FROZEN HEAT | Volume 1

3.5 RESPONSE OF GAS HYDRATES TO CLIMATE CHANGE

3.5.1 Oceanic response to climate change

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).

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

In addition to changes in ocean temperature, the global sea level will rise in response to global warming. Sea level rise in-

Future change in bottom-water temperatures at the sea floor Future change in bottom-water temperatures at the sea floor

Temperature change °C Temperature change °C

2- 2-

-1 -1

0 0

1 1

2 2

Source: adapted from Park etal. 2009 Source: adapted from Park etal. 2009

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.

A GLOBAL OUTLOOK ON METHANE GAS HYDRATES 57

Made with