Feedback mechanisms can, as already stated, lead to a whole range of new possibilities which we never considered before.
If we suppose that we have a 'perfect' planet; one in which there is no atmosphere other than carbon dioxide and the star around which it orbits with no change in angle never changes in its activity then, by adding carbon dioxide, we will increase the temperature (to be more accurate, it increases the radiative forcing of carbon dioxide in the atmosphere; the effective additional wattage of energy in or out provided by its presence).
The forcing given by additional carbon dioxide is positive (it effectively increases the energy coming in or, at least, not leaving) logarithmic in scale; if we continue adding the same absolute amount then it will have less of an effect each time. After a while we will reach a theoretical 'maximum' at which point any additional carbon dioxide will have a negligible effect, negligible in this case meaning 'so close to zero it doesn't really matter any more'.
In our hypothetical planet, then, carbon dioxide can only provide so much heating before it makes no difference to add any more, and it will probably end at some reasonable temperature.
However, this is when we strip out any factors which might change things. When we take into account other factors, such as feedback mechanisms, it looks a little different.
Suppose we have a few trillion (10^18) tonnes of methane in a frozen form on this planet. They will have little or no effect on radiative forcing when frozen, so there's little need to worry if the temperature either remains stable or decreases.
However, if the temperature increases enough then we may see this methane being unfrozen and reverting to gaseous form.
It will then enter the atmosphere and cause a further positive shift in radiative forcing; methane is a very powerful greenhouse gas.
We then see a higher temperature as a result of this methane than we would see from carbon dioxide or the methane alone; we have the radiative forcing effect of first carbon dioxide and then the methane on top of it.
We have a scenario which couldn't have been arrived at without the methane feedback mechanism, hence the new possibilities.
Of course, it works both ways. For example, an increase in carbon dioxide which could cause an explosion in the plant population sufficient to withdraw all carbon dioxide from the atmosphere; this would cause a massive cooling as the carbon dioxide's forcing effect is lost.
This is another scenario which can't be arrived at if feedback mechanisms are not considered.
If we suppose that we have a 'perfect' planet; one in which there is no atmosphere other than carbon dioxide and the star around which it orbits with no change in angle never changes in its activity then, by adding carbon dioxide, we will increase the temperature (to be more accurate, it increases the radiative forcing of carbon dioxide in the atmosphere; the effective additional wattage of energy in or out provided by its presence).
The forcing given by additional carbon dioxide is positive (it effectively increases the energy coming in or, at least, not leaving) logarithmic in scale; if we continue adding the same absolute amount then it will have less of an effect each time. After a while we will reach a theoretical 'maximum' at which point any additional carbon dioxide will have a negligible effect, negligible in this case meaning 'so close to zero it doesn't really matter any more'.
In our hypothetical planet, then, carbon dioxide can only provide so much heating before it makes no difference to add any more, and it will probably end at some reasonable temperature.
However, this is when we strip out any factors which might change things. When we take into account other factors, such as feedback mechanisms, it looks a little different.
Suppose we have a few trillion (10^18) tonnes of methane in a frozen form on this planet. They will have little or no effect on radiative forcing when frozen, so there's little need to worry if the temperature either remains stable or decreases.
However, if the temperature increases enough then we may see this methane being unfrozen and reverting to gaseous form.
It will then enter the atmosphere and cause a further positive shift in radiative forcing; methane is a very powerful greenhouse gas.
We then see a higher temperature as a result of this methane than we would see from carbon dioxide or the methane alone; we have the radiative forcing effect of first carbon dioxide and then the methane on top of it.
We have a scenario which couldn't have been arrived at without the methane feedback mechanism, hence the new possibilities.
Of course, it works both ways. For example, an increase in carbon dioxide which could cause an explosion in the plant population sufficient to withdraw all carbon dioxide from the atmosphere; this would cause a massive cooling as the carbon dioxide's forcing effect is lost.
This is another scenario which can't be arrived at if feedback mechanisms are not considered.
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