Positive feedbacks that amplify climate change

 

The water vapour feedback

Water vapour is a greenhouse gas (GHG), as it absorbs outgoing longwave radiation (heat radiation) from Earth. Unlike other GHGs such as carbon dioxide (CO2) and methane, water vapour levels in the atmosphere cannot be controlled or altered directly by human activity. Instead, the amount of water vapour in the atmosphere is a function of the temperature of the atmosphere. There is a physical limit to how much water vapour air can hold at a given temperature, with warmer air able to hold more moisture than cooler air. For every additional degree Celsius in air temperature, the atmosphere can hold about 7% more water vapour. When air becomes saturated with water vapour, the water vapour condenses and falls as rain or snow, which means that water vapour does not reside for long in the atmosphere. When an external forcing agent, such as increases in atmospheric CO2, causes climate warming, the rise in temperature both increases the evaporation of water from the surface of the Earth and increases the atmospheric water vapour concentrations. This increased water vapour, in turn, amplifies the warming from the initial CO2-induced forcing. Therefore, water vapour provides a strong positive climate feedback in response to changes initiated by human emissions of other GHGs (Boucher et al., 2013).

 

The snow/ice albedo feedback

Snow and ice are bright, highly reflective surfaces. While open water reflects only about 6% of incoming solar radiation and absorbs the rest, snow-covered sea ice reflects as much as 90% of incoming radiation. This value decreases to 40%–70% during the melt season, due to melt ponds on the ice surface (see Figure 2.4; Perovich et al., 1998; Perovich et al., 2007). Climate warming decreases the amount of snow and ice cover on Earth, reducing the Earth’s albedo (reflectivity). Darker land and water surfaces exposed by melting snow and ice absorb more incoming solar radiation, adding more heat to the climate system and amplifying the initial warming, in turn causing further melting of snow and ice. Increased absorption of solar energy over the ocean is particularly important, as this additional heat must be dissipated in the autumn before ice can form again, thereby delaying the date of freeze-up. This positive climate feedback is particularly important in the Northern Hemisphere, where declines in Arctic Ocean sea ice and snow cover have been strong (see Chapter 5, Sections 5.2 and 5.3). In combination with other feedbacks involving the ocean, atmosphere, and clouds, the snow/ice albedo feedback explains why temperatures across the Arctic have warmed at approximately twice the rate of the rest of planet (Overland et al., 2017; Pithan and Mauritsen, 2014; Serreze and Barry, 2011).