One of the most important areas of question is if the clouds and/or water vapor are a negative or positive feedback overall. In other words, do they warm or cool the earth?

In AR4 (2007) the IPCC stated that “much research” is currently being done “to understand how clouds change in response to climate warming, and how these changes affect climate through various feedback mechanisms”. (AR4 FAQ 1.3 Page 115). 

This research continues in AR5 as it dedicates chapter 7 to clouds and aerosols to the fact there exist the “largest uncertainty to estimates and interpretations of the Earth’s changing energy budget”. (AR5 Chapter 7 Executive Summary Page 573) 

Their research in AR5 focuses on the fact clouds and water vapor can be both a positive and a negative feedback.

So, how can clouds both cool or warm the earth? The IPCC notes there are many reasons why depending on many characteristics like different cloud altitudes, water content, size, shape, and more. (AR4 FAQ 1.3 Page 115)

The IPCC tells us in AR5 of these the biggest question is the effect of “warming on low clouds”. (AR5 Section 7.2.5.4 Page 590)

The IPCC notes it has been suggested a negative feedback forms when there is more moisture in the atmosphere resulting in the creation of even more clouds, giving us cooler and cooler temperatures. (AR5 Section 7.2.5.4 Page 590)

However, the IPCC in AR5 makes an interesting statement pushing the idea clouds are a positive feedback warming the earth more and more. This is a change from AR4 as the IPCC now notes two reasons.

First, they note a couple studies in early 1980s showing clouds were a negative feedback ignored the following argument “the physics of crucial cloud-regulating processes such as precipitation formation and turbulence”. (AR5 Section 7.2.5.4 Page 590) 

On this maybe someone can explain to me the “physics of crucial cloud-regulating processes”? The IPCC does not, and to date I have found no explanation on how this proves a positive feedback. 

Second, they state “Observational evidence discounting a large effect of this kind was reported in AR4”. I am not sure what they mean here because I did not find anything in AR4 regarding this. (AR5 Section 7.2.5.4 Page 590) 

The IPCC explains here they are making improvements with clouds. However, they note “biases in cloud simulation led to regional errors on cloud radiative effect of several tens of watts per square meter.” (AR5 Chapter 9 Executive Summary Page 743) 

However, the IPCC admits they have ‘low confidence remains in the observation of global-scale cloud variability and trends.” (AR5 Section 2.5.6.1 Page 208) 

Seems to me the other stance by the IPCC that clouds and water vapor are the “largest uncertainty to estimates and interpretations of the Earth’s changing energy budget” is a more accurate statement.

All this is an important in other ways because it is believed for every 1°C rise in temperature this will increase moisture by 7% in the atmosphere making even more clouds, causing even more feedback.

Here in AR5 the IPCC states moisture “should go up 7% for every 1°C increase in temperature.” (AR5 Section 2.5.5.3 Page 207)