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).
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