Why do we assume that positive feedback mechanisms are driving global warming?

Generally, the temperature increase due to a doubling of CO2 (280 ppm to 560 ppm) should result in a global average temperature increase of about 1.5 degrees C. Most of the predictions for future warming lie in the 3 to 11 degree C range, although a few people predict that we could have uncontrolled... show more Generally, the temperature increase due to a doubling of CO2 (280 ppm to 560 ppm) should result in a global average temperature increase of about 1.5 degrees C.

Most of the predictions for future warming lie in the 3 to 11 degree C range, although a few people predict that we could have uncontrolled warming.

Given this difference between CO2 forcing and predicted temperature increases, there is an implication that there is some sort of positive feedback mechanism.

Is it reasonable to assume that there is a positive feedback mechanism at work? Especially in light of the fact that almost all natural systems exhibit negative feedback mechanisms (broad generalization) that tend to dampen the effects of perturbations to a system.

I realize that H2O is often cited as the mechanism, but there is a high degree of uncertainty due to the fact that water vapor can be a driver of cooling AND warming.

Is there historical precedence for assuming that these positive feedbacks exist?
Update: 1.5 degrees C pops into my head as the number that I see associated with CO2 only forcing. Here is a source that puts it at 1.2 degrees C "The sensitivity of the climate system to a forcing is commonly expressed in terms of the global mean temperature change that would be expected after a time sufficiently... show more 1.5 degrees C pops into my head as the number that I see associated with CO2 only forcing. Here is a source that puts it at 1.2 degrees C

"The sensitivity of the climate system to a forcing is commonly expressed in terms of the global mean temperature change that would be expected after a time sufficiently long for both the atmosphere and ocean to come to equilibrium with the change in climate forcing. If there were no climate feedbacks, the response of Earth's mean temperature to a forcing of 4 W/m2 (the forcing for a doubled atmospheric CO2) would be an increase of about 1.2 °C (about 2.2 °F). However, the total climate change is affected not only by the immediate direct forcing, but also by climate “feedbacks” that come into play in response to the forcing."

Climate Change Science: An Analysis of Some Key Questions, pp 6-7,
Committee on the Science of Climate Change
National Research Council

This document is a little old (2001) but it should still be relevant.
Update 2: If the IPCC doesn't take feedback mechanisms into account, how can you be sure that their estimates are conservative? There should be negative feedbacks as well as positive feedbacks. Not including any feedbacks tells me that they are neither conservative nor excessive in their estimates, but rather they are... show more If the IPCC doesn't take feedback mechanisms into account, how can you be sure that their estimates are conservative? There should be negative feedbacks as well as positive feedbacks. Not including any feedbacks tells me that they are neither conservative nor excessive in their estimates, but rather they are unable to make reliable estimates at this time.
Update 3: Keith-->

Thanks for the links, I hadn't seen those before.

I like the UMich site, it tries to lay out the interplay of a number of feedbacks.
Update 4: Cosmos: I thought volcanic events were cooling events, no warming events. I also thought that the permian extinction was due to either vulcanism or large meteor impact choking out the sun and causing a precipitous decrease in temperature. Could you send me a link regarding this warming volcanic event and its... show more Cosmos:

I thought volcanic events were cooling events, no warming events. I also thought that the permian extinction was due to either vulcanism or large meteor impact choking out the sun and causing a precipitous decrease in temperature.

Could you send me a link regarding this warming volcanic event and its rapid reversal?
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