt change to a new stable state has occurred.
Note that the new “stable state is at a higher level (temperature) than the former stable state.
An example of a simple positive feedback in everyday life is the growth of an interest-earning savings account. As interest is accrued the principal will begin to grow. As the principal grows, even more interest will be accrued, quickening the rate of principal growth.
An example of a simple negative feedback is your body's cooling mechanism. When your body temperature rises, you begin to sweat. The evaporation of this sweat from your skin cools your body and your temperature returns to normal.
Examples of Feedbacks when atmospheric temperature increase
Recall that methane is 60 times more powerful than CO 2 as a greenhouse gas but only remains in the atmosphere for about ten years and so looses it's greenhouse effect quickly compared to CO 2 which remains in the atmosphere for 100 years.
Methane can be “frozen” as methane hydrate deposits in the cold, high pressure environment at the bottom of the ocean.
It is estimated that there is more carbon locked away as ocean methane hydrates than all of the oil and gas reserves of the world combined.
Methane can also be trapped by permafrost layers which over-lay lower unfrozen layers of vegetable material that is decaying and producing methane which remains trapped by the frozen permafrost on top.
If the permafrost layer were to melt then the methane in the layers below would escape into the atmosphere.
Given the vast areas of permafrost in the Arctic Regions there is a significant potential for methane to be released if the permafrost melted as a result of global warming.
Clouds can produce both positive and negative feedback in the climate system
An increase in global temperature will increase evaporating from the oceans which leads to the formation of clouds.
Negative Feedback : Low, thick clouds primarily reflect solar radiation and cool the surface of the Earth.
Positive Feedback : High, thin clouds primarily transmit incoming solar radiation; at the same time, they trap some of the outgoing infrared radiation emitted by the Earth and radiate it back downward, thereby warming the surface of the Earth.
The thermohaline circulation is the part of the global ocean circulation that is driven by geographic differences in the density of sea water, which are controlled by temperature (thermal) and salinity (haline).
In the North Atlantic this circulation transports warm and salty water from the tropics to the north (See Next Slide).
There, the water cools and releases heat to the atmosphere, warming the North Atlantic region.
Once the water loses heat, it becomes cooler and more dense, sinking into the deep ocean.
This deepwater flows slowly southward (~0.1 m/s) near the bottom of the ocean basins and gradually returns to the surface as a result of wind-driven upwelling near Antarctica and slow diffusive upwelling over the rest of the global ocean.
It then joins near-surface currents to be returned to the areas of deepwater formation.