1. Cloud Feedbacks

 

Climate models unanimously predict an increasing trend for the atmospheric CO2 concentration, and therefore, a warming climate. Five major cloud feedbacks, as well as the water vapor and lapse rate feedbacks on a warming climate are assessed in this chapter. It is illustrated in Figure 7.11 (above).

 

• Changes in high-level cloud altitude

 

    A warming climate will result in more water vapor in the atmosphere due to stronger convection and reduced lapse rate in tropical area. This indicates rising of high cloud tops and melting level, associated with a drying cloud base. Based on the ‘fixed anvil-temperature’ theory, the top of the cloud layer remain at ~200 K (Earth’s brightness temperature) no matter how much the global surface warms. In other words, the Earth’s outgoing energy is insensitive to the temperature increase at the Earth’s surface (high confidence).

 

• Effects of hydrological cycle and storm track changes on cloud systems

 

    Most models simulate a pole-ward shift of large-scale circulation in a warmer climate (high confidence). This means the clouds shift to higher latitudes that receive less solar radiation and consequently less planetary albedo. In addition, models also simulate reduced upward mass flux in deep clouds, which then indicates decreased cloudiness in storm tracks and in tropical ocean rainfall zones (medium confidence).

 

• Changes in low-level cloud amount

 

    As in AR4, modeled low-cloud feedbacks contain the greatest uncertainty in cloud feedbacks. It is likely to be a positive feedback, but mechanisms unclear (low confidence).

 

• Microphysically induced opacity changes

 

    Model results indicate a latitude-dependent opacity change – cloud optical depth slightly decreasing at low and middle latitudes, while increasing pole-ward. This is near-zero net feedback, caused by model parameterization of phase changes in high latitude clouds and model simulated pole-ward moisture transport and circulation shift. However, due to our poor knowledge on cloud physics, especially for mixed-phase and ice clouds, the simulation is associated with low confidence.

   

• Changes in high-latitude clouds

 

    The pole-ward shift of clouds simulated by models suggests increased cloudiness in high-latitude. Cirrus clouds are the major cloud type at high latitude. As discussed before, cirrus has a net warming effect due to its weaker reflection on shortwave radiation compared with its downward emission of longwave radiation. Thus, this is a positive feedback (high confidence).

 

• Water vapor and lapse rate feedbacks

 

    Compared with AR4, much more confidence has been gained on the water vapor and lapse rate feedbacks on warming climates. It has been recognized that the traditional (adopted in AR4) derivation of significant feedbacks of water vapor (positive) and lapse rate (negative) based on specific humidity is way too arbitrary. In the troposphere of a warmer climate, the enhanced outgoing longwave radiation (OLR) due to water vapor will be mostly counteracted by the reduce lapse rate, arriving at a near-zero net feedback. However, the enhanced OLR due to other greenhouse gases and clouds doesn’t have a counteracting part like the lapse rate to water vapor. A new framework based on relative humidity rather than absolute water vapor mixing ratio has been established in AR5 to re-evaluate the water vapor/lapse rate feedback, which turns out to be more robust [Ingram 2010; Ingram, 2013a,b]. Regardless of these advances, the water vapor feedback in the stratosphere still remains greatly uncertain.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

    In summary, the net cloud feedbacks from the five major feedback estimates is +0.6 (-0.2 to +2.0) W m^-2 °C^-1, with a 17% probability of a negative feedback (very likely, 90% probability) (Figure 7.10, above). The water vapor and lapse rate feedback as a whole is estimated to be +1.1 (+0.9 to +1.3) W m^-2 °C^-1 (90% probability) (Figure 7.9, below).