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A GLOBAL OUTLOOK ON METHANE GAS HYDRATES
57
Figure 3.4:
Future change in bottom-water temperatures at the
sea floor. Changes are given in °C per 100 years as predicted
by the Kiel Climate Model (KCM) (Park
et al.
2009), for a pCO
2
increase scenario (1 per cent increase until current-day values
are doubled). Values are an ensemble average of eight individual
model realizations starting at different initial states.
Future change in bottom-water
temperatures at the sea floor
Source: adapted from Park
etal.
2009
0
-1
2
1
2-
Temperature change °C
Future change in bottom-water
temperatures at the sea floor
Source: adapted from Park
etal.
2009
0
-1
2
1
2-
Temperature change °C
3.5.1
Oceanic response to climate
change
Marine gas-hydrate deposits occur in sediments under 300-500
metres or more of water and at a significant depth beneath the
sea floor. As a result, the most important climate change con-
sideration for hydrate dissociation is the possible warming of
bottom waters. Heat conduction is the primary heat transfer
process from the atmosphere into the ground in terrestrial set-
tings, but a number of processes can transport heat from the sea
surface into the ocean’s interior. These include vertical mixing,
convection of water masses and changes in ocean circulation.
First-order predicted trends in bottom-water temperatures over
the next 100 years are shown in Figure 3.4. Bottom-water tem-
peratures could increase by up to 2 °C in shallow water along
continental margins by the end of this century, but significantly
smaller temperature changes are predicted for deep-sea set-
tings. However, new result show that even during cold stadials,
persistent intermediate water warming existed (Ezat
et al.
, 2014)
making future scenarios more difficult to predict. Gas hydrates
occurring at shallow burial depths or as outcrops around the
continental margins could experience significant warming over
the coming decades and centuries. The largest bottom-water
warming is predicted for the Arctic Ocean, where large areas
of sea floor are affected by changes in the relatively warm Atlan-
tic waters flowing into the European Nordic seas and the Arctic
Ocean (Biastoch
et al.
2011). In some Arctic locations, shallow
bottom waters may warm by up to 5 °C by 2100 (Fig. 3.5).
The increase in bottom-water temperatures is slowed by the high
heat capacity of seawater and by slow communication between
surface waters and the deep ocean. Atmospheric temperature in-
creases will however, over the coming centuries and millennia,
raise bottom-water temperatures. The long-term effect of global
warming on sea-floor temperatures has been evaluated by Fyke
and Weaver (2006). According to their model, the bottom-water
temperature at continental margins will eventually increase by
about 4 °C, and as reported by Ruppel (2011), approximately 3.5
per cent of world’s gas hydrate could be dissociated over the next
century due to bottom-water warming (see Section 3.6).
In addition to changes in ocean temperature, the global sea
level will rise in response to global warming. Sea level rise in-
3.5
RESPONSE OF GAS HYDRATES
TO CLIMATE CHANGE